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- 1. Chapter 8 The Cellular Basis of Reproduction and Inheritance 0
- 4. 0
- 5. 0
- 9. 0 Prokaryotic chromosome Duplication of chromosome and separation of copies Cell wall Plasma membrane 1
- 10. 0 Prokaryotic chromosome Duplication of chromosome and separation of copies Cell wall Plasma membrane 1 Continued elongation of the cell and movement of copies 2
- 11. 0 Prokaryotic chromosome Duplication of chromosome and separation of copies Cell wall Plasma membrane 1 Continued elongation of the cell and movement of copies 2 Division into two daughter cells 3
- 12. 0 Prokaryotic chromosomes Binary fission of a dividing bacterium
- 15. 0 Plant cell just before division
- 16. 0 Centromere Chromosome duplication Sister chromatids Chromosome distribution to daughter cells Sister chromatids Centromere
- 18. 0 S (DNA synthesis) G 1 G 2 Cytokinesis Mitosis I NTERPHASE M ITOTIC PHASE (M)
- 22. 0 INTERPHASE
- 25. 0 PROPHASE
- 27. 0 PROMETAPHASE
- 28. 0 Centrosomes (with centriole pairs) Kinetochore Early mitotic spindle Chromatin INTERPHASE PROMETAPHASE PROPHASE Centrosome Fragments of nuclear envelope Plasma membrane Chromosome, consisting of two sister chromatids Nuclear envelope Spindle microtubules Nucleolus Centromere
- 30. 0 METAPHASE
- 32. 0 ANAPHASE
- 34. 0 TELOPHASE AND CYTOKINESIS
- 35. 0 Metaphase plate Nucleolus forming METAPHASE TELOPHASE AND CYTOKINESIS ANAPHASE Cleavage furrow Daughter chromosomes Nuclear envelope forming Spindle
- 37. 0 Cleavage furrow Contracting ring of microfilaments Daughter cells Cleavage furrow
- 38. 0 Cell plate Daughter cells Cell wall Vesicles containing cell wall material Daughter nucleus Cell plate forming Wall of parent cell New cell wall
- 41. 0 G 1 checkpoint Control system M S G 2 G 1 M checkpoint G 2 checkpoint G 0
- 43. 0 G 1 checkpoint Control system M S G 2 G 1 Receptor protein Signal transduction pathway Relay proteins Plasma membrane Growth factor
- 46. 0 A tumor grows from a single cancer cell. Cancer cells spread through lymph and blood vessels to other parts of the body. Cancer cells invade neighboring tissue. Tumor Glandular tissue Lymph vessels Blood vessel
- 51. 0 Sister chromatids One duplicated chromosome Centromere Homologous pair of chromosomes
- 53. 0 Haploid gametes ( n = 23) n Egg cell Sperm cell Fertilization Meiosis Multicellular diploid adults (2 n = 46) Mitosis and development n 2 n Diploid zygote (2 n = 46)
- 61. 0 Centrosomes (with centriole pairs) PROPHASE I Microtubules attached to kinetochore INTERPHASE Sites of crossing over Metaphase plate Spindle MEIOSIS I : Homologous chromosomes separate METAPHASE I Sister chromatids remain attached ANAPHASE I Nuclear envelope Sister chromatids Centromere (with kinetochore) Homologous chromosomes separate Chromatin Tetrad
- 62. 0 Cleavage furrow TELOPHASE II AND CYTOKINESIS
- 67. 0 PROPHASE I MEIOSIS II : Sister chromatids separate METAPHASE II ANAPHASE II Cleavage furrow TELOPHASE II AND CYTOKINESIS Sister chromatids separate Haploid daughter cells forming TELOPHASE II AND CYTOKINESIS
- 70. 0 Prophase Metaphase I Metaphase 2 n = 4 Tetrads align at the metaphase plate Duplicated chromosome (two sister chromatids) Parent cell (before chromosome duplication) Chromosome duplication Chromosomes align at the metaphase plate Anaphase Telophase Sister chromatids separate during anaphase Daughter cells of mitosis 2 n 2 n n Chromosome duplication Site of crossing over Tetrad formed by synapsis of homologous chromosomes M EIOSIS Prophase I Anaphase I Telophase I M ITOSIS M EIOSIS I Haploid n = 2 Daughter cells of meiosis I M EIOSIS II n n n Daughter cells of meiosis II Homologous chromosomes separate ( anaphase I ); sister chroma- tids remain together No further chromosomal duplication; sister chromatids separate ( anaphase II )
- 72. 0 Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2
- 73. 0 Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Metaphase II
- 74. 0 Two equally probable arrangements of chromosomes at metaphase I Possibility 1 Possibility 2 Metaphase II Combination 1 Gametes Combination 2 Combination 3 Combination 4
- 78. 0 Centromere Chiasma Tetrad
- 79. 0 Breakage of homologous chromatids Coat-color genes Eye-color genes C (homologous pair of chromosomes in synapsis) E c e Tetrad C E c e Joining of homologous chromatids 2 C E c e Chiasma 1
- 80. 0 Separation of homologous chromosomes at anaphase I C E c e Chiasma Separation of chromatids at anaphase II and completion of meiosis C E c e c E C e c e c E C E C e Parental type of chromosome Gametes of four genetic types Recombinant chromosome Parental type of chromosome Recombinant chromosome 4 3
- 83. 0 Packed red and white blood cells Centrifuge Blood culture Fluid 1 Blood culture is centrifuged to separate the blood cells From the culture fluid
- 84. 0 Packed red and white blood cells Centrifuge Blood culture Fluid 1 Hypotonic solution 2 Fluid is discarded, and a hypotonic solution is mixed with The cells. This makes the RBC burst, the WBC swell but Do not burst, and their chromosomes spread out
- 85. 0 Packed red and white blood cells Centrifuge Blood culture Fluid 1 Hypotonic solution 2 3 Fixative White blood cells Stain Another centrifugation separates the WBC. The fluid Containing the remnants of RBCs is discarded. A preservative Is mixed w/ the WBC and a drop of the cell suspension is Spread on a microscope slide, dried, and stained.
- 86. 0 4
- 87. 0 Centromere Sister chromatids Pair of homologous chromosomes 5 Digital photograph of chromosomes is obtained and a computer Sorts them by size and shape. Resulting karyotype is below. 22 pairs of autosomes, 2 sex chromosomes
- 89. 0
- 90. 0
- 91. 0 Infants with Down syndrome (per 1,000 births) Age of mother 90 70 60 50 40 30 20 10 0 80 20 40 35 30 25 50 45
- 93. 0 Nondisjunction in meiosis I
- 94. 0 Nondisjunction in meiosis I Normal meiosis II
- 95. 0 Nondisjunction in meiosis I Normal meiosis II n + 1 Gametes Number of chromosomes n + 1 n – 1 n – 1
- 96. 0 Normal meiosis I
- 97. 0 Nondisjunction in meiosis II Normal meiosis I
- 98. 0 Nondisjunction in meiosis II Normal meiosis I Gametes Number of chromosomes n + 1 n – 1 n n
- 99. 0
- 103. 0 Deletion Inversion Duplication Homologous chromosomes
- 104. 0 Reciprocal translocation Nonhomologous chromosomes
- 105. 0 Chromosome 9 “ Philadelphia chromosome” Activated cancer-causing gene Reciprocal translocation Chromosome 22
- 106. Need Chemistry Help? Contact Veronica Walker for FREE tutoring! [email_address] Need Biology Help?? Contact Veronica Walker for FREE tutoring! [email_address]
Hinweis der Redaktion
- Most eukaryotic organisms are capable of both asexual and sexual reproduction. Students may be surprised to learn that asexual reproduction plays a major role in the life cycles of many organisms. For example, the unicellular algae Chlamydomonas generates an increased population by asexual reproduction when conditions are favorable for cell division. In unfavorable conditions, the organism undergoes sexual reproduction. This has the advantage of producing a new combination of genes and traits that could be advantageous for survival under new environmental conditions. Student Misconceptions and Concerns 1. As the authors note in Module 8.1, biologists use the term daughter to indicate offspring and not gender. Students with little experience in this terminology can easily become confused. 2. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips 1. Sometimes the most basic questions can challenge students and get them focused on the subject at hand. Consider asking your students why we expect that dogs will produce dogs, cats will produce more cats, and chickens will only produce chickens. Why does like produce like?
- Figure 8.1A A single-celled amoeba producing a genetically identical offspring cell through asexual reproduction. This figure shows an amoeba reproducing asexually. Ask students to suggest other examples of asexual reproduction. Possibilities include growth of a plant from a seed, replacement of skin cells, growth of an embryo or fetus from a fertilized egg.
- Figure 8.1B Sexual reproduction produces offspring with unique combinations of genes. Members of this family show similar traits but have differences in appearance. Variations include skin color and hair curling. Many genes contribute to skin color, influencing the number, size, and shape of melanosomes, organelles that synthesize and store the pigment melanin, as well as the type of melanin produced. Hair curling is related to at least one gene with incomplete dominance. When alleles for curly and straight are combined in a heterozygous individual, wavy hair is observed.
- Student Misconceptions and Concerns 1. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips 1. Virchow’s principle of “every cell from a cell” is worth thinking through with your class. Students might expect that, like automobiles, computers, and cell phones, parts are constructed and cells are assembled. In our society, few nonliving products are generated only from existing products (try to think of such examples). For example, you do not need a painting to paint or a house to construct a house. Yet this is a common expectation in biology. 2. Students who think through Virchow’s principle might ask how the first cells formed. They might wonder further whether the same environments that produced these cells are still in existence. The conditions on Earth when life first formed were very different from those we know today. Chapter 15 addresses the origin and early evolution of life on Earth.
- The process of binary fission is very rapid. E. coli cells divide every 20 minutes under optimal environmental conditions. The antibiotic penicillin inhibits the growth of the bacterial cell wall. Cells can duplicate their internal contents, including the chromosome, but will burst when they become too large for the existing cell wall. Student Misconceptions and Concerns 1. Some basic familiarity or faint memory of mitosis and meiosis might result in a lack of enthusiasm for mitosis and meiosis in some of your students. Consider beginning such lectures with important topics related to cellular reproduction. For example, cancer cells reproduce uncontrollably, stem cells have the capacity to regenerate lost or damaged tissues, and the study of embryonic stem cells is variously restricted and regulated. Teaching Tips Consider contrasting the timing of DNA replication and cytokinesis in prokaryotes and eukaryotes. In prokaryotes, addressed in Module 8.3, these processes are over-lapping. However, as revealed in the next few modules, these events are separate in eukaryotes.
- Figure 8.3A Binary fission of a prokaryotic cell. This figure shows the steps in binary fission.
- Figure 8.3A Binary fission of a prokaryotic cell. This figure shows the steps in binary fission.
- Figure 8.3A Binary fission of a prokaryotic cell. This figure shows the steps in binary fission.
- Figure 8.3B Electron micrograph of a dividing bacterium. The chromosomes of each of the two identical cells are visible in this electron micrograph.
- Chromatin is compacted about 100,000 fold to produce the interphase/metaphase chromosome. If all the DNA in the human chromosomes were aligned, it would stretch for one meter. All of this DNA is condensed to fit into a nucleus that can only be seen with the aid of a microscope. The centromere has a unique DNA sequence involving repeated stretches of nucleotides. In biotechnological applications, artificial chromosomes can be produced that have a centromeric sequence. This chromosome will be properly distributed during cell division because the spindle fibers attach to the artificial centromeric sequence. Student Misconceptions and Concerns 1. Students often seem confused by the difference between a DNA molecule and a chromosome. This is especially problematic when discussing DNA replication. 2. Students are often confused by photographs of chromosomes. Such photographs, such as Figure 8.4B, typically show duplicated chromosomes during some aspect of cell division. It remains unclear to many why (a) chromosome structure is typically different between interphase G1 and the stages of division and (b) why chromosomes are not photographed during interphase (the stage in which chromosomes are typically first discussed) before the chromosomes duplicate. Teaching Tips 1. Figure 8.4C is an important point of reference for some basic terminology. Consider referring to it as you distinguish between a DNA molecule and a chromosome, unreplicated and replicated chromosomes, and the nature of sister chromatids.
- Figure 8.4A A plant cell (from an African blood lily) just before division. This cell shows the coiling or compaction of chromatin, in prophase. Although the chromosomes have already duplicated, higher resolution is required to distinguish sister chromatids.
- Figure 8.4B Electron micrograph of a duplicated chromosome. This electron micrograph provides a view of sister chromatids.
- Differences in the length of the cell cycle can be instructive. Yeast cells have a 2-hour life cycle, while human cells in culture take about 24 hours to divide. Mitosis and cytokinesis represent a shorter section of the cycle, lasting one hour for cultured human cells. Teaching Tips 1.The authors note in Module 8.5 that each of your students consists of about 100 trillion cells. It is likely that this number is beyond comprehension for most of your students. Consider sharing several simple examples of the enormity of that number to try to make it more meaningful. For example, the U.S. population in 2008 is about 310 million people. To give every one of those people about $323,000, we will need a total of $100 trillion. Here is another example. If we give you $31,688 every second of your life, and you lived for 100 years, you would receive $100 trillion dollars. 2.The concepts of DNA replication and sister chromatids are often obstacles for many students. If you can find twist ties or other bendable wire, you can demonstrate or have students model the difference between (1) a chromosome before DNA replication and (2) sister chromatids after DNA replication. One piece of wire will represent a chromosome before replication. Two twist ties twisted about each other can represent sister chromatids. We have doubled the DNA, but the molecules remain attached (although not attached in the same way as the wire). You might also want to point out that when sister chromatids are separated, they are considered separate chromosomes. 3. In G1, the chromosomes have not duplicated. But by G2, chromosomes consist of sister chromatids. If you have created a demonstration of sister chromatids, relate DNA replication and sister chromatids to the cell cycle.
- Figure 8.5 The eukaryotic cell cycle.
- Teaching Tips 1. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. 2. The authors note that animals, but not plants, have a pair of centrioles in their centrosomes. They add that the role of centrioles in cell division is a mystery. Students might not appreciate all that remains to be explained in biology. Sharing the existence of such mysteries with them promotes critical thinking skills and helps them imagine a place for themselves in future research.
- Centrioles give rise to basal bodies that are the foundations for cilia and flagella. They are found in animal cells but also in plants such as mosses and ferns that have swimming sperm. They are not found in flowering plants, showing that centrioles are not essential for spindle formation. There is other evidence, however, that suggests centrioles may influence progression through alternative stages in the cell cycle, including entry into the S phase and completion of cytokinesis. (Reviewed in A. W. Murray, 2001, “Centrioles at the Checkpoint,” Science , 291:1499–1501.) For the BioFlix Animation Mitosis, go to Animation and Video Files. Teaching Tips 1. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. 2. The authors note that animals, but not plants, have a pair of centrioles in their centrosomes. They add that the role of centrioles in cell division is a mystery. Students might not appreciate all that remains to be explained in biology. Sharing the existence of such mysteries with them promotes critical thinking skills and helps them imagine a place for themselves in future research.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Applying Your Knowledge Human cells have 46 chromosomes. By the end of interphase, How many chromosomes are present in one cell? 46 How many chromatids are present in one cell? 92 (Each chromosome has been duplicated and consists of a pair of chromatids joined at the centromere.) Teaching Tips 1. Students might keep better track of the sequence of events in a cell cycle by simply memorizing the letters IPPMAT: the first letters of interphase, prophase, prometaphase, metaphase, anaphase, and telophase are represented in this acronym. 2. The authors note that animals, but not plants, have a pair of centrioles in their centrosomes. They add that the role of centrioles in cell division is a mystery. Students might not appreciate all that remains to be explained in biology. Sharing the existence of such mysteries with them promotes critical thinking skills and helps them imagine a place for themselves in future research.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- Figure 8.6 The stages of cell division.
- This material allows a review of cellular components. Students can be reminded that microtubules are composed of actin molecules and that actin and myosin work in concert for muscle cell contraction. They can also be reminded that vesicles have a lipid boundary that will contribute to the plasma membrane of the new plant cells. For the BLAST Animation Cytokinesis in Plants, go to Animation and Video Files. Teaching Tips 1. Many students think of mitosis and cytokinesis as one process. In some situations, mitosis occurs without subsequent cytokinesis. Challenge your students to predict the outcome of mitosis without cytokinesis (multinuclear cells called a syncytium). This occurs in human development during the formation of the placenta. 2. The authors make an analogy between a drawstring and the mechanism of cytokinesis in animal cells. Students seem to appreciate this association. Have your students think of a person tightening the drawstring of sweatpants so tight that they pinch themselves in two, or perhaps nearly so! The analogy is especially good because, like the drawstring just beneath the surface of the sweat pants, the microfilaments are just beneath the surface of the cell’s plasma membrane.
- Figure 8.7A Cleavage of an animal cell.
- Figure 8.7B Cell plate formation in a plant cell.
- There is promising research on the use of growth factors to stimulate the regeneration of nerves by promoting axon growth. For example, the macrophage-derived protein oncomodulin has been shown to increase axon length in the optic nerve nearly twofold in vitro. (Laboratory of L. Benowitz, in Yin, et al., 2006, Nature Neuroscience 9, 843–852.) Teaching Tips 1. Students who closely examine a small abrasion on their skin might notice that the wound tends to heal from the outer edges inward. This space-filling mechanism is a natural example of density-dependant inhibition, which is also seen when cells in a cell culture dish stop dividing when they have formed a complete layer.
- Student Misconceptions and Concerns 1. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer. Teaching Tips 1. The authors make an analogy between the cell cycle control system and the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, as the authors note, unlike a washing machine, the components of the control system of a cell cycle are not all located in one place.
- Figure 8.9A Mechanical model for the cell cycle control system.
- Student Misconceptions and Concerns 1. Students do not typically know that all cancers are genetically based. Consider making this clear early in your discussions. Challenge your students to explain how certain viruses can lead to cancer. Teaching Tips 1. The authors make an analogy between the cell cycle control system and the control device of an automatic washing machine. Each has a control system that triggers and coordinates key events in the cycle. However, as the authors note, unlike a washing machine, the components of the control system of a cell cycle are not all located in one place.
- Figure 8.9B How a growth factor signals the cell cycle control system.
- Teaching Tips 1. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection. Many other approaches are under consideration to attack cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.
- Teaching Tips 1. Chemotherapy has some disastrous side effects. The drugs used to fight cancer attack rapidly dividing cells. Unfortunately for men, the cells that make sperm are also rapidly dividing. In some circumstances, chemotherapy can leave a man infertile (unable to produce viable sperm) but still able to produce an erection. Many other approaches are under consideration to attack cancers. You may wish to explore these as sidelights to your lecture. Good resources include cell biology and development textbooks.
- Figure 8.10 Growth and metastasis of a malignant (cancerous) tumor of the breast.
- Teaching Tips 1. Figure 8.11 visually summarizes key functions of mitosis. It is an important image to introduce mitosis or summarize mitosis after addressing its details.
- Student Misconceptions and Concerns 1. Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips 1. Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait discussed in Module 8.12. 2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes. 3. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . Maybe a sandal and a sneaker!
- Applying Your Knowledge Humans have 46 chromosomes; how many homologous pairs does that represent? 23 If there is one pair of sex chromosomes, how many pairs of autosomes are found in humans? 22 Student Misconceptions and Concerns 1. Some students might conclude that sex chromosomes function only in determining the sex of the individual. As the authors note, sex chromosomes contain genes not involved in sex determination. Teaching Tips 1. Students might recall some basic genetics, remembering that for many traits a person receives a separate “signal” from mom and dad. These separate signals for the same trait are carried on the same portion of homologous chromosomes, such as the freckle trait discussed in Module 8.12. 2. Consider helping students through mitosis and meiosis by developing an analogy to pairs of shoes. In this case, any given species has a certain number of pairs of shoes, or homologous chromosomes. 3. In the shoe analogy, females have 23 pairs of matching shoes, while males have 22 matching pairs and 1 odd pair . . . Maybe a sandal and a sneaker!
- Figure 8.12 A homologous pair of chromosomes. This figure shows a pair of homologous chromosomes in their duplicated state. Each homologue is represented as a pair of chromatids. These two contexts for the word pair often confuse students. It would be helpful to introduce the term tetrad at this point, to emphasize both uses of the term pair .
- Teaching Tips 1. You might want to get your students thinking by asking them why eggs and sperm are different. (This depends upon the species, but within vertebrates, eggs, and sperm are specialized for different tasks. Sperm are adapted to move to an egg and donate a nucleus. Eggs contain a nucleus and most of the cytoplasm of the future zygote. Thus eggs are typically larger, nonmotile, and full of cellular resources to sustain cell division and growth.)
- Figure 8.13 The human life cycle.
- Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Sister chromatids are exact duplicates, but nonsister chromatids belong to different members of the homologous pair. Any one of the maternal chromatids is a nonsister to any of the paternal chromatids. Since maternal and paternal chromatids can have different versions of genes (alleles) at corresponding loci, crossing over potentially produces new genetic combinations, mixing maternal and paternal versions on the same chromatid. Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Sister chromatids are exact duplicates, but nonsister chromatids belong to different members of the homologous pair. Any one of the maternal chromatids is a nonsister to any of the paternal chromatids. Since maternal and paternal chromatids can have different versions of genes (alleles) at corresponding loci, crossing over potentially produces new genetic combinations, mixing maternal and paternal versions on the same chromatid. Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Applying Your Knowledge Human cells have 46 chromosomes. At the end of Metaphase I, How many chromosomes are present in one cell? 46 How many chromatids are present in one cell? 92 Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Applying Your Knowledge After telophase I and cytokinesis, How many chromosomes are present in one human cell? 23 How many chromatids are present in one human cell? 46 For the BioFlix Animation Meiosis, go to Animation and Video Files. Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Figure 8.14 The stages of meiosis.
- Figure 8.14 The stages of meiosis.
- Meiosis II is similar to mitosis, starting with a haploid cell. In evolutionary terms, mitosis is the earlier process, being observed for haploid cells in the protist kingdom, for example. Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Meiosis II is similar to mitosis, starting with a haploid cell. In evolutionary terms, mitosis is the earlier process, being observed for haploid cells in the protist kingdom, for example. Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Applying Your Knowledge After telophase II and cytokinesis, How many chromosomes are present in one human cell? 23 How many chromatids are present in one human cell? 0 Student Misconceptions and Concerns 1. Students might not immediately see the need for meiosis in sexual reproduction. Consider an example of what would happen over many generations if gametes were produced by mitosis. The resulting genetic doubling is prevented if each gamete has only half the genetic material of the adult cells. 2. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. In meiosis I and meiosis II, the processes begin with duplicated pairs of chromosomes. This pair becomes two pairs. The two pairs include four items. Sort this group into two subgroups, and you are back to two pairs. Divide again, and you have separated four items into four groups of one. This can work with the shoe analogy if you wish to continue the reference. A pair of shoes is “reproduced” and becomes two pairs. Mitosis sorts them back into two pairs of shoes. However, meiosis keeps sorting, eventually isolating each shoe. Each solitary shoe would then represent a gamete, which would then be matched with another similar shoe (gamete) to make a new pair of shoes (organism).
- Figure 8.14 The stages of meiosis.
- This slide can be used to generate class discussion in a variety of ways. The answers can be brought in by animation after each question is considered by the class. Or the answers on the slide can be deleted and responses generated by the class can be added to the slide during the presentation. Which characteristics are similar for mitosis and meiosis? One duplication of chromosomes Which characteristics are unique to meiosis? Two divisions of chromosomes Pairing of homologous chromosomes Exchange of genetic material by crossing over Student Misconceptions and Concerns 1. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. Consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous pairs of chromosomes separate. Consider sketching a comparison of the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I. Figure 8.15 helps to make this important distinction. You might create a test question in which you ask students to draw several pairs of homologous chromosomes lined up at metaphase in mitosis versus meiosis I.
- This slide can be used to generate class discussion in a variety of ways. The answers can be brought in by animation after each question is considered by the class. Or the answers on the slide can be deleted and responses generated by the class can be added to the slide during the presentation. What is the outcome of each process? Mitosis: two genetically identical cells, with the same chromosome number as the original cell Meiosis: four genetically different cells, with half the chromosome number of the original cell The reference to genetic differences between meiotic products assumes both crossing over and independent orientation of multiple pairs of chromosomes. These processes are detailed on slides that follow. Student Misconceptions and Concerns 1. How meiosis results in four haploid cells, yet mitosis yields two diploid cells, is often memorized but seldom understood. It can be explained like this. Consider a pair of chromosomes in a cell before any cell divisions. This pair of chromosomes duplicates such that two chromosomes become four (although each pair of sister chromatids are joined at their centromeres). Therefore, mitosis and meiosis each typically begin with four chromosomes. Mitosis divides once, producing two cells, each with two chromosomes. Meiosis divides twice, sorting the four chromosomes into four separate cells. Teaching Tips 1. Consider emphasizing a crucial difference between the processes of mitosis and meiosis. In mitosis, sister chromatids separate at metaphase. In meiosis I metaphase, sister chromatids stay together, and homologous pairs of chromosomes separate. Consider sketching a comparison of the alignment of the chromosomes at mitosis metaphase and meiosis metaphase I. Figure 8.15 helps to make this important distinction. You might create a test question in which you ask students to draw several pairs of homologous chromosomes lined up at metaphase in mitosis versus meiosis I.
- Figure 8.15 Comparison of mitosis and meiosis.
- The amount of genetic diversity possible for humans can be emphasized by the calculation described in the text. Based on independent orientation at metaphase I, the number of different eggs (and sperm) for humans is 2 23 = 8 million. So with the union of unique gametes, the probability of two siblings having exactly the same genetic profile is 1 out of 64 trillion possibilities. (Identical twins are not considered, as both individuals result from the same fertilization event.) This is an underestimate because it does not include the additional variability generated by crossing over. For the BLAST Animation Genetic Variation: Fusion of Gametes, go to Animation and Video Files. Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase is 2 23 or 8,388,698. This number squared is more than 70 trillion. The authors rounded down to 8 million for 2 23 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization is over 70 trillion! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. There are four possible pairs that can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations!
- Figure 8.16 Results of the independent orientation of chromosomes at metaphase I.
- Figure 8.16 Results of the independent orientation of chromosomes at metaphase I.
- Figure 8.16 Results of the independent orientation of chromosomes at metaphase I.
- The amount of genetic diversity possible for humans can be emphasized by the calculation described in the text. Based on independent orientation at metaphase I, the number of different eggs (and sperm) for humans is 2 23 = 8 million. So with the union of unique gametes, the probability of two siblings having exactly the same genetic profile is 1 out of 64 trillion possibilities. (Identical twins are not considered, as both individuals result from the same fertilization event.) This is an underestimate because it does not include the additional variability generated by crossing over. For the BLAST Animation Genetic Variation: Fusion of Gametes, go to Animation and Video Files. Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase is 2 23 or 8,388,698. This number squared is more than 70 trillion. The authors rounded down to 8 million for 2 23 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization is over 70 trillion! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. There are four possible pairs that can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations!
- Teaching Tips 1. The possible number of combinations produced by independent orientation of human chromosomes at meiosis metaphase is 2 23 or 8,388,698. This number squared is more than 70 trillion. The authors rounded down to 8 million for 2 23 and squared this to estimate 64 trillion possible combinations. But more precisely, the number of possible zygotes produced by a single pair of reproducing humans, based solely on independent assortment and random fertilization is over 70 trillion! 2. Another way to represent the various combinations produced by independent orientation of chromosomes at meiosis metaphase I continues the shoe analogy. Imagine that you have two pairs of shoes. One pair is black, the other is white. You want to make a new pair drawing one shoe from each original pair. There are four possible pairs that can be made. You can have (1) the left black and left white, (2) the right black and right white, (3) the left black and right white, or (4) the right black and left white. Actually using two pairs of shoes from your students can inject humor and create a concrete example that reduces confusion. For an additional bit of humor, ask the class if 46 students want to contribute their shoes as you try to demonstrate all 8,388,608 combinations!
- Students are often surprised at the frequency of recombination, so emphasizing the average of one to three crossover events per chromosome is instructive. This shows the likelihood that chromosomes with combinations of traits from both parents can be passed to offspring. Teaching Tips 1. If you wish to continue the shoe analogy, crossing over is somewhat like exchanging the shoelaces in a pair of shoes (although this analogy is quite limited). A point to make is that the shoes (chromosomes) before crossing over are what you inherited . . . either from the sperm or the egg; but, as a result of crossing over, you no longer pass along exactly what you inherited. Instead, you pass along a combination of homologous chromosomes (think of shoes with switched shoelaces). Critiquing this limited analogy may also help students to think through the process of crossing over. 2. In the shoe analogy, after exchanging shoelaces, we have “recombinant shoes”! 3. Challenge students to consider the number of unique humans that can be formed by the processes of the independent orientation of chromosomes, random fertilization, and crossing over. Without crossing over, we already calculated over 70 trillion possibilities. But as the text notes in Module 8.18, there are typically one to three crossover events for each human chromosome, and these can occur at many different places along the length of the chromosome. The potential number of combinations far exceeds any number that humans can comprehend, representing the truly unique nature of each human being (an important point that delights many students!)
- Figure 8.18A Chiasmata.
- Figure 8.18B How crossing over leads to genetic recombination. This figure shows the outcome of crossing over between the coat and eye color genes for the mice introduced in module 8.17. One of the parents contributes the genes for brown coat with black eyes, while the other parent donates the genes for white coat with pink eyes. Four genetically different chromosomes are produced, two parental versions carrying genes for brown coat with black eyes and white coat with pink eyes, and two recombinant versions carrying genes for brown coat with pink eyes and white coat with black eyes.
- Figure 8.18B How crossing over leads to genetic recombination. This figure shows the outcome of crossing over between the coat and eye color genes for the mice introduced in module 8.17. One of the parents contributes the genes for brown coat with black eyes, while the other parent donates the genes for white coat with pink eyes. Four genetically different chromosomes are produced, two parental versions carrying genes for brown coat with black eyes and white coat with pink eyes, and two recombinant versions carrying genes for brown coat with pink eyes and white coat with black eyes.
- Student Misconceptions and Concerns 1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23. Teaching Tips 1. The Genetic Interest Group has a website devoted to human genetic disorders at www.gig.org.uk / . Its long list of Internet links is both extensive and comprehensive.
- Figure 8.19 Preparation of a karyotype from a blood sample.
- Figure 8.19 Preparation of a karyotype from a blood sample.
- Figure 8.19 Preparation of a karyotype from a blood sample.
- Figure 8.19 Preparation of a karyotype from a blood sample.
- Figure 8.19 Preparation of a karyotype from a blood sample.
- Student Misconceptions and Concerns 1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23. Teaching Tips 1. The Genetic Interest Group has a website devoted to human genetic disorders at www.gig.org.uk / . Its long list of Internet links is both extensive and comprehensive.
- Figure 8.20A A karyotype for trisomy 21 (Down syndrome). This figure shows the karyotype of a female with Down syndrome. Chromosomes 1–20, 22, and 23 are shown in pairs while chromosome 21 is present in three copies.
- Figure 8.20B A child with Down syndrome. This child shows characteristic facial features associated with Down syndrome, including a round face and flattened nose bridge.
- Figure 8.20C Maternal age and incidence of Down syndrome. This figure shows the rise in incidence of Down syndrome with increasing maternal age. Studies demonstrate that a high frequency of cases is related to nondisjunction during meiosis I, but the mechanism for this increased occurrence in aging eggs has not yet been elucidated. As described in the text, there may be an age-related error in one of the checkpoints that coordinate the meiotic process. There is also some evidence for nondisjunction during sperm production since the incidence of Down syndrome is further increased when both the mother and father are over age 40.
- Student Misconceptions and Concerns 1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23. Teaching Tips 1. The Genetic Interest Group has a website devoted to human genetic disorders at www.gig.org.uk / . Its long list of Internet links is both extensive and comprehensive. 2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.
- Figure 8.21A Nondisjunction in meiosis I. If nondisjunction occurs during meiosis I, all gametes will have an abnormal number of chromosomes. Half will have two copies of the chromosome pair that failed to separate, and the other half will be missing copies of that chromosome. For Down syndrome, an egg with two copies of chromosome 21 can be fertilized by a sperm carrying one copy of chromosome 21. For the sex-chromosome abnormality Turner syndrome, an egg lacking a copy of the X or Y chromosome can be fertilized by a sperm carrying a copy of the X chromosome.
- Figure 8.21A Nondisjunction in meiosis I. If nondisjunction occurs during meiosis I, all gametes will have an abnormal number of chromosomes. Half will have two copies of the chromosome pair that failed to separate, and the other half will be missing copies of that chromosome. For Down syndrome, an egg with two copies of chromosome 21 can be fertilized by a sperm carrying one copy of chromosome 21. For the sex-chromosome abnormality Turner syndrome, an egg lacking a copy of the X or Y chromosome can be fertilized by a sperm carrying a copy of the X chromosome.
- Figure 8.21A Nondisjunction in meiosis I. If nondisjunction occurs during meiosis I, all gametes will have an abnormal number of chromosomes. Half will have two copies of the chromosome pair that failed to separate, and the other half will be missing copies of that chromosome. For Down syndrome, an egg with two copies of chromosome 21 can be fertilized by a sperm carrying one copy of chromosome 21. For the sex-chromosome abnormality Turner syndrome, an egg lacking a copy of the X or Y chromosome can be fertilized by a sperm carrying a copy of the X chromosome.
- Figure 8.21B Nondisjunction in meiosis II. If nondisjunction occurs during meiosis II, two of the four products will have a balanced number of chromosomes. Of the other two products, one will have two copies of the same chromosome, and the other will be missing any copy of that chromosome.
- Figure 8.21B Nondisjunction in meiosis II. If nondisjunction occurs during meiosis II, two of the four products will have a balanced number of chromosomes. Of the other two products, one will have two copies of the same chromosome, and the other will be missing any copy of that chromosome.
- Figure 8.21B Nondisjunction in meiosis II. If nondisjunction occurs during meiosis II, two of the four products will have a balanced number of chromosomes. Of the other two products, one will have two copies of the same chromosome, and the other will be missing any copy of that chromosome.
- Table 8.22 Abnormalities of Sex Chromosome Number in Humans. At fertilization, humans are chromosomally male or female but have presumptive gonads that can be influenced to become testes or ovaries. If the Y chromosome has been inherited, a series of genetic changes influences testis development. In the absence of a Y chromosome, the gonads become ovaries. Klinefelter syndrome results from two or more X chromosomes with one Y chromosome. Individuals with Turner syndrome (XO) are sterile, showing the importance of two X chromosomes during early development for the formation of functional sex organs. This influence must be exerted before X-chromosome inactivation, otherwise XX females would also be sterile.
- Student Misconceptions and Concerns 1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23. Teaching Tips 1. The Genetic Interest Group has a website devoted to human genetic disorders at www.gig.org.uk / . Its long list of Internet links is both extensive and comprehensive. 2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.
- Animal examples include fish, amphibians, and one species of rat. Student Misconceptions and Concerns 1. Before addressing karyotyping and nondisjunction events, consider reviewing the general structure and terminology associated with replicated chromosomes and the arrangement of chromosomes during metaphase of mitosis, meiosis I, and meiosis II. Figures 8.4B and 8.15 will be particularly helpful. A firm foundation in chromosome basics is necessary to understand the irregularities discussed in Modules 8.20–8.23. Teaching Tips 1. The Genetic Interest Group has a website devoted to human genetic disorders at www.gig.org.uk / . Its long list of Internet links is both extensive and comprehensive. 2. 4. Students might be confused by the term nondisjunction. But simply put, it is an error in the sorting of chromosomes during mitosis or meiosis. Figure 8.21 illustrates two types of nondisjunction errors in meiosis.
- Reciprocal translocations involve exchange of segments between nonhomologous chromosomes, but the sizes of the segments do not need to be the same. Teaching Tips 1. Challenge students to create a sentence and then modify that sentence to represent (a) a deletion, (b) a duplication, and (c) an inversion as an analogy to these changes to a chromosome.
- Figure 8.24A Alterations of chromosome structure involving one chromosome or a homologous pair.
- Figure 8.24B Chromosomal translocation between nonhomologous chromosomes.
- Figure 8.24C The translocation associated with chronic myelogenous leukemia. Familial Down syndrome is the result of a Robertsonian translocation. This is when the long arms of two nonhomologous chromosomes are joined to the same centromere. A translocation carrier for Down syndrome would have a translocated chromosome that has the long arm of chromosome 21 attached to another chromosome, such as chromosome 15. That individual would also have one complete copy of chromosome 21 and one complete copy of chromosome 15. Due to the translocated chromosome, this individual would have 45 chromosomes. If this parent produces a gamete containing the translocated chromosome along with the complete copy of 21, and the other parent provides single copies of 15 and 21, the offspring will have Down syndrome. The translocated chromosome provides a nearly complete third copy of chromosome 21.