1. The Cell Cycle, Mitosis, and Meiosis
I. Cell Division
A. The Nature of Cell Division
1. Multicellular organisms need new cells to grow, develop and to replace
worn out and dead cells.
2. 2. When a cell is getting ready to divide, one trigger to initiate this event is
the balance between the ratio of cytoplasmic volume to plasma
membrane area.
3. The cell membrane grows in two dimensions because the membrane
width never changes- it is always two cells thick: A = l x h
Area (cell membrane) = length x width
4. The cytoplasm volume grows in three dimensions: V = l x h x w
Volume (cytoplasm) = length x width x height
5. So for cytoplasm V = l x h x w and for the cell membrane is A = l x h
3. 6. Let’s look at this mathematically:
2
Cell Membrane
Cytoplasm
ratio
A = 1 mm x 1 mm = 1 mm
3
V = 1mm x 1 mm x 1 mm = 1 mm
1/1 = 1 : 1
2
Cell Membrane A = 2 mm x 2 mm = 4 mm
3
Cytoplasm
V = 2 mm x 2 mm x 2 mm = 8 mm
ratio
2/4 = 1 :2
2
Cell Membrane A = 3 mm x 3 mm = 9 mm
3
Cytoplasm
V = 3 mm x 3 mm x 3 mm = 27 mm
ratio
1/3 = 1 : 3
7. As the cell grows, the cell membrane material increases at a slower rate
than the cytoplasm, triggering cell division.
4. 8. Cells may be programmed to stop mitosis after 52 divisions.
9. When this occurs the cell is said to be senescent and has reached its Hayflick
limit.
10. Cells stop dividing because the telomeres, protective bits of DNA on the end
of each chromosome required for replication, shorten with each copy.
11. As telomeres shorten, the cell dies.
6. III. A Closer Look at the Nucleus
A. Chromatin
1. Inside the nucleus there are 46 pieces of chromatin.
2. Chromatin is threadlike and composed of DNA, proteins, and a trace
of RNA.
3. DNA is so abundant that it must be tightly packed in the nucleus.
7. 4. The nucleosome structure compacts DNA in the nucleus
-Chromatin compacts the DNA in the nucleus by forming the nucleosome
-Each nucleosome is composed of 8 positively charged histone proteins,
core DNA, and one H1 histone
-Linker DNA connects the many nucleosomes together like beads on a
string
9. 5. Further chromatin compaction occurs when nucleosomes combine to
form the solenoid structure.
6. The solenoids then compact into chromosomes.
10. B. Sister Chromatids
1. When chromatids become visible during mitosis and meiosis, they
appear as connected sister chromatids.
2. Sister chromatids are identicle and connected by a centromere.
3. Each sister chromatid represents an individual chromosome.
4. The kinetochore area of the centromere connects with kinetochore
fibers during mitosis.
5. The kinetochore fibers are composed of microtubules and function to
move the chromosomes during mitosis.
11. C. Homologous Chromosomes
1. Homologous chromosomes are chromosome pairs that are similar but
not identical.
2. Humans have 46 chromosomes.
3. The 46 chromosomes make up 23 pairsof homologues.
4. One homologue is inherited from dad and the other from mom during
fertilization.
12. D. Human Karyotype
1. A karyotype represents the 46 human chromosomes lined up by
homologues.
2. The sister chromatids are barely visible.
3. Karyotypes are used to study chromosomes.
13. IV. Mitosis
A. Mitosis Characteristics
1. After the cell leaves G2 , it enters into mitosis.
2. Mitosis is nuclear division.
3. The purpose of mitosis is to copy the cell’s DNA so that each
daughter cell, upon cell division, has an identical copy of its DNA.
4. After mitosis, the cell divides.
5. Cell division is called cytokinesis.
6. The stages of mitosis are:
Interphase (not part of mitosis)
Prophase
Metaphase
Anaphase
Telophase
7. Sometimes mitotic stages are represented by IPMAT.
22. V. Abnormal Cell Division
A. Malignant Tumors as a Result of Mutations
1. The tumor begins to develop when a cell experiences a mutation that
makes the cell more likely to divide than it normally would.
2. The altered cell and its descendants grow and divide too often, a
condition called hyperplasia. At some point, one of these cells
experiences another mutation that further increases its tendency to divide.
3. This cell's descendants divide excessively and look abnormal, a condition
called dysplasia.
4. As time passes, one of the cells experiences yet another mutation. This cell
and its descendants are very abnormal in both growth and appearance. If
the tumor that has formed from these cells is still contained within its
tissue
of origin, it is called in situ cancer.
5. If some cells experience additional mutations that allow the tumor to
invade neighboring tissues and shed cells into the blood or lymph, the
tumor is said to be malignant. The escaped cells may establish new tumors
23.
24. B. Cancer Cells are Immortal
1. Cancer cells have unique features that make them "immortal."
2. The enzyme telomerase is used to extend the cancer cell's life span.
3. Telomeres of cells shortens after each division eventually causing the
cell to die, telomerase extends the cell's telomeres. This is a major
reason that cancer cells can accumulate over time creating tumors.
25. C. Cancer Cells Do Not Display Contact Inhibition
1. The cells in our bodies are governed by growth control mechanisms
cell senescence (aging).
2. Cell aging puts a limit on the number of times a cell can divide: the
more a cell has divided, the less likely it will be to divide again.
3. Contact inhibition is a growth mechanism that will cell growth
4. Cancerous cells typically lose this property and thus grow in an
uncontrolled manner even when in contact with neighboring cells.
26. D. Types of Cancer Cells
Breast cancer cells
Lung cancer cells
Cervical cancer cells
Prostate cancer cell
27. VI. Male and Female Gametes
A. Male Sperms
1. Haploid cell - 23 chromosomes
2. Produced in great numbers in the seminiferous tubules of the
paired testes
3. Composed of the heat, midpiece and tail
4. Short-lived and highly motile
28. B. Female Eggs (Ova)
1. Haploid cell - 23 chromosomes
2. Produced in paired ovaries
3. Large cells containing what is needed to produce a new individual
4. Nonmotile
29. VII. Meiosis
A. Meiosis I (cells are diploid)
1. Prophase I
-homologues synapse- synapsis
-synaptonemal complex
-chiasmata
-crossing over
2. Metaphase I
-paired homologues move to the equator
3. Anaphase I
-homologues begin to separate
4. Telophase I
-homologues separate into separate cells
B. Meiosis II (cells are haploid)
1. Meiosis II
-meiosis II occurs the same manner as mitosis
2. End result of meiosis II is the generation of haploid gametes
30.
31.
32. Homologous pairs of chromosomes in prophase I
synapsis
chiasmata
crossing-over
