Meiosis

 In humans, somatic cells (body cells) have:
• 23 pairs of homologous chromosomes and
• one member of each pair from each parent.
 The human sex chromosomes (Gonosomes)
X and Y differ in size and genetic composition.
 The other 22 pairs of chromosomes are
autosomes with the same size and genetic
composition.
UNIT 3: MEIOSIS
REDUCTION DIVISION
1. CHROMOSOMES ARE MATCHED IN HOMOLOGOUS PAIRS
© 2012 Pearson Education, Inc.

 Homologous chromosomes are matched in:
• length,
• centromere position, and
• gene locations (locus).
 A locus (plural, loci) is the position of a gene.
 Different versions (alleles) of a gene may be
found at the same locus on maternal and
paternal chromosomes.
Centromere

 Homologous chromosome pair
Centromere

 Humans and most animals and plants have diploid
body cells.
 That means they have two sets of chromosomes
(homologous chromosome pair) one from each
parent.
 Diploid is written 2n.
 It refers to the total number of chromosomes
a cell can have.
2. GAMETES HAVE A SINGLE SET OF
CHROMOSOMES

 Meiosis is a process that converts diploid
nuclei to haploid nuclei.
• Diploid cells have 2 sets of chromosomes.
• Haploid cells have 1 set of chromosomes.
• Meiosis occurs in the sex organs,
producing gametes—sperm and eggs.
 Fertilization is the fusion of a sperm and egg
cell.
 The zygote has a diploid chromosome
number, one set from each parent.

Haploid gametes (n  23)
Egg cell
Sperm cell
Fertilization
n
n
Meiosis
Ovary Testis
Diploid
zygote
(2n  46)
2n
Mitosis
Key
Haploid stage (n)
Diploid stage (2n)
Multicellular diploid
adults (2n  46)
A life cycle

 All sexual life cycles include an alternation
between
• a diploid stage and
• a haploid stage.
 Why is meiosis so important? It produces
haploid gametes which prevents the
chromosome number from doubling in every
generation. Produce gametes for fertilization.

3
Meiosis is a type of cell division that
produces haploid gametes from diploid
cells.
Two haploid gametes combine in
fertilization to restore the diploid state in
the zygote.
3. MEIOSIS

3
SUMMERY OF THE MEIOSIS PROCESS

MEIOSIS I consisting of 5 phases:
Interphase I, Prophase I, Metaphase I,
Anaphase I, Telophase I.
MEIOSIS II consisting of 4 phases
Prophase II, Metaphase II, Anaphase II,
Telophase II.
MEIOSIS HAS 2 STAGES:

Cell build up energy
DNA Replication (to make
duplicated chromosomes
Cell doesn’t change
structurally.
MEIOSIS I : INTERPHASE

 Events occurring in the nucleus:
• Chromosomes coil and become individual chromo-
somes, nucleolus and nuclear envelope disappear.
• Homologous chromosomes come together as pairs by
synapsis forming a tetrad (Each pair, with four
chromatids)
• Non-sister chromatids exchange genetic material through
the process of crossing over to ensure genetic
variation.
• Centrioli move to opposite poles with spindle fibers
between them.
MEIOSIS I : PROPHASE I

MEIOSIS I : PROPHASE I

 Genetic recombination is the production of new
combinations of genes due to crossing over.
 Crossing over is an exchange of genesbetween
separate (non-sister) chromatids on homologous
chromosomes.
• Non-sister chromatids join at a chiasma
(plural, chiasmata), the site of attachment.
• Genetic material are exchanged between
maternal and paternal (nonsister) chromatids.
CROSSING OVER

CROSSING OVER

 Centrioli has reached the
poles.
 Homologous pairs align at
the cell equator.
 The two chromosomes attach
to one spindle fiber by means
of the kinetochore of the
centromere.
.
MEIOSIS I: METAPHASE I

 Spindle fibers contract.
 Duplicated chromosomes
move to opposite poles.
.
MEIOSIS I: ANAPHASE I

• Duplicated
chromosomes have
reached the poles.
• A nuclear envelope and
nucleolus re-forms
around chromosomes.
• Each nucleus now has
the haploid number of
chromosomes.
• Cell invaginates forming
a cleavage furrow, which
extends to for 2 separate
haploid cells.
MEIOSIS I: TELOPHASE I

 Follows meiosis I without chromosome
duplication.
 Each of the two haploid products enters
meiosis II.
MEIOSIS II

• Chromosomes coil and
become compact (if
uncoiled after telophase I).
• Nuclear envelope and
nucleolus, if re-formed,
dissappears again.
• Centrioli move to opposite
poles, forming spindle
fibers between them.
MEIOSIS II: PROPHASE II

• Individual duplicated
chromosomes align on the
equator.
• One chromosome per spindle
fiber attached by means of
kinetochore of centromere.
• Centrioli has reached the
poles.
MEIOSIS II: METAPHASE II

• Spindle fibers contract.
• Duplicated chromosomes
split in half (centromere
dividing in 2)
• Daughter chromosomes
move to opposite poles.
MEIOSIS II: ANAPHASE II

• Daughter chromosomes has
reached the poles.
• Two cells invaginate and form 4
daughter haploid cells
(gametes)
• They uncoil and form
chromatin.
• Nuclear envelope and
nucleolus for around chromatin
again.
• Centrioli for centrosome.
MEIOSIS II: TELOPHASE II

SUMMERY OF MEIOSIS II
Prophase II Metaphase II Anaphase II
Haploid daughter
cells forming
Telophase II
and Cytokinesis

 Mitosis and meiosis both
• begin with diploid parent cells that
• have chromosomes duplicated during the
previous interphase.
 However the end products differ.
• Mitosis produces two genetically identical
diploid somatic daughter cells.
• Meiosis produces four genetically unique
haploid gametes.
4. SIMILARITIES AND DIFFERENCES BETWEEN
MITOSIS AND MEIOSIS

• Independent orientation at metaphase
I
• Random fertilization.
• Crossing over of genes during
prophase I
5. GENETIC VARIATION IN GAMETES
RESULTS FROM:

6. KARYOTYPE
• A karyotype is an ordered display of
magnified images of an individual’s
chromosomes arranged in pairs.
• Karyotypes allow for the observation of :
 homologous chromosome pairs,
 chromosome number, and
 chromosome structure.

SCIENTIST OBSERVING A HUMAN
KARYOTYPE

Centromere
Sister
chromatids
Pair of
homologous
chromosomes
Sex chromosomes

 An extra copy of chromosome 21 causes
Down syndrome or also known as TRISOMY
21.
 A. Trisomy 21
• involves the inheritance of three copies of
chromosome 21 and
• is the most common human chromosome
abnormality.
7. ALTERATION IN CHROMOSOME NUMBER

 Trisomy 21 produces a characteristic set of symptoms,
which include:
• mental retardation,
• characteristic facial features,
• short stature,
• heart defects,
• susceptibility to respiratory infections, leukemia,
and Alzheimer’s disease, and
• shortened life span.
 The incidence increases with the age of the mother.

 Nondisjunction is the failure of chromosomes or
chromatids to separate normally during meiosis. This
can happen during:
• meiosis I, if both members of a homologous pair go
to one pole or
• meiosis II if both sister chromatids go to one pole.
 Fertilization after nondisjunction yields zygotes with
altered numbers of chromosomes.
B. ACCIDENTS DURING MEIOSIS CAN
ALTER CHROMOSOME NUMBER

Nondisjunction
MEIOSIS I
MEIOSIS II
Normal
meiosis II
Gametes
Number of
chromosomes
Abnormal gametes
n  1 n  1 n  1 n  1

Normal
meiosis I
MEIOSIS I
MEIOSIS II
Nondisjunction
Abnormal gametes Normal gametes
n  1 n  1 n n

Sex chromosome abnormalities tend to
be less severe, perhaps because of
• the small size of the Y chromosome
or
• X-chromosome inactivation.
C. ABNORMAL NUMBERS OF SEX
CHROMOSOMES

 In general,
• a single Y chromosome is enough to produce
“maleness,” even in combination with several
X chromosomes, and
• the absence of a Y chromosome yields
“femaleness.”

 The following table lists the most common human sex
chromosome abnormalities.

 Errors in mitosis or meiosis may produce
polyploid species, with more than two
chromosome sets.
 .
D. NEW SPECIES CAN ARISE FROM
ERRORS IN CELL DIVISION

 Chromosome breakage can lead to
rearrangements that can produce:
• genetic disorders or,
• if changes occur in somatic cells, cancer.
8. ALTERATIONS OF CHROMOSOME
STRUCTURE

• a deletion, the loss of a chromosome
segment,
• a duplication, the repeat of a chromosome
segment,
• an inversion, the reversal of a chromosome
segment, or
• a translocation, the attachment of a segment
to a nonhomologous chromosome that can be
reciprocal.
THESE REARRANGEMENTS MAY INCLUDE:

THESE REARRANGEMENTS MAY INCLUDE:
Deletion
Duplication
Inversion
Reciprocal translocation
Homologous
chromosomes Nonhomologous
chromosomes

• 1ste: 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. 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 noted in Module 8.11.
• 4de: Student 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.1.
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. 2. 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! 3. 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.)

• 10de: 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. Challenge students to identify which stage of meiosis is most like mitosis.
Comparing the specific events of mitosis, meiosis I, and meiosis II to each other allows students
to identify essential differences.
• 14de: 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 8.17, 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!)

• 26ste: 1. The possible number of combinations produced by independent
orientation of human chromosomes at meiosis metaphase I is 223 or
8,388,608. This number squared is more than 70 trillion. The authors
rounded down to 8 million for 223 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. Four possible pairs 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!

14de: 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 8.17, 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!)

• 26ste: 1. The possible number of combinations produced by independent
orientation of human chromosomes at meiosis metaphase I is 223 or
8,388,608. This number squared is more than 70 trillion. The authors
rounded down to 8 million for 223 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. Four possible pairs 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!

• 30ste: 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 8.3B and 8.14 will be particularly helpful. A firm
foundation in chromosome basics is necessary to understand the
irregularities discussed in Modules 8.19–8. 1. The Human
Genome Website is a tremendous asset for nearly every
discussion related to human genetics. It can be accessed at
www.genomics.energy.gov. 2. If you have several hundred
students or more in your class, it is likely that at least one of your
students has a sibling with Down syndrome. The authors note
that, overall, about one in every 700 babies are born with Down
syndrome. 3. The National Down Syndrome Society has a
website at www.ndss.org. It is a wonderful resource.

Meiosis

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