Meiosis
Producing Gametes
(eggs & sperm)
Another form of cell division?!
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Mitosis produces two cells with the same
number of chromosomes as the parent cell.
So -- Mitosis of a diploid cell (2n) produces
two diploid daughter cells.
If two diploid cells went on to participate in
sexual reproduction, they would produce a
tetraploid (4n) zygote (which would be a
problem!).
The Solution…
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Meiosis is a process of cell division in
eukaryotes characterized by:
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Two divisions in a row: meiosis I and meiosis II
no DNA synthesis / replication (no S phase)
between the two divisions
Result: 4 cells with half the number of
chromosomes as the starting cell (2n → n)
Animation – Mitosis vs. Meiosis
Gamete + Gamete = Zygote
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Fusion of two n (haploid) cells produces a
2n zygote.
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So – Meiosis is used to produce the
gametes: sperm and eggs
Interphase
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The parent cell is diploid (2n) – it has one
copy of a chromosome from “mom” and
another copy of that chromosome from “dad”.
Before Meiosis I, the cell prepares for
division (G1, S, G2); most importantly,
chromosomes are copied in S phase.
After S phase, the parent cell is still diploid,
but now with replicated chromosomes.
Homologous Chromosomes
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Homologous chromosomes are similar but
not identical; a pair may carry different
versions of the same genetic information.
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Ex: A pair carries information about hair color;
one homologous chromosome may be for blond
hair while the other homologous chromosome
may be for black hair.
One chromosome was inherited from “mom”,
the other from “dad”
Chromosomes and Chromatids
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As a cell prepares to enter meiosis, each of
its chromosomes has duplicated, as in
mitosis.
Each chromosome thus consists of two sister
chromatids, joined at the centromere.
Meiosis I
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At the beginning of Meiosis I, a human cell
contains:
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46 chromosomes, or
92 chromatids.
This is the same number as during mitosis.
So, it starts with one diploid (2n) cell.
Prophase I
Early Prophase
Late Prophase
Video clip – 1:09
Prophase I (cont’d)
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Nuclear membrane dissolves, spindle fibers
form
Chromatids shorten and thicken, become
visible
Synapsis occurs -- the two homologous
chromosomes come near each other.
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So -- there are actually four chromatids aligned
next to one another. This combination of four
chromatids is called a tetrad
Prophase I (cont’d)
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Crossing over occurs
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segments of DNA from one chromatid in the
tetrad pass to another chromatid in the tetrad.
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result = a genetically new chromatid.
Metaphase I
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Video clip – 0:17
Tetrads / pairs of homologous chromosomes
line up at equator
Spindle fibers attach to centromeres
Anaphase I
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Homologous chromosome pairs separate.
One homologous chromosome (consisting of
two chromatids) moves to one side / pole of
the cell, while the other homologous
chromosome (consisting of two chromatids)
moves to the other side of the cell.
Video clip – 0:24
Anaphase I
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Result in humans = 23 chromosomes (each
consisting of two chromatids) move to one
pole, and 23 chromosomes (each consisting
of two chromatids) move to the other pole.
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Essentially, the chromosome number of the cell is
halved.
So the cells are considered haploid (n).
For this reason the process is a reductiondivision.
Telophase I
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New nuclear membrane forms around each
set of chromosomes (still consisting of 2
sister chromatids)
Cytokinesis occurs -- each daughter cell
(with 23 chromosomes each consisting of
two chromatids) then enters interphase,
during which there is no duplication of the
DNA
Video clip – 0:13
Meiosis II
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occurs in essentially the same way as
mitosis.
a cell containing 46 chromatids undergoes
division into two cells, each with 23
chromosomes.
Starts with two haploid cells; ends with four
haploid cells.
Prophase II
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Video clip – 0:30
chromosomes condense
each chromosome contains two chromatids
attached by the centromere
Metaphase II
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Video clip – 0:13
chromatid pairs (chromosomes) gather at the
center of the cell
identical to metaphase in mitosis
Anaphase II
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Video clip – 0:14
Spindle fibers pull chromosomes apart at the
centromeres
Spindle fibers move sister chromatids to
opposite poles (sides) of the cell
Video clip – 0:44
Telophase II
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At end: animation
Chromatids – now called chromosomes
again - gather at the poles of the cells
A new nuclear envelope forms around each
set of chromosomes
Usually, cytokinesis happens.
The Results of Meiosis
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During meiosis II, each cell containing 46
chromatids produces two cells, each with 23
chromosomes.
Originally, there were two cells that
underwent meiosis II; therefore, the result of
meiosis II is four cells, each with 23
chromosomes.
Each of the four cells is haploid; that is, each
cell contains a single set of chromosomes.
The Results of Meiosis (cont’d)
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The 23 chromosomes in the four cells from
meiosis are not identical because crossing
over has taken place in prophase 1.
The crossing over creates variation, so that
each of the four resulting cells from meiosis
differs from the other three.
The Results of Meiosis (cont’d)
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Thus, meiosis provides a mechanism for
producing variations in the chromosomes.
Also, it accounts for the formation of four
haploid cells from a single diploid cell.
Video clip – The Importance of Meiosis – 1:19
Animation – Cells Alive
Spermatogenesis
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Meiosis that produces sperm (male animals)
Occurs in testes
The four haploid cells at the end of meiosis II
will all become sperm
Oogenesis
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Meiosis that produces eggs (ovum =
singular; ova = plural)
Occurs in ovaries
Only one of the four haploid cells at the end
of meiosis II becomes an egg; the other three
cells – all smaller than the egg because they
get less cytoplasm – are called polar bodies
Video clip – 0:28
Errors in Meiosis
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It is estimated that 10–20% of all human
fertilized eggs contain chromosomal
abnormalities, and these are the most
common cause of pregnancy failure /
miscarriage / spontaneous abortion (35% of
miscarriages).
About Chromosomal Abnormalities
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arise from errors in meiosis, usually meiosis
I;
occur more often (90% of the time) during
egg formation than during sperm formation;
become more frequent as a woman ages.
Chromosomal Abnormalities (cont’d)
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Aneuploidy — the gain or loss of whole
chromosomes — is the most common
chromosome abnormality. It is caused by
nondisjunction, the failure of chromosomes
to correctly separate:
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homologues during meiosis I or
sister chromatids during meiosis II
Video clip – 3:50
Chromosomal Abnormalities (cont’d)
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Zygotes missing one chromosome
("monosomy") cannot develop to birth (except
for females with a single X chromosome).
Three of the same chromosome ("trisomy") is
also lethal except for chromosomes 13, 18, and
21 (trisomy 21 is the cause of Down syndrome).
Three or more X chromosomes are viable
because all but one of them are inactivated.
Video clip – 4:19
Klinefelter Syndrome (XXY)
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Trisomy of sex chromosomes
Male
Infertile (sterile)
Klinefelter’s Syndrome (cont’d)
Turner Syndrome (XO)
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Monosomy of sex chromosomes
Female (“O” indicates no second X
chromosome)
Infertile
Turner Syndrome (cont’d)
Down Syndrome (Trisomy 21)
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Trisomy of 21st pair of chromosomes
Mental retardation
Characteristic facial features
Physiological problems – esp. with the heart
Down Syndrome (cont’d)