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Meiosis-An Introduction
Unit 8: Cell Reproduction
Quiz 3 Meiosis
Meiosis
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Meiosis is a process of nuclear division
that reduces the number of chromosomes
in new cells to half the number in the
original cell.
Why is there a need for MEIOSIS?
Most eukaryotes reproduce sexually.
 Sexual reproduction involves the union of
two reproductive cells to produce a new
individual.
 The reproductive cells must be haploid so
that when they unite to form the new
individual the chromosome number is
restored.
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Overview
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Start with a diploid cell, with 2
copies of each chromosome, one
form each parent. The two copies
are called homologues.
Chromosomes each with 2
chromatids attached at the
centromere.
Use 2 cell divisions:
Meiosis 1. First separate the
homologues
Meiosis 2. Then separate the 2
chromatids.
The stages of meiosis have the
same names as in mitosis:
prophase, metaphase, anaphase,
telophase. Each of the 2 cell
divisions has all of these stages.
Meiosis 1 is unusual and needs a
bit of study, but meiosis 2 is just
like mitosis
Meiosis 1
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Two important events in
meiosis 1: crossing over in
prophase, and the pairing of
homologues in metaphase.
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Crossing over. Homologues
break at identical locations,
then rejoin opposite partners.
This creates new
combinations of the alleles on
each chromosome. Occurs
randomly several times on
every chromosome. Results
in mixing of the genes you
inherited from your parents.
More Meiosis 1
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The main event in meiosis 1
is the pairing and separation
of the homologues.
At metaphase, the pairs of
homologous chromosomes
line up side by side. This
does not happen in mitosis of
meiosis 2, but only in meiosis
1.
At anaphase of meiosis 1, the
pairs of homologues are
pulled to opposite poles by
the spindle.
Note: the centromeres do
NOT divide; the chromosomes
remain in the 2-chromatid Xshaped state.
Result of Meiosis 1
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Go from 1 cell to 2 cells. Each
daughter cell contains 1 copy
of each chromosome: they
are haploid, with the
chromosomes still having 2
chromatids each.
For humans: start with one
cell containing 46
chromosomes (23 pairs) to 2
cells containing 23
chromosomes.
As a result of crossing over,
each chromosome is the
mixture of the original
homologues.
Meiosis 2
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Meiosis 2 is just like mitosis
No replication of DNA
between meiosis 1 and
meiosis 2.
Chromosomes line up
individually on the equator of
the spindle at metaphase.
At anaphase the centromeres
divide, splitting the 2
chromatids.
The one-chromatid
chromosomes are pulled to
opposite poles.
More Meiosis 2
Summary of Meiosis
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2 cell divisions.
Start with 2 copies of each
chromosome (homologues), each
with 2 chromatids.
In meiosis 1, crossing over in
prophase mixes alleles between
the homologues.
In metaphase of meiosis 1,
homologues pair up, and in
anaphase the homologues are
separated into 2 cells.
Meiosis 2 is just like mitosis. The
centromeres divide in anaphase,
giving rise to a total of 4 cells,
each with 1 copy of each
chromosome, and each
chromosome with only 1
chromatid.
Meiosis I:
1.
2.
Chromosomes are reduced from diploid (2N) to haploid (1N).
Four stages
Prophase I
Similar to prophase of mitosis, except that
homologous chromosomes pair and crossover. Spindle apparatus begins to form, and
nuclear envelope disappears.
Metaphase I
Chromosome pairs (bivalents) align across
equatorial plane. Random assortment of
maternal/paternal homologs occurs (different
from metaphase of mitosis).
Anaphase I
Homologous chromosome pairs separate and
migrate toward opposite poles.
Telophase I
Chromosomes complete migration, and new
nuclear envelopes form, followed by cell
division.
Meiosis II:
1.
2.
Similar to mitotic division.
Also four stages:
Prophase II
Chromosomes condense.
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Metaphase II
Spindle forms and centromeres align on the
equatorial plane.
Anaphase II
Centromeres split and chromatids are pulled
to opposite poles of the spindle (one sister
chromatid from each pair goes to each pole).
Telophase II
Chromatids complete migration, nuclear
envelope forms, and cells divide, resulting in
4 haploid cells. Each progeny cell has has
one chromosome from each homologous
pair, but these are not exact copies due to
crossing-over.
Fig. 1.20 The stages of meiosis in an animal cell
Crossing-over
Random
Assortment
Peter J. Russell, iGenetics: Copyright © Pearson Education, Inc., publishing as Benjamin Cummings.
Significant results of meiosis:
1.
Haploid cells are produced because two rounds of division follow one
round of chromosome replication.
2.
Alignment of paternally and maternally inherited chromosomes is
random in metaphase I, resulting in random combinations of
chromosomes in each gamete. Number of possible chromosome
arrangements = 2n-1.
3.
Crossing-over between maternal and paternal chromatids during
meiosis I provides still more variation. Moreover, the crossing-over
sites vary from one meiosis to another.