Transcript Meiosis - NIU Department of Biological Sciences

Meiosis
Meiosis
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Cell division to form the gametes, sperm
(male gamete) and egg (female gamete).
Characteristic of eukaryotes only: not in
prokaryotes.
Normal cells are diploid: 2 copies of every
gene.
Diploidy is useful because 2 copies of every
gene means that there a backup copy if one
gets mutated. Mutations are very frequent in
the cells of large organisms. We wouldn’t
survive with just one copy of each gene.
Gametes are haploid: 1 copy of every gene
Need to choose 1 copy of each gene
randomly.
Why have sexual reproduction? Shuffling of
alleles between parents and offspring leads
to new combinations. Bad combinations die
without reproducing; good combinations
survive and reproduce more offspring.
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
• Two important events in
meiosis 1: crossing over in
prophase, and the pairing of
homologues in metaphase.
• 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
• 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
• 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
• 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.
Life Cycles
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Meiosis converts a diploid cell into
haploid cells. Fertilization combines
the 2 haploid gamete cells (sperm and
egg) back into a diploid cell.
Eukaryotes alternate between diploid
and haploid stages. This is called the
life cycle of the organism.
In plants, the haploid cells grow into
multicellular organisms. Unicellular
eukaryotes and many of the more
primitive plants (such as mosses and
ferns) spend most of their time as
haploids. The diploid stage in these
organisms is quite short.
In higher plants (the conifers and
flowering plants), the haploid stage is
very short and small. Pollen grains,
for instance, have a total of 3 haploid
cells in them, and the ovules contain 8
haploid cells. The rest of the plant
body is diploid.
Gamete Formation in Animals
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Differences between male and
female gametes.
In males, all 4 products of meiosis
develop into sperm cells. They
lose most of their cytoplasm,
remodel their cell shape, and grow
a long flagellum (tail).
In females, the cell divisions of
meiosis are asymmetric: most of
the cytoplasm goes into 1 of the 4
meiotic products, which becomes
the egg. Eggs also develop large
amounts of yolk proteins, which
are used to feed the developing
embryo. The other 3 meiotic cells
are small “polar bodies”, which
degenerate.