Meiosis
What is Meiosis?
• Meiosis produces daughter cells that have one
half the number of chromosomes as the parent
cell.
»2N  N
Meiosis enables organisms to reproduce sexually.
Gametes (sperm and eggs) are haploid.
Meiosis involves two divisions producing a total of
four daughter cells.
What is Meiosis?
• Mitosis is normal cell division, which
goes on throughout life in all parts of
the body.
• Meiosis is the special cell division that
creates the sperm and eggs, the
gametes. We will discuss meiosis
separately.
Meiosis
•Cell division to form the gametes,
sperm (male gamete) and egg (female
gamete).
•Characteristic of eukaryotes only: not
• Normal cells are diploid: 2 copies of every
in prokaryotes.
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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.
Summary of the Phases of
Meiosis
• 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.
Summary of the Phases of
Meiosis
• Use 2 cell divisions:
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Meiosis 1. First
separate the
homologues
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Meiosis 2. Then
separate the 2
chromatids.
Summary of the Phases of
Meiosis
• 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
•A cell undergoing meiosis will divide two times; the first
division is meiosis 1 and the second is meiosis 2. The phases
have the same names as those of mitosis. A number indicates
the division number (1st or 2nd):
• meiosis 1: prophase 1, metaphase 1, anaphase 1, and telophase 1
• meiosis 2: prophase 2, metaphase 2, anaphase 2, and telophase 2
Meiosis 1
In the first meiotic division, the number
of cells is doubled but the number of
chromosomes is not. This results in 1/2 as
many chromosomes per cell.
•Two important events in meiosis
1: crossing over in prophase, and
the pairing of homologues in
metaphase.
Meiosis 1
Prophase I
•Homologous chromosomes
become paired.
•Crossing-over occurs
between homologous
chromosomes.
Meiosis 1
Metaphase I
• Homologous pairs
become aligned in the
center of the cell.
Meiosis 1
Anaphase I
• Homologous chromosomes
separate.
• Homologous Chromosomes
• Diploid organisms have two copies of
each chromosome (except the sex
chromosomes). Each pair of
chromosomes is homologous. For
example, the two #7 chromosomes are
homologous. The homologue to the #3
chromosome would be the other #3
chromosome.
Meiosis 1
Telophase I
• This stage is absent in some
species
• The nuclear membrane
reappears.
The chromosomes uncoil.
The spindle apparatus breaks
down.
The cell divides into two.
Meiosis 1
Interkinesis
• Interkinesis is similar to
interphase except DNA
synthesis does not occur.
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
• 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.