Summary of chromosome
characteristics
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1.
2.
Diploid set for humans; 2n = 46
Autosomes; homologous chromosomes, one
from each parent (humans = 22 sets of 2)
Sex chromosomes (humans have 1 set of 2)
Female-sex chromosomes are homologous
(XX)
Male-sex chromosomes are non-homologous
(XY)
Ploidy: Number of sets of chromosomes
in a cell
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Haploid (n)-- one set chromosomes
Diploid (2n)-- two sets
chromosomes
Most plant and animal adults are
diploid (2n)
Eggs and sperm are haploid (n)
Meiosis
Meiosis has some similarities to mitosis
 Meiosis has some very important
differences too!
 During meiosis and the cytokinesis that
follow, one diploid mother cell (2n)
makes four unique daughter cells and
they are all haploid (1n)!
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Meiosis
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Meiosis is often called "reduction
division" because the genetic material
is reduced - by half.

Meiosis is extremely important not
only for sexual reproduction, but also
for creating the diversity upon which
natural selection operates.
Sexual reproduction
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Combines the genetic information from two
different cells that are usually from two
different individuals.
Each parent contributes a haploid (n) gamete
(sex cell).
These two gametes, a female egg and a
male sperm, fuse to produce a genetically
unique single cell called a zygote.
The zygote is diploid (2n) having
combined the genetic information
provided by the gametes (n) of both
parents.
In a very real genetic sense, all offspring
of sexual reproduction are "half identical"
to each parent!
If it were not for meiosis, sexual
reproduction would double
chromosome sets every
generation!
That's deadly and that's why
meiosis is important.
That new individual will be diploid
having been created by the fusion of
two haploid (n) cells.
It goes from generation to generation.
Our genetic material has been handed
down in "half loads" (haploid gametes =
n) from our ancestors and fused into
"full loads" (diploid zygotes = 2n).
Each individual is unique but is also a mix.
We are "half like" (½) each of our (two)
parents, a "quarter like" (¼) each of our
(four) grandparents, an "eight like" (1/8)
each of our (eight) great grandparents, a
"sixteenth like" (1/16) each of our (sixteen)
great, great grandparents, etc.
But meiosis is not "simply" a halving
of the genetic materials.
Chromosomes have to be
distributed in a precise manner to
make sure that a full set (n) of
chromosomes is in the gametes.
Meiosis has three purposes:
1St: reduction of the chromosomes'
number from diploid (2n) to haploid (n).
2nd: ensure the haploids are composed
of a complete set of chromosomes.
3rd promote genetic diversity among
the products (gametes).
The "trick" to meiosis is to understand that
the DNA is replicated only once, during S
phase in interphase, BUT the nucleus
divides TWICE!
It is also important to understand that a
diploid cell, containing its normal 2n
complement of chromosomes, is composed
of a set (n) from each parent and they can
be thought of as pairs of chromosomes.
What is meiosis I?
In meiosis I, chromosomes in a
diploid cell resegregate, producing
four haploid daughter cells.
 It is this step in meiosis that
generates genetic diversity.
 The phases of meiosis I & II
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Prophase I
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DNA replication
precedes the start
of meiosis I.
During prophase I,
homologous
chromosomes pair
and form synapses,
a step unique to
meiosis.
Prophase I
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The paired
chromosomes
are called
tetrads. Here
genetic
recombination
can occur by a
process called
crossing over.
Prophase I
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Chromosomal
condensation allows
these to be viewed in
the microscope.
Tetrads have two
chromosomes and
four chromatids,
One chromosome
coming from each
parent.
Late Prophase I/Prometaphase I
The nuclear
membrane
disappears.
 The
chromosomes
attached to
spindle fibers
begin to move.
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Metaphase I
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Tetrads, each
composed of two
chromosomes (four
chromatids) align at
the metaphase plate.
The orientation is
random, with either
parental homologue
on a side.
Metaphase I
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This means that
there is a 50-50
chance for the
daughter cells to get
either the mother's
or father's
homologue for each
chromosome.
Anaphase I
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Tetrads separate.
Chromosomes, each
with two chromatids,
move to separate
poles.
Each of the daughter
cells is now haploid
(23 chromosomes),
but each
chromosome has two
chromatids.
Telophase I
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Nuclear
envelopes may
reform, or the
cell may quickly
start meiosis II.
Cytokinesis
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Analogous to
mitosis where
two complete
daughter cells
form.
Meiosis II
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Meiosis II is similar to mitosis.
However, there is no "S" phase.
The chromatids of each chromosome
are no longer identical because of
recombination.
Meiosis II separates the chromatids
producing two daughter cells each
with 23 chromosomes (haploid), and
each chromosome has only one
chromatid.