Meiosis is similar in many ways to mitosis
However, it is also very different…
–Involves 2 cell divisions
–Each parent cell results in 4 cells
with 1/2 the normal genetic
information, not identical
daughter cells
•Meiosis results in the
formation of haploid (n) cells.
–In Humans, these are the Ova
(egg) and sperm.
–Ova are produced in the ovaries
in females
• Process called oogenesis
–Sperm are produced in the
testes of males.
•Process called spermatogenesis
•Meiosis occurs in 2 phases: Meiosis 1 & Meiosis 2
• Prior to the first division, the amount of DNA
doubles
What stage of the cell cycle would this “doubling” take place
in?
What stage
of interphase
would this
doubling take
place?
the largest differences between mitosis and meiosis occur in
prophase I.
• Chromosomes condense and attach
to the nuclear envelope
Chiasma
• Synapsis is when a pairing of
homologous chromosomes occurs and
a tetrad is formed. Each tetrad is
composed of four chromatids.
• Crossing over may occur, and
chiasmata become apparent.
• Each chromosome gets attached
to the spindle fiber in transition
to metaphase.
Synapsis is time
consuming! It is
estimated that prophase
accounts for some 85% 95% of the total time for
meiosis.
All chromosomes are now positioned at the spindle’s
equator.
The orientation of the
tetrads is random, with
either parental homologue
on a side.
As there are 46 chromosomes in
human cells, that means that 23
chromosomes end up on either side.
This means that there is a 5050 chance for the daughter
cells to get either the
mother's or father's
homologue for each
chromosome.Because of the exchange during
crossing over, these chromosomes
are different than they were
Each chromosome is separated from its homologue,
and they are moved to opposite poles of the spindle.
Chiasmata separate.
Chromosomes, each
with two chromatids,
move to separate poles.
This is the point when many
chromosomal abnormalities
occur, due to incomplete
separation.
(Nondisjunction)
•The cytoplasm of
the germ cell
divides.
“Cytokinesis”
• Each of the daughter cells is now haploid, but each
chromosome still has two chromatids.
Remember…each new cell has duplicates of either mom’s or dad’s
chromosomes…but not both, like what happens in mitosis.
• Nuclear envelopes may reform, or the cell may
quickly start meiosis II.
In what two ways does the Meiotic Cell cycle differ from
the Mitotic Cell cycle?
What happens just prior to Meiosis I?
Which stage of the meiotic cell cycle is most different from
the mitotic cell cycle?
What happens during synapsis, after tetrads form?
How is the chromosomal arrangement during Metaphase I
different than the chromosomal arrangement during
Metaphase?
At what point during the Meiotic cycle can many
chromosomal abnormalities occur?
What process divides the cytoplasm at the end of Meiosis I?
Meiosis II is the second part of the meiotic process.
Much of the process is similar to mitosis and meiosis
I.
Microtubules have moved one
member of the centriole pair
(from the centrosome) to the
opposite pole of each
daughter cell.
Microtubules attach from
centriole to the chromosomes,
and they are driven toward
the spindle’s equator.
Chromosomes are positioned at the spindle equator,
midway between the two poles.
The kinetochores of the
sister chromatids point
toward opposite poles.
The kinetochores
assemble on the
centromere, and are areas
responsible for joining
the chromosome to the
microtubule from the
spindle fiber.
The attachment between the two chromatids of each
chromosome breaks.
Each of the former
sisters, is now a separate
chromosome in its own
right.
(Similar to anaphase of
mitosis)
Distinct nuclei form
at the opposite
poles and
cytokinesis occurs.
At the end of
meiosis II, there
are four daughter
cells each with one
half the number of
chromosomes of the
original parent cell.
It is really this “second division” of Meiosis II that
is responsible for reducing the number of
chromosomes in each daughter.
What is a kinetochore?
How do chromosomes line up at the
metaphase plate during Meiosis II,
as opposed to Meiosis I?
Which part of the Meiotic Cell
cycle is mostly responsible for the
reduction in chromosome number
to ½ in each daughter cell?
In human female embryos, primary oocytes complete
interphase and prophase I, where they are frozen and remain
this way until the female reaches puberty.
A female is born with about 2
million primary oocytes. By
the time she reaches
puberty, about 400,000 are
left (most of them die
through the years). This is
still way more than enough!
If a woman is fertile on
average for 40 years, then
40 X 12 = 580 eggs in her
lifetime. Thus, 400,000 eggs
is plenty!
Asymmetrical cytokinesis leads to
the production of polar bodies
during oogenesis.
To conserve nutrients, the
majority of cytoplasm is
segregated into the secondary
oocyte during meiosis I , when the
secondary oocyte is formed.
The remaining daughter cells generated from the
meiotic events contain relatively little cytoplasm and
are referred to as polar bodies. Eventually, the polar
bodies degenerate.
Of the four haploid cells that form by way of meiosis
and cytoplasmic divisions, one or all may develop into
gametes and function in sexual reproduction.
Haploid
Haploid, but
duplicated
Diploid
Diploid
When the haploid egg, fuses with a sperm to become
fertilized, the normal complement of chromosomes is again
restored.
Spermatogonia (precursors to sperm)
divide by mitosis to produce more
spermatogonia or
spermatocytes.
Meiosis of each spermatocyte
produces 4 haploid spermatids.
This process takes over three
weeks to complete.
Then the spermatids form sperm,
losing most of their cytoplasm in
the process.
With 22 pairs of autosomes and an
average of two crossovers between
each pair, there is huge variation
among sperm.
When do primary oocytes form in human females?
How is cytokinesis different in oogenesis than
spermatogenesis?
When is the normal complement of chromosomes
again achieved?
What is a precursor to sperm called?
How are additional spermatogonia produced?
Why are sperm so small compared to eggs?