Meiosis I- Halving the
Chromosomes
Meiosis is a part of the sexual reproductive
life cycle. Its purpose is to take a diploid cell
(2N) and make 4 haploid (1N) cells. It is also
to produce gametes with diversity.
The cell divides twice - Meiosis I and Meiosis
II
Meiosis I
The purpose of meiosis I is to take a
diploid cell with double stranded
chromosomes and produce two haploid
cells with double stranded chromosomes
and to achieve genetic diversity.
Meiosis IIIt is just like mitosis but the DNA has
already been replicated. It purpose is
to separate double stranded
chromosomes into single stranded
chromosomes.
DefinitionsDiploid-Having two sets of chromosomes.
Advantage-hides deliterous genes.
Haploid-Having one set of chromosomes.
"What you see is what you get".
Advantage-easier to reproduce
Homologous chromosomesChromosomes that have the genes
(coding for the same trait) in the
same order.
Interphase- G1,S,G2.
DNA has been
replicated.
Centrosomes with
centrioles repli-cate.
Cetrosomes are
microtubules
organizing centers
Prophase I- lasts
longer and is more
complicated then
prophase of mitosis.
Early prophaseChromosomes
become visible as
long well separated
filaments.
Middle prophase homologous chromosomes
become thicker and shorter, and they synapse.
Crossing over occurs.
The 2 homologs find one another and long
protein axes join the two homologs forming a
tetrad. Recombination nodules (proteins) .
Each nodule will clip the homologs at
precisely the same place and the resulting
fragments are spliced to one another forming
recombined chromatids or hybrid
chromatids. The point of crossing over
is called a chiasma. When the two
homologs are paired, they are called a
tetrad.
Late Prophase I The
tetrad structure and
chiasmata are visible. Nuclear
membrane disappears. Kinetochore microtubules attach chromosomes to poles of the
centrosome.
In mitosis,
kinetochore
microtubules attach
to both sides of the
centromere, but in
meiosis I they on
attach to one side.
Metaphase I- The
homolgous
chromosomes move
as unit to the
metaphase plate.
The chromsomes line
up on the plate two
by two instead of
single file.
Which side of the metaphase plate, the
chromosomes line up on is a matter of chance.
This allows different combinations of
chromosomal arrangements.
The previous card shows that having two
homologous pairs of chromosomes will
result in 4 different gametes. The next card
will show that having three homologous
pairs can result in 8 different gametes. The
equation for this is
2n where n is equal to the haploid number of
chromosomes. Human have the possibility
of making 223 different types of gametes
which is why two siblings are never
identical unless they are identical twins.
Anaphase I-Centromeres do not split.
Homologous
chromosomes move
apart to opposite poles.
Spindle fibers from
opposite poles interact
to force poles apart.
Two sets of homologous chromosomes are
nearing poles. The resulting nuclei will become
double stranded haploid cells.
Telophase I- New
nuclear membrane
begins to form.
Chormosomes
become longer and
thinner and as they
uncoil.
Cells finishing telophase I resulting in two
haploid cells that have double stranded
chromosomes.