Meiosis
Meiosis – M phase that occurs in the ovaries
(females) or testes (males) of an organism to
create gametes or sex cells
• Produces 4 haploid cells using 2 stages of meiosis
(Meiosis I and Meiosis II)
• Makes 4 n sperm cells in males (spermatogenesis)
• Makes 1 n egg cell and 3 n polar bodies in females
(oogenesis)
Meiosis I
Prophase I
• Nuclear membrane and
nucleolus dissolves
• Chromatin finishes coiling into
Chromosomes
• Centrioles move to opposite poles and
begin forming spindle fibers
• Homologous chromosomes pair together through
synapsis
– Independent assortment occurs
(no way to know which cell will get which
chromosome from each pair)
– Crossing over of genes may occur with
the tetrad pair creating genetic recombination
Homologous chromosomes - chromosome pairs,
one from each parent, that are the same in
length, gene position, and centromere location
• The position of the genes on each
homologous chromosome is the same, however
the genes may contain different alleles
• Called a tetrad
Metaphase I
• Spindle fibers are attached to the
chromosomes at their centromeres
• Tetrads (homologous chromosome
pairs) are pulled to the metaphase plate
or center of the cell be the spindle fibers
Anaphase I
• Spindle fibers pull the homologous chromosomes to
opposite sides of the cell
• Each chromosome still has two sister
chromatids
• Nondisjunction can occur here
Nondisjunction - failure of homologous chromosomes
or sister chromatids to
separate properly during cell division
Telophase I
• Chromosomes stay in their coiled up shape
• Nuclear membrane may or may not reform; depends
on the species
• Cytokinesis occurs (Cleavage furrow forms in animal
cells; cell plate forms in plant cells)
• Produces 2 diploid daughter cells
The cells will enter a brief interphase but they DO NOT
replicate their DNA again. It is just long enough to
prepare for Meiosis II. How long depends on the
species (can be minutes to hours)
Meiosis II (Looks just like mitosis but there are two
cells)
Prophase II
• Spindle fibers reattach to the
centromeres
• If nuclear membrane reformed, it dissolves again
• Cenrioles move to opposite sides of the cell
Metaphase II
• Individual chromosomes line up on the
metaphase plate or equator of each
cell
Anaphase II
• Spindle fibers pull the chromosomes
apart causing sister chromatids to go to opposite
sides of the cell
Telophase II
• Nuclear membrane and nucleolus reappears
• Spindle fibers dissolve
• Chromatids uncoil into chromatin
• Cytokinesis occurs (Cleavage furrow
forms in animal cells; cell plate forms in
plant cells)
• Produces 4 different haploid cells
Meiosis provides variation in sexual reproduction
increasing species ability to evolve and survive
(Natural selection; survival of the fittest)
• Caused by independent assortment during the
formation of a tetrad
• Caused by crossing-over within
homologous chromosomes
Genetic Recombination
nonsister chromatids
chiasmata: site
of crossing over
Tetrad
variation
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