Download Meiosis II

Meiosis
A double cell division to produce sex
cells (sperm and egg)
Meiosis
• The form of cell division by which gametes,
with half the number of chromosomes, are
produced.
• diploid (2n)  haploid (1n)
• Two divisions (meiosis I and meiosis II)
Meiosis
• Sex cells divide to produce gametes (sperm or
egg)
• Gametes have half the number of
chromosomes
• Occurs in the gonads (testes or ovaries)
– Male: spermatogenesis
– Female: oogenesis
• Meiosis is similar to mitosis with some
chromosomal differences
Interphase I
• Similar to mitosis interphase
• Chromosome replicated (S phase)
• Each duplicated chromosome consist of two
identical sister chromatids attached at their
centromeres
• Centriole pairs also replicate
Prophase I
• Chromosomes condense
• Synapis occurs: homologous chromosomes
come together to form a tetrad
• Tetrad is four chromatids (sister and nonsister
chromatids)
Homologous chromosomes
• Pair of chromosomes (material and paternal) that
are similar in shape and size.
• Homologous pairs (tetrads) carry genes
controlling the same inherited traits.
• Each locus (position of the gene) is in the same
position on the homologous chromosomes.
• Humans have 23 pairs of homologous
chromosomes
– 22 pairs of autosomes
– 1 pair of sex chromosomes
Crossing Over
• Crossing over (variation) may occur between
nonsister chromatids at the chismata.
• Crossing over: segments of nonsister
chromatids break and reattach to the other
chromatids
• Chismata (chiasma) are the sites of crossing
over.
Metaphase I
• Tetrads align on the metaphase plate (middle)
Anaphase I
• Homologous chromosomes separate and
move towards the poles.
• Sister chromatids remain attached at their
centromere
Telophase I
• Each pole now has sister chromatids that will
be separated
Meiosis II
• No interphase II
– Very short: no need for DNA replication
• Remember: Meiosis II is similar to mitosis
Telophase II
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•
•
•
Same as telophase in mitosis
Nuclei form
Cytokinesis will occur at the end
The final product should be four haploid
daughter cells
Gametes = sperm or egg
Animation Link
• Mitosis vs. Meiosis