Meiosis

• The form of cell division by which gametes, with half the number of chromosomes, are produced.
• Diploid (2n) --------> haploid (n)
• Meiosis is sexual reproduction.
• Two divisions (meiosis I and meiosis II).

• Sex cells divide to produce gametes (sperm or egg).
• Gametes have half the # of chromosomes.
• Occurs only in gonads (testes or ovaries).
• Male: spermatogenesis
• Female: oogenesis
• Meiosis is similar to mitosis with some chromosomal differences.

• Similar to mitosis interphase.
• Chromosomes replicate (S phase).
• Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres.
• Centriole pairs also replicate.

• Nucleus and nucleolus visible.

Cell division that reduces the chromosome number by one-half.

four phases:

a. prophase I

b. metaphase I

c. anaphase I

d. telophase I

• Longest and most complex phase (90%).
• Chromosomes condense.
• Synapsis occurs: homologous chromosomes come together to form a tetrad.
• Tetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

• Pair of chromosomes (maternal and paternal) that are similar in shape and size.
• Homologous pairs (tetrads) carry genes controlling the same inherited traits.
• Each locus (position of a gene) is in the same position on homologues.
• Humans have 23 pairs of homologous chromosomes:
• 22 pairs of autosomes
• 1 pair of sex chromosomes

• Crossing over (variation) may occur between nonsister chromatids at the chiasmata.
• Crossing over: segments of nonsister chromatids break and reattach to the other chromatid.
• Chiasmata (chiasma) are the sites of crossing over.

• In terms of Independent Assortment -how many different combinations of sperm could a human male produce?

• Homologous chromosomes separate and move towards the poles.
• Sister chromatids remain attached at their centromeres.

• Each pole now has haploid set of chromosomes.
• Cytokinesis occurs and two haploid daughter cells are formed.

• No interphase II
• (or very short - no more DNA replication)
• Remember: Meiosis II is similar to mitosis

• same as prophase in mitosis

• Same as telophase in mitosis.
• Nuclei form.
• Cytokinesis occurs.
• Remember: four haploid daughter cells produced.
• gametes = sperm or egg

1. crossing over (prophase I)

2. independent assortment (metaphase I)

3. random fertilization

Remember: variation is good!

A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?

10 chromosomes (haploid)

A method of organizing the chromosomes of a cell in relation to number, size, and type.

• The fusion of a sperm and egg to form a zygote.
• A zygote is a fertilized egg