Warm Up #5 Thinking back to MITOSIS: Are haploid or diploid cells formed? When in DNA replicated? During what phase do chromosomes line up on the equator? How many daughter cells are formed?

MEIOSIS

Meiosis The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n) Involves two divisions (meiosis I and meiosis II).

Meiosis 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.

Spermatogenesis n=23 n=23 2n=46 sperm haploid (n) meiosis II human sex cell diploid (2n) n=23 meiosis I

Interphase I Similar to mitosis interphase. DNA replicates (S phase). Each duplicated DNA strand consist of two identical sister chromatids attached at their centromere DNA is loosely coiled--chromatin

Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus

Meiosis I (four phases) a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I Longest and most complex phase (90%). Homologous chromosomes come together to form a tetrad. Tetrad is two chromosomes or four chromatids.

Prophase I Homologous chromosomes sister chromatids Tetrad

Prophase I Chromosomes condense. Tetrads form.

Metaphase I Shortest phase Tetrads align (straddle) on the equator.

Metaphase I INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random. Leads to Variation Formula: 2n Example: if 2n = 4 then n = 2 thus 22 = 4 possible combinations

Question: 223 = ~8 million combinations In terms of Independent Assortment -how many different combinations of sperm could a human male produce? Formula: 2n Human chromosomes: 2n = 46 n = 23 223 = ~8 million combinations

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

Telophase I Each pole now has haploid set of replicated chromosomes. The chromosomes still consist of two chromatids.

Meiosis II Very short interphase II - no more DNA replication The next set of cell divisions will separate the chromatids. Meiosis II is similar to mitosis

Meiosis II (four phases) a. prophase 2 b. metaphase 2 c. anaphase 2 d. telophase 2

Prophase II same as prophase in mitosis

Metaphase II same as metaphase in mitosis The chromosomes line up at the equator

Anaphase II same as anaphase in mitosis The chromatids separate and move towards opposite poles

Telophase II Same as telophase in mitosis. Nuclei form. Four haploid daughter cells produced.

Meiosis n=2 n=2 2n=4 sperm haploid (n) meiosis II meiosis I sex cell diploid (2n)

The End Product of Meiosis In the end from one cell in the gonads (testes or ovary), 4 new cells will be formed. Each of the new cells will contain only one member from each homologous pair.

Question: 10 chromosomes (haploid) A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 chromosomes (haploid)

Question: A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? 10 chromosomes