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 meiosis II

Meiosis Sex cells divide to produce gametes sperm or egg Gametes have half the # of chromosomes.

Meiosis Occurs only in gonads (testes or ovaries). Processes: Male: spermatogenesis -making of sperm Female: oogenesis -making of eggs 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

Stages: Interphase I Meiosis I Interphase II Meiosis II

Interphase I Similar to mitosis interphase. DNA replicates in the S phase Each duplicated chromosome will eventually (prophase I) consist of two identical sister chromatids attached at their centromere.

Interphase I Nucleus and nucleolus are 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 total. One of the chromosomes comes from Mom and one from Dad

Prophase I Homologous chromosomes sister chromatids Tetrad

Prophase I Chromosomes condense. Tetrads form.

Metaphase I Shortest phase Tetrads align on the equator. Chromosomes are lined up with their homologue.

Metaphase I INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pairs (to poles) is random. In the tetrad, Mom’s chromosome could be on the left or the right with Dad’s on the other side. Leads to Genetic Variation

Metaphase I

Metaphase I Independent Assortment: Humans have 46 chromosomes or 23 pairs of chromosomes. Since each pair lines up randomly from every other pair, there are 8 million different combinations of the 23 pairs!!!

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. BUT 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. Can you figure out how?

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

Prophase II same as prophase in mitosis (Hint: you already Know this picture! But now BOTH cells go on together…)

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

Anaphase II same as anaphase in mitosis The chromatids separate at their centromeres and move towards the 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 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. Either the chromosome from Mom or Dad but NOT both!!!---Haploid!

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

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome (double stranded) # of Chromosomes in the end 1 of each kind of chromosome 1 of each kind of chromosome (single stranded) How they line up on the equator Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 kind of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell (2 identical cells) Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell Not Done Yet Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell Not Done Yet Gamete Haploid vs Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell Not Done Yet Gamete Haploid vs Diploid Diploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell Not Done Yet Gamete Haploid vs Diploid Diploid Haploid

Comparing Mitosis and Meiosis Characteristic Mitosis Meiosis I Meiosis II # of Chromosomes in the beginning 2 of each kind of chromosome 1 of each kind of chromosome # of Chromosomes in the end How they line up on the equator Individually In homologous pairs tetrads Replication of DNA S phase of interphase S phase of interphase I No replication in interphase II Separation of Chromosomes during Anaphase Separate 2 chromatids at the centromere Separate Homologous Pairs End Product Somatic Cell Not Done Yet Gamete Haploid vs Diploid Diploid Haploid