MEIOSIS

Meiosis cell divisiongametes, halfchromosomes,The form of cell division by which gametes, with half the number of chromosomes, are produced. Diploid (2n)  haploid (n)Diploid (2n)  haploid (n) Meiosissexual reproduction.Meiosis is sexual reproduction. Two divisionsmeiosis I meiosis IITwo divisions (meiosis I and meiosis II).

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

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

Interphase I mitosisSimilar to mitosis interphase. ChromosomesChromosomes replicate. chromosome chromatids centromeresEach duplicated chromosome consist of two identical sister chromatids attached at their centromeres. CentrioleCentriole pairs also replicate.

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

Meiosis I (four phases) Cell division chromosome one-half.Cell division that reduces the chromosome number by one-half. four phasesfour phases: a.prophase I b.metaphase I c.anaphase I d.telophase I

Prophase I Longest and most complex phase (90%).Longest and most complex phase (90%). ChromosomesChromosomes condense. Synapsis homologous chromosomes tetradSynapsis occurs: homologous chromosomes come together to form a tetrad. Tetradchromosomes chromatidsTetrad is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Homologous Chromosomes chromosomesmaternalpaternalPair of chromosomes (maternal and paternal) that are similar in shape and size. (tetrads)Homologous pairs (tetrads) carry genes controlling the same inherited traits. locus(position of a gene)Each locus (position of a gene) is in the same position on homologues. homologous chromosomes.Humans have 23 pairs of homologous chromosomes. autosomes a.22 pairs of autosomes sex chromosomes b.01 pair of sex chromosomes

Homologous Chromosomes PaternalMaternal eye color locus eye color locus hair color locus hair color locus

Prophase I centrioles spindle fiber aster fibers

Metaphase I Shortest phaseShortest phase Tetradsmetaphase plateTetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS:INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example:2n = 4 then n = 2 thus 2 2 = 4 combinations thus 2 2 = 4 combinations

Metaphase I metaphase plate OR metaphase plate

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

Anaphase I

Telophase I haploid chromosomesEach pole now has haploid set of chromosomes. CytokinesisCytokinesis occurs and two haploid daughter cells are formed.

Telophase I

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

Prophase II prophasemitosissame as prophase in mitosis

Metaphase II metaphasemitosissame as metaphase in mitosis metaphase plate

Anaphase II anaphasemitosissame as anaphase in mitosis sister chromatids separatesister chromatids separate

Telophase II telophasemitosisSame as telophase in mitosis. Nuclei form. CytokinesisCytokinesis occurs. Remember:four haploid daughter cells produced.Remember:four haploid daughter cells produced. gametes = sperm or egg

Telophase II

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