MEIOSIS

Meiosis cell divisiongametes, half chromosomes, 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.

Meiosis SEX CELLS gametes(sperm or egg) SEX CELLS divide to produce gametes (sperm or egg). GAMETES halfchromosomes GAMETES have half the # of chromosomes. Occurs only in gonads (testes or ovaries). Occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Two divisionsmeiosis I meiosis II Two divisions (meiosis I and meiosis II). (Meiosismitosis (Meiosis is similar to mitosis with some chromosomal differences.)

Overview Start with a diploid cell, with 2 copies of each chromosome, one form each parent. The two copies are called homologues. Chromosomes each with 2 chromatids attached at the centromere. Use 2 cell divisions: Meiosis 1. First separate the homologues Meiosis 2. Then separate the 2 chromatids. The stages of meiosis have the same names as in mitosis: prophase, metaphase, anaphase, telophase. Each of the 2 cell divisions has all of these stages. Meiosis 1 is unusual and needs a bit of study, but meiosis 2 is just like mitosis

Meiosis I (four phases) Cell division chromosome one-half. Cell division that reduces the chromosome number by one-half. four phases four 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%). Chromosomes Chromosomes condense. SYNAPSIS homologous chromosomes tetrad SYNAPSIS occurs: homologous chromosomes come together to form a tetrad. TETRADchromosomeschromatids TETRAD is two chromosomes or four chromatids (sister and nonsister chromatids).

Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad

Metaphase I Shortest phase Shortest phase Tetradsmetaphase plate Tetrads 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 How many combinations are there for humans????

Metaphase I metaphase plate OR metaphase plate

Answer Formula: 2 n Formula: 2 n Human chromosomes:2n = 46 Human chromosomes:2n = 46 n = 23 n = = ~8 million combinations 2 23 = ~8 million combinations

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

Anaphase I

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

Telophase I

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

Prophase II prophasemitosis same as prophase in mitosis

Metaphase II metaphasemitosis same as metaphase in mitosis metaphase plate

Anaphase II anaphasemitosis same as anaphase in mitosis sister chromatids separate sister chromatids separate

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

MEIOSIS II

SPERMATOGENESIS Differences between male and female gametes. In males, all 4 products of meiosis develop into sperm cells. They lose most of their cytoplasm, remodel their cell shape, and grow a long flagellum (tail).

OOGENESIS In females, the cell divisions of meiosis are asymmetric: most of the cytoplasm goes into 1 of the 4 meiotic products, which becomes the egg. Eggs also develop large amounts of yolk proteins, which are used to feed the developing embryo. The other 3 meiotic cells are small “polar bodies”, which degenerate.

Fertilization spermeggZYGOTE The fusion of a sperm and egg to form a ZYGOTE. A zygote is a fertilized egg n=23 egg sperm n=23 2n=46 zygote