MEIOSIS Meiosis The form of cell division by
MEIOSIS
Meiosis • The form of cell division by which gametes, with half the number of chromosomes, are produced. • Diploid (2 n) haploid (n) • Meiosis is sexual reproduction.
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 Two divisions (meiosis I and meiosis II). II • (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 that reduces the chromosome number by one-half. • four phases: phases a. prophase I b. metaphase I c. anaphase I d. telophase I
Prophase 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).
Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids
Metaphase I • Shortest phase • Tetrads align on the metaphase plate • INDEPENDENT ASSORTMENT OCCURS: 1. Orientation of homologous pair to poles is random. 2. Variation 3. Formula: 2 n Example: 2 n = 4 then n=2 thus 22 = 4 combinations How many combinations are there for humans? ?
Metaphase I OR metaphase plate
Answer • Formula: 2 n • Human chromosomes: 2 n = 46 n = 23 • 223 = ~8 million combinations
Anaphase I • Homologous chromosomes separate and move towards the poles. • Sister chromatids remain attached at their centromeres
Anaphase I
Telophase I • Each pole now has haploid set of chromosomes • Cytokinesis occurs and two haploid daughter cells are formed.
Telophase I
Meiosis II • No interphase II (or very short - no more DNA replication) replication • Remember: Meiosis II is similar to mitosis
Prophase II • same as prophase in mitosis
Metaphase II • same as metaphase in mitosis metaphase plate
Anaphase II • same as anaphase in mitosis • sister chromatids separate
Telophase II • Same as telophase in mitosis • Nuclei form. • Cytokinesis occurs. • 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 • The fusion of a sperm and egg to form a ZYGOTE • A zygote is a fertilized egg n=23 egg sperm n=23 2 n=46 zygote
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