- Slides: 38
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. • Two divisions (meiosis I and meiosis II). 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 human sex cell sperm n=23 2 n=46 haploid (n) diploid (2 n) n=23 meiosis II
Interphase I • Similar to mitosis interphase. • Chromosomes replicate (S phase). • Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres • Centriole pairs also replicate.
Interphase I • Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus
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
Homologous Chromosomes • Pair of chromosomes (maternal and paternal) paternal that are similar in shape and size. • Homologous pairs (tetrads) carry genes controlling the same inherited traits. • Each locus (position of a gene) is in the same position on homologues. • Humans have 23 pairs of homologous chromosomes. a. 22 pairs of autosomes b. 01 pair of sex chromosomes
Homologous Chromosomes eye color locus hair color locus Paternal Maternal
Crossing Over • Crossing over (variation) may occur between nonsister chromatids at the chiasmata • Crossing over: over segments of nonsister chromatids break and reattach to the other chromatid • Chiasmata (chiasma) are the sites of crossing over
Crossing Over - variation nonsister chromatids chiasmata: site of crossing over Tetrad variation
Sex Chromosomes XX chromosome - female XY chromosome - male
Prophase I spindle fiber aster fibers centrioles
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
Metaphase I OR metaphase plate
Question: • In terms of Independent Assortment how many different combinations of sperm could a human male produce?
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
Telophase I • Each pole now has haploid set of chromosomes • Cytokinesis occurs and two haploid daughter cells are formed.
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 sex cell sperm n=46 n=23 2 n=46 (2 X in S Phase) diploid (2 n) n=23 haploid (n) n=23 n=46 n=23 meiosis II
Variation • Important to population as the raw material for natural selection. • Question: What are three sexual sources of genetic variation?
Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!
Question: • A cell containing 20 chromosomes (diploid number before S phase) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes
Answer: • 10 chromosomes (haploid)
Karyotype • A method of organizing the chromosomes of a cell in relation to number, size, and type.
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
Question: • A cell containing 40 chromatids at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes? chromosomes
Answer: • 10 chromosomes