MEIOSIS
Meiosis The form of cell division by which gametes, with half the regular number of chromosomes, are produced. diploid (2n) haploid (n) (complete set of chromosomes) (half the regular number of chromosomes) Meiosis is needed for sexual reproduction. Two divisions (meiosis I and meiosis II).
Meiosis Reproductive cells divide to produce gametes (sperm or egg). Gametes have half the regular # of chromosomes. This process occurs only in gonads (testes or ovaries). Male: spermatogenesis Female: oogenesis Meiosis is similar to mitosis with some chromosomal differences.
Spermatogenesis human sex cell ll llll llll llll ll sperm haploid (n) diploid (2n) llll ll haploid (n) ll meiosis I meiosis II
Interphase I Similar to mitotic interphase. Chromosomes replicate (S phase). Each duplicated chromosome consist of two identical sister chromatids attached at their centromeres. Centriole pairs also replicate (in animal cells).
Interphase I Nucleus and nucleolus visible. chromatin nuclear membrane cell membrane nucleolus
Meiosis I (four phases) Cell division reduces the chromosome number in daughter cells produced by one-half. four 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 a pair of homologous chromosomes or four chromatids (sister and non sister chromatids) that align in Prophase 1
Prophase I - Synapsis Homologous chromosomes sister chromatids Tetrad sister chromatids
Homologous Chromosomes Pair of chromosomes (maternal and paternal) that are similar in shape and size, and which 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. 1 pair of sex chromosomes
Homologous Chromosomes eye color locus eye color locus hair color locus hair color locus Paternal Maternal
Sex Chromosomes XX chromosome - female XY chromosome - male
Crossing Over Crossing over may occur between non sister chromatids at the chiasmata during Prophase 1. During crossing over segments of nonsister chromatids overlap, break, and reattach to the other Crossing over increases variation in the gametes produced Chiasmata (chiasma) are the sites of crossing over.
Crossing Over - variation nonsister chromatids Tetrad chiasmata: site of crossing over variation
Prophase I spindle fiber centrioles aster fibers
Metaphase I Shortest phase Tetrads align on the metaphase plate. INDEPENDENT ASSORTMENT OCCURS: Orientation of homologous pair to poles is random.
Metaphase I OR metaphase plate metaphase plate
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 There is no interphase II (or, if it does occur, it is very short) There is no more DNA replication Remember: Meiosis II is similar to mitosis
Prophase II same as prophase in mitosis
Metaphase II same as metaphase in mitosis metaphase plate metaphase plate
Anaphase II same as anaphase in mitosis sister chromatids separate
Telophase II Same as telophase in mitosis. Nuclei form. Cytokinesis occurs. Four haploid daughter cells (gametes) are produced. gametes = sperm or egg
Telophase II
Genetic Variation Important to population as the raw material for natural selection (evolution). Question: What are the three sexual sources of genetic variation?
Answer: 1. crossing over (prophase I) 2. independent assortment (metaphase I) 3. random fertilization Remember: variation is good!
Fertilization 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
Question: A cell containing 20 chromosomes (diploid) at the beginning of meiosis would, at its completion, produce cells containing how many chromosomes?
Answer: 10 chromosomes (haploid)
Meiosis Video http://highered.mcgrawhill.com/sites/0072495855/student_view0/ch apter28/animation how_meiosis_works.htm l