Biology Chapter 10

“Meiosis”_______________________Biology

10th Chapter

A. Types of Reproduction 1. Asexual Reproduction • involves only one parent • offspring are genetically identical to parents • advantageous when organism is well-adapted to a stable environment 2. Sexual Reproduction • involves two parents • offspring are genetically diverse • advantageous in a changing environment Ploidy – degree of repetition of the basic number of chromosomes Example: Human gametes (eggs and sperm) each contain 23 chromosomes (haploid) Human body cells (somatic) cells contain 46 chromosomes (two sets of 23 – diploid) Haploid gametes are produced by meiosis. Diploid cells are produced by mitosis or when gametes fuse. B. Meiosis – type of cell that halves the number of chromosomes • occurs in germ cells • Sperm and egg • Germ cells replicate chromosomes in S phase of the cell cycle 1. Meiosis 1 (reduction division) • Prophase 1 a. Steps are the same as prophase in mitosis except: synapsis and crossing over occurs Synapsis – homogolous chromosomes pair up Homologs – chromosomes that carry genes for the same traits in sequence Crossing Over – non sister chromatids of homologs exchange chromosomal material which leads to new genetic combinations Chiasma – visible point of cross over between homologs

a. Metaphase •

paired homogolous chromosome lines up along equator of spindle

b. Anaphase 1 • homogolous chromosomes separate and move to opposite poles c. Telophase 1 * spindle apparatus breaks down * chromosomes decondense * nuclear membranes partially reform around two nuclei 2. Meiosis 2 3. Anaphase 2 4. Telophase 2 Cytokinesis divides two cells into four non-identical cells

Differences between mitosis and meiosis Mitosis *1 nuclear division *results in 2 diploid cells that are genetically identical *no synopsis and crossing over of homologs *chromosomes align Individually (metaphase) *occurs in somatic cells *occurs throughout life cycle *used for growth, repair, and asexual reproduction

Meiosis *2 nuclear divisions *results in 4 haploid cells that are genetically diverse * synopsis and crossing over of homologs (during Prophase 1) *homologous pairs align (metaphase 1) *occurs in germ cells *in humans, completed only after sexual maturity *used for sexual reproduction