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MEIOSIS: PROCESS OF CELL DIVISION THAT REDUCES THE # OF CHROMOSOMES IN THE NEW
CELLS TO HALF
produces “gametes”: sex cellsMale: sperm X and Y
Female: eggs
Meiosis: divided into 2 stages:
Meiosis I Meiosis II
Meiosis I
PROPHASE I:spindle fibers appear
homologous chromosomes pair
**this happens only in meiosis!
Synapsis: the pairing of homologous chromosomes
Tetrad: each pair of homologous chromosomes
Aligned so that genes line up
Prophase I
alignment
Prophase I (con’t)
Crossing-over occurs: Chromatids twist
around each other Portions of
chromatids break off and attach to the adjacent chromosome
Creates “genetic recombination”: new mix of genetic material
Metaphase I
Tetrads line up along equator of cell
Spindle fibers are attached to centromere
Anaphase I
Each homologous chromosome moves to the opposite pole
“INDEPENDENT ASSORTMENT”: random separation of chromosomes
Telophase I
Chromosomes reach opposite ends
Cytokinesis I begins
These cells are HAPLOID! But ….. Each cell contain 2 copies (chromatids)
Meisosis II
Some organsism, interphase first occurs
In other organisms, you go straight from telophase I to prophase II
Prophase II
Spindle fibers form
Metaphase II
Chromosomes line up along the midline
Each chromatid faces the opposite poles
Anaphase II
Chromatids separate and move toward opposite poles
Telophase II
Nuclear memb forms around the chromosome in the 4 new cells
Cytokinesis II occurs
Each cell is haploid
Mitosis and Meiosis
Spermatogenesis: the production of sperm
Occurs in the testes
4 sperm created
Oogenesis: production of “ova” or eggs
3 polar bodies made and 1 egg