Meiosisvs.
Mitosis
Chromosome Number
Chromosome Number
• Species have characteristic numbers of chromosomes
• Eukaryotic cells have pairs of homologous chromosomes
• Cells with a double set of chromosomes are diploid (2n)
• Cells with one set of chromosomes are • haploid (n)
• Humans body cells are diploid (2N)– 2N = 46
• Gametes - sex cells– Males – sperm– Females – eggs
• Gametes are HAPLOID– N = 23
• Fertilization restores the diploid state
• (N + N = 2N)• Zygote - a fertilized
egg (2N)
Meiosis…bottom line• Process to make haploid
gametes• Chromosome # is cut in
half– 2N N
• Occurs in ovaries & testes• TWO cell divisions• Increases genetic
variation
Prior to Meiosis….INTERPHASE• What happens during Interphase???
– DNA REPLICATION!!!
2N
2N
Meiosis I• Prophase I – homologous chromosomes form tetrads & crossing over occurs• Metaphase I – tetrads line up along the middle
• Anaphase I – homologous chromosomes separate• Telophase I – two nuclei form• Cytokinesis I – cells separate
2N
N N
Crossing Over• Occurs in Prophase I• Chromosomes exchange portions of their
chromatids • Increases genetic variation
Meiosis II
• Prophase II – spindle forms, prepares for division
• DNA replication DOES NOT occur between Meiosis I & II
• Metaphase II – chromosomes align along the center• Anaphase II – sister chromatids separate
• Telophase II – nuclei reform; cytokinesis
N N
N N NN
Looks A LOT like MITOSIS!
Meiosis in Men vs. Women
• Men: 4 sperm are made
• Women: only 1 egg is made
Meiosis vs. Mitosis
Meiosis Summary
• Chromosome number is cut in half!• Produces haploid gametes – egg and sperm• Crossing-over occurs during Prophase I
– leads to increased genetic variation• Two cell divisions, but DNA does not
replicate between divisions• Results: 4 sperm in men and 1 egg in
women