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