Meiosis

Introduction to Meiosis Mitosis= reproduction of body cells (SOMATIC CELLS) Meiosis= reproduction of sex cells (sperm and eggs) Sex Cells (GAMETES) = reproductive cells produced in sex organs (testes/ovaries) Reduces chromosome number from DIPLOID (2n) down to HAPLOID (n)

Comparing Mitosis vs. Meiosis Body Cells One Division Create 2 identical cells Copies have the exact same number of chromosomes 46 chromosomes in every body cell Paired chromosomes Sex Cells (egg and sperm) Two Divisions Creates 4 cells Copies have half the number of chromosomes 23 chromosomes in a sperm or egg cell NOT in pairs

Meiosis Overview Meiosis Overview Video

Homologous Chromosomes HOMOLOGOUS (“same”) pairs of chromosomes = same sized chromosomes that carry the same genes

Meiosis I – First of 2 Divisions Prophase I Chromosomes coil up and become visible Nuclear membrane and the nucleolus disappear Spindle fibers form Centrioles begin to move to opposite sides HOMOLOGOUS CHROMOSOMES pair up to form a TETRAD ~ this pairing process is called SYNAPSIS

Meiosis I Metaphase I Chromosomes pairs will line up in the middle/equator as TETRADS CROSSING OVER may occur! Chromosomal material is exchanged between sister chromatids Allows for genetic variation!

Meiosis I ANAPHASE I Homologous chromosomes split and move to opposite ends This is where the chromosome number is divided in half (2n  n)

Meiosis I Cleavage furrow forms Cytokinesis occurs Telophase I Cleavage furrow forms Cytokinesis occurs Nuclear membrane DOES NOT reform Chromosomes DO NOT go back to chromatin 2 daughter cells have been created with the haploid number of chromosomes Meiosis I Animation

Meiosis II – Mitosis in each daughter cell Centrioles move apart Spindle forms between centrioles Fibers connect to sister chromatids Sister chromatids line up in the middle of cell Fibers pull sister chromatids apart Nuclear membrane and nucleolus reform Cell membrane pinches in and divides in the cytoplasm End Result: 4 sex cells (different than what started out with!!)

Meiosis II - Review

Why is Meiosis Important? Keeps the number of chromosomes the same from one generation to the next Human sperm cell + Human egg cell= zygote 23 chromosomes + 23 Chromosomes= 46 chromosomes Provides genetic variation Why you don’t look exactly like your siblings

Meisosis Animations http://www.cellsalive.com/meiosis.htm http://www.youtube.com/watch?v=D1_-mQS_FZ0&feature=related http://www.youtube.com/watch?v=MyXvx5FkfTE&feature=related

Meiosis: Males vs. Females Males – Meiosis is called SPERMATOGENESIS 4 equally sized sperm produced Cytoplasm divides evenly Ongoing in testes throughout a male’s life Human males produce approximately 1000 sperm per second (30 billion/year). Each ejaculation should contain 200 - 300 million sperm

Meiosis: Males vs. Females Females – Meiosis is called Oogenesis 4 cells, but only 1 becomes an egg Uneven cytokinesis – 1 cell receives most of the cytoplasm = EGG (needs extra cytoplasm to accommodate fertilization) Other 3 cells receive very little cytoplasm and die off = POLAR BODIES

Meiosis: Males vs. Females Meiosis in human females begins before person is born but stops in prophase I and does not resume until after puberty. Each month, approximately 1000 primary oocytes will mature but most will die. Ovulation occurs approximately once every 28 days. Females ovulate approximately 400 times during their lifetime Women are born with all of the primary oocytes that they will ever have (2 million). At puberty, there are approximately 400,000 left.

Final Comparison – Mitosis vs. Meiosis Compare Mitosis and Meiosis http://www.pbs.org/wgbh/nova/miracle/divi_flash.html