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)

Why do sex cells have only half the genetic information of body cells?

46Body Cell

46 46

92!

MITOSIS

Body Cells One Division Create 2 identical

cells Copies have the exact

same number of chromosomes

46 chromosomes in every body cell

Paired chromosomes

MEIOSIS

Sex Cells (egg and sperm)

Two Divisions Creates 4 cells Copies has half the

number of chromosomes

23 chromosomes in a sperm or egg cell

NOT in pairs

Meiosis Overview Video

Chromosomes

Chromatids

• All male sperm is genetically different from one another

• All female eggs are genetically different from one another

• When they fuse in fertilization, the chromatids combine to make a new individual!

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

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 and CROSSING OVER occurs.

CROSSING OVER

Chromosomal material is exchanged between sister chromatidsAllows for genetic variation!

Then, paired chromosomes line up in the center for metaphase I

ANAPHASE I

Homologous chromosomes split and move to opposite ends

This is where the chromosome number is divided in half (2n n)

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

Centrioles move apart Spindle forms between

centrioles 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!!)

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

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

Can you find the homologous pairs?

How many homologous pairs do you see?

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).

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 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.

Compare Mitosis and Meiosis http://www.pbs.org/wgbh/nova/

miracle/divi_flash.html