1. Meiosis

2. 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)

3. 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

4. Meiosis Overview
Meiosis Overview Video

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

6. 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

7. 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!

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

9. 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

10. 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!!)

11. Meiosis II - Review

12. 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

13. Meisosis Animations http://www.cellsalive.com/meiosis.htm

14. 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 million sperm

15. 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

16. 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.

17. Final Comparison – Mitosis vs. Meiosis
Compare Mitosis and Meiosis