Boo-Yah Biology!

MeiosisMolecular Biology

Sumner HS

Quick Mitosis Review

Mitosis important in growth & repair

Produces genetically identical daughter cells.

DNA replicated in S phase

Copies divided into daughter cells during mitosis (PMAT) & cytokinesis.

Daughter cells have the diploid number of chromosomes. (humans = 46, or 23 pair)

http://www.youtube.com/watch?v=O3_PNiLWBjY

MeiosisMaking Reproductive Cells

(Sexual Reproduction)

Meiosis Fundamentals

• Meiosis is a process of cell division that reduces the number of chromosome sets from diploid to haploid, producing 4 cells.

• Takes place in two sets of divisions, meiosis I and meiosis II

• It is this process that organisms use to make sperm and egg cells

Meiosis Fundamentals

Chromosomes replicate once.

Two consecutive nuclear divisions (similar stages to mitosis)

Meiosis 1

Meiosis 2

Results in four haploid cells.

Reduces Chromosome number in half.

Meiosis Fundamentals

• Why do we want reproductive cells to be haploid?

Two stages of Meiosis

• Meiosis I

• Reduces the number of chromosomes from 4n to 2n, producing 2 cells.

• Homologous chromosomes separate

• Meiosis II

• Sister chromatids separate

• Produces four haploid (n) daughter cells

Chromosome Accounting

9

Chromosome Accounting

Regular human body cell = 46 total chromosomes

The total number of chromosomes in a regular cell is the diploid number (46 in humans).

Or 2n = 46

Or 23 pairs

The number of chromosomes received from each parent is the haploid number (23 in humans)

Or n = 23

Or 23 total

Organism Life Cycles

At sexual maturity, the ovaries and testes produce haploid gametes by meiosis.

n = 23

Not genetically identical to parent cell due to only having half of the total chromosome set

During fertilization these gametes (sperm and ovum) fuse, forming a diploid zygote

The zygote develops into an adult organism

Homologous Chromosome

What is a homologous chromosome?

The pair of chromosomes from each parent

Ex: Homologous chromosome 1: 1 from Mom and 1 from Dad

• The human genome: http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/chooser.shtml

• Meiosis: http://www.youtube.com/watch?v=kVMb4Js99tA

Meiosis: The Steps

The Steps of Meiosis IInterphase

Chromosomes duplicate (4n)

Prophase I

Homologous replicated chromosomes pair forming tetrads (four chromatids) and exchange gene segments = crossing over

Steps of Meiosis I

Metaphase 1

Tetrads line up on cell equator

Spindles attached to centromere

Steps of Meiosis I

Anaphase I

Homologous chromosome pairs separate

Sister chromatids still attached

Steps of Meiosis I

Telophase I/Cytokinesis

New nuclei form

Cytokinesis produces two non-identical diploid cells

Chromosomes still as sister chromatids.

Steps of Meiosis II

Prophase II

Much like prophase in mitosis

Preparation for division

Steps of Meiosis II

Metaphase II

Homologous pairs line up on the metaphase plate (equator)

Spindle attaches to centromere

Steps of Meiosis II

Anaphase II

Sister chromatids separate

Pulled to opposite ends of the cell.

Steps of Meiosis IITelophase II/Cytokinesis

New nuclei forming

New cells are haploid, not diploid

Cells divide

Four non-identical daughter cells formed.

Meiosis Accounting

During G1 = 2n = 46

During S & G2 = 4n = 92 (DNA is replicated)

After Cytokinesis I = 2n = 46

After Cytokinesis II = n = 23

Meiosis animation

http://www.cellsalive.com/meiosis.htm

The Big Picture

Comparison

Mitosis/Meiosis Animation

• Mitosis vs. Meiosis: http://www.youtube.com/watch?v=C6hn3sA0ip0&feature=related