Meiosis Creating Cells for Sexual Reproduction

Meiosis Takes place in Gametes(sex cells) Produces a haploid cell (represented by N) People have 46 Chromosome or 23 pairs (2N) When an egg joins a sperm the count must stay at 46 to remain human egg can only have 23 sperm can only have 23 How does this happen?

Definitions Diploid Cells- (2N)cells that have pairs of chromosomes. There are 2 of each chromosome in diploid cells. Haploid Cells- (N)cells that have only one of each chromosome. Haploid cells are used for sexual reproduction. Somatic Cells – body cells that are Diploid. Gametes or germ cells – sex cells that are Haploid.

During Meiosis gamete (sex) cells undergo a “double division”, called Meiosis I and Meiosis II. This maintains the DNA, but reducing the chromosomal count to 23. + = Sperm (23) + Egg (23) = Fertilized Cell (46) Haploid Cells (N) Haploid Cells (N) Diploid Cells (2N)

Meiosis I

Prophase I Chromosomes already copied Centrioles separate Nuclear membrane breaks down Crossing over occurs here

Crossing Over Synapsis- Homologous Pairs line up = Tetrad chromatids of homologous pair may twist around each other and break off and attach to the other homologous pair = Crossing Over Results in exchange of genetic material between maternal and paternal chromosomes = genetic recombination

Metaphase I Chromosome pairs line up at equator

Anaphase I Chromosome pairs split Sister chromatids stay connected

Telophase I Cell starts to divide in two Nuclear membranes start to form again

Meiosis II

Prophase II Chromosomes do not replicate again Cells have one set of sister chromatids Nuclear membrane breaks down Centrioles separate

Metaphase II Sister chromatids line up at equator

Anaphase II Sister chromatids divide

Telophase II Cells start to split in two Each cell has one set of chromosomes—it is now haploid 4 cells form

Original Gamete Prophase Metaphase Anaphase Telophase/Cytokinesis 2 Daughter Cells Prophase 2 Metaphase 2 Anaphase 2 Telophase 2/Cytokinesis 4 Gametes

OOGENESIS – Meiosis in females SPERMATOGENESIS - Meiosis in Males

Meiosis ensures that all living organisms maintain both Genetic Diversity and Genetic Integrity

Cell Division in Prokaryotes Lack a nucleus Have a single chromosome Reproduce by binary fission Include bacteria

Steps in Binary Fission Cells increase cell mass the single, circular bacterial chromosome is replicated Each cell divides into 2 daughter cells