Meiosis

Division of sex cells

Mitosis vs. Meiosis• Mitosis: start with one diploid cell, end with

two identical diploid cells– Somatic cells (most cells in your body)

• Meiosis: start with one diploid cell, end with four different haploid cells– Gametic cells (sperm and egg)

Humans are diploid• Ploidy refers to sets of

chromosomes in an organism

• Diploids have two sets of chromosomes, one from each parent

• Each homologous pair contain the same genes in the same loci (locus) or location

Gametes are haploid

Why?

Meiosis proceeds through 2 divisions

Tetrad

Spermatogenesis

• Each primary spermatocyte divides to form 4 mature sperm

• This process is ongoing in reproductive aged men

• A mature sperm cell has shed cytoplasm

• The parts of the sperm are developed to assist in delivering the DNA to an oocyte (egg)

Oogenesis

• One primary oocyte divides and produces only one mature egg• Primary oocytes arrest at Prophase 1 by birth – meiosis I continues

after puberty, but meiosis II only proceeds if fertilization occurs

• Meiosis is uneven- most of the cytoplasm is passed on to the oocyte.

• Cell structures from this cytoplasm are responsible for cell function for the first few days of development.

• Mitochondria is only inherited from mother cell.

Independent Assortment

Homologous chromosomes carry different versions of genes (alleles)

During meiosis, homologous chromosomes can exchange pieces

Crossing over, results in recombination of genes