Meiosis

Purpose of Meiosis

• To make gametes for sexual reproduction

–To create diversity

What are gametes?

– Gametes are “sex cells.”• Sperm in males • Ova (eggs) in females.

– Each gamete is haploid (contains only one copy of each chromosome instead of two).

– Every gamete is completely unique.

Starting point

• Only special cells can go through meiosis. – Spermatogonia– Oogonia

Where does meiosis occur?

• In males, sperm are made in the testes.

• In females, ova (eggs) are made in the ovaries.

Stages of Meiosis

Interphase

• The stage between divisions

• Consists of three steps:

– G1 – Cell growth– S – copying the chromosomes– G2 – preparing to divide

Prophase 1

• Chromosomes condense

• Nuclear envelope begins to dissolve

• Nucleolus disappears

• Tetrads form and crossing over occurs

Crossing Over

• The process in which homologous chromosomes trade parts.

Homologous Chromosomes

Matching chromosomes.

• One from mom; one from dad.

Metaphase 1

• Homologous chromosomes line up in the middle

Anaphase 1

• Homologous chromosomes are pulled apart.

Telophase 1

• Two new nuclei form

Prophase 2

• Nuclear envelopes break down again

Metaphase 2

• Chromosomes line up in the middle

Anaphase 2

• Sister chromatids are pulled apart

Telophase 2

• 4 new nuclei are formed

Product of Meiosis

• Each spermatagonia makes 4 sperm.

• Each oogonia makes one ovum and 3 polar bodies.

Mendel’s Law of Segregation

• An individual’s alleles separate randomly during meiosis.