Lesson 5.5: Meiosis

Purpose Meiosis is a process of cellular division that is used to make gametes (eggs and sperm) Egg cells from the ovaries Sperm cells from the testes

Chromosome number Diploid Haploid All cells in your body but sex cells Have twice the number of chromosomes as haploid cells Only sex cells, including egg and sperm cells Have half the number of chromosomes as normal cells.

Prophase I Chromatin condenses (coil) into chromosomes Centrosomes form the mitotic spindle Homologous chromosomes pair up (1 maternal/1 paternal) Nuclear envelope breaks down

Prophase I – Crossing Over Crossing over may occur = Homologous chromosomes exchange genetic material. (Notice the colors have switched places)

Metaphase I Spindles are attached to the Homologous chromosomes that are lined up on the equatorial plate Chromosomes assort independently (different colors can go to different sides of the plate)

Anaphase I Homologous chromosomes separate and move to opposite poles Each chromosome now consists of sister chromatids They may not be identical due to crossing over Sister Chromatids

Telophase I Chromosomes decondense or uncoil Nuclear membranes reform Cytokinesis occurs DNA replication does not occur (it did before Prophase I)

Prophase II Chromatin again condenses into chromosomes Centrosomes form the mitotic spindle The cells are haploid Remember there were 4 total chromosomes (2 pairs of 2) in prophase I Now there is 1 of each pair in prophase II Nuclear envelope breaks down

Metaphase II Spindles are attached to the chromosomes that are lined up on the equatorial plate

Anaphase II Centromeres divide and each chromatid moves to opposite poles of the cell

Telophase II Chromosomes uncoil Nuclear membranes reform. Cytokinesis occurs Result are 4 haploid gametes (either egg or sperm cells) Each of four daughter cells are genetically unique due to independent assortment and crossing over.