Meiosis Chapter 10.1

Cell reproduction review Cell cycle Interphase – DNA replicated Mitosis – nuclear division Cytokinesis – cytoplasm divided Results in 2 cells that are exact copies of the original cell.

Sexual Reproduction Humans and other animals reproduce sexually Two specialized cells form to make a new organism. Gametes or sex cells Human gametes: eggs and sperm

Chromosomes Humans have 46 chromosomes Each parent contributes 23 chromosomes Homologous chromosomes: Two paired chromosomes, one from each parent 23 sets of homologous chromosomes

Haploid and Diploid Cells Haploid – (N) number chromosomes Diploid – (2N) number chromosomes Gametes are haploid Gametes maintain the same number of chromosomes from generation to generation

Sexual Reproduction Cell Cycle Interphase Meiosis I Prophase I Metaphase I Anaphase I Telophase I Meiosis II Prophase II Metaphase II Anaphase II Telophase II

Meiosis Cell division to make four gametes with half the number of chromosomes Two divisions : Meiosis I and Meiosis II Male: Spermatogenesis Female: Oogenesis

Interphase Chromosomes replicate Each duplicated chromosome consists of two identical sister chromatids attached at centromeres.

Meiosis I (four phases) Cell division that reduces the chromosome number by one-half. (Diploid  haploid) four phases: a. prophase I b. metaphase I c. anaphase I d. telophase I

Prophase I – Meiosis I Chromosomes condense Synapsis – Homologous chromosomes come together to form a tetrad. Crossing over occurs (variation)

Prophase I – Crossing over Segments of non-sister chromatids break and reattach to the other chromatid Chiasmata – site of crossing over

Prophase I

Metaphase I Tetrads align on the metaphase plate Independent assortment occurs – orientation of homologous chromosomes is random. Creates variation

Metaphase I

Anaphase I Homologous Chromosomes separate Sister chromatids remain attached at centromere.

Telophase I Homologous chromosomes reach the poles The poles have haploid set of chromosomes Cytokinesis – cytoplasm splits. Two haploid daughter cells are made.

Telophase I Cytokinesis

Meiosis I Overview Meiosis I Summary

Meiosis II No interphase II (or very short - no more DNA replication) Remember: Meiosis II is similar to mitosis four phases: a. prophase II b. metaphase II c. anaphase II d. telophase II

Meiosis II No Interphase – no replication Meiosis II is very similar to mitosis

Prophase II Chromosomes condense Nucleus disappears Spindle fibers form & attach again

Metaphase II Chromosomes line up on metaphase plate

Anaphase II Sister chromatids separate

Telophase II Chromosomes arrive at poles Nucleus reforms Cytokinesis occurs – produces 4 haploid daughter cells

Telophase II Cytokinesis

Oogenesis Female – begins before birth until menopause

Spermatogenesis Male – begins at puberty continues until death

Meiosis Overview VIDEO

Karyotype A method of organizing the chromosomes of a cell in relation to number, size, and type. SKIP

Fertilization Fusion of Egg and sperm to form a Zygote. Egg – haploid Sperm – haploid Zygote – diploid

Genetic variation Meiosis creates variation within a population 1) Crossing over (prophase I) 2) Independent assortment (Metaphase I) 3) Random fertilization Variation allows for natural selection and evolution to occur