Meiosis Chapter 13

What you need to know! The role of meiosis and fertilization in sexually reproducing organisms The importance of homologous chromosomes to meiosis How the chromosome number is reduced from diploid to haploid through the stages of meiosis Three important differences between mitosis and meiosis The importance of crossing over, independent assortment, and random fertilization to increasing genetic variability

Meiosis A process by which an organism produces haploid (n) sex cells from diploid (2n) germ cells Meiosis is divided into meiosis 1 and meiosis 2 Meiosis 1 = separation of homologous chromosomes Meiosis 2 = separation of sister chromatids

Interphase G1, S, G2 S Phase: Duplication of DNA 2n  4n Assembly of centrioles

Prophase I 90% of meiotic time Condensation of chromosomes Nuclear envolope disappears Spindles form Synapsis: –Homologous chromosomes pair up as tetrads –The chiasmata are where crossing-over takes place

Crossing Over Leads to the creation of novel chromosomes Genes from homologous maternal and paternal chromosomes mix a)Synapsis: homologous chromosomes form tetrads b)Chiasmata: homologous chromosomes overlap on multiple points c)Crossing Over: identical parts break off and reattach

Crossing Over Frequency The recombination frequency between two genes on one chromosome is mathematically related to the distance between them The further apart 2 genes are the higher the probability of a crossing over event separating them Recombination frequencies vary between 0% and 50% This method is useful for establishing gene maps (location of genes on a chromosome Example: Recombination frequencies between genes A and B is 45% while A and C is only 20%. Map the genes.

Metaphase 1 Homologous chromosomes line up at the equator side by side One spindle attaches to each chromosome 4n = 92 chromosomes in humans

Anaphase 1 Homologous chromosomes separate (sister chromatids remain together)

Telophase 1, Cytokinesis, and Interkinesis Telophase 1/cytokinesis: Two cells separate Each contains one homologous chromosome (haploid) with two sister chromatids Often no nuclei form and the DNA remains condensed 2 x (n) = 46 (no pairs) Interkinesis: Short interphase with little growth Sometimes partial uncoiling of the chromosomes Duplication of centrioles (no S phase)

Prophase 2 Spindle fibers form 2 kinetochore spindle fibers attach to each chromosome

Metaphase II Chromosomes line up at the equator of the cell Spindle fibers attach to each Centromere of the sister chromatids

Anaphase 2 Sister chromatids are pulled apart Sister chromatids are not identical (crossing over)

Telophase 2 and Cytokinesis Four new cells form All are genetically different All are haploid with 1n chromosomes –Humans 23