Chapter 10 Sexual Reproduction & Genetics Part 1: Meiosis

Concepts for Review Chromatin is the relaxed form of DNA Chromosome is the coiled form of DNA Sister chromatids are identical copies of DNA (after S-phase/replication)

Key Terms Homologous chromosomes are chromosomes that make up a pair, one from each parent – Not necessarily identical but… – Are the same length – Have the same genes

Key Terms Diploid refers to complete chromosome sets in a cell (or 2N) – N= the number of chromosomes in a gamete (sex cell) – N chromosomes from the female parent + N chromosomes from the male parent= 2N – For humans, 2N = 46

Key Terms Haploid refers to single chromosomes in a cell (or N) – One-half of the 2N set (the maternal N or the paternal N) – Gametes (i.e. sperm, egg sex cells) are haploid cells – For humans, N = 23

Bottom Line diploid means that there are the full number of chromosomes where as haploid means that there are half the number of chromosomes.

Key Terms Meiosis is a type of cell division that reduces the number of chromosomes – Forms gametes (i.e. sex cells) – Occurs in the reproductive structures of organisms that reproduce sexually – Occurs in 2 consecutive cell divisions: meiosis I and meiosis II

Before meiosis Interphase occurs…. Replication of DNA occurs. Chromosomes Condense Sounds familiar right???

Meiosis I: “Reduction Division” Reducing 2N cells to… N cells

PROPHASE I Homologous chromosomes pair up Crossing over occurs Nuclear envelope breaks down Spindles form

Crossing Over Chromosomal segments (genetic information) are exchanged between a pair of homologous chromosomes

METAPHASE I Spindle fibers attach to the centromeres Homologous Chromosomes Line up at the equator

ANAPHASE I Homologous chromosomes separate and move to the opposite poles of the cell

TELOPHASE I The cell divides The result is two daughter cells. Each daughter cell is haploid. The daughter cells are not genetically identical **During Telophase I Cytokinesis occurs at the same time!!**

Meiosis II: “Mitotic Division” Separating sister chromatids

Nucleus breaks up Spindles form and attach to centromere

Sister chromatids randomly align at middle of cell

Sister chromatids separate Chromosomes move to opposite poles of cell

Cells divide 4 haploid daughter cells result Each cell is genetically different

Summary of Key Concepts Chromosomes contain genes (humans have 46 chromosomes or 23 homologous pair) Homologous chromosomes may contain different forms of the same genes Meiosis reduces chromosome # by ½ and results in sex cells (gametes) Crossing-over increasing genetic variation Click here for detailed animation of meiosis

which set of chromatids illustrates the result of a single crossover of the homologous chromosomes?