Chromosomes and Meiosis

Somatic Cells Also called Body Cells Make up most of your body tissues and organs

Germ Cells Are cells in your reproductive organs—ovaries and testes that develop into gametes. Gametes are sex cells -ova or eggs in female -sperm in male

Chromosome

Homologous Chromosomes “having the same structure” Two chromosomes—one from father, one from mother that have the same length and general appearance

Chromosomes 1-22 Autosomes---chromosomes that contdain genes for characteristics not directly related to the sex of an organizme. What about chromosome pair #23?

Sex Chromosomes

Unsorted Human Chromosomes

Human Male Karyotype

Human Female Karyotype

Diploid & Haploid Cells Diploid—a cell that has two copies of each chromosome; one from mother, one from father---chromosomes #1-#22 Haploid—a cell that has only one copy of each chromosome (also called sec cells); gametes are haploid; chromosome #23

First Division of Meiosis Prophase 1: Duplicated chromatin condenses. Each chromosome consists of two, closely associated sister chromatids. Crossing-over can occur during the latter part of this stage. Metaphase 1: Homologous chromosomes align at the equatorial plate. Anaphase 1: Homologous pairs separate with sister chromatids remaining together. Telophase 1: Two daughter cells are formed with each daughter containing only one chromosome of the homologous pair.

Second Division of Meiosis Second division of meiosis: Gamete formation Prophase 2: DNA does not replicate. Metaphase 2: Chromosomes align at the equatorial plate. Anaphase 2: Centromeres divide and sister chromatids migrate separately to each pole. Telophase 2: Cell division is complete. Four haploid daughter cells are obtained.

Mutations

Crossing Over Exchange of chromosome segments between homologous chromosomes during prophase 1 of meiosis.

Crossing Over & Recombination

Genetic Linkage The top diagram shows paternal (blue) and maternal (red) chromosomes aligned in a germ cell, a cell that gives rise to eggs or sperm. Three DNA sequences are shown, labelled A, B and C. The capital letters represent the paternal alleles and the lower case letters represent the maternal alleles. The middle panel shows the physical process of recombination, which involves crossing over of DNA strands between the paired chromosomes. The bottom panel shows what happens when the crossover is resolved. The maternal and paternal alleles are mixed (recombined) and these mixed chromosomes are passed to the sperms or eggs. If A is the disease gene and B and C are genetic markers, recombination is likely to occur much more frequently between A and C than it is between A and B. This allows the disease gene to be mapped relative to the markers B and C.