Meiosis and Sexual Reproduction

Asexual Reproduction Sexual Reproduction “Germ cells” undergo meiosis and cytokinesis Daughter cells become gametes (sex cells) Gametes meet (hopefully!) at fertilization Produces genetic variation among offspring This variation in traits is the basis for evolutionary change Single “parent” produces offspring Growth and repair Mitosis produces daughter cells genetically identical to one another and to the parent = “clones”

Chromosome Structure Explore the “chromosome viewer” at this site based on information from the Human Genome project (& the source of this figure): http://www.ornl.gov/sci/techresources/Human_Genome/posters/chromosome/index.shtml

Homologous Chromosomes may carry different Alleles Each diploid cell has two of each chromosome (23 pairs in humans) One chromosome in each pair from mother, other from father Paternal and maternal chromosomes may carry different alleles Mom Dad Perhaps different forms of the same gene?

Ploidy: Chromosome Number Sum total of chromosomes in a cell Germ cells are diploid (2n) Gametes are haploid (n) Meiosis halves chromosome number, leaving one complete set of 23 in each gamete Are these chromosomes replicated? 1-5 6-12 13-18 19-22 23: XX

Meiosis: Two Divisions Two successive nuclear divisions Meiosis I Meiosis II Four haploid nuclei are formed All (potentially) genetically different Events are similar to those in mitosis (prophase, metaphase, anaphase, telophase) (Without DNA replication in between)

Meiosis I (one homologous pair of chromosomes) What is the ploidy level of this nucleus? Are the chromosomes replicated? Each homologue in the cell pairs with its partner, then the partners separate What is the ploidy level of these nuclei after meiosis I?

Meiosis II The two sister chromatids of each duplicated chromosome are separated from each other two chromosomes (unduplicated) one chromosome (duplicated)

Meiosis I - Stages Prophase I Metaphase I Anaphase I Telophase I

Meiosis II - Stages Prophase II Metaphase II Anaphase II Telophase II

3 main reasons for genetic variability as a result of sexual reproduction: Crossing over Prophase I Independent Assortment (of chromosomes) Metaphase I Random Fertilization random re-combination of genes in fertilized gamete

Crossing Over All four chromatids are closely aligned p. 144 E E e e All four chromatids are closely aligned Homologous chromosomes exchange segments New combinations of alleles created What would result if sister chromatids exchanged segments? l L L l Germ cell  “synapsis” big eyes, small eyes no light light Big Eyes (E) small eyes (e) Light (L) no light (l) Bioluminescenc and eye size in viper fish. What traits would be linked after crossing over?

Independent Assortment random alignment of chromosomes during Metaphase number of possible combinations of chromosomes in a gamete is: 2n n = # chromosome types p. 145