Meiosis and Sexual Reproduction

Sex and Alleles

Meiosis in Reproduction

Stages of Meiosis

Shuffling Parental DNA

Mitosis and Meiosis Compared

Sexual Reproduction

Involves the fusion of reproductive cells

Involves two individuals and mixes their genetic material

Ø Somatic (body) cells of sexually-reproducing eukaryotes contain pairs of homologous chromosomes: Ø One from the mother and one from the

father/ carry the same genes

Ø Different forms of the same gene are called alleles

Introducing Alleles

A

B

Genes occur in pairs on homologous chromosomes.

The members of each pair of genes may be identical, or they may differ slightly, as alleles.

The Advantages of Sexual Reproduction

Ø Sexual reproduction mixes up the genes of two parents, so offspring have unique combinations of traits

Ø Diversity offers a better chance of surviving environmental changes than clones

Meiosis

Nuclear division associated with the formation of gametes (reproductive cells) All gametes are haploid (n), but they differ in other details Gamete formation differs among plants and animals

Meiosis Halves the Chromosome Number

Meiosis Halves the Chromosome Number

Fertilization Restores the Chromosome Number

Ø Haploid gametes form by meiosis

Ø The diploid chromosome number is restored at fertilization

Ø Two haploid gametes fuse a zygote is formed

Stages of Meiosis

Crossing Over in Prohase I

Independent Assortment of Chromosomes at metaphase I

Two events in meiosis introduce novel combinations of alleles into gametes:

Ø Crossing over in prophase I

Ø Segregation of chromosomes into gametes § For each chromosome pair, the maternal or paternal version is equally likely to end up in either nucleus

Sources of Genetic Variation in Sexual Reproduction

Random fertilization

Sources of Genetic Variation in Sexual Reproduction

Are Mitosis and Meiosis Related?

Homologous chromosome pairs in a human cell during prophase I of meiosis Blue/ centromeres Yellow/ MLH1 protein assisting with crossing over Same protein involved repairing mismatched bases during mitosis Meiosis may have evolved by the remodeling of existing mechanisms of mitosis