Meiosis and Sexual Reproduction

• Meiosis involves sexual reproduction through the formation of gametes

Why Aren't We Clones?

• Somatic cells reproduce via mitosis
• Mitosis creates new daughter cells that are genetic copies of the parent cells
• Asexual reproduction requires one parent
• Asexual reproduction produces genetically identical offspring, for example in bacteria
• Mitosis in humans would lead to exact clones
• Humans reproduce sexually, involving two parents and leading to genetically distinct offspring

Sexual Reproduction Terminology

• Sexual reproduction involves the fusion of male and female reproductive cells
• Gametes are male or female reproductive cells that are haploid, symbolized as n = 23
• Male gametes are sperm
• Female gametes are egg cells, or ovum
• Zygotes are cells formed by the fusion of two gametes
• Zygotes are diploid cells with 2n chromosomes
• n chromosomes come from the female
• n chromosomes come from the male
• Fertilization is the joining of male and female haploid gametes to produce a zygote
• Haploid cells contain half the number of chromosomes as the parent cell
• Diploid cells contain pairs of homologous chromosomes

Meiosis and Gametes

• Meiosis is the process that produces gametes with a haploid number of chromosomes
• Meiosis results in two outcomes:
• Genetic reduction, a form of cell division that produces daughter cells with half the number of chromosomes of the parent cell
• Genetic recombination produces products with different combinations of alleles

Haploid and Diploid Cells

• Zygotes have the same number of chromosomes as a somatic cell
• Gametes carry one set of homologous chromosomes
• Cells with one set of chromosomes are haploid, symbolized as (n)
• Cells with two sets of chromosomes are diploid, symbolized as (2n)
• The human diploid number is 2n = 46

Meisosis

• Meiosis involves two complete cycles of four phases
• Each cell contains some chromosomes from the mother, some from the father, and some chromosomes with segments that have been exchanged
• Observed under a magnification of 200x

Gamete Formation in Animals

• Spermatogenesis is the process that produces sperm in male animals
• Oogenesis produces eggs in females
• In males, spermatogonia reproduce by mitosis and then meiosis, starting at puberty
• In females, oogonia reproduce by mitosis before birth
• Oogonia begin meiosis but stop at prophase I
• After puberty, one cell completes meiosis each month
• Cytoplasm is unequally distributed, and only one cell matures

Meiosis and Genetic Variation

• Genetic variation is ensured in two ways: independent assortment and crossing over
• Independent assortment results gametes that carry different combinations of maternal and paternal chromosomes
• Independent assortment occurs during metaphase I, when each homologous chromosome is randomly oriented towards one of the poles
• Over 8 million different chromosome combinations can be produced through independent assortment
• Crossing over exchanges genetic material between maternal and paternal chromosomes
• Crossing over occurs during prophase I, where non-sister chromatids exchange genetic material in multiple sections