BSC1010 Lecture 11b

Introduction to Meiosis and Sexual Reproduction

• Sexual reproduction has evolved to ensure proximity of sperm to egg.

• Involves courtship rituals and mating behaviors.

• Objective: To perpetuate species by producing genetically unique offspring.

Functions of Sexual Reproduction

• Formation of Reproductive Cells:

  • Half of the genetic makeup (haploid).

• Creation of Genetic Variability:

  • Crossing Over: Exchange of genetic material between homologous chromosomes during meiosis.

  • Independent Assortment: Random distribution of maternal and paternal chromosomes during gamete formation.

Human Karyotype

• Composition:

  • 46 chromosomes total: 22 pairs of autosomes and 1 pair of sex chromosomes.

  • Each somatic cell contains 23 pairs making a total of 46.

• Homologous Chromosome Pairs:

  • Carriers of the same inherited traits, providing genetic consistency except for variation in sex chromosomes (XX for female, XY for male).

Meiosis vs. Mitosis

• Why Meiosis Instead of Mitosis for Gametes?:

  • Meiosis involves two rounds of division reducing chromosome number.

  • Produces haploid cells (n=23) instead of diploid cells (2n=46).

  • Gamete Types:

    • Egg cells and sperm cells are both haploid, containing one chromosome of each type.

Ploidy of Cells

• Somatic Cells:

  • Diploid (2n), contain two copies of each gene (22 autosomal pairs + 1 sex pair).

• Gametes:

  • Haploid (n), carry half the genetic information, with only one copy of each gene.

Meiosis Process

Meiosis Overview

• Comprises two main stages: Meiosis I and Meiosis II, proceeding after a regular cell cycle that includes interphase.

• Purpose:

  • Halving chromosome number while ensuring genetic diversity.

• Stages of Meiosis:

  • Meiosis I: Separates homologous chromosomes.

  • Meiosis II: Separates sister chromatids.

Stages of Meiosis I

• Prophase I:

  • Homologous chromosomes pair (synapsis) and crossing over occurs, increasing genetic variability.

• Metaphase I:

  • Homologous pairs align at the equatorial plane (metaphase plate).

• Anaphase I:

  • Homologous chromosomes are pulled apart to opposite ends of the cell.

• Telophase I and Cytokinesis:

  • Two haploid cells result, each with half the chromosome count.

Importance of Crossing Over and Independent Assortment

• Crossing Over:

  • Exchange of segments between homologous chromosomes during Prophase I creates new allele combinations.

• Independent Assortment:

  • Random orientation of chromosome pairs leads to genetic variation in gametes.

Meiosis II Overview

• Resembles mitotic division but involves haploid cells to separate sister chromatids.

• Stages:

  • Prophase II: Chromosomes condense, and nuclear envelope dissolves.

  • Metaphase II: Chromosomes align at the metaphase plate.

  • Anaphase II: Sister chromatids are pulled apart.

  • Telophase II and Cytokinesis: Results in four genetically unique haploid cells.

Genetic Variability in Meiosis

• Recombination:

  • Results from crossing over, promotes genetic diversity among offspring.

• Independent Assortment:

  • Variations in gamete formation due to random arrangement of homologous pairs during Metaphase I leads to diverse combinations in offspring.

Problems in Meiosis

• Non-disjunction:

  • Failure of homologous chromosomes to separate properly can create gametes with irregular chromosome numbers, potentially leading to genetic disorders such as Down syndrome.

Conclusion

• Meiosis is essential for sexual reproduction and genetic diversity.

• Understanding the processes and mechanisms involved in meiosis can provide insight into heredity and evolution.