Biol 103: Introductory Biology I Lecture 9 - Meiosis

Introduction

  • Course: Biol 103: Introductory Biology I Lecture 9 Meiosis
  • Instructor: Dr. Michael D. Preston, Assistant Professor, Ecosystem Science and Management
  • Contact Information: michael.preston@unbc.ca
  • Office Hours: 12:20-1:00 PM Mon/Wed/Fri or by appointment

Learning Objectives

  • Outline the mechanism for genetic recombination
  • Describe genetic recombination in eukaryotes
  • Summarize the phases of meiosis
  • Note: Focus is primarily on eukaryotic organisms, with Bacteria and Archaea covered in Biol 203 - Microbiology
  • Readings: Chapter 8 – Genetic Recombination

Genetic Recombination

  • Purpose of Genetic Change:
    • Offspring inherit genes from both parents yet exhibit unique genetic profiles.
    • Genetic Variation:
    • Labelled as bad for individual health, yet beneficial for population survival and evolution.
    • Populations evolve through genetic variability.

Mechanism of Genetic Recombination

  • Occurs between similar regions of DNA, termed homologous regions (not identical).
    • Homologous regions can vary from a few base pairs to entire chromosomes.
    • Each genetic recombination event involves “cutting and pasting” of four DNA backbones.

Eukaryotic Genetic Recombination

  • Meiotic Division:
    • Essential for sexual reproduction, which results from the union of male and female gametes (sperm and egg).
    • Meiosis reduces the chromosome number by half, producing haploid gametes.
    • Greek origin: means “diminish” or “lessening.”
    • Evolutionary Advantage:
    • Fosters genetic shuffling through sexual reproduction, resulting in diverse offspring, none of which are genetically identical.

Homologous Chromosomes

  • Definition: Homologous chromosomes carry the same set of genes but may have different alleles of those genes.
    • Terms:
    • Genes: Units of heredity; sections of DNA coding for a single trait or polypeptide.
    • Alleles: Distinct variations of the same gene.
    • Karyotyping: A process used to study the number and structure of chromosomes.

Haploidy and Diploidy

  • Diploid (2n):
    • Cells containing pairs of homologous chromosomes.
  • Haploid (1n):
    • Cells that contain only one of each homologous pair.
  • Example:
    • Dad and Mom are diploid (2n), their offspring (Junior) are also diploid (2n), while gametes (sperm and egg) are haploid (1n).

Meiotic Process

  • Two meiotic divisions lead to the formation of four haploid nuclei.
  • Divisions Explained:
    • Chromosome replication occurs during premeiotic interphase.
    • Chromosome pairing occurs during Prophase I of meiosis.
    • The process consists of two meiotic divisions.

Phases of Meiosis

  • Prophase I:
    • Chromosomes condense and homologous chromosomes pair up (forming tetrads).
  • Metaphase I:
    • Homologous pairs align at the metaphase plate.
  • Anaphase I:
    • Homologous chromosomes are pulled apart.
  • Telophase I:
    • Two new nuclei form, with cells now haploid (1n).
  • Interkinesis:
    • A brief interphase between Meiosis I and II.
  • Meiosis II:
    • Sister chromatids separate, with additional phases mirroring mitosis (Prophase II, Metaphase II, Anaphase II, Telophase II).

Comparison of Mitosis and Meiosis

  • Mitosis:
    • A single division resulting in two genetically identical diploid daughter cells.
  • Meiosis:
    • Two divisions resulting in four genetically dissimilar haploid cells.
  • Key differences:
    • In meiosis, homologous chromosomes pair up and crossing-over occurs during Prophase I, enhancing genetic diversity.

Crossing Over

  • Definition: Crossing over leads to the formation of recombinant chromosomes, combining genes from both parents.
    • Begins in Prophase I as homologous chromosomes pair (synapsis) and trade portions at chiasmata.
    • Contributes greatly to genetic variation by mixing parental DNA into a single chromosome.

Mechanisms of Genetic Diversity

  • Genetic diversity arises from:
    1. Genetic recombination during Prophase I.
    2. Random segregation of maternal/paternal chromosomes in Anaphase I.
    3. Segregation of recombinant vs. parental chromatids in Anaphase II.
    4. Random fertilization of gametes.
  • These mechanisms bolster the genetic variability within populations.

Animal Life Cycles

  • Characterized by a diploid phase where meiosis is followed directly by gamete formation, with a reduced and short haploid phase lacking mitosis.
    • In males, all four meiotic products become sperm cells.
    • In females, only one nucleus becomes an egg; others become polar bodies.

Plant and Fungal Life Cycles

  • Alternation of Generations:
    • Life cycles alternate between haploid (gametophyte) and diploid (sporophyte) phases.
    • Fertilization leads to sporophytes, which produce spores via meiosis and generate gametophytes through mitosis.

Other Life Cycles in Fungi and Algae

  • In some species, the diploid phase exists solely as a single-celled zygote that will undergo meiosis.
    • Mitosis is only observed in haploid cells, where gametes may be denoted as distinct (+) or (−).