A

Meiosis, Genetic Variation, and Inheritance Notes

Learning Objectives and Big Picture

  • Understand how offspring acquire genes from parents by inheriting chromosomes.
  • Understand how meiosis and fertilization bridge one generation to the next (life cycle).
  • Understand how meiosis reduces the number of chromosome sets from diploid to haploid (overview of stages of meiosis).
  • Understand the different ways that sexual reproduction creates genetic variation.

Inheritance, Genes, and Chromosomes

  • Inheritance (heredity) is the transmission of genes and associated traits from one generation to the next.
  • Siblings are not identical copies of either parent or of their siblings, reflecting genetic variation.
  • Genetics is the study of heredity and inherited variation.

Chromosomes, Genes, Loci, and Ploidy

  • Genes are passed to the next generation via reproductive cells called gametes (sperm and eggs).
  • DNA is packaged into chromosomes that contain genes.
  • Humans have 23 pairs of chromosomes (46 total) in the nuclei of somatic (body) cells.
  • A gene’s specific position along a chromosome is called its locus.
  • We have two copies of each chromosome (two copies of each gene) in somatic cells, except for the X and Y chromosome pair which have a more complex relationship (noted for later lectures).

The Structure of the Human Genome in Context of Reproduction

  • Humans have 23 pairs of chromosomes, meaning two copies of each chromosome and thus two copies of each gene.
  • Chromosome pairs are homologous (except for the sex chromosomes where XX or XY differ in males and females).
  • Genes reside at specific loci on chromosomes; homologous chromosomes carry the same genes at the same loci but can carry different alleles.

Meiosis and the Life Cycle: Overview

  • Meiosis reduces chromosome number by half, producing haploid gametes (n) from diploid (2n) parent cells.
  • Meiosis occurs in specialized cells (gonads): testes in males, ovaries in females.
  • In humans, somatic cells are diploid with 46 chromosomes (23 pairs).
  • Meiosis I and II are two consecutive divisions that yield four haploid daughter cells, each with half as many chromosomes as the parent.
  • Fertilization restores diploidy by joining a sperm (n) and an egg (n) to form a zygote (2n).

Life Cycle: From Gamete to Zygote

  • Gamete (n): produced by meiosis in ovaries/testes; in humans, n = 23, consisting of 22 autosomes and 1 sex chromosome.
    • Egg (ovum) sex chromosome is X in humans.
    • Sperm sex chromosome can be X or Y.
  • Zygote (2n): union of gametes (fertilization) forms a zygote with 2n = 46; has one set of chromosomes from each parent.
  • The zygote then undergoes mitosis to form somatic cells and develop into an adult.

Meiosis: IPMAT and the Four Daughter Cells

  • Meiosis consists of two consecutive divisions: Meiosis I and Meiosis II.
  • The two divisions produce four daughter cells, each with half the number of chromosomes of the parent.
  • In meiosis I, homologous chromosomes separate; in meiosis II, sister chromatids separate.
  • Each daughter cell ends up haploid (n) after meiosis II.

Meiosis I: Prophase I to Telophase I

  • Prophase I:
    • Centrosome movement and spindle formation occur.
    • Nuclear envelope breaks down.
    • Chromosomes condense.
    • Homologous chromosomes pair up and crossing over occurs.
    • Chiasmata (X-shaped regions) are the sites of crossing over.
  • Metaphase I:
    • Pairs of homologous chromosomes line up at the metaphase plate in pairs.
    • One chromosome from each pair faces each pole.
    • Microtubules attach to kinetochores of chromosomes from opposite poles.
  • Anaphase I:
    • Homologous chromosomes separate and move toward opposite poles.
    • Sister chromatid cohesions persist at the centromere, enabling each chromatid pair to move as a unit toward the same pole.
  • Telophase I and Cytokinesis:
    • Each half of the cell has a haploid set of duplicated chromosomes.
    • Each chromosome still consists of two sister chromatids.
    • Cytokinesis produces two haploid daughter cells.

Important Centromere, Centriole, and Centrosome Concepts

  • Centromere: the DNA region where sister chromatids are held together and where the kinetochore forms.
  • Kinetochore: a protein complex at the centromere that attaches chromosomes to spindle microtubules to guide movement during mitosis and meiosis.
  • Centriole: a barrel-shaped organelle that helps organize spindle microtubules; pairs with the centrosome during cell division.
  • Centrosome: the major microtubule-organizing center during mitosis.

Meiosis I: Prophase I Details and Crossing Over

  • Crossing over during prophase I: non-sister chromatids exchange genetic material, producing non-identical chromatids.
  • Result: recombinant chromosomes that carry DNA from both parental origins.
  • The chiasmata are the visible manifestations of crossing over.

Meiosis I: Metaphase I and Anaphase I Details

  • Metaphase I: homologous chromosomes align at the metaphase plate as pairs; each pair is oriented independently relative to the poles.
  • Anaphase I: homologous chromosomes separate and move toward opposite poles; sister chromatids stay together at this stage.

Meiosis I: Telophase I and Cytokinesis

  • Telophase I: each daughter cell has a haploid set of duplicated chromosomes.
  • Cytokinesis: typically occurs, producing two haploid daughter cells.

Meiosis II: Overview and Key Steps

  • Meiosis II resembles mitosis in that sister chromatids separate.
  • Prophase II: spindle apparatus forms again; chromosomes, each still composed of two sister chromatids, begin migrating toward the metaphase II plate.
  • Metaphase II: chromosomes align at the metaphase plate; kinetochores attach to microtubules from opposite poles.
  • Anaphase II: centromere proteins are cleaved, chromatids separate and move toward opposite poles; each chromatid is now an individual chromosome.
  • Telophase II and Cytokinesis: nuclei form, chromosomes decondense, cytokinesis occurs, producing four haploid daughter cells.
  • Note: there is no crossing over in Meiosis II.

Mitosis vs. Meiosis: A Quick Comparison

  • Mitosis yields two genetically identical diploid daughter cells (2n) from one parent cell.
  • Meiosis yields four genetically unique haploid daughter cells (n) from one parent cell.
  • In Meiosis I, homologous chromosomes separate; in Mitosis, sister chromatids separate in one division.
  • In Meiosis II, sister chromatids separate; in Mitosis, this separation occurs in one division after a single replication.

Genetic Variation: How Offspring Differ

  • Mutations are a source of genetic diversity (new alleles).
  • Sexual reproduction reshuffles alleles to create variation through:
    • Independent assortment of chromosomes during Meiosis I and II.
    • Crossing over during prophase I.
    • Random fertilization of gametes produced by meiosis.

Independent Assortment of Chromosomes

  • During metaphase I, homologous chromosome pairs orient randomly with respect to the poles.
  • This independent assortment means the maternal and paternal homologs sort into daughter cells independently of other pairs.
  • The number of possible chromosome combinations is given by 2^{n}, where n is the haploid number.
  • For humans, n=23, so there are 2^{23} = 8{,}388{,}608 possible chromosome combinations per gamete.
  • The combinations from both parents multiply: 2^{23} \times 2^{23} = 2^{46} \approx 7.04 \times 10^{13} possible diploid combinations for zygotes (roughly 70 trillion).

Crossing Over and Recombinant Chromosomes

  • Crossing over during prophase I creates recombinant chromosomes that mix DNA from both parents.
  • This increases genetic diversity by producing chromosomes with new allele combinations.
  • On average, each chromosome undergoes about 1 to 3 crossover events in humans.
  • These crossovers contribute to non-identical chromatids and genetic variation among gametes.

Random Fertilization and Its Contribution to Variation

  • Any sperm can fuse with any ovum, dramatically increasing variation.
  • Each gamete has about 2^{23} = 8{,}388{,}608 possible chromosome combinations due to independent assortment.
  • The fusion of an egg and a sperm yields a zygote with about 2^{46} \approx 7.04 \times 10^{13} possible diploid combinations, far more than the number of humans that have ever lived.
  • The combination possibilities from independent assortment and random fertilization contribute to the enormous genetic diversity observed in populations.

Real-world Example of Genetic Variation

  • Spirit bears: mother black bears can have cubs that are black or all-white due to genetic variation, illustrating how genetic diversity can manifest phenotypically in offspring.

Summary of Key Terms and Concepts

  • Allele: different versions of a gene.
  • Gene: a unit of heredity that encodes a trait.
  • Locus: a gene’s fixed position on a chromosome.
  • Chromosome: DNA molecule with many genes; humans have 46 in somatic cells (23 pairs).
  • Homologous chromosomes: paired chromosomes (one from each parent) that carry the same genes at the same loci.
  • Sister chromatids: identical copies formed by DNA replication, held together at the centromere.
  • Centromere: DNA region where sister chromatids are held together and where kinetochores form.
  • Kinetochores: protein structures at centromeres that attach chromosomes to spindle fibers.
  • Chiasmata: X-shaped regions where crossing over occurred between homologous chromosomes.
  • Haploid (n): a cell containing a single set of chromosomes.
  • Diploid (2n): a cell containing two sets of chromosomes.
  • Zygote: the diploid cell formed when a sperm fertilizes an egg; 2n = 46 in humans.
  • Gamete: a haploid sex cell (sperm or egg).
  • Meiosis I and Meiosis II: two consecutive cell divisions that produce four haploid gametes; homologs separate in I, sister chromatids separate in II.

Numerical and Statistical References (Summary)

  • Haploid number in humans: n = 23.
  • Number of chromosome combinations per gamete due to independent assortment in humans: 2^{23} = 8{,}388{,}608.
  • Number of possible diploid zygote combinations due to independent assortment in both parents: 2^{46} = 7.04 \times 10^{13}\, approximately 70 trillion.
  • Crossing over per chromosome: average of 1 \sim 3 crossovers.
  • Zygote chromosome count: 2n = 46; Gametes: n = 23.

Connections to Broader Principles and Real-World Relevance

  • The inheritance of genes through chromosomal transmission underpins all heredity and genetic variation observed in populations.
  • Genetic variation through meiosis and fertilization provides raw material for evolution by natural selection.
  • Ethical and philosophical implications relate to genetic diversity, heredity, and how we understand individual differences within populations.