Chromosomal Basis of Heredity: Cell Division, Mitosis, and Meiosis

Chromosomal Basis of Heredity

Overview

The chromosomal basis of heredity explains how genetic information is transmitted from one generation to the next through the structure and behavior of chromosomes during cell division. This section covers cell architecture, chromosome features, the cell cycle, mitosis, meiosis, and the comparison between mitosis and meiosis.

Cell Architecture: Prokaryotes and Eukaryotes

Key Differences

• Prokaryotes lack a nucleus and membrane-bound organelles; their genetic material is typically a single circular DNA molecule.

• Eukaryotes possess a nucleus and multiple linear chromosomes contained within the nucleus.

Additional info: Eukaryotic cells also contain organelles such as mitochondria and, in plants, chloroplasts, each with their own DNA.

General Features of Chromosomes

Eukaryotic Chromosome Terminology

• Chromosomes have two arms: the long arm (q) and the short arm (p).

• Centromeres are specialized regions that help with chromosome segregation during cell division.

• Telomeres protect the ends of chromosomes from degradation.

• A karyotype is a visual representation of the complete set of chromosomes in a cell, arranged by size and shape.

• Diploid cells (2N) have two copies of each chromosome, one from each parent.

Chromosome Structure and Replication

• Before cell division, chromosomes are replicated, resulting in two identical sister chromatids held together at the centromere.

Ploidy and Homology

• N = number of complete sets of chromosomes in a cell.

• Diploid (2N): Two homologous chromosomes per set.

• Haploid (1N): One chromosome per set (e.g., gametes).

• Homology: Similarity due to common origin; homologous chromosomes carry the same genes but may have different alleles.

The Cell Cycle and Mitosis

Cell Cycle

• The cell cycle is a regulated process ensuring the production of two identical daughter cells.

• Phases: G1 (growth), S (DNA synthesis), G2 (preparation for division), and M (mitosis).

Mitosis

• Mitosis is the division of a somatic cell into two genetically identical daughter cells.

• During mitosis, sister chromatids are separated, ensuring each daughter cell receives a complete genome.

Sexual Reproduction and Meiosis

Sexual Reproduction

• Most animals and plants reproduce sexually, merging genetic material from two different individuals.

• This process increases genetic diversity, which is essential for evolution through natural selection and genetic drift.

• The cellular division that generates genetic diversity is meiosis.

Sexual Life Cycle

• Composed of meiosis and fertilization.

• Diploid cells (somatic): Two sets of chromosomes.

• Haploid cells (gametes): One set of chromosomes.

• Meiosis reduces chromosome number by half, preventing chromosome doubling at fertilization.

• Offspring inherit genetic material from both parents.

Alternation of Haploid and Diploid Stages

• Sexually reproducing organisms alternate between haploid and diploid stages.

• In animals, the diploid state dominates; the zygote undergoes mitosis to produce diploid cells, some of which later undergo meiosis to produce haploid gametes.

• Germ-line cells undergo meiosis to produce gametes.

Meiosis

Overview

• Meiosis consists of two rounds of cell division: Meiosis I (reduction division) and Meiosis II (equational division).

• Meiosis I reduces chromosome number by half, producing haploid gametes.

• Meiosis II is similar to mitosis, separating sister chromatids.

• Results in four genetically unique haploid cells from one diploid parent cell.

Sources of Genetic Variation in Meiosis

• Recombination (crossing over): Exchange of genetic material between non-sister chromatids during Prophase I.

• Independent assortment: Random orientation and separation of homologous chromosomes during Metaphase I and Anaphase I.

Summary of Meiosis Stages

Stage	Main Events
Prophase I	Homologous chromosomes pair (synapsis), crossing over occurs, chromosomes condense, nuclear envelope breaks down
Metaphase I	Homologous pairs align at metaphase plate, spindle fibers attach
Anaphase I	Homologous chromosomes separate to opposite poles (independent assortment)
Telophase I & Cytokinesis	Two haploid cells form, chromosomes may decondense
Meiosis II	Similar to mitosis; sister chromatids separate, resulting in four haploid cells

Key Events in Meiosis I

• Homologous chromosome pairing (synapsis) forms bivalents/tetrads.

• Crossing over between non-sister chromatids at chiasmata creates recombinant chromatids.

• Independent assortment of homologous chromosomes increases genetic diversity.

Mechanistic Basis of Genetic Diversity

• Meiosis is a major source of genetic diversity due to crossing over and independent assortment.

• Mutations also contribute to genetic variation.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Number of Divisions	One	Two
DNA Replication	Once per cell cycle	Once prior to Meiosis I
Synapsis & Crossing Over	No	Yes, during Prophase I
Number of Daughter Cells	Two	Four
Genetic Identity	Identical	Genetically unique
Ploidy of Daughter Cells	Diploid (2N)	Haploid (1N)
Cell Types	Somatic cells	Gametes (germ cells)

Summary

• Mitosis produces two identical diploid cells for growth and repair.

• Meiosis produces four genetically diverse haploid gametes for sexual reproduction.

• Genetic diversity arises from crossing over, independent assortment, and mutation.