Chromosomal Basis of Heredity: Cell Division, Mitosis & Meiosis

Chromosomal Basis of Heredity

Introduction

The chromosomal basis of heredity explores how genetic information is stored, organized, and transmitted through cell division. This module covers the architecture of prokaryotic and eukaryotic cells, the structure and function of chromosomes, and the processes of mitosis and meiosis.

Cell Architecture: Prokaryotes vs. Eukaryotes

Overview

Cells are classified as prokaryotic or eukaryotic based on their structural features, which impact their genetic organization and cell division mechanisms.

• Prokaryotic Cells: Small (1-10 μm), simple structure, single circular chromosome, lack a defined nucleus and organelles.

• Eukaryotic Cells: Larger (10-100 μm), complex structure, multiple linear chromosomes, defined nucleus, specialized organelles.

Feature	Prokaryotic Cells	Eukaryotic Cells
Size	1-10 μm	10-100 μm
Chromosome Type	Single, circular	Multiple, linear
Genome Size	1-10 Mb	10 Mb - >100 Gb
Organelles	Absent	Present (mitochondria, chloroplasts)
Cytoskeleton	Simple	Complex (protein filaments)

Additional info: These differences influence how each cell type divides and organizes its genetic material.

General Features of the Chromosome

Definition and Structure

Chromosomes are structures found in cells that carry genetic information in the form of DNA. They are organized packages of DNA complexed with proteins, allowing efficient storage and accurate duplication during cell division.

• One or more chromosomes make up the genome.

• Key differences exist in chromosome structure and number between prokaryotes and eukaryotes.

Prokaryotic Chromosomes

• Bacteria typically have a single, circular DNA molecule.

• DNA length is ~1 mm when stretched; condensation (looping and supercoiling) reduces it to ~1 μm.

• Lack histone proteins; use bacterial-specific proteins for DNA packaging.

Eukaryotic Chromosomes

• Long, linear DNA molecules associated with histone proteins.

• Histones condense and organize DNA into chromatin.

• Organization starts with nucleosomes (8 histone proteins) and progresses to higher-order compaction.

Additional info: Chromatin structure is dynamic, allowing regions to be more or less condensed depending on gene activity.

Eukaryotic Chromosome Terminology

Key Terms

• Centromere: Specialized region for chromosome segregation during cell division.

• Telomere: Protects chromosome ends from degradation.

• Karyotype: Visual representation of the complete set of chromosomes in a cell, arranged by size and shape.

• Diploid (2N): Two copies of each chromosome (one from each parent).

• Haploid (1N): One copy of each chromosome.

• Homologous Chromosomes: Chromosome pairs with the same genes but possibly different alleles.

• Sister Chromatids: Identical copies of a chromosome held together at the centromere after replication.

Ploidy and Chromosome Number

• Ploidy level (N): Number of sets of chromosomes in a cell.

• Examples: 1N=5 (haploid, 5 chromosomes), 2N=4 (diploid, 4 chromosomes), 4N=16 (tetraploid, 16 chromosomes).

Ploidy	Chromosome Sets	Example
1N	1 set	Haploid, 1N=5
2N	2 sets	Diploid, 2N=4
4N	4 sets	Tetraploid, 4N=16

Homozygous vs. Heterozygous

Definitions

• Homozygous: Both alleles at a gene locus are identical (e.g., AA or cc).

• Heterozygous: Two different alleles at a gene locus (e.g., Bb).

• Dominant allele: Expressed over the recessive trait.

• Recessive allele: Masked by the dominant variant.

Genotype	Description
AA	Homozygous for dominant allele
Bb	Heterozygous (one dominant, one recessive)
cc	Homozygous for recessive allele

Human Chromosomes

Karyotype and Ploidy

• Humans have 2 sets of 23 chromosomes (total 46): 22 autosomes and 1 pair of sex chromosomes (X, Y).

• Somatic cells: 2N=46 (diploid).

• Gametes (egg/sperm): 1N=23 (haploid).

Cell Cycle and Mitosis

Importance

Cell division is essential for growth, development, and repair in multicellular organisms, and for reproduction in single-celled organisms.

• Prokaryotes: Divide by binary fission (asexual reproduction).

• Eukaryotes: Divide by the cell cycle and mitosis.

Bacterial Cell Division: Binary Fission

• Replication begins at the origin of replication (oriC).

• Chromosomes move to opposite ends; cell wall forms, producing two daughter cells.

• Results in genetically identical clones.

Eukaryotic Cell Cycle

• Consists of growth, DNA replication, and division.

• Mitosis: Division of the nucleus, separating duplicated chromosomes into two new nuclei.

• Regulated by cyclins and cyclin-dependent kinases (Cdks).

• Loss of cell cycle control can lead to cancer (oncogenes).

Stages of Mitosis

• Prophase: Chromosomes condense, spindle forms, nuclear envelope breaks down.

• Prometaphase: Spindle microtubules attach to kinetochores.

• Metaphase: Chromosomes align at the metaphase plate.

• Anaphase: Sister chromatids are pulled apart to opposite poles.

• Telophase: Nuclear envelopes reform, chromosomes de-condense.

• Cytokinesis: Division of cytoplasm and organelles; cleavage furrow in animals, cell plate in plants.

Additional info: Cohesin proteins hold sister chromatids together; separase hydrolyzes cohesin during anaphase.

Summary of Mitosis

• Produces two identical daughter cells.

• Each receives a complete set of genetic material (same chromosome number and ploidy as the mother cell).

• Essential for growth, repair, and asexual reproduction.