3. Cell Division and Chromosome Heredity

3. Cell Division and Chromosome Heredity

Introduction

This chapter explores the fundamental processes of cell division—mitosis and meiosis—and their roles in genetic inheritance and chromosome behavior. Understanding these mechanisms is essential for grasping how genetic information is faithfully transmitted and how variation arises in sexually reproducing organisms.

Cell Division

Mitosis

Mitosis is the process by which somatic (nonreproductive) cells divide to produce two genetically identical daughter cells. This ensures the maintenance of the diploid chromosome number across cell generations.

• Somatic cells: Body cells with chromosomes present in pairs (diploid, 2n).

• Diploid number (2n): Total number of chromosomes in a somatic cell.

• Haploid number (n): Number of chromosomes in gametes, containing one of each chromosome pair.

• Daughter cells: Genetically identical replicas of the parental cell.

Example: Human somatic cells have 46 chromosomes (2n = 46).

Meiosis

Meiosis occurs in germ-line cells to produce gametes (sperm and eggs) with half the chromosome number of the original cell. This process introduces genetic diversity among gametes.

• Gametes: Reproductive cells produced by meiosis.

• Gametes are not genetically identical to one another due to crossing over and independent assortment.

Example: Human gametes have 23 chromosomes (n = 23).

Chromosomes

Sex Chromosomes and Heredity

Sex chromosomes (X and Y) determine the sex of an organism and differ between males and females. Certain mechanisms equalize gene expression from sex chromosomes, and the discovery of genes on the X chromosome supported the chromosome theory of heredity.

• Chromosome theory of heredity: Genes are carried on chromosomes, and their behavior during meiosis explains inheritance patterns.

The Cell Cycle

Phases of the Cell Cycle

The cell cycle is a regulated sequence of events involving DNA replication and cell division. All eukaryotes share similar cell cycles, which are divided into two principal phases:

• M phase: Short period during which cells divide (mitosis or meiosis).

• Interphase: Longer period between M phases, including cell growth and DNA replication.

Interphase

Interphase consists of three subphases:

• G1 (Gap 1): High gene expression and cell activity; duration varies by cell type.

• S phase (Synthesis): DNA replication occurs, doubling the DNA content and producing two sister chromatids per chromosome.

• G2 (Gap 2): Preparation for cell division.

• Some cells enter G0 after G1, becoming specialized and ceasing to divide.

M Phase Substages

M phase is divided into:

• Prophase

• Prometaphase

• Metaphase

• Anaphase

• Telophase

Karyokinesis refers to the partitioning of DNA into daughter nuclei, while cytokinesis is the division of cytoplasmic contents.

Chromosome Behavior During Mitosis

Chromosome Condensation and Structure

• Chromosomes are diffuse before mitosis and condense during prophase, reaching maximum condensation at metaphase.

• Centromeres: Specialized DNA sequences joining sister chromatids; bind protein complexes called kinetochores.

Chromosome Movement and Distribution

• Animal cells have two centrosomes that migrate to opposite poles.

• Centrosomes generate spindle fiber microtubules (minus end at centrosome, plus end grows outward).

• Spindle fibers form an aster pattern.

Metaphase Chromosomes

• Kinetochore microtubules attach to each kinetochore by the end of prometaphase.

• Chromosomes are highly condensed and aligned along the metaphase plate.

Anaphase

• Sister chromatids separate and move to opposite poles (chromosome disjunction).

• Separase enzyme cleaves cohesin proteins, allowing separation.

• Kinetochore microtubules depolymerize, moving chromatids.

• Polar microtubules elongate the cell, facilitating cytokinesis.

Completion of Cell Division

• In telophase, nuclear membranes reassemble and chromosomes decondense.

• Two identical nuclei are formed; cytokinesis divides the cell into two daughter cells.

Cytokinesis

• Animal cells: Contractile ring of actin forms a cleavage furrow.

• Plant cells: New cell wall (cell plate) forms along the midline.

• Cytoplasmic contents and organelles are divided between daughter cells.

Mitosis Produces Identical Daughter Cells

• Replicated sister chromatids are separated into identical nuclei.

• The diploid chromosome number (2n) is maintained.

Cell Cycle Checkpoints

Regulation and Genetic Control

Cell cycle progression is regulated by genetically controlled signals and monitored by cell cycle checkpoints:

• Checkpoints ensure readiness for the next stage (e.g., DNA replication, spindle attachment).

• Mutations in checkpoint control can lead to abnormal cell growth, such as cancer.

Meiosis and Sexual Reproduction

Types of Reproduction

• Asexual reproduction: Produces genetically identical offspring without mating.

• Sexual reproduction: Involves fusion of haploid gametes to produce diploid progeny.

Meiosis vs. Mitosis

• Both processes share interphase, but meiosis includes two division stages (meiosis I and II) without intervening DNA replication.

• Meiosis produces four genetically distinct haploid gametes.

Table: Comparison of Mitosis and Meiosis

Characteristic	Mitosis	Meiosis
Purpose	Growth and maintenance; identical cells	Sexual reproduction; genetically different gametes
Location	Somatic cells	Germ-line cells
Mechanics	One division after one DNA replication	Two divisions after one DNA replication
Homologous chromosomes	Do not pair	Pair and undergo recombination
Sister chromatids	Attach to spindle fibers from opposite poles	Attach to same pole in meiosis I, opposite poles in meiosis II
Product	Two identical diploid daughter cells	Four genetically unique haploid gametes

Meiosis I and II

• Meiosis I: Homologous chromosomes separate, reducing chromosome number from diploid to haploid.

• Meiosis II: Sister chromatids separate, producing four haploid gametes.

Meiosis I: Hallmark Events

1. Homologous chromosome pairing

2. Crossing over between homologous chromosomes

3. Segregation of homologous chromosomes

Stages of Meiosis I

• Prophase I (subdivided into leptotene, zygotene, pachytene, diplotene, diakinesis)

• Metaphase I

• Anaphase I

• Telophase I

Prophase I Substages

• Leptotene: Chromosome condensation begins.

• Zygotene: Meiotic spindle forms; nuclear envelope disintegrates; homologous chromosomes undergo synapsis.

• Synaptonemal complex: Protein bridge forms between homologs, tightly binding nonsister chromatids.

Additional info: The synaptonemal complex is essential for crossing over and genetic recombination.