Cell Cycle & Cell Division: Mitosis and Meiosis

Introduction

Every living organism, from the simplest unicellular bacteria to complex multicellular plants and animals, is composed of cells. These cells not only function as the basic structural and functional units of life but also possess the remarkable ability to divide and generate new cells. Cell division is the process by which a parent cell divides into two or more daughter cells. It is fundamental for growth, development, tissue repair, and reproduction.

The sequence of events by which a cell grows, prepares for division, and divides into daughter cells is called the cell cycle. The cell cycle consists of phases that ensure the proper duplication of genetic material (DNA) and its equal distribution into daughter cells.

There are two major types of cell division in eukaryotes:

1. Mitosis – also called equational division, where the chromosome number of the daughter cells remains the same as the parent cell. It occurs in somatic (body) cells.
2. Meiosis – also called reduction division, where the chromosome number is halved (haploid) in the daughter cells. It occurs in germ cells during gamete formation.

Both processes are tightly regulated by genetic controls to maintain genomic stability and ensure faithful transmission of genetic material.

Mitosis

Mitosis is a type of cell division in which a single cell divides into two identical daughter cells, each having the same number of chromosomes as the parent cell. It is the primary mode of division for somatic cells and is responsible for growth, development, repair, and asexual reproduction.

Mitosis occurs in two major phases:

1. Interphase – the preparatory phase of the cell cycle.
2. Mitotic phase (M-phase) – the actual process of division, subdivided into prophase, metaphase, anaphase, and telophase.

Cell Cycle

The cell cycle consists of a repeating sequence of events that prepares the cell for division. It is divided into interphase and the mitotic phase.

1. Interphase

Interphase is the longest phase of the cell cycle, accounting for nearly 90% of the cycle. It is a metabolically active stage where the cell grows, synthesizes proteins, and replicates DNA. It is divided into three subphases:

• G1 phase (Gap 1): The cell grows in size, increases protein synthesis, and prepares for DNA replication.
• S phase (Synthesis): DNA replication occurs, and each chromosome is duplicated to form two sister chromatids.
• G2 phase (Gap 2): Final preparation for mitosis, with further synthesis of proteins required for spindle formation and chromosome condensation.

Cells that exit the cycle enter a resting stage called G0 phase, where they remain metabolically active but do not divide.

2. Mitotic Phase

The mitotic phase is where the actual division takes place, ensuring equal distribution of replicated chromosomes into daughter cells.

• Prophase: Chromosomes condense, spindle fibers form, and the nuclear envelope begins to disintegrate.
• Metaphase: Chromosomes align at the equatorial plate; spindle fibers attach to centromeres.
• Anaphase: Sister chromatids separate and move to opposite poles.
• Telophase: Chromosomes de-condense, nuclear envelope reforms, and the cell prepares for cytokinesis.

Finally, cytokinesis divides the cytoplasm, resulting in two genetically identical daughter cells.

Genetic Control of Mitosis

Mitosis is regulated by a complex network of genes and proteins that act as checkpoints to ensure accuracy.

• Cyclins and Cyclin-Dependent Kinases (CDKs) control progression through G1, S, G2, and M phases.
• Checkpoints:
  • G1/S checkpoint ensures DNA is intact before replication.
  • G2/M checkpoint ensures DNA replication is complete.
  • Spindle checkpoint ensures chromosomes are correctly attached to spindle fibers before anaphase.
• Tumor suppressor genes (e.g., p53): Halt division when DNA is damaged.
• Proto-oncogenes (e.g., c-myc): Promote progression of the cycle. Mutations can lead to uncontrolled mitosis (cancer).

Significance of Mitosis

• Maintains genetic stability by producing identical cells.
• Enables growth and development in multicellular organisms.
• Allows tissue repair and wound healing.
• Basis of asexual reproduction in unicellular and some multicellular organisms.
• Prevents genetic variation, maintaining uniformity of somatic cells.

Meiosis

Meiosis is a specialized form of cell division that reduces the chromosome number by half, producing four haploid daughter cells from one diploid parent cell. It occurs only in germ cells during gametogenesis. Meiosis ensures genetic variation through processes such as crossing-over and independent assortment.

Meiotic Division

Meiosis consists of two successive divisions:

1. First Meiotic Division (Meiosis I – Reductional Division)

• Chromosome number is reduced from diploid (2n) to haploid (n).
• Homologous chromosomes separate, but sister chromatids remain attached.
• Substages: Prophase I, Metaphase I, Anaphase I, Telophase I.

Prophase I is further divided into:

• Leptotene: Chromosomes become visible as thin threads.
• Zygotene: Pairing of homologous chromosomes (synapsis) begins.
• Pachytene: Crossing-over occurs at chiasmata.
• Diplotene: Homologs begin to separate but remain attached at chiasmata.
• Diakinesis: Chromosomes fully condensed, spindle fibers form.

2. Second Meiotic Division (Meiosis II – Equational Division)

• Resembles mitosis.
• Sister chromatids separate, leading to four haploid cells.
• Substages: Prophase II, Metaphase II, Anaphase II, Telophase II.

Synaptonemal Complex

The synaptonemal complex is a protein structure formed between homologous chromosomes during zygotene of prophase I. It resembles a ladder-like structure, stabilizes pairing, and facilitates crossing-over.

Genetic Control of Meiosis

• Meiotic-specific cohesins and synaptonemal proteins ensure proper homolog pairing and separation.
• Recombination genes (e.g., Spo11, RecA homologs) initiate and regulate crossing-over.
• Checkpoints such as the pachytene checkpoint monitor synapsis and recombination.
• Cyclins/CDKs are modified to allow two consecutive divisions without an intervening S phase.

Significance of Meiosis

• Maintains chromosome number across generations.
• Introduces genetic variation through crossing-over and independent assortment.
• Provides raw material for evolution and adaptation.
• Essential for sexual reproduction.

Comparison of Mitosis and Meiosis

Feature	Mitosis	Meiosis
Type of cells	Occurs in somatic cells	Occurs in germ cells
Number of divisions	Single division	Two successive divisions
Number of daughter cells	Two	Four
Chromosome number	Diploid → Diploid (2n → 2n)	Diploid → Haploid (2n → n)
Genetic identity	Daughter cells are genetically identical	Daughter cells are genetically variable
Crossing-over	Absent	Present during Prophase I
Synaptonemal complex	Not formed	Formed in Prophase I
Significance	Growth, repair, asexual reproduction	Sexual reproduction, genetic variation