Chapter 6: Cell Cycle and Cell Division

Introduction

The cell cycle refers to the series of events that takes place in a cell leading to its division and duplication. Cell division is the process by which a parent cell divides into two or more daughter cells. It is essential for growth, development, repair, and reproduction in living organisms.

Cell Cycle

Phases of Cell Cycle

The cell cycle consists of two major phases:

1. Interphase: Period between two successive cell divisions
2. M-Phase (Mitotic Phase): Actual cell division

Interphase

Interphase is further divided into three sub-phases:

G₁ Phase (First Gap Phase)

• Cell increases in size
• Synthesis of RNA, proteins, and cellular organelles
• Preparation for DNA replication
• Duration: Variable (longest phase)
• Certain cells temporarily or permanently stop dividing and enter G₀ phase

S Phase (Synthesis Phase)

• Replication of DNA (chromosome duplication)
• Synthesis of histone proteins
• Duration: 6-8 hours
• Each chromosome has two sister chromatids after this phase

G₂ Phase (Second Gap Phase)

• Continued growth of cell
• Protein synthesis
• Preparation for mitosis
• Duration: 3-5 hours

M-Phase (Mitotic Phase)

M-Phase includes:

• Karyokinesis (Nuclear division)
• Cytokinesis (Cytoplasmic division)

Cell Cycle Regulation

• Controlled by cyclins and cyclin-dependent kinases (CDKs)
• Checkpoints ensure proper completion of each phase:
  • G₁ checkpoint (restriction point)
  • G₂ checkpoint
  • M checkpoint (spindle assembly checkpoint)
• External factors affecting cell cycle:
  • Growth factors
  • Nutrients
  • Cell density
  • Environmental conditions

Types of Cell Division

Mitosis

Mitosis is the type of cell division that results in two genetically identical daughter cells. It is important for growth, development, and repair of tissues.

Phases of Mitosis

Prophase

• Chromatin condenses to form distinct chromosomes
• Nuclear membrane begins to disintegrate
• Nucleolus disappears
• Centrosomes move to opposite poles and form spindle fibers
• In plant cells, preprophase band of microtubules forms at future division site

Prometaphase

• Nuclear envelope completely breaks down
• Kinetochores develop on centromeres
• Spindle fibers attach to kinetochores

Metaphase

• Chromosomes align at the equatorial plate (metaphase plate)
• Each chromosome is attached to spindle fibers from both poles
• Maximum condensation of chromosomes

Anaphase

• Sister chromatids separate and move towards opposite poles
• Movement facilitated by shortening of spindle fibers
• Begins with the division of centromere

Telophase

• Chromosomes reach opposite poles and begin to decondense
• Nuclear envelope reforms around each set of chromosomes
• Nucleoli reappear
• Spindle fibers disappear

Cytokinesis

Cytokinesis is the division of cytoplasm following nuclear division.

In Animal Cells

• Cleavage furrow formation begins at the cell equator
• Actomyosin contractile ring pinches the cell into two
• Process: Cleavage

In Plant Cells

• Cell plate formation starts at the center and grows outward
• Vesicles from Golgi apparatus form phragmoplast
• Cell plate develops into middle lamella and primary cell walls
• Process: Cell plate formation

Meiosis

Meiosis is a specialized type of cell division that reduces the chromosome number by half. It is important for sexual reproduction and genetic diversity.

Significance of Meiosis

• Maintains chromosome number across generations
• Introduces genetic variation through:
  • Crossing over (genetic recombination)
  • Random alignment of homologous chromosomes
  • Random fertilization

Phases of Meiosis

Meiosis consists of two successive divisions: Meiosis I and Meiosis II.

Meiosis I (Reduction Division)

Prophase I

• Longest and most complex phase
• Subdivided into five stages:
  • Leptotene: Chromosome condensation begins
  • Zygotene: Synapsis (pairing of homologous chromosomes)
  • Pachytene: Crossing over between non-sister chromatids
  • Diplotene: Separation of homologous chromosomes except at chiasmata
  • Diakinesis: Complete terminalization of chiasmata

Metaphase I

• Bivalents (tetrads) align at the equatorial plate
• Homologous chromosomes attach to spindle fibers from opposite poles

Anaphase I

• Homologous chromosomes separate and move to opposite poles
• Sister chromatids remain attached at centromere
• Reduction in chromosome number occurs at this stage

Telophase I

• Chromosomes reach opposite poles
• Nuclear envelope may or may not form
• Brief interkinesis period (no DNA replication)

Meiosis II (Equational Division)

Prophase II

• Brief phase
• Spindle apparatus forms
• Nuclear envelope disintegrates (if reformed during interkinesis)

Metaphase II

• Chromosomes align at the equatorial plate
• Each chromosome attaches to spindle fibers from opposite poles

Anaphase II

• Sister chromatids separate and move to opposite poles
• Similar to mitotic anaphase

Telophase II

• Chromosomes reach opposite poles and decondense
• Nuclear envelope reforms
• Cytokinesis follows, resulting in four haploid cells

Comparison Between Mitosis and Meiosis

Similarities

• Both involve DNA replication before division
• Both involve nuclear and cytoplasmic division
• Both involve spindle formation
• Both proceed through prophase, metaphase, anaphase, and telophase

Differences

Number of Divisions

• Mitosis: Single division
• Meiosis: Two successive divisions

Number of Daughter Cells

• Mitosis: Two daughter cells
• Meiosis: Four daughter cells

Chromosome Number in Daughter Cells

• Mitosis: Diploid (2n), identical to parent cell
• Meiosis: Haploid (n), half the number of parent cell

Genetic Composition

• Mitosis: Daughter cells genetically identical to parent cell
• Meiosis: Daughter cells genetically different due to crossing over and random assortment

Occurrence

• Mitosis: Somatic cells, asexual reproduction
• Meiosis: Germ cells, sexual reproduction

Prophase

• Mitosis: Simple, no synapsis or crossing over
• Meiosis: Complex, involves synapsis and crossing over

Metaphase

• Mitosis: Individual chromosomes align at metaphase plate
• Meiosis I: Homologous pairs (bivalents) align at metaphase plate

Anaphase

• Mitosis: Sister chromatids separate
• Meiosis I: Homologous chromosomes separate
• Meiosis II: Sister chromatids separate

Significance of Mitosis and Meiosis

Significance of Mitosis

• Growth and development of multicellular organisms
• Tissue repair and regeneration
• Asexual reproduction in some organisms
• Maintenance of chromosome number
• Genetic stability across somatic cell lineages

Significance of Meiosis

• Reduction of chromosome number by half
• Maintenance of species chromosome number during sexual reproduction
• Introduction of genetic variation through:
  • Crossing over during prophase I
  • Random alignment of homologous chromosomes in metaphase I
  • Random fertilization
• Formation of gametes (eggs and sperm)
• Evolutionary significance through increased genetic diversity