Chapter 1: The Cell Cycle

The cell cycle is the complete sequence of events from the birth of a cell to its division into two daughter cells. It is divided into two major phases: interphase and the mitotic (M) phase.

Interphase (~95% of cycle) comprises three sequential stages. G1 phase is the first growth phase — the cell increases in size, synthesises RNA and proteins, and makes the "decision" to divide or enter G0. S phase (6–8 hours) is when DNA replication occurs, doubling DNA content from 2C to 4C while chromosome number stays 2n. G2 phase (3–4 hours) involves synthesis of tubulin and other proteins needed for spindle assembly. G0 phase is a quiescent exit from G1 for non-dividing cells; neurons and mature RBCs are permanently in G0, while some cells (liver) can re-enter the cycle.

The M phase (~1 hour) consists of karyokinesis (nuclear division via mitosis) followed by cytokinesis (cytoplasmic division).

Chapter 2: Mitosis

Mitosis (equational division) produces two genetically identical diploid daughter cells. The four stages are:

• Prophase: Chromatin condenses into visible chromosomes (2 chromatids per chromosome); nucleolus disappears; spindle begins to form.
• Metaphase: Chromosomes align at the metaphase plate; spindle fibres attach via kinetochores; maximum condensation.
• Anaphase: Centromeres split; sister chromatids separate and move to opposite poles.
• Telophase: Nuclear envelope reforms; chromosomes decondense; nucleolus reappears.

Cytokinesis follows — cleavage furrow (centripetal, actin-myosin) in animal cells; cell plate (centrifugal, Golgi vesicles) in plant cells. Functions: growth, tissue repair, asexual reproduction.

Chapter 3: Meiosis — Overview and Division I

Meiosis produces four haploid (n) cells from one diploid (2n) cell through two sequential divisions, occurring only in germ cells for gamete formation.

Meiosis I is the reductional division: homologous chromosomes separate, halving chromosome number from 2n to n. Prophase I is its most distinctive and longest stage, with five substages (LZPDD):

• Leptotene: Chromosomes begin condensing (thin threads).
• Zygotene: Synapsis — homologues pair via synaptonemal complex forming bivalents.
• Pachytene: Crossing over between non-sister chromatids; recombination nodules active.
• Diplotene: Synaptonemal complex dissolves; chiasmata become visible; homologues start separating.
• Diakinesis: Chiasmata terminalize; nuclear membrane breaks down; full condensation complete.

In Metaphase I, bivalents align at equatorial plate (random orientation = independent assortment). In Anaphase I, homologous chromosomes separate (centromeres do NOT split — sister chromatids remain joined). Result: 2 cells, each n chromosomes and 2C DNA.

Chapter 4: Meiosis II and Significance

Meiosis II is equational (like mitosis): sister chromatids separate in Anaphase II, producing four haploid gametes each with n chromosomes and 1C DNA. No S phase (DNA replication) occurs between meiosis I and II.

The significance of meiosis: (1) maintains constant chromosome number across generations; (2) generates genetic variation through crossing over (intrachromosomal recombination) and independent assortment (interchromosomal recombination). These two mechanisms collectively produce gametes with unique genetic compositions, driving biological diversity.

Key Comparison Summary