Structure of Chromosomes, Cell Cycle and Cell Division

Chromosome: Thread-like structure made of DNA and proteins (histones) carrying genetic information. Visible during cell division.

Discovery: Discovered by Waldeyer (1888). Name derived from 'chroma' (color) and 'soma' (body) as they take up stains.

Structure: Each chromosome has: (1) Chromatids - two identical halves joined at centromere, (2) Centromere - constriction point holding chromatids, (3) Telomeres - protective caps at ends.

Chromatin vs Chromosome: Chromatin is loosely coiled DNA-protein complex in non-dividing cells. During division, chromatin condenses to form chromosomes.

Types by Centromere Position: (1) Metacentric - centromere in middle (V-shaped), (2) Sub-metacentric - centromere slightly off-center (L-shaped), (3) Acrocentric - centromere near one end (J-shaped), (4) Telocentric - centromere at end (I-shaped).

Chromosome Number: Species-specific. Humans: 46 (23 pairs), Fruit fly: 8, Rice: 24, Dog: 78.

Autosomes vs Sex Chromosomes: Autosomes control body characteristics (44 in humans). Sex chromosomes determine sex (XX in females, XY in males).

Diagram Importance: Structure of chromosome showing chromatids, centromere, and arms is crucial for ICSE.

DNA (Deoxyribonucleic Acid): Hereditary material carrying genetic information. Located in nucleus (also in mitochondria and chloroplasts).

Structure: Double helix discovered by Watson and Crick (1953). Two polynucleotide chains twisted around each other.

Components: (1) Sugar - deoxyribose, (2) Phosphate group, (3) Nitrogenous bases - Purines (Adenine, Guanine) and Pyrimidines (Thymine, Cytosine).

Base Pairing: Chargaff's Rule - A pairs with T (2 hydrogen bonds), G pairs with C (3 hydrogen bonds). Complementary base pairing.

Functions: (1) Storage of genetic information, (2) Replication - making copies for cell division, (3) Transmission of traits from parents to offspring, (4) Protein synthesis through transcription and translation.

Gene: Segment of DNA coding for a specific protein or trait. Humans have ~20,000-25,000 genes.

Cell Cycle: Series of events from one cell division to the next. Divided into Interphase and M phase (Mitotic phase).

Interphase (90% of cycle): Cell grows and prepares for division. Subdivided into: (1) G₁ phase (Gap 1) - cell growth, protein synthesis, (2) S phase (Synthesis) - DNA replication, chromosome duplication, (3) G₂ phase (Gap 2) - further growth, organelle duplication, preparation for mitosis.

M Phase (10% of cycle): Mitosis (nuclear division) + Cytokinesis (cytoplasmic division).

G₀ Phase: Quiescent phase. Cells exit cycle and stop dividing. Example: Nerve cells, muscle cells.

Checkpoints: Control points ensuring proper cell division: (1) G₁ checkpoint - checks cell size and nutrients, (2) G₂ checkpoint - checks DNA replication, (3) M checkpoint - checks chromosome attachment to spindle.

Duration: Varies by cell type. Human cells: 24 hours (G₁: 11h, S: 8h, G₂: 4h, M: 1h).

Importance: Growth, repair, reproduction, maintaining chromosome number.

Mitosis: Type of cell division producing two identical daughter cells with same chromosome number as parent. Occurs in somatic (body) cells.

Significance: (1) Growth of organism, (2) Repair and regeneration of tissues, (3) Asexual reproduction, (4) Maintains chromosome number.

Phases of Mitosis: Prophase → Metaphase → Anaphase → Telophase (PMAT).

Prophase: (1) Chromatin condenses into chromosomes, (2) Each chromosome has 2 sister chromatids, (3) Centrioles move to opposite poles, (4) Nuclear membrane disintegrates, (5) Spindle fibers form.

Metaphase: (1) Chromosomes align at cell equator (metaphase plate), (2) Spindle fibers attach to centromeres, (3) Chromosomes most visible and condensed.

Anaphase: (1) Sister chromatids separate and move to opposite poles, (2) Centromeres divide, (3) Spindle fibers shorten, (4) Cell elongates.

Telophase: (1) Chromosomes reach poles and decondense, (2) Nuclear membrane reforms around each set, (3) Spindle disappears, (4) Two nuclei form.

Cytokinesis: Division of cytoplasm. In animal cells: cleavage furrow forms. In plant cells: cell plate forms.

Diagram Importance: Stages of mitosis with labeled diagrams are frequently asked in ICSE exams.

Meiosis: Type of cell division producing four daughter cells with half the chromosome number. Occurs in reproductive cells to form gametes.

Significance: (1) Maintains chromosome number across generations, (2) Produces genetic variation through crossing over and random assortment, (3) Essential for sexual reproduction.

Two Divisions: Meiosis I (reductional) and Meiosis II (equational).

Meiosis I: (1) Prophase I - longest phase, homologous chromosomes pair (synapsis), crossing over occurs, (2) Metaphase I - bivalents align at equator, (3) Anaphase I - homologous chromosomes separate, (4) Telophase I - two haploid cells form.

Crossing Over: Exchange of genetic material between non-sister chromatids of homologous chromosomes during Prophase I. Increases genetic variation.

Meiosis II: Similar to mitosis. Sister chromatids separate. Results in four haploid cells.

Comparison: Meiosis I reduces chromosome number (diploid to haploid). Meiosis II is like mitosis (sister chromatids separate).

Diagram Importance: Stages of meiosis showing chromosome behavior and crossing over are crucial.

Location: Mitosis in somatic cells; Meiosis in germ cells (reproductive organs).

Number of Divisions: Mitosis - one division; Meiosis - two divisions.

Daughter Cells: Mitosis - 2 identical diploid cells; Meiosis - 4 non-identical haploid cells.

Chromosome Number: Mitosis maintains (2n → 2n); Meiosis reduces (2n → n).

Genetic Variation: Mitosis - no variation (clones); Meiosis - variation through crossing over and random assortment.

Function: Mitosis for growth and repair; Meiosis for gamete formation.

Synapsis: Absent in mitosis; Present in meiosis (pairing of homologous chromosomes).

Crossing Over: Does not occur in mitosis; Occurs in Prophase I of meiosis.

Duration: Mitosis is shorter; Meiosis is longer (especially Prophase I).

Growth: Multicellular organisms grow by increasing cell number through mitosis. Single cell (zygote) → trillions of cells.

Repair and Regeneration: Damaged tissues repaired by mitosis. Example: Wound healing, bone fracture repair.

Reproduction: (1) Asexual reproduction through mitosis (binary fission, budding, fragmentation), (2) Sexual reproduction through meiosis (gamete formation).

Maintaining Chromosome Number: Mitosis maintains constant chromosome number. Meiosis reduces it in gametes; fertilization restores it.

Genetic Variation: Meiosis creates variation essential for evolution and adaptation through crossing over and independent assortment.

Cell Replacement: Old and dead cells replaced. Example: RBCs replaced every 120 days, skin cells every 2-4 weeks.

Cancer: Uncontrolled cell division due to loss of cell cycle regulation. Cells divide continuously forming tumors.

Tumor: Mass of cells. Benign (non-cancerous, localized) or Malignant (cancerous, spreads).

Causes: Mutations in genes controlling cell cycle (oncogenes, tumor suppressor genes), carcinogens (tobacco, UV radiation, chemicals).

Chromosomal Abnormalities: Errors during meiosis causing: (1) Aneuploidy - abnormal chromosome number, (2) Down syndrome (Trisomy 21) - extra chromosome 21, (3) Turner syndrome (45,X) - missing X chromosome in females, (4) Klinefelter syndrome (47,XXY) - extra X chromosome in males.

Non-disjunction: Failure of chromosomes to separate during meiosis, causing aneuploidy.

Polyploidy: More than two sets of chromosomes. Common in plants, rare in animals.