Cell Cycle and Cell Division

The sequence of events by which a cell duplicates its genome, synthesizes other constituents of the cell and eventually divides into two daughter cells.

Generation Time: Human cells ~24 hours (Interphase ~95%). Yeast ~90 minutes.

A. Interphase (Resting Phase?)

Actually the most active phase. Preparation for division.

• G1 Phase (Gap 1):
  Cell is metabolically active and grows continuously. No DNA replication. Synthesis of RNA and Proteins.
• S Phase (Synthesis):
  DNA Replication: Amount of DNA doubles (2C → 4C).
  Chromosome Number: Remains SAME (2n).
  Centriole Duplication: Occurs in cytoplasm (Animal cells). Histone synthesis occurs.
• G2 Phase (Gap 2):
  Proteins for spindle fibres (Tubulin) are synthesized. Cell growth continues.
• G0 Phase (Quiescent Stage):
  Cells that do not divide further (e.g., Heart cells, Nerve cells) exit G1 and enter G0. Metabolically active but no proliferation.

Equational Division. Occurs in Somatic cells.

• Prophase:
  Chromosomal material condenses. Centrosomes move to opposite poles.
  Disappearance: Golgi, ER, Nucleolus, Nuclear envelope.
• Metaphase:
  Nuclear envelope completely gone. Chromosomes highly condensed (Best for morphology study).
  Spindle Fibres: Attach to Kinetochores (disc structures at centromere).
  Metaphase Plate: Alignment of chromosomes at the equator.
• Anaphase:
  Splitting: Centromeres split and chromatids separate.
  Chromatids (now daughter chromosomes) move to opposite poles. Arms trail behind.
• Telophase:
  Chromosomes decondense (lose individuality). Nuclear envelope reforms. Nucleolus, Golgi, ER reappear.
• Cytokinesis:
  Animal: Cell furrow (Centripetal).
  Plant: Cell plate (Centrifugal). Precursor is 'Phragmoplast'. Represents middle lamella.
  Syncytium: Karyokinesis not followed by Cytokinesis (e.g., Liquid Endosperm in Coconut).

Occurs in reproductive cells (Gametes). Reduces chromosome number by half (2n → n). Involves two cycles of division (Meiosis I & II) but only one cycle of DNA replication.

Meiosis I (Prophase I - Complex)

• Leptotene: Chromosomes become visible (Beaded appearance).
• Zygotene: Synapsis (Pairing of homologous chromosomes). Formation of Synaptonemal complex. Pairs called Bivalents (Tetrads).
• Pachytene: Crossing Over occurs directly. Exchange of genetic material. Enzyme: Recombinase. Recombination nodules appear.
• Diplotene: Dissolution of synaptonemal complex. Separation starts except at crossover sites (Chiasmata - X shaped). Oocytes of some vertebrates can stay here for years.
• Diakinesis: Terminalisation of Chiasmata. Nucleolus disappears. Transition to Metaphase.

Other Stages

• Metaphase I: Bivalents align at equator. Spindle attaches to homologous pair.
• Anaphase I: Homologous chromosomes separate while sister chromatids remain associated at their centromeres. (Actual reduction step).
• Meiosis II: Resembles Mitosis. Sister chromatids separate in Anaphase II. Results in 4 haploid daughter cells.

• Mitosis: Growth (Zygote to Adult), Cell Repair (Epidermis, Blood cells), Maintenance of Nucleo-cytoplasmic ratio.
• Meiosis: Conservation of specific chromosome number across generations. Increases genetic variability (Evolution).

The sequence of events including genome duplication, synthesis of constituents, and division.

Human cells divide once in approximately 24 hours. Yeast divides in 90 minutes.

Lasts for more than 95% of the duration of basic cell cycle.

Interval between mitosis and initiation of DNA replication. Cell metabolically active and grows.

DNA replication takes place. Amount of DNA doubles (2C to 4C). Chromosome number remains same.

In animal cells, during S phase, centriole duplicates in the cytoplasm.

Proteins are synthesized (Tubulin) in preparation for mitosis. Cell growth continues.

Quiescent stage. Cells that do not divide further exit G1 phase. Metabolically active but no proliferation.

Chromosomal material condenses. Centrosome moves to opposite poles. Nuclear envelope disappears.

Best stage to study morphology of chromosomes.

Small disc-shaped structures at the surface of the centromeres. Spindle fibres attach here.

Plane of alignment of the chromosomes at the equator during metaphase.

Splitting of centromere and separation of sister chromatids (now daughter chromosomes).

Chromosomes decondense. Nuclear envelope develops. Nucleolus, Golgi, ER reappear.

Multinucleate condition when karyokinesis is not followed by cytokinesis (e.g., coconut liquid endosperm).

In plant cells, new cell wall formation starts in center (Centrifugal). Precursor: Phragmoplast. Represents Middle Lamella.

Reductional division. Specific to gametes. Reduces chromosome number by half (2n to n).

Two sequential cycles of division (Meiosis I & II) but only ONE cycle of DNA replication (before Meiosis I).

Chromosomes become gradually visible under light microscope. Beaded appearance.

Chromosomes start pairing together (Synapsis). Homologous chromosomes. Formation of Synaptonemal Complex.

Paired homologous chromosomes (Tetrads). Clearly visible at Pachytene.

Crossing over occurs between non-sister chromatids of homologous chromosomes. Appearance of Recombination Nodules.

Enzyme involved in crossing over.

Dissolution of Synaptonemal complex. Bivalents separate except at crossover sites (Chiasmata).

X-shaped structures where crossing over occurred. Seen in Diplotene.

Terminalisation of chiasmata. Nucleolus disappears. Transition to Metaphase I.

Homologous chromosomes separate, while sister chromatids remain associated at their centromere.

Stage between Meiosis I and Meiosis II. Short lived. No DNA replication.

Spliting of centromere. Sister chromatids move to opposite poles (just like Mitosis).

Growth (Zygote to adult), Cell repair (Epidermis, Gut lining, Blood cells).

Conservation of specific chromosome number. Increases genetic variability (Evolution).

Mitotic poison. Inhibits spindle formation. Arrests cell in Metaphase (induces Polyploidy).

G1/S (Restriction Point) and G2/M. Cyclins and CDKs regulate the cell cycle.

Prolonged Diplotene stage in Oocytes of vertebrates (lasts months or years).

Precursor of Cell Plate. Formed by Golgi vesicles and microtubules.

Most dramatic period phase. Lasts only about an hour in the 24 hour human cell cycle.

Division of nucleus. Followed by Cytokinesis (Division of cytoplasm).

Pair of chromosomes (one from each parent) similar in shape, size and gene position.

Another name for Leptotene where chromosomes converge towards centrioles.

Direct cell division without spindle formation. Seen in Bacteria, Yeast, Amoeba.

Plants can show mitosis in haploid cells (e.g., Male Honey Bees, Gametophyte of plants).

Identical copies of a chromosome connected at centromere. Produced in S phase.

Does not fully occur at end of Telophase I (chromosomes remain somewhat condensed).

Movement of chromosomes to the equator during Prometaphase/Metaphase.

Microtubules radiating from centrioles. Aster + Spindle Fibres = Mitotic Apparatus.

Mitosis without asters. Found in higher plants (no centrioles).

Mitosis with two asters. Found in animals.

A bivalent consists of 4 chromatids, hence called a Tetrad.

Tumor suppressor protein. "Guardian of the genome". Regulates G1 checkpoint.

Cells that have lost contact inhibition and show uncontrolled division (Mitosis).