Cell Cycle and Cell Division - Premium Notes

I. The Cell Cycle

Sequence of events by which a cell duplicates its genome and eventually divides into two daughter cells.

Interphase: Resting phase (95% of cycle).
--> G1 (Gap 1): Metabolic growth; cell prepares for DNA replication.
--> S (Synthesis): DNA Duplicates (2C to 4C); Centriole duplicates in animal cells.
--> G2 (Gap 2): Protein synthesis for tubulin/spindle.
G0 Phase: Cells that don't divide further (e.g., heart cells) enter an inactive quiescent stage.

II. Mitosis (Equational Division)

Occurs in somatic cells. Maintains the chromosome number.

Prophase: Chromatin condenses; Nucleolus/Nuclear envelope disappear.
Metaphase: Chromosomes align at Metaphase Plate. Spindle fibers attach to Kinetochores.
Anaphase: Centromeres split; chromatids move to opposite poles.
Telophase: Reverse of Prophase. Chromosomes decondense; ER/Golgi reform.

[!] Metaphase: This is the best stage to study the Morphology and number of chromosomes.

III. Meiosis (Reductional Division)

Occurs in germ cells during sexual reproduction. Reduces chromosome number by half (2n to n).

Prophase I Stages:

Leptotene: Chromosomes become thin threads.
Zygotene: Synapsis occurs; formation of Bivalent/Tetrad.
Pachytene: Crossing Over between non-sister chromatids via Recombinase enzyme.
Diplotene: Dissolution of synaptonemal complex; Chiasmata (X-shape) visible.
Diakinesis: Terminalisation of chiasmata; spindle assembly.

[i] Significance: Crossing over causes Genetic Variation, which is the basis of evolution.

IV. Cytokinesis

Animal Cells: Cleavage furrow (centripetal - outside to inside).
Plant Cells: Cell Plate formation (centrifugal - inside to outside).

[Q] Why is the S-phase transition critical for the cell?

[A] Explanation:

During S-phase (Synthesis), the amount of DNA per cell doubles (from 2C to 4C). However, the number of chromosomes remains the same (2n). This ensures that the cell has two sets of instructions to split between daughter cells during mitosis. If DNA doesn't double correctly, the daughter cells will have chromosomal abnormalities.

[Q] Differentiate between open and closed Mitosis.

[A] Explanation:

Open Mitosis (seen in humans) involves the complete disintegration of the nuclear envelope before chromosomes separate. Closed Mitosis (seen in some fungi/yeast) occurs entirely inside an intact nucleus with the spindle forming within the nuclear membrane.

[Q] What is the significance of the G0 phase (Quiescent stage)?

[A] Explanation:

Cells in G0 are metabolically active but have exited the cycle and stopped dividing. This is a protective state for specialized cells like heart muscles or neurons. They only re-enter G1 if specifically triggered by growth factors or tissue damage (in cells like Liver).

[Q] How does Meiosis ensure genetic variation over generations?

[A] Explanation:

Through two processes: 1) Crossing over in Pachytene (Meiosis I), where genetic material is swapped between non-sister chromatids. 2) Independent assortment in Metaphase I, where homologous pairs line up randomly, leading to unique combinations of maternal and paternal chromosomes in gametes.

[Q] "Mitosis is equational while Meiosis I is reductional." Justify.

[A] Explanation:

In Mitosis, the chromosome number is maintained (2n -> 2n). In Meiosis I, the homologous chromosomes separate, reducing the chromosome count to half (2n -> n). Note: Meiosis II is equational (n -> n).

[Q6] Why is Pachytene important?

Crossing over occurs; mediated by recombinase.

[Q7] Chiasmata definition.

X-shaped junctions where crossing over took place; visible in Diplotene.

[Q8] Kinetochore role.

Protein structure on centromere where spindle fibers attach.

[Q9] Cleavage furrow vs Cell plate.

Animal (Centripetal) vs Plant (Centrifugal) cytokinesis.

[Q10] Interkinesis.

Gap between Meiosis I and II; NO DNA replication occurs.

[Q11] Syncytium example.

Liquid endosperm in coconut (karyokinesis without cytokinesis).

[Q12] Synaptonemal complex.

Structure that holds homologous chromosomes together during Zygotene.

[Q13] Every mitosis produces?

Two identical daughter cells.

[Q14] Every meiosis produces?

Four haploid (and varied) daughter cells.

[Q15] When do sister chromatids separate?

Anaphase (Mitosis) and Anaphase II (Meiosis).

[Q16] Metaphase plate significance.

Ensures equal distribution of chromosomes to poles.

[Q17] Spindle fibers are made of?

Microtubules (Tubulin protein).

[Q18] Why is yeast used in cell cycle study?

Divides every 90 minutes; simple model organism.

[Q19] Human cell cycle duration.

Approximately 24 hours.

[Q20] Most persistent phase of prophase I?

Diplotene (can last years in some vertebrate oocytes).

Cell Cycle - 50 Premium Facts

Cell Cycle: 50 Mastery Facts

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Cell Cycle: Sequence of events including DNA replication and growth.

Interphase: Phase between two M-phases; lasts ~95% of cycle.

M phase: Actual cell division (Mitosis/Meiosis) occurs here.

G1 phase: Interval between mitosis and initiation of DNA replication.

S phase: Synthesis phase where DNA amount per cell doubles.

No Chromosome doubling: Chromosome number stays same in S phase.

G2 phase: Proteins for spindle formation and mitosis are synthesized.

G0 phase: Quiescent stage; cells stay metabolically active but don't divide.

Mitosis: Also called Equational division.

Karyokinesis: Separation of daughter chromosomes (nuclear division).

Cytokinesis: Division of cytoplasm into two cells.

Prophase: Marked by chromosomal material condensation.

Mitotic spindle: Formed by microtubules during prophase.

Metaphase: Chromosomes align at the equator (Metaphase plate).

Kinetochores: Small disc-shaped structures at the surface of centromeres.

Anaphase: Centromeres split; sister chromatids move to opposite poles.

Telophase: Nuclear envelope develops; nucleolus and ER reform.

Syncytium: Multinucleate condition (e.g., coconut liquid endosperm).

Meiosis: Reductional division; occurs in sexual reproduction.

Meiosis I: Reduces chromosome number from 2n to n.

Prophase I: Most complex/longest phase with 5 substages.

Zygotene: Pairing of homologous chromosomes called Synapsis.

Bivalents: Pairs of synapsed homologous chromosomes.

Pachytene: Crossing over occurs between non-sister chromatids.

Recombinase: Enzyme that facilitates crossing over.

Diplotene: Chiasmata (X-shaped structures) are formed.

Diakinesis: Terminalisation of chiasmata and end of Prophase I.

Metaphase I: Bivalents align on the equatorial plate.

Anaphase I: Homologous chromosomes separate; sister chromatids stay together.

Telophase I: Results in dyad of cells.

Interkinesis: Short interval between Meiosis I and II.

Anaphase II: Splitting of centromeres and sister chromatid separation.

Meiosis result: Four varied haploid cells.

Genetic variation: Produced by meiosis; essential for evolution.

Centriole duplication: Occurs during S-phase in animal cells only.

Cell plate: Precursor of middle lamella in plant cytokinesis.

Syncytium: Caused when karyokinesis is NOT followed by cytokinesis.

Crossing over: Reciprocal exchange of genetic material.

Tetrad: Another name for a bivalent showing 4 chromatids.

Morphology: Best studied during Metaphase.

Tissue Repair: A primary function of mitotic division.

Reductional: Meiosis I reduces 2n to n.

Synaptonemal complex: Dissolves during Diplotene.

Centromere: Holds sister chromatids together.

Fragile X: Example of a chromosomal error during division.

Asters: Spindle fibers and centrioles together in animal cells.

Tubulin: Protein synthesized in G2 phase for spindle.

90 minutes: Total cycle duration for Yeast.

24 hours: Average duration of cell cycle in human culture.

Variation: The hallmark of Meiosis and sexual reproduction.

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