Cell Cycle — Revision Notes

⚡ 30-Second Revision

Interphase — G1 $ightarrow$ S $ightarrow$ G2

- G1: Cell growth, protein/RNA synthesis, organelle duplication. DNA = 2C, Chromosomes = 2n. - S: DNA replication. DNA = 4C, Chromosomes = 2n (each with 2 chromatids). - G2: Further growth, protein synthesis (tubulin), DNA repair. DNA = 4C, Chromosomes = 2n.

M Phase — Karyokinesis (Mitosis) + Cytokinesis

- Prophase: Chromatin condenses, nucleolus disappears, nuclear envelope breaks, spindle forms. - Metaphase: Chromosomes align at equatorial plate, kinetochores attach to spindle. - Anaphase: Centromeres split, sister chromatids separate, move to opposite poles.

DNA = 4C, Chromosomes = 4n (temporarily). - Telophase: Chromosomes decondense, nuclear envelope reforms, nucleolus reappears, spindle disappears. DNA = 2C, Chromosomes = 2n (in each forming daughter nucleus).

- Cytokinesis: Cytoplasm divides. - Animal: Cleavage furrow (actin/myosin contractile ring). - Plant: Cell plate (Golgi vesicles).

Regulation — Checkpoints (G1, G2, M), Cyclins, Cyclin-Dependent Kinases (CDKs).

2-Minute Revision

The cell cycle is the life journey of a cell, from its origin to its division into two daughter cells, crucial for growth and repair. It's broadly divided into Interphase and M phase. Interphase, the longest phase, includes G1 (growth, protein synthesis), S (DNA replication, doubling DNA content from 2C to 4C while chromosome number remains 2n), and G2 (further growth, preparation for mitosis).

The M phase involves karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). Karyokinesis proceeds through Prophase (chromosome condensation, spindle formation), Metaphase (chromosomes align at the equatorial plate), Anaphase (sister chromatids separate, moving to poles, temporarily doubling chromosome number to 4n), and Telophase (chromosomes decondense, nuclear envelopes reform).

Cytokinesis differs: animal cells form a cleavage furrow, while plant cells build a cell plate. The entire process is tightly controlled by checkpoints (G1, G2, M) and regulatory molecules like cyclins and Cyclin-Dependent Kinases (CDKs), ensuring fidelity and preventing errors that could lead to diseases like cancer.

5-Minute Revision

The cell cycle is the fundamental process governing cell proliferation, essential for growth, development, and tissue repair. It consists of two major phases: Interphase and the M phase. Interphase is a period of intense cellular activity and growth, subdivided into G1, S, and G2.

In G1, the cell grows, synthesizes proteins and RNA, and duplicates organelles, preparing for DNA replication. The S phase is the 'synthesis' phase where DNA replication occurs, doubling the DNA content from 2C to 4C, but the chromosome number remains 2n (as each chromosome now consists of two sister chromatids).

G2 involves further growth, synthesis of proteins required for mitosis (like tubulin), and crucial DNA damage checks.

The M phase, or mitotic phase, is the actual division phase, comprising karyokinesis (nuclear division) and cytokinesis (cytoplasmic division). Karyokinesis is divided into four stages: Prophase sees chromatin condense into visible chromosomes, the nucleolus disappears, and the nuclear envelope disintegrates while the mitotic spindle begins to form.

In Metaphase, chromosomes align perfectly at the cell's equatorial plate, with kinetochores attached to spindle fibers from opposite poles. Anaphase is characterized by the splitting of centromeres and the rapid movement of sister chromatids (now considered individual chromosomes) to opposite poles, temporarily doubling the chromosome number to 4n.

Finally, Telophase reverses prophase events: chromosomes decondense, nuclear envelopes reform around the two sets of chromosomes, and nucleoli reappear. Cytokinesis, the division of the cytoplasm, typically overlaps with telophase.

In animal cells, a cleavage furrow forms due to a contractile ring of actin and myosin. In plant cells, a cell plate forms from Golgi vesicles, growing outwards to create a new cell wall. The entire cell cycle is meticulously regulated by checkpoints (G1, G2, M) and molecular players like cyclins and Cyclin-Dependent Kinases (CDKs), which form active complexes to drive progression and ensure genetic integrity.

Errors in this regulation can lead to severe consequences, including cancer.

Prelims Revision Notes

Cell Cycle: NEET Quick Facts

I. Phases of Cell Cycle:

1
Interphase (95% of cell cycle) — Preparatory phase.

* G1 Phase (First Gap): * Cell growth, increase in size. * Synthesis of proteins, RNA, enzymes. * Duplication of organelles (mitochondria, ER). * DNA content: 2C (diploid cell). Chromosome number: 2n.

* G0 Phase: Quiescent stage, cells exit cycle (e.g., neurons, muscle cells). * S Phase (Synthesis): * DNA Replication: DNA content doubles (2C $\rightarrow$ 4C). * Histone synthesis.

* Centriole duplication (animal cells). * Chromosome number: 2n (each chromosome now has 2 sister chromatids). * G2 Phase (Second Gap): * Further cell growth. * Synthesis of proteins (e.g., tubulin for spindle).

* Checks for DNA damage/replication errors. * DNA content: 4C. Chromosome number: 2n.

1
M Phase (Mitotic Phase - <5% of cell cycle) — Actual cell division.

* Karyokinesis (Nuclear Division - Mitosis): * Prophase: * Chromatin condensation $\rightarrow$ visible chromosomes (2 chromatids). * Nucleolus disappears. * Nuclear envelope disintegrates.

* Spindle fibers (microtubules) begin to form (from centrioles in animals). * Metaphase: * Chromosomes maximally condensed. * Align at equatorial plate (metaphase plate). * Kinetochores (on centromeres) attach to spindle fibers from opposite poles.

* Anaphase: * Centromeres split. * Sister chromatids separate $\rightarrow$ individual chromosomes. * Move to opposite poles (pulled by shortening kinetochore microtubules). * DNA content: 4C (temporarily).

Chromosome number: 4n (temporarily, at poles). * Telophase: * Chromosomes decondense. * Nuclear envelope reforms around each set of chromosomes. * Nucleolus reappears. * Spindle fibers disappear.

* DNA content: 2C (in each forming nucleus). Chromosome number: 2n (in each forming nucleus). * Cytokinesis (Cytoplasmic Division): * Animal Cells: Cleavage furrow forms (contractile ring of actin/myosin) $ightarrow$ pinches cell into two.

* Plant Cells: Cell plate forms (from Golgi vesicles) in center $ightarrow$ grows outwards $ightarrow$ forms new cell wall.

II. Cell Cycle Regulation:

Checkpoints — Control points ensuring proper progression.

* G1 Checkpoint (Restriction Point): Most important. Checks cell size, nutrients, growth factors, DNA damage. * G2 Checkpoint: Checks DNA replication completion, DNA damage. * M Checkpoint (Spindle Assembly Checkpoint): During metaphase. Ensures all kinetochores are attached to spindle fibers.

Regulatory Molecules

* Cyclins: Proteins whose concentration fluctuates, activating CDKs. * Cyclin-Dependent Kinases (CDKs): Enzymes activated by cyclins, phosphorylate target proteins to drive cycle progression.

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Interphase
Prophase
Metaphase
Anaphase
Telophase
Cytokinesis