Phases of Cell Cycle — Revision Notes

⚡ 30-Second Revision

Interphase: — G1 (growth, protein synthesis), S (DNA replication, $2C \to 4C$, $2n$ chromosomes), G2 (growth, preparation for M-phase).
M-Phase (Karyokinesis):

- Prophase: Chromatin condenses, nucleolus disappears, nuclear envelope breaks down, spindle forms. - Metaphase: Chromosomes fully condensed, align at metaphase plate, kinetochores attach to spindle fibers. - Anaphase: Centromeres split, sister chromatids separate (now daughter chromosomes), move to opposite poles ($2n \to 4n$ temporarily). - Telophase: Chromosomes decondense, nuclear envelope reforms, nucleolus reappears, spindle disappears.

Cytokinesis: — Cytoplasmic division.

- Animals: Cleavage furrow (contractile ring). - Plants: Cell plate (Golgi vesicles).

G0 Phase: — Quiescent stage, cells exit G1, metabolically active but non-dividing.

2-Minute Revision

The cell cycle is divided into Interphase and M-phase. Interphase, the longest phase, includes G1, S, and G2. In G1, the cell grows and synthesizes proteins. S phase is crucial for DNA replication, doubling the DNA content from $2C$ to $4C$, while the chromosome number ($2n$) remains constant.

G2 involves further growth and preparation for division. M-phase, the division phase, starts with karyokinesis (nuclear division) comprising Prophase, Metaphase, Anaphase, and Telophase. Prophase sees chromosome condensation and nuclear envelope breakdown.

Metaphase is marked by chromosome alignment at the equatorial plate. Anaphase involves the splitting of centromeres and separation of sister chromatids to opposite poles, temporarily doubling the chromosome number within the cell.

Telophase reverses prophase events, forming two new nuclei. Finally, cytokinesis divides the cytoplasm, either by a cleavage furrow in animals or a cell plate in plants. Cells can exit G1 to enter the non-dividing G0 phase.

5-Minute Revision

The cell cycle is a meticulously regulated sequence of events enabling cell growth and division. It begins with Interphase, a period of intense activity, subdivided into three stages:

1
G1 Phase: — The cell grows, synthesizes proteins and RNA, and prepares for DNA replication. DNA content is $2C$, chromosome number is $2n$.
2
S Phase: — DNA replication occurs. DNA content doubles from $2C$ to $4C$, but the chromosome number remains $2n$ as sister chromatids are still joined.
3
G2 Phase: — The cell continues to grow, synthesizes proteins (e.g., tubulin), and checks for DNA integrity. DNA content is $4C$, chromosome number is $2n$.

Following Interphase is the M-Phase (Mitotic Phase), which includes Karyokinesis (nuclear division) and Cytokinesis (cytoplasmic division).

Karyokinesis Stages:

Prophase: — Chromatin condenses into visible chromosomes (each with two sister chromatids). Nucleolus disappears, nuclear envelope disintegrates, and the mitotic spindle begins to form.
Metaphase: — Chromosomes are maximally condensed and align precisely at the metaphase plate. Spindle fibers attach to kinetochores on centromeres.
Anaphase: — Centromeres split, separating sister chromatids. These are now considered individual daughter chromosomes and are pulled to opposite poles. The chromosome number within the cell temporarily doubles (e.g., $2n \to 4n$), while DNA content remains $4C$.
Telophase: — Daughter chromosomes arrive at poles, decondense. Nuclear envelopes reform around each set, nucleoli reappear, and spindle fibers disappear. Two nuclei are formed, each with $2n$ chromosomes and $2C$ DNA.

Cytokinesis:

Animal Cells: — A cleavage furrow forms by the constriction of a contractile ring.
Plant Cells: — A cell plate forms in the center, growing outwards to divide the cell.

Some cells may exit the G1 phase and enter a quiescent state called G0 phase, where they are metabolically active but do not divide, like mature nerve cells. Understanding the precise changes in DNA content and chromosome number across these phases is critical for NEET.

Prelims Revision Notes

Phases of Cell Cycle: NEET Revision Notes

I. Interphase (Preparatory Phase)

Longest phase: — ~90-95% of cell cycle.
G1 Phase (First Gap):

* Interval between mitosis and DNA replication. * Cell grows, synthesizes proteins, RNA, and organelles. * Metabolically active. * DNA content: $2C$. Chromosome number: $2n$. * Decision point: proceed, G0, or apoptosis.

S Phase (Synthesis):

* DNA replication occurs. * DNA content doubles: $2C \to 4C$. * Chromosome number remains constant: $2n$ (each chromosome now has two sister chromatids). * Histone synthesis occurs. * Centriole duplication in animal cells.

G2 Phase (Second Gap):

* Cell grows further, synthesizes proteins for mitosis (e.g., tubulin). * Prepares for M-phase, checks DNA integrity. * DNA content: $4C$. Chromosome number: $2n$.

II. M-Phase (Mitotic Phase / Division Phase)

Relatively short phase.
Comprises Karyokinesis (nuclear division) and Cytokinesis (cytoplasmic division).

A. Karyokinesis (Nuclear Division)

Prophase:

* Chromatin condensation begins, chromosomes become visible (each with 2 chromatids). * Nucleolus disappears. * Nuclear envelope disintegrates (late prophase/prometaphase). * Centrioles move to poles, spindle fibers form.

Metaphase:

* Chromosomes maximally condensed. * Align at the metaphase plate (equatorial plane). * Kinetochores (on centromeres) attach to spindle fibers from opposite poles.

Anaphase:

* Centromeres split simultaneously. * Sister chromatids separate, become daughter chromosomes. * Daughter chromosomes move to opposite poles (centromere leading). * Chromosome number temporarily doubles within the cell (e.g., $2n \to 4n$). DNA content remains $4C$.

Telophase:

* Daughter chromosomes reach poles, decondense. * Nuclear envelope reforms around each set. * Nucleolus reappears. * Spindle fibers disappear. * Two nuclei formed, each with $2n$ chromosomes and $2C$ DNA.

B. Cytokinesis (Cytoplasmic Division)

Animal Cells: — Formation of a cleavage furrow by a contractile ring of actin and myosin.
Plant Cells: — Formation of a cell plate in the center, growing outwards, derived from Golgi vesicles.

III. G0 Phase (Quiescent Stage)

Cells exit G1 phase.
Metabolically active but do not proliferate.
Perform specialized functions (e.g., nerve cells, heart cells).
Can re-enter cell cycle if stimulated (e.g., liver cells).

Vyyuha Quick Recall

To remember the phases of M-phase: Please Make A Tasty Curry.

Prophase
Metaphase
Anaphase
Telophase
Cytokinesis