Скачать или смотреть Cell cycle important questions | complete ଓଡ଼ିଆରେ🥳CHSE ବି. ଏଡ଼ ouat ojee ପାଈଁ 🥰

Cell cycle is the ordered sequence of events by which a cell grows, duplicates its DNA and divides to form two daughter cells.


---

PHASES OF CELL CYCLE

Total cycle = Interphase (95%) + M Phase (5%)


---

1. INTERPHASE (G1 → S → G2)

Interphase is the period of growth and DNA replication.

A. G₁ Phase (Gap 1 / Post-mitotic phase)

Cell grows in size

Proteins and enzymes required for DNA replication synthesized

Number of organelles doubles

High metabolic activity

Restriction/Check point present (G1 checkpoint)

Cell may exit cycle → G₀ phase (nerve cells, cardiac cells)


B. S Phase (Synthesis phase)

DNA replication occurs

Chromosomes become double-stranded (two sister chromatids)

Centrosome duplicates (in animal cells)

Amount of DNA doubles (2C → 4C) but chromosome number remains same


C. G₂ Phase (Gap 2 / Pre-mitotic phase)

Further growth

Protein synthesis for mitosis (spindle protein, tubulin etc.)

Cell checks DNA for errors (G2 checkpoint)

Mitochondria and chloroplasts divide



---

2. M PHASE (MITOTIC PHASE)

Includes Karyokinesis + Cytokinesis

(A) Karyokinesis – Division of Nucleus

1. Prophase

Chromatin → thick chromosomes

Nuclear membrane and nucleolus disappear

Spindle fibers appear

Centrosomes move to opposite poles


2. Metaphase

Chromosomes arranged on equatorial plate

Maximum condensation

Spindle fibers attach to centromeres at kinetochore

Karyotype prepared in this stage


3. Anaphase

Centromere splits

Sister chromatids separate → move to opposite poles

Shortening of spindle fibers


4. Telophase

Chromatids reach poles

Nuclear membrane and nucleolus reappear

Chromosomes → chromatin

Spindle disappears

Cytokinesis usually begins



---

(B) Cytokinesis – Division of Cytoplasm

Animal Cells

By cleavage furrow (constriction)


Plant Cells

By cell plate formation (from Golgi vesicles)



---

CELL CYCLE CONTROL

Key regulators

Cyclins

Proteins that rise & fall during cell cycle

Types: G1 cyclin, S cyclin, M cyclin


CDKs (Cyclin-Dependent Kinases)

Enzymes activated by cyclins

Control progression from one phase to another


Checkpoints

1. G₁ Checkpoint → Start checkpoint
Checks: size, nutrients, DNA damage


2. G₂ Checkpoint
Checks: DNA replication accuracy


3. M Checkpoint (Spindle checkpoint)
Checks: chromosomes attached to spindle




---

CELL CYCLE DURATION

Human cell cycle: ~24 hours

G1: ~10–12h

S: ~6–8h

G2: ~4–6h

M phase: ~1 hour




---

SIGNIFICANCE OF CELL CYCLE

Growth of organism

Tissue repair & regeneration

Asexual reproduction

Maintains chromosome number

Ensures accurate DNA transmission



---

QUESTIONS FOR EXAM (Short/Board/NEET)

1. During which phase does DNA replication occur? → S phase


2. What happens at G1 checkpoint?


3. Why chromosome number does not double in S phase?


4. In which stage is a karyotype prepared? → Metaphase


5. Why plant cells cannot undergo cleavage ଫୁରରବ



thanks ଦେଖିବା pai🥰
ଜୟ ଜଗନ୍ନାଥ 🙏🙏🙏
କମେଣ୍ଟସ କରିବେ 😍
9827965699❤️