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Biology · Class 9 · Chapter 5: Cell Cycle

8 min

Key points

Cell cycle is the sequence of events that occurs in a cell leading to its division and the production of two daughter cells
Cell cycle consists of two main phases: Interphase (G1, S, G2) and M phase (Mitosis and Cytokinesis)
G1 phase is the gap phase where cell grows in size and organelles are duplicated
S phase is synthesis phase where DNA replication occurs and chromosome number doubles
G2 phase is second gap phase where proteins necessary for chromosome condensation are produced
M phase includes mitosis (nuclear division) and cytokinesis (cytoplasmic division)
Checkpoints control the cell cycle progression at G1/S, G2/M, and metaphase/anaphase transitions
Cancer cells have defective cell cycle control mechanisms leading to uncontrolled division
Cell cycle duration varies in different cell types - rapidly dividing cells like in bone marrow complete cycle in 24 hours

Definitions

Cell Cycle: The sequence of events that occurs in a cell leading to its division and the production of two daughter cells
Interphase: The phase of cell cycle during which the cell grows and DNA is replicated, consisting of G1, S and G2 phases
Mitosis: The process of nuclear division in which the nucleus divides to form two nuclei with identical sets of chromosomes
Cytokinesis: The division of cytoplasm that follows nuclear division during cell division
Checkpoint: Control mechanisms that ensure proper progression of cell cycle by checking for DNA damage and proper chromosome attachment

Worked example

Define cell cycle and explain the phases of mitosis. (6 marks)

Cell cycle is the sequence of events that occurs in a cell leading to its division and production of two daughter cells. The phases of mitosis are: (i) Prophase - chromosomes condense and nuclear membrane disappears, (ii) Metaphase - chromosomes align at cell equator, (iii) Anaphase - sister chromatids separate and move to opposite poles, (iv) Telophase - nuclear membranes reform around each set of chromosomes. Mitosis ensures growth and repair in multicellular organisms.

Common mistakes

Students confuse S phase with synthesis of proteins instead of DNA synthesis
Often mix up the order of mitotic phases - remember PMAT (Prophase, Metaphase, Anaphase, Telophase)
Forget that cytokinesis is separate from mitosis and is part of M phase
Confuse cell cycle with cell division - cell cycle includes both growth and division phases
Don't mention that checkpoints prevent damaged cells from dividing

Quick recap

Cell cycle is the sequence of events leading to cell division producing two daughter cells. It includes Interphase (G1, S, G2 phases) and M-phase (mitosis + cytokinesis). Mitosis has four phases: Prophase (chromosome condensation), Metaphase (chromosome alignment), Anaphase (chromatid separation), Telophase (nuclear reformation). Essential for growth, repair and asexual reproduction in organisms.

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