Cell Cycle, Division & Chromosomes

ICSE Class 10 Biology • Chapter 01

1. The Cell Cycle

Cell Cycle: The sequence of events involving growth and division of a cell. It consists of two main phases: Interphase (Non-dividing phase) and M-Phase (Dividing phase).

A. Interphase (Resting Phase)

Although called "resting", the cell is metabolically very active preparing for division. It has three sub-phases:

First Growth Phase ($G_1$):
- RNA and proteins are synthesized.
- Volume of cytoplasm increases.
- Mitochondria (and Chloroplasts in plants) divide.
Synthesis Phase ($S$):
- DNA replication occurs. The amount of DNA doubles, but chromosome number remains the same.
- Centrosome divides (in animal cells).
Second Growth Phase ($G_2$):
- More RNA and proteins are synthesized.
- Cell ready for division.

[IMAGE PLACEHOLDER: CELL CYCLE DIAGRAM]

Circular pie-chart like diagram of Cell Cycle. Show Interphase occupying 95% (G1 -> S -> G2). Show M-Phase (Mitosis) occupying 5% (P -> M -> A -> T). Label 'G0' phase as an exit from G1 (Quiescent stage).

Practice Q1: Cell Cycle Stages

BOARD QUESTION In which phase of the cell cycle does DNA replication take place?

Ans: Synthesis Phase (S-Phase).

2. Structure of Chromosomes

A. Chromatin vs Chromosomes

Feature	Chromatin Fibres	Chromosomes
Appearance	Long, thin, thread-like network.	Short, thick, rod-like structures.
Visibility	Visible during Interphase.	Visible during Cell Division (Prophase/Metaphase).
State	Uncoiled and decondensed DNA.	Supercoiled and condensed DNA.

B. Molecular Structure (The Nucleosome)

Chromosomes are made of DNA (40%) and Histone proteins (60%).

Nucleosome: The basic structural unit of chromatin. It consists of a DNA strand wound around a core of 8 Histone molecules (Histone Octamer).
This gives a "Beads on a String" appearance.

[IMAGE PLACEHOLDER: NUCLEOSOME STRUCTURE]

Diagram showing DNA double helix wrapping around a cluster of 8 histone balls. Label: "DNA", "Histone Core", "Nucleosome". Show how nucleosomes coil to form Chromatin fibres.

Practice Q2: Structure Analysis

CONCEPT CHECK What are the two main chemical constituents of a chromosome?

Ans: DNA (Deoxyribonucleic Acid) and Histone Proteins.

3. Mitosis (Karyokinesis)

Mitosis: A type of cell division resulting in two daughter cells each having the same number and kind of chromosomes as the parent nucleus ($2n \rightarrow 2n$). Occurs in somatic (body) cells for growth and repair.

Phases of Mitosis

1. Prophase (First Phase)

Chromatin condenses to form chromosomes.
Nucleolus and Nuclear membrane disappear.
Centrosomes separate and move to opposite poles (in animals).
Spindle fibers begin to appear.

2. Metaphase (After Phase)

Chromosomes align at the equatorial plane (metaphase plate).
Each chromosome attaches to spindle fibers by its centromere (specifically at the kinetochore).

Master Tip: Metaphase is the best stage to study the size, shape, and number of chromosomes because they are most condensed.

3. Anaphase (Up Phase)

Centromere splits: The two sister chromatids separate.
Spindle fibers shorten, pulling chromatids (now called daughter chromosomes) to opposite poles.
A furrow starts appearing in the cell membrane (in animals).

4. Telophase (End Phase)

Reverse of Prophase.
Chromosomes uncoil back into chromatin.
Nuclear membrane and Nucleolus reappear.
Two daughter nuclei are formed.

Practice Q3: Identifying Stages

DIAGRAM BASED A student observes a slide where chromosomes are aligned at the center of the cell. Which stage is this?

Ans: Metaphase.

4. Cytokinesis (Division of Cytoplasm)

Feature	Animal Cytokinesis	Plant Cytokinesis
Method	Cell Furrowing (Cleavage).	Cell Plate Formation.
Direction	Centripetal (Periphery to Center).	Centrifugal (Center to Periphery).

5. Mitosis vs Meiosis

Feature	Mitosis	Meiosis
Occurrence	Somatic (Body) cells.	Reproductive (Germ) cells.
Purpose	Growth, Repair, Replacement.	Gamete formation (Sperm/Egg).
Daughter Cells	Two ($2n$ - Diploid).	Four ($n$ - Haploid).
Genetic Identity	Identical to parent.	Genetically different (due to Crossing Over).

Exam Practice Questions (PYQ Trends)

Year: 2019

BOARD QUESTION Name the specific region of the chromosome to which spindle fibers attach.

Ans: Centromere (or Kinetochore).

Reasoning

THINKING Why does meiosis occur only in reproductive parts?

Ans: To reduce the chromosome number to half ($n$) so that when fertilization occurs ($n+n$), the normal diploid number ($2n$) of the species is restored.

Diagram Identification

VISUAL If a diagram shows homologous chromosomes sticking together (synapsis), which stage is it?

Ans: Prophase I of Meiosis.