Structure of Chromosomes, Cell Cycle and Cell Division

ICSE Class 10 Biology Detailed Master Notes (Chapter 2)

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation. It is equally true for simple organisms like Amoeba as it is for complex ones like humans or giant elephants.

1. Structure of Chromosomes

The most conspicuous events occurring during cell division are all related to chromosomes. The duplicated chromosomes get evenly distributed into daughter cells during mitosis, ensuring normal functioning of the cells.

1.1 What are Chromosomes?

• When a normal, unstained living cell is observed under a light microscope, its nucleus may appear empty.
• When stained with suitable dyes, structures become noticeable. In the interphase (non-dividing stage), the nucleus shows a network of very long, extremely thin dark-staining fibres called chromatin fibres.
• As the cell enters the first stage of division (prophase), the chromatin fibres condense to form chromosomes.
• The name comes from their ability to pick up dyes easily: chroma = coloured, soma = body.

1.2 Discovery of Chromosomes

• Discovered in 1882 by German Scientist Walther Fleming.
• He noticed them in the rapidly dividing cells of salamander larvae (an amphibian).
• Using an old-type microscope, he saw minute threads appearing to divide lengthwise. He coined the term mitosis (literally meaning "thread").

1.3 Gross Structure of a Chromosome

Each chromosome, in its condensed form at the start of cell division, consists of:

• Two Chromatids: The two identical halves of a duplicated chromosome.
• Centromere: The point of attachment where the two sister chromatids are joined. It appears as a small constricted region. The centromere is also the exact site that attaches to the spindle fibre during cell division.

As the spindle fibre contracts, the sister chromatids are separated at the centromere and pulled towards opposite poles. After division, chromatids (now individual chromosomes) decondense back into chromatin fibres.

1.4 Molecular Structure (The Nucleosome & DNA)

The Nucleosome: The DNA strand winds around a core of exactly eight histone molecules. This core can be imagined like a football around which a long rope is wound with one or two loops. Each such complex is called a nucleosome.

• A single human chromosome may contain about a million nucleosomes.
• The entire chromatin fibre undergoes extensive coiling and supercoiling (similar to a typical telephone cord).

Structure of DNA

(Note: Shape studied by Rosalind Franklin in 1953; structure worked out by Watson and Crick in 1953.)

DNA is a very large single molecule, described as a macromolecule. It is composed of two complementary strands wound around each other in a double helix.

Each single DNA strand is composed of repeating structural units called Nucleotides. A nucleotide consists of three components:

1. Phosphate
2. Sugar (pentose)
3. Nitrogenous Base (attached to the sugar inwards)

The two strands make a ladder-like arrangement. The alternating sugar and phosphate form the uprights, and the nitrogenous bases form the "rungs" by joining via Hydrogen Bonds.

Replication of DNA

During the interphase of the cell cycle, DNA duplicates (replicates) to prepare for equitable distribution during mitosis. The DNA double helix unzips (opens) at one end, exposing free strands. New strands are built against each original strand simultaneously. The result is two identical DNA molecules, each containing one "old" strand and one "new" strand.

1.5 What are Genes?

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2. Cell Division - New Cells from Existing Ones

New cells are required in the body for four primary reasons:

1. For Growth: Every organism begins life as a single cell (the fertilized egg). This cell divides repeatedly to form a cluster, differentiating into tissues and organs.
2. For Replacement: Wear and tear of cells occurs daily. Example: 20 million red blood cells are destroyed every minute in our body and are constantly replaced by new cells from the bone marrow.
3. For Repair: Cells divide to cover up gaps from accidental injuries, cuts in the skin, or fractures in bones.
4. For Reproduction: Unicellular organisms (Amoeba, yeast) divide to form independent cells. In higher forms, special cells undergo meiosis to produce gametes (sperms and eggs).

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3. Types of Cell Division

There are two primary types of cell division:

• Mitosis: Cell division leading to growth, repair, and replacement. Occurs in somatic (body) cells.
• Meiosis: Cell division leading to the production of gametes (sex cells). Occurs in reproductive organs.

3.1 Mitosis (mitos: thread)

Before division, the cell prepares by doubling the quantity of DNA during the Interphase (the apparently resting phase, though internally very active).

Phases of Mitosis

Mitosis is broadly divided into Karyokinesis (division of the nucleus) and Cytokinesis (division of the cytoplasm). Karyokinesis occurs in four continuous phases:

Cytokinesis (Division of Cytoplasm)

At the end of telophase, cytokinesis completes the process. In animal cells, the cleavage furrow deepens totally and splits the cell into two. In plant cells, a cell plate is laid down in the cytoplasm at the equatorial plane, growing from the center to the periphery.

Differences between Mitosis in Animal and Plant Cells

Feature	Animal Cell Mitosis	Plant Cell Mitosis
Asters	Asters are formed around centrioles.	Asters are not formed (no centrosome).
Cytokinesis	By furrowing of the cytoplasm.	By cell plate formation.
Location of occurrence	Occurs in most tissues throughout the body (for growth and replacement).	Occurs mainly at growing tips (meristems) for lengthening and sides for increasing girth.

Significance of Mitosis

1. Growth: Increase in body size due to new cell formation.
2. Repair: Healing of damaged and wounded tissues.
3. Replacement: Replacing old/dead cells (e.g., blood cells, epidermal cells).
4. Asexual Reproduction: Used by unicellular organisms like Amoeba to divide into two.
5. Genetic Stability: Maintains the exact same chromosome number in daughter cells.

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4. The Cell Cycle - "Divide, grow and redivide"

The interphase prepares the cell for division and consists of three distinct phases:

1. First Growth Phase ($G_1$): RNA and proteins are synthesised, the volume of cytoplasm increases. Mitochondria and chloroplasts divide.
2. Synthesis Phase ($S$): More DNA is synthesised. The chromosomes strictly duplicate but remain attached at the centromere.
3. Second Growth Phase ($G_2$): A shorter growth phase where RNA and proteins necessary for cell division continue to be synthesised.

In late $G_1$ phase, cells may withdraw from the cycle and enter a Resting phase (R), or proceed to the S phase.

Can the cell cycle go on endlessly?

No. It stops permanently in some cells (like nerve cells), temporarily in others (like liver cells dividing once every 1-2 years), and runs continuously in others (skin surface cells, plant meristems). Uncontrolled, non-stop cell cycles lead to tumours/cancer.

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5. Meiosis (Reduction Division)

Meiosis occurs in the reproductive organs (testis and ovary in humans, anthers and ovary in plants) to produce gametes.

The most significant aspect of meiosis is that the chromosome number is halved. From 23 pairs (diploid, 2n) in human body cells, only single chromosomes (haploid, n) are passed to sperms/eggs. Upon fertilization, the diploid number ($n + n = 2n$) is restored.

Significance of Meiosis

1. Halves Chromosome Number: Essential so fertilization restores the normal double (diploid) number.
2. Mixing of Genes (Variations): Occurs in two ways:
   • Maternal and paternal chromosomes mix up randomly as they separate from homologous pairs.
   • Crossing Over: While separating, chromatid material often gets exchanged between members of a homologous pair at a point called the chiasma. This results in genetic recombination and variation in progeny.

Difference Between Mitosis and Meiosis

Feature	Mitosis	Meiosis
1. Where it occurs	In the somatic (body) cells.	In reproductive cells.
2. What for	Growth, repair, and replacement.	Only for gamete formation.
3. When it occurs	Continuously throughout life.	Only in reproductively active age.
4. Number of daughter cells	Two daughter cells.	Four daughter cells.
5. Chromosome number	Full set (Diploid, 2n).	Half the number (Haploid, n).
6. Number of nuclear divisions	A single nuclear division.	Two nuclear divisions.
7. Identity of genes	Identical to the parent cell.	Randomly assorted (results in variations).

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6. ICSE Board Exam Practice Questions

Mastering these questions directly from the syllabus exercises guarantees you are fully prepared!