Revision: Basic Biology >> Structure of Chromosomes, Cell Cycle and Cell Division Biology (English Medium) ICSE Class 10 CISCE

The DNA strand winds around a core of eight histone proteins (called the histone octamer). Each such complex is called a nucleosome.

Histones are the proteins that help in the coiling and packaging of DNA into structural units called nucleosomes.

Genes are specific sequences of nucleotides on a chromosome that encode particular proteins which are expressed in the form of some particular feature of the body. They are the units of heredity which are transferred from parents to offspring and are responsible for some specific characteristics of the offspring.

Genetics is the study of heredity i.e. transmission of body features (both similarities and differences) from parents to offspring and the laws relating to such transmission. 

GENOME is the full complement of DNA (including all genes and the intergenic regions) of an organism.

The cell cycle is a series of events that take place in a cell leading to the duplication of its DNA and the subsequent division of the cell to produce two daughter cells.

The two daughter cells produced from a mother cell are relatively small, with a full-sized nucleus but relatively little cytoplasm. These cells are said to be in interphase.

Cell division is one of the most fundamental characteristics of life. This is the method which enables life to perpetuate generation after generation.

The centrosome (in animal cell) splits into two along with the simultaneous duplication of the centrioles contained in it. The daughter centrioles move apart and occupy opposite "poles" of the cell. Each centriole is surrounded by radiating rays and is termed an aster (aster : star).

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

The two sister chromatids remain attached to each other at a small region called centromere.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

A number of fibres appear between the two daughter centrioles, which are called the spindle fibres.

All the nuclear changes that occur during cell division are collectively termed karyokinesis (karyo: nucleus).

The two sister chromatids remain attached to each other at a small region called centromere.

1. Chromatin is the thread-like material present in the nucleus.
2. It is made up of DNA (40%) and histone proteins (60%).
3. DNA carries genetic information, while histones help package and organize DNA.
4. Chromatin condenses to form chromosomes during cell division.
5. It remains as long, thin fibres when the cell is not dividing.

• Miescher (1869) isolated a substance from white blood cell nuclei (pus from bandages) and called it nuclein — the first discovery of nucleic acid.
• Nuclein properties — High phosphorus content + acidic nature → renamed nucleic acid.
• Two types of nucleic acids: DNA and RNA.
• Early belief — Scientists thought proteins were the genetic material (large, complex, varied). DNA was wrongly considered simple and unimportant.
• 1928–1952 — Over 25 years, three key experiments proved that DNA (not protein) is the genetic material.
• Role of DNA — It is stable, can replicate accurately, and passes traits to the next generation — making it the true genetic material.

1. DNA structure was first studied by Rosalind Franklin (1953); later explained by Watson and Crick, who proposed the double helix model (Nobel Prize, 1962).

2. DNA is a macromolecule made of two complementary strands twisted into a double helix.

3. Each strand is made up of nucleotides, which include phosphate, sugar (pentose), and a nitrogenous base.

4. There are four nitrogenous bases:

• Adenine (A) pairs with Thymine (T) (2 hydrogen bonds)
• Guanine (G) pairs with Cytosine (C) (3 hydrogen bonds)

5. The two strands form a ladder-like structure, with bases as rungs and sugar-phosphate as the backbone.

1. Histones are proteins that help coil and package DNA into structural units called nucleosomes.
2. Each nucleosome consists of DNA wrapped around an octamer of 8 histone proteins (histone octamer).
3. A single human chromosome may contain about a million nucleosomes.
4. Human DNA is about 2 meters long, but the nucleus is only 6 micrometres in diameter, so DNA must be tightly packed.
5. DNA is coiled and supercoiled (like a telephone cord) to fit into the nucleus and eventually form chromosomes.

1. For Growth: A single cell (zygote) divides repeatedly to form tissues and organs. Cell division is essential for an organism’s growth.

2. For Replacement: Old or damaged cells (e.g. red blood cells) are replaced by new ones formed through cell division.

3. For Repair: Injured tissues are repaired when new cells fill gaps and heal cuts or fractures.

4. For Reproduction:

• In simple organisms, reproduction occurs by mitosis.
• In higher organisms, meiosis produces sperm and eggs, each with half the chromosome number (23 in humans).
• At fertilisation, the zygote gets 23 chromosomes from each parent, restoring the full set (46 chromosomes).

1. The cell cycle is a series of steps by which a cell grows, duplicates its DNA, and divides to form two daughter cells.
2. It has two main phases: Interphase (growth and preparation) and M-phase (mitosis or actual division).
3. This cycle ensures orderly cell growth, DNA replication, and equal distribution of genetic material.

1. Interphase is the longest phase of the cell cycle, where the cell grows, synthesizes proteins and RNA, and duplicates DNA.

2. It has three stages:

• G₁ (First Growth Phase) – Cell grows and organelles divide.
• S (Synthesis Phase) – DNA replication occurs.
• G₂ (Second Growth Phase) – Final preparation for division.

3. In the S-phase, the DNA double helix unwinds and new complementary strands form, resulting in two identical DNA molecules.

4. The cell cycle is tightly regulated and can stop temporarily, permanently, or continue as needed.

5. In plants, cell division occurs actively in meristematic tissues; in animals, germinal cells undergo meiosis to form sex cells.

6. Uncontrolled cell cycles may lead to tumours, while cell production balances cell death in adults and declines with age.

• Cell division is a vital process for growth, repair, and the formation of new organisms, helping maintain life in all living beings.
• It occurs in two forms: mitosis (in somatic and stem cells) for producing diploid identical cells, and meiosis (in germ cells) for forming haploid gametes.
• Mitosis supports body growth and tissue repair, while meiosis ensures genetic variation and maintains chromosome number in reproduction.
• Before division, the cell’s chromosome number doubles (e.g., from 2n to 4n) to ensure accurate distribution during mitosis or meiosis.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

Karyokinesis is the division of the nucleus during mitosis, ensuring equal distribution of chromosomes into two daughter nuclei.
It occurs in four continuous phases:

1. Prophase – Chromosomes condense and become visible; nuclear membrane and nucleolus disappear; spindle fibres form.
2. Metaphase – Chromosomes align at the cell's equator and attach to spindle fibres via centromeres.
3. Anaphase – Centromeres split; sister chromatids separate and move to opposite poles.
4. Telophase – Chromatids decondense into chromatin; nuclear envelope and nucleolus reappear around each set of chromosomes.

• Cytokinesis is the division of the cytoplasm that follows nuclear division (karyokinesis), resulting in the formation of two separate daughter cells.
• In animal cells, it occurs by the formation of a cleavage furrow, while in plant cells, a cell plate forms at the centre to divide the cytoplasm.

• Meiosis is a reduction division that results in haploid gametes (sex cells) with half the chromosome number.
• In humans, it occurs in the testes (to form sperm) and ovaries (to form ova).
• In flowering plants, it occurs in the anthers (to form pollen grains) and ovaries (to form ovules).
• Fertilisation restores the diploid (2n) number, maintaining chromosome count across generations.

• Meiosis is a two-stage process: Meiosis I and Meiosis II, responsible for forming haploid gametes from diploid germ cells.
• Prophase I is a complex phase subdivided into five stages where homologous chromosomes pair up and undergo crossing over, enabling genetic recombination.
• In Metaphase I, homologous pairs (not sister chromatids) align along the equatorial plate, attached to spindle fibers from opposite poles.
• During Anaphase I, homologous chromosomes are separated and pulled to opposite poles, reducing the chromosome number by half.
• Telophase I results in the formation of two haploid daughter cells, each with half the chromosome number, setting the stage for Meiosis II.

• Meiosis II is similar to mitosis and occurs in both haploid cells formed after Meiosis I, dividing the recombined sister chromatids.
• It includes four stages: Prophase II, Metaphase II, Anaphase II, and Telophase II, resulting in four haploid daughter cells.
• The process ensures genetic variation, as each resulting cell is genetically different from the parent and from each other.
• Meiosis II plays a vital role in sexual reproduction, producing gametes (in animals) or spores (in plants) with half the chromosome number.