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).

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

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.

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

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

• Chromatin is the thread-like material present in the nucleus.
• It is made up of DNA (40%) and histone proteins (60%).
• DNA carries genetic information, while histones help package and organize DNA.
• Chromatin condenses to form chromosomes during cell division.
• 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).

• The cell cycle is the sequence of events in which a cell grows, duplicates its DNA, and divides into two daughter cells.
• Cell division is essential for growth and reproduction in all living organisms, starting from a single cell.
• A single cell forms a large organism through repeated cycles of growth and division, producing many cells.
• DNA replication and cell growth must be coordinated to ensure that daughter cells receive a complete and correct genome.
• Cell growth is continuous, but DNA synthesis occurs only at a specific stage of the cell cycle, followed by the distribution of chromosomes during division.

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 cytoplasm into two daughter cells after karyokinesis (nuclear division).
• In animal cells, a furrow appears in the plasma membrane, deepens, and joins in the centre to divide the cytoplasm.
• In plant cells, a cell plate forms in the centre and grows outward to meet the lateral walls, forming a new cell wall. The cell plate represents the middle lamella between two adjacent cells.
• During cytokinesis, organelles like mitochondria and plastids are distributed between the two daughter cells.
• In some organisms, karyokinesis is not followed by cytokinesis, resulting in a multinucleate condition called syncytium (e.g., liquid endosperm in coconut).

• Mitosis is an equational division that produces genetically identical daughter cells, maintaining chromosome number.
• It is responsible for growth in multicellular organisms by increasing the number of cells.
• Mitosis helps in repair, regeneration, and wound healing by replacing damaged or lost cells.
• It maintains the nucleo-cytoplasmic (surface/volume) ratio, which gets disturbed during cell growth.
• Mitosis also enables asexual reproduction in unicellular organisms and continuous cell replacement (e.g., skin, blood, plant meristems).

• Meiosis is a specialised reduction division that produces four haploid daughter cells with half the chromosome number.
• It involves two sequential divisions (Meiosis I and II) but only one round of DNA replication during interphase.
• Occurs during gametogenesis — in humans (testes and ovaries) and in plants (anthers and ovaries).
• It includes pairing of homologous chromosomes and crossing over, leading to genetic recombination.
• Maintains chromosome number across generations by alternating haploid and diploid phases; fertilisation restores the diploid (2n) condition.

• Meiosis I is a reductional division, where the chromosome number reduces from diploid to haploid.
• Prophase I is the longest and most complex stage, divided into five sub-stages: leptotene, zygotene, pachytene, diplotene, and diakinesis.
• In Leptotene, chromosomes become visible and start condensing.
• In Zygotene, homologous chromosomes pair (synapsis), forming bivalents/tetrads with the help of the synaptonemal complex.
• In Pachytene, crossing over occurs between non-sister chromatids, causing genetic recombination.
• In Diplotene and Diakinesis, homologous chromosomes begin to separate; chiasmata appear, chromosomes fully condense, and the nuclear membrane breaks down.
• During Metaphase I, Anaphase I, and Telophase I:
  • Chromosomes align at the equator (Metaphase I)
  • Homologous chromosomes separate (Anaphase I)
  • Two haploid cells (dyads) are formed after Telophase I and cytokinesis

• Formation of Gametes - Meiosis is responsible for the formation of gametes in sexually reproducing organisms.
• Reduces Chromosome Number - Meiosis reduces the chromosome number by half in gametes, preventing doubling of chromosomes in each generation.
• Restores Chromosome Number - The original chromosome number is restored when gametes fuse during fertilisation.
• Conserves Species-Specific Number - Meiosis ensures that each species maintains its specific chromosome number across generations.
• Genetic Variability - Meiosis increases genetic variability in the population from one generation to the next, which is crucial for the process of evolution.