Biology for Class 10 ICSE (2025-26)

Unit 1: Basic Biology – The Blueprint of Life

A Comprehensive Lecture by Prof. Anil Tyagi, Ph.D.

My dear students, the human body is a universe of roughly 37 trillion cells. Each of these cells knows its purpose, its function, and its lifespan. This incredible organization, the transmission of traits from one generation to the next, and the very differentiation of male and female—all of it is governed by the principles we will study today. We will delve into the cell’s life cycle, the architecture of genetic information, and the elegant rules of heredity laid down by a monk in a garden. This is where your journey into medicine truly begins.

1. The Cell Cycle and Cell Division: The Rhythm of Life

Every living organism, from a single-celled amoeba to a complex human, grows and reproduces through cell division. It is a tightly regulated process, not a chaotic event. This ordered sequence of events is called the Cell Cycle.

The cell cycle is divided into two primary phases:

Interphase (The Preparatory Phase)
M-phase (The Division Phase)

Interphase: The Busy Resting Period

Do not be fooled by the old term “resting phase.” Interphase is a period of intense biochemical activity and preparation. It is the longest phase of the cell cycle and is subdivided into three distinct stages:

G1 Phase (First Gap or Post-mitotic Gap):
- What happens? Immediately after a cell divides, it enters G1. This is a period of rapid growth and metabolic activity. The cell increases in size, produces new proteins and organelles (like mitochondria and ribosomes), and carries out its designated functions.
- The Decision Point: Towards the end of G1, the cell reaches a critical checkpoint. If conditions are favourable (e.g., adequate nutrients, cell size is sufficient, DNA is undamaged), the cell commits to dividing and proceeds to the S phase. If not, it may exit the cycle and enter a quiescent state called the G0 phase. Many cells in the human body, like mature neurons and cardiac muscle cells, are permanently in G0.
S Phase (Synthesis Phase):
- What happens? This is the most crucial event of interphase. The synthesis or replication of DNA occurs. Each strand of the chromatin (DNA-protein complex) serves as a template to create an identical copy.
- The Outcome: By the end of the S phase, each chromosome has been duplicated. It now consists of two identical DNA molecules, called sister chromatids, which are joined together at a region called the centromere. The chromosome number remains the same (e.g., 46 in humans), but the amount of DNA has doubled.
G2 Phase (Second Gap or Pre-mitotic Gap):
- What happens? The cell prepares for the actual division. It produces the necessary proteins and enzymes required for mitosis. There is additional growth and a final check to ensure all DNA has been replicated accurately without any damage.
- The Final Check: The G2 checkpoint ensures the cell is fully prepared to undergo the intricate process of mitosis.

M-Phase: The Phase of Division

This is where the cell divides its already duplicated contents to form two daughter cells. M-phase consists of two processes:

Karyokinesis: Division of the nucleus.
Cytokinesis: Division of the cytoplasm, resulting in two separate cells.

Mitosis (Equational Division)

Mitosis is the process of nuclear division that ensures each daughter cell receives an exact copy of the parent cell’s genetic material. It is used for growth, repair, and asexual reproduction. It consists of four stages:

Prophase:
- Chromatin condenses and coils to become short, thick, distinct chromosomes (each with two sister chromatids).
- The nuclear membrane and nucleolus start to disappear.
- In animal cells, two centrioles move to opposite poles of the cell and form spindle fibres (achromatic spindle).
Metaphase:
- The chromosomes, attached to spindle fibres at their centromeres, line up perfectly along the equator (metaphase plate) of the cell. This is the best stage to study the morphology and number of chromosomes.
Anaphase:
- The centromeres split, separating the sister chromatids.
- These now-separated chromatids (called daughter chromosomes) are pulled by the spindle fibres towards opposite poles of the cell.
Telophase:
- The chromosomes reach the poles and begin to uncoil back into chromatin.
- The nuclear membrane and nucleolus reappear, forming two new nuclei.
- Cytokinesis begins simultaneously. In animal cells, a cleavage furrow forms and pinches the cell in two. In plant cells, a cell plate forms that becomes the new cell wall.

The result of mitosis is two daughter cells, each genetically identical to the parent cell and to each other, with the same diploid number of chromosomes.

Meiosis (Reductional Division) – A Basic Idea

Meiosis is a specialized type of cell division that occurs only in the germ cells of the reproductive organs (ovaries and testes) to produce gametes (sperms and eggs).

Key Purpose: To halve the chromosome number. A diploid (2n) parent cell undergoes two sequential divisions (Meiosis I and Meiosis II) to produce four haploid (n) daughter cells.
Why is this important? When two haploid gametes fuse during fertilization, the original diploid number is restored. This prevents the chromosome number from doubling with each generation.
Crucial for Genetic Variation: Meiosis introduces genetic variation through two key processes:
1. Crossing Over: During Prophase I, homologous chromosomes (pairs of similar chromosomes from each parent) exchange segments of DNA. This creates new combinations of genes on a chromosome.
2. Random Assortment: During Metaphase I, the pairs of homologous chromosomes line up at the equator randomly. This random orientation leads to a multitude of possible combinations of chromosomes in the resulting gametes.

2. Structure of Chromosome: The Packaging of Heredity

How is meters-long DNA packed into a microscopic nucleus? The answer lies in the incredible packaging of chromosomes.

DNA (Deoxyribonucleic Acid): The fundamental hereditary material. It is a double-stranded helical molecule that carries the genetic code in the sequence of its nitrogenous bases (A, T, G, C).
Histones: Proteins around which DNA winds. DNA wrapped around a core of eight histone proteins is called a nucleosome. Nucleosomes look like “beads on a string.”
Chromatin: The complex of DNA and histone proteins. This is the form DNA takes during interphase for efficient transcription and replication. It appears as a diffuse, granular mass in the nucleus.
Chromatid: As the cell prepares to divide, the chromatin condenses and coils further. After DNA replication in the S phase, each chromosome consists of two identical strands of condensed chromatin, called sister chromatids.
Chromosome: The highly condensed, compact form of a chromatid (or paired chromatids) that becomes visible during cell division. It is the vehicle for transporting genetic information during cell division.
Centromere: The specialized constricted region of a chromosome where the two sister chromatids are joined. It is the attachment site for spindle fibres during cell division.
Telomere: The protective, cap-like structure at the end of each chromosome arm. It prevents chromosomes from fraying or sticking to each other. Telomeres shorten with each cell division and are associated with aging.

Think of it like this: DNA is a long, intricate thread. Winding it around histones (nucleosomes) makes it manageable like a string of beads. Coiling this string further creates the thick, rod-like structures we see as chromosomes during cell division. This packaging is essential for the accurate segregation of genetic material.

3. Genetics: The Science of Heredity

Genetics is the study of heredity and variation. It explains how traits are passed from parents to offspring.

Mendel’s Laws: The Foundation

Gregor Mendel, the father of genetics, discovered the fundamental principles of inheritance through his meticulous pea plant experiments.

Law of Dominance:
- Statement: In a cross between two purebred organisms for contrasting traits, only one trait (the dominant trait) appears in the F1 generation. The trait that remains hidden is the recessive trait.
- Example: Crossing a pure tall (TT) plant with a pure dwarf (tt) plant produces all tall (Tt) plants in the F1 generation. ‘T’ (tall) is dominant over ‘t’ (dwarf).
Law of Segregation:
- Statement: During gamete formation, the two alleles for a trait separate (segregate) so that each gamete carries only one allele for that trait.
- Basis: This occurs during Anaphase I of Meiosis when homologous chromosomes are separated.
Law of Independent Assortment:
- Statement: The alleles for different traits separate independently of one another during gamete formation.
- Basis: This occurs due to the random alignment of homologous chromosome pairs at Metaphase I of Meiosis. The way one pair lines up does not influence how another pair lines up.
- Note: This law holds true only for genes located on different chromosomes.

Monohybrid and Dihybrid Crosses

Monohybrid Cross: A cross that studies the inheritance of one pair of contrasting traits (e.g., Tall vs. Dwarf).
- The phenotypic ratio in the F2 generation is 3 : 1 (3 Dominant : 1 Recessive).
- The genotypic ratio is 1 : 2 : 1 (1 Pure Dominant : 2 Hybrid : 1 Pure Recessive).
Dihybrid Cross: A cross that studies the inheritance of two pairs of contrasting traits simultaneously (e.g., Seed shape: Round vs. Wrinkled and Seed colour: Yellow vs. Green).
- The phenotypic ratio in the F2 generation is 9 : 3 : 3 : 1.
- (9 Round-Yellow : 3 Round-Green : 3 Wrinkled-Yellow : 1 Wrinkled-Green).

Sex Determination

This is the biological system that determines the development of sexual characteristics in an organism. In humans, it is chromosomal.

Human Chromosomes: Humans have 23 pairs of chromosomes (46 total). 22 pairs are autosomes (same in males and females). The 23rd pair is the sex chromosomes.
Female: The sex chromosomes are XX.
Male: The sex chromosomes are XY.
The Mechanism: During gamete formation, all female eggs carry an X chromosome. Male sperm can carry either an X or a Y chromosome.
- If a sperm carrying an X chromosome fertilizes the egg (X), the zygote is XX (female).
- If a sperm carrying a Y chromosome fertilizes the egg (X), the zygote is XY (male).
Conclusion: The sex of the child is determined by the type of sperm (X or Y) that fertilizes the ovum. The father is therefore genetically responsible for the sex of the child.

Sex-linked Inheritance

Genes located on the sex chromosomes (mostly the X-chromosome) show a specific pattern of inheritance called sex-linked inheritance.

Why X-chromosome? The X chromosome is larger and carries many more genes than the smaller Y chromosome. Many genes on the X have no corresponding allele on the Y. In males, a single recessive allele on the unpaired X chromosome will express the disorder because there is no dominant allele on the Y to mask it. Males are said to be hemizygous for X-linked traits.
Examples:
1. Colour Blindness: A recessive disorder where an individual cannot distinguish between certain colours, most commonly red and green.
  - Genotype: Affected Male (X^cY), Carrier Female (X^CX^c), Affected Female (X^cX^c – very rare).
  - Inheritance Pattern: from carrier mother to son.
2. Haemophilia: A recessive disorder where the blood fails to clot properly after an injury, leading to excessive bleeding.
  - Genotype: Affected Male (X^hY), Carrier Female (X^HX^h), Affected Female (X^hX^h – very rare).
  - It was famously present in the European royal families.

Professor Tyagi’s Key Takeaways for NEET Aspirants:

Link the Processes: Connect the S phase of interphase to the appearance of chromatids in prophase. Connect Anaphase of mitosis to the Law of Segregation, and Metaphase I of meiosis to the Law of Independent Assortment.
Visualize the Chromosome: Draw the structure of a chromosome repeatedly until you can label its parts blindfolded. Understand the hierarchy: DNA -> Nucleosome -> Chromatin fibre -> Chromatid -> Chromosome.
Practice Crosses: Genetics is not a spectator sport. Solve monohybrid and dihybrid cross problems until the ratios become second nature. For sex-linked crosses, always remember that males never pass an X-linked allele to their sons (they give them the Y).
Think Clinically: When you study disorders like haemophilia, think about the clinical implications. This will make the theory stick and prepare you for medical case studies.

This unit is the bedrock of modern biology and medicine. From understanding cancer (uncontrolled cell division) to genetic counseling and gene therapy, it all starts here.

Internalize these concepts. Build your foundation strong. Your future patients are counting on the knowledge you build today.

– Prof. Anil Tyagi

Biology for Class 10 ICSE – Unit 1: Basic Biology – The Blueprint of Life