Cell Division and Reproduction

Why Cell Division Matters

The cell is the basic unit of life. A grown-up human body has trillions of cells, yet life began from just one fertilised cell. How did that single cell become so many? Through cell division — the process by which one cell splits to form new cells.

Cell division does three big jobs:

• Growth — more cells mean a bigger body, from a baby to an adult.
• Repair — new cells replace dead or injured ones, such as healing a cut.
• Reproduction — special cell divisions make the cells needed to produce offspring.

For NDA, you do not need to memorise every step of the cycle. You need the clear definitions, the key differences and a handful of striking facts that examiners love to test.

Chromosomes, Genes and DNA

Inside the nucleus of a cell are thread-like structures called chromosomes. They carry the instructions for building and running the body. These instructions are stored in DNA (deoxyribonucleic acid), and a small section of DNA that controls one feature — such as eye colour — is called a gene.

Body cells (also called somatic cells) carry the full set of chromosomes — this is the diploid number, written as 2n. Sex cells (sperm and egg, called gametes) carry only half the set — the haploid number, written as n.

The father provides the Y or X chromosome through sperm, so the father’s gamete decides the baby’s sex. This is a frequently asked fact.

The Cell Cycle in Brief

A cell does not divide all the time. It follows a repeating sequence called the cell cycle, which has two main stages.

• Interphase — the long ‘preparation’ phase. The cell grows, makes proteins and copies its DNA so each new cell will get a full set.
• M phase (division) — the actual splitting, which includes division of the nucleus and then the cytoplasm.

The division of the nucleus is called karyokinesis, and the division of the cytoplasm is called cytokinesis. Together they complete the formation of new cells. In animal cells, cytokinesis happens by a furrow that pinches the cell from outside, while in plant cells a new cell plate forms in the middle and grows outward to make a wall. This difference is worth a quick note for the exam.

A useful way to picture the cell cycle is a clock: most of the time the cell sits quietly in interphase doing its routine work and stocking up for division, and only a short slice of the clock is the dramatic splitting of the M phase. If something goes wrong with the controls of this cycle, cells may divide without limit — this uncontrolled division is the basis of cancer, a point general-science papers sometimes touch upon.

Mitosis: Copying Cells

Mitosis is the division that produces two new cells identical to the parent cell. It happens in body (somatic) cells and is responsible for growth and repair.

Mitosis goes through four phases, easy to remember as PMAT:

1. Prophase — chromosomes become visible; nuclear membrane starts to disappear.
2. Metaphase — chromosomes line up in the middle of the cell.
3. Anaphase — chromosome halves move to opposite ends.
4. Telophase — two new nuclei form; the cell prepares to split.

After telophase, cytokinesis divides the cytoplasm and two complete cells are formed.

Meiosis: Making Gametes

Meiosis is the special division that makes gametes (sperm and egg). It happens only in the reproductive organs. Its big job is to halve the chromosome number so that when two gametes join, the offspring gets the correct full number again.

Because the chromosome number is halved, meiosis is also called reductional division. It involves two rounds — Meiosis I and Meiosis II.

During meiosis, matching chromosomes exchange small pieces in a process called crossing over. This shuffles features and is a key reason why children differ from their parents and from each other. Along with the random way chromosomes get distributed into the gametes, crossing over makes sure that no two gametes are exactly alike. This is why brothers and sisters from the same parents still look different from one another.

Think of meiosis as nature’s way of dealing a fresh hand of cards each time. The parents’ chromosomes are shuffled, cut and re-dealt, so every child receives a unique combination. This built-in variation is precious: it gives a species the raw material to adapt when the environment changes, and over very long periods it drives evolution.

Mitosis vs Meiosis at a Glance

This comparison is a near-guaranteed source of NDA questions, so fix it firmly.

• Where: Mitosis in body cells; meiosis in reproductive cells.
• Daughter cells: Mitosis → 2; meiosis → 4.
• Chromosome number: Mitosis keeps it same (2n); meiosis halves it (n).
• Result: Mitosis → identical cells; meiosis → varied cells.
• Purpose: Mitosis for growth and repair; meiosis for gamete formation.

Two Ways to Reproduce

Reproduction is the process by which living organisms produce new individuals of their own kind. It ensures the continuation of a species. There are two broad types.

Asexual reproduction

Only one parent is involved; no gametes fuse. The offspring are genetically identical to the parent (called clones). It is quick and common in simpler organisms.

Sexual reproduction

Two parents are involved. Male and female gametes fuse (fertilisation) to form a new individual that carries a mix of features from both parents.

Common Methods of Asexual Reproduction

NDA often asks ‘which organism reproduces by which method’, so link each method to a clear example.

• Binary fission — the cell splits into two. Example: Amoeba, bacteria.
• Multiple fission — the cell splits into many at once. Example: Plasmodium (malaria parasite).
• Budding — a small outgrowth (bud) grows and detaches. Example: Yeast, Hydra.
• Spore formation — tiny spores grow into new organisms. Example: Rhizopus (bread mould), ferns.
• Regeneration — a body part grows into a whole organism. Example: Planaria, Hydra.
• Fragmentation — the body breaks into pieces, each growing anew. Example: Spirogyra.
• Vegetative propagation — new plants from roots, stems or leaves. Example: potato (eyes), Bryophyllum (leaf buds).

Sexual Reproduction Basics

In sexual reproduction, the male gamete and female gamete join in a step called fertilisation. The fused cell is the zygote, which divides by mitosis to grow into a new individual.

• In flowering plants, the male part is the stamen and the female part is the carpel (pistil). Transfer of pollen is pollination, which leads to fertilisation and seed formation.
• In humans, the male gamete is the sperm and the female gamete is the egg (ovum). They are made by meiosis in the testes and ovaries.

Fertilisation can be external (outside the body, as in fish and frogs) or internal (inside the body, as in humans and birds). External fertilisation usually involves large numbers of eggs because many are lost in the open water, whereas internal fertilisation needs fewer eggs and offers better protection to the developing young.

Animals can also be grouped by how the young are produced. Oviparous animals lay eggs that hatch outside the mother’s body, such as birds and most reptiles. Viviparous animals give birth to live young that develop inside the mother, such as humans and most mammals. These two terms are simple but frequently confused, so keep them clearly separated.

Worked Example

Let us apply the chromosome rules to a simple numerical question, the kind NDA frames around human cells.

Notice how meiosis halving the number and fertilisation doubling it back keeps the species count stable at 46 from one generation to the next. That balance is the whole logic of the chapter.

Previous-Year Style Question

Quick Revision

Revise the mitosis-versus-meiosis contrast and the asexual examples the night before your exam — together they account for most questions from this chapter.