Why cells divide and the types of cell division: WASSCE Biology

Every living thing started as a single cell. A baby's body has trillions of cells; a tree has millions. All of those cells came from earlier cells that divided. Cell division is how living things grow, repair damage, and reproduce. WAEC tests this topic almost every year because it links cells to genetics, growth, and inheritance.

Definition

Cell division — the process by which one cell splits to form two or more new cells.

Parent cell — the original cell that is about to divide.

Daughter cells — the new cells produced by cell division.

Mitosis — the type of cell division that produces two genetically identical daughter cells. Used for growth and repair.

Meiosis — the type of cell division that produces four daughter cells, each with half the number of chromosomes. Used to make gametes (sex cells).

Gamete — a sex cell. In animals: sperm (male) and egg / ovum (female). In plants: pollen (male) and ovule (female).

Why cells divide

There are three big reasons cells divide:

1. Growth. A fertilised egg is a single cell. To become a baby, it must divide millions of times. Every part of your body — bones, muscles, skin, brain — grew because cells kept dividing.

2. Repair. When you cut your skin or break a bone, cells around the injury divide to make new cells that fill the gap. This is why wounds heal.

3. Reproduction. Sexual reproduction depends on sex cells (gametes). The only way to make a gamete is through a special type of cell division called meiosis.

Cells do not divide endlessly. Most body cells divide only when they need to. When growth is finished or repair is complete, the cells stop dividing. Loss of this control is what causes cancer — cells that should have stopped dividing keep going.

Two main types of cell division

There are two main kinds of cell division, with very different purposes.

Feature	Mitosis	Meiosis
Where it happens	All body cells	Only in reproductive organs (testes, ovaries)
Purpose	Growth and repair	Make sex cells (gametes)
Number of daughter cells	2	4
Chromosomes in each daughter	Same as parent (full set)	Half of parent (half set)
Are daughters identical?	Yes, identical to parent	No, all different from each other
Number of divisions	One	Two (meiosis I and meiosis II)

The most important thing to remember: mitosis keeps the chromosome number the same; meiosis halves it. That single difference shapes everything else.

Why halving the chromosome number matters

A human body cell has 46 chromosomes. When a sperm fertilises an egg, the chromosomes from both combine to form the new individual. If both sperm and egg had 46 chromosomes, the baby would have 92. The next generation would have 184. The number would double every generation.

Meiosis solves this problem. It halves the chromosome number when making gametes:

Body cells in a parent: 46 chromosomes (full set, called diploid)
Gametes (sperm, egg): 23 chromosomes (half set, called haploid)
Sperm (23) + egg (23) = 46 chromosomes in the new baby

The chromosome number stays the same across generations. This is why meiosis is sometimes called reduction division.

A third type: binary fission

Bacteria and other simple, single-celled organisms divide by a process called binary fission. This is much simpler than mitosis. The single cell just splits in two. You may see this in objective questions on bacterial reproduction.

WAEC mostly tests mitosis and meiosis. Binary fission is mentioned in the context of bacteria.

Common mistake

Students sometimes write that meiosis happens in every cell of the body. It does not. Meiosis only happens in the reproductive organs (testes in men, ovaries in women, or anther and ovary in plants). Every other cell in the body uses mitosis.

Worked example

Question: Explain why meiosis is sometimes called "reduction division." Use the example of human cells in your answer.

Solution:

Meiosis is called reduction division because it reduces (halves) the chromosome number from the parent cell to each daughter cell.

In humans:

A normal body cell has 46 chromosomes (called the diploid number).
During meiosis in the testes or ovaries, a parent cell with 46 chromosomes divides twice to produce four gametes, each with 23 chromosomes (called the haploid number).

This halving is essential because when fertilisation happens, the sperm (23 chromosomes) joins the egg (23 chromosomes) to form a fertilised egg with 46 chromosomes — the same as the parent. If meiosis did not halve the chromosome number, every generation would have double the chromosomes of the previous one, which would not be compatible with life.

WAEC tests this

Reasons why cells divide — easy 3-mark question. Always say growth, repair, reproduction.
Differences between mitosis and meiosis — the comparison table is the standard answer.
Why meiosis is called reduction division — direct explain question.
The chromosome number in human gametes vs body cells — almost guaranteed objective question. 23 vs 46.
Where in the body meiosis happens — testes and ovaries (and the anther and ovary in plants).

Summary

Cells divide for three reasons: growth, repair, and reproduction.
Mitosis makes two identical daughter cells with the same chromosome number — used for growth and repair.
Meiosis makes four different daughter cells with half the chromosome number — used to make gametes.
Halving the chromosome number in meiosis is essential. Otherwise the number would double each generation.
Human body cells have 46 chromosomes; human gametes have 23.
Meiosis only happens in the reproductive organs.

Why cells divide and the types of cell division