2.2.2Variation, Homeostasis and Micro-organisms

Types of Cell Division

5 min read 8 parts Lesson 11 of 51 1 quiz

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Section 1 of 8What mitosis is and why it matters

Mitosis is a nuclear division that gives rise to two genetically identical diploid daughter cells.

The diploid number in a human is 23 pairs of chromosomes (46 in total), so each daughter cell has the same full set as the parent.

All body cells (but not gametes) are produced by mitosis of the zygote, the fertilised egg.

Cells in the body divide by mitosis to produce more cells, which is essential for growth, for repairing damaged tissue and for replacing cells that are lost.

The cells lining the intestines, for example, are constantly dividing to replace those shed as food moves through the digestive system.

Mitosis is also the type of cell division that produces new individuals in asexual reproduction.

Section 2 of 8The cell cycle and the stages of mitosis

Cell cycle: interphase (G1, S, G2) and mitosis

Before dividing, a cell goes through interphase, when it grows, increases the number of organelles and replicates (copies) its DNA so that each chromosome is made of two identical chromatids joined together. Mitosis itself then follows in a series of stages:

1Prophase

Prophase the DNA condenses, chromosomes become visible and the nuclear membrane breaks down.

2Metaphase

Metaphase the chromosomes line up on the equator of the cell (the metaphase plate), attached to spindle fibres.

3Anaphase

Anaphase spindle fibres split the chromosomes down the centre and pull one chromatid to either side of the cell.

4Telophase

Telophase nuclear membranes re-form around the two sets of chromosomes, and the cell divides its cytoplasm (cytokinesis), producing two identical cells.

Mitosis is only one part of the cell cycle, and the cycle is regulated by many different genes so that cells divide only when they need to and stop when required.

Section 3 of 8Calculating the number of cells

Each division doubles the number, so after n divisions one cell becomes 2ⁿ cells. If a cell divides 7 times, it produces 2⁷ = 128 cells. If you start with more than one cell, multiply the starting number by 2ⁿ.

Number of divisions	Cells from 1 starting cell
1	2
2	4
3	8
4	16
7	128

Section 4 of 8Uncontrolled cell division

THE PROCESS OF CELL DIVISION BY MITOSIS — INTERACTIVE SIMULATION

Cell division by mitosis in five labelled steps

If the genes that regulate the cell cycle are damaged (mutated), a cell may divide repeatedly without control.

Tumour

This produces a mass of abnormal cells called a tumour.

A tumour that stays in one place is benign; if its cells spread through the blood to other parts of the body and form new tumours, it is malignant, or cancer.

Factors that increase the risk of cancer include smoking, ultraviolet radiation, ionising radiation and some viruses, as well as inherited genes.

Comparison of mitosis and uncontrolled division: during normal division the number of cells is controlled and matched to the body's needs, but in cancer the controls have been lost, so the cells keep dividing and may invade and damage surrounding tissue.

Section 5 of 8Observing mitosis and exam application

Mitosis can be observed in the lab by examining a root tip squash.

Observing mitosis and exam application

The tip of an onion or garlic root is placed in warm hydrochloric acid to separate the cells, stained (for example with toluidine blue or acetic orcein), and squashed under a coverslip so that a single layer of cells is seen under the microscope.

Cells in the dividing region show chromosomes in different stages, and counting cells in each stage gives an idea of how long each stage lasts: the stage with the most cells is the longest.

The mitotic index is the number of cells in mitosis divided by the total number of cells observed.

In exam answers on mitosis, use correct vocabulary: chromosomes, chromatids, spindle fibres and equator.

Observing mitosis and exam application

State clearly that the daughter cells have the same number of chromosomes as the parent and are genetically identical, because this is what makes mitosis suitable for growth, repair and asexual reproduction.

If asked about cancer, link the loss of control of the cell cycle to damaged genes and the formation of tumours.

Section 6 of 8Worked examples

Mitosis: Producing Two Genetically Identical Diploid Daughter Cells

A single cell divides by mitosis 7 times. How many cells are produced? [2]2 marks
Describe what happens to the chromosomes during anaphase. [2]2 marks
Explain why cells lining the intestine divide by mitosis constantly. [2]2 marks

Section 7 of 8Key terms

Term	Meaning
Mitosis	Nuclear division that produces two genetically identical diploid daughter cells.
Diploid	Having two sets of chromosomes, one from each parent; 46 chromosomes in humans.
Chromatid	One of the two identical halves of a replicated chromosome.
Interphase	The part of the cell cycle in which a cell grows and replicates its DNA.
Tumour	A mass of abnormal cells caused by uncontrolled cell division.
Spindle fibres	Protein fibres that move chromosomes during cell division.

Section 8 of 8Exam technique

Thinking mitosis halves the chromosome number — the correct idea is that daughter cells are diploid, identical to the parent.

Mixing up the order of the stages — the correct order is prophase, metaphase, anaphase, telophase.

Calculating cell number as n × 2 — the correct idea is 2 to the power n.

Saying all tumours are cancerous — the correct idea is that benign tumours do not spread; malignant ones do.

Cambridge 9700 AS Biology Core Formula

Magnification Triangle & Unit Conversion

Formula: M = I / A ⇔ A = I / M ⇔ I = M × A

Units: 1 mm = 1,000 μm = 1,000,000 nm.
Rule: Convert Image size (ruler mm) to μm by ×1,000 before dividing by Actual size.