AQA A-level Biology: The Cell Cycle, Mitosis and Required Practical 2 (7402) – Revision Notes

Short notes for the final weeks. These notes follow section 3.2.2, All cells arise from other cells, in the AQA AS and A-level Biology specification (A-level 7402), for first teaching 2015, exams June 2016 onwards. The section is part of the AS content, so nothing here is marked A-level only. Find the AQA A-level Biology hub, tick off topics on the checklist, and use the free diagnostics to find weak spots. Meiosis is in a different section: see the mutation and meiosis notes. Measuring cells under the microscope is in the microscopy notes.

Key definitions

Term	Meaning
Cell cycle	The repeating sequence of growth, DNA replication and division shown by eukaryotic cells that can still divide
Interphase	The part of the cycle between divisions; the cell grows and its DNA is replicated
Mitosis	Nuclear division that gives two daughter nuclei, each with an identical copy of the parent cell’s DNA
Cytokinesis	Division of the cytoplasm, which usually follows mitosis and gives two new cells
Chromatid	One of the two identical copies of a chromosome made by replication, joined to its sister at the centromere
Centromere	The region holding two sister chromatids together, where spindle fibres attach
Spindle fibres	Protein fibres that attach to centromeres and pull chromatids to opposite poles
Mitotic index	The proportion of cells in a sample that are in mitosis
Binary fission	The way prokaryotic cells divide

Who divides?

• In a multicellular organism, not every cell keeps the ability to divide. Many specialised cells never divide again.
• Eukaryotic cells that can divide go through the cell cycle: interphase, then mitosis, then (usually) cytokinesis.
• DNA replication happens in interphase, not during mitosis. By the start of mitosis each chromosome is two sister chromatids.
• Mitosis gives two daughter cells that are genetically identical to each other and to the parent.

The stages of mitosis

Stage	What the chromosomes do	What you see in a root tip squash
Interphase	DNA replicates; chromosomes are long and uncoiled, so they cannot be seen individually	A clear nucleus, often with a nucleolus; no separate chromosomes
Prophase	Chromosomes condense (shorten and thicken); the nuclear envelope breaks down; the spindle starts to form	Thick, visible threads inside the region where the nucleus was
Metaphase	Chromosomes line up on the equator, each attached to spindle fibres at its centromere	A band of chromosomes across the middle of the cell
Anaphase	Centromeres divide; spindle fibres shorten and pull sister chromatids to opposite poles	Two groups of chromatids moving apart, often V-shaped as they are pulled centromere first
Telophase	Chromatids reach the poles, uncoil, and nuclear envelopes form around each group	Two nuclei at opposite ends of one cell
Cytokinesis	The cytoplasm divides	Two cells; in plants a new cell wall forms between them

Learn the stage order as I-P-M-A-T, then cytokinesis.

Explain the appearance (not just name it)

• Chromosomes are visible in prophase because they have condensed. In interphase they are too long and thin to see.
• In metaphase they sit in a line because spindle fibres from each pole are attached to every centromere and pull equally.
• In anaphase the chromatids look V-shaped because the spindle fibre pulls at the centromere and the arms trail behind.
• Each daughter nucleus gets one chromatid from every chromosome, so both have the full, identical set.

Mitosis, tumours and cancer

• Mitosis is normally a controlled process.
• Uncontrolled cell division can produce a tumour, and some tumours become cancers.
• Many cancer treatments aim to control the rate of cell division. For example, some drugs stop DNA from replicating or stop the spindle from working, so cells are stopped in the cycle. Because rapidly dividing normal cells (such as hair follicle and gut lining cells) are affected too, these treatments have side effects.
• How genes control the cycle, and why tumours form, is A-level content in section 3.8: see the regulation of gene expression and cancer notes.

Binary fission in prokaryotes

1. The single circular DNA molecule replicates.
2. Any plasmids replicate too.
3. The cytoplasm divides, giving two daughter cells.
4. Each daughter cell has one copy of the circular DNA but a variable number of copies of plasmids.

There is no spindle and no nucleus in a prokaryote, so binary fission is not mitosis.

Viruses do not divide

• Viruses are non-living and do not undergo cell division.
• A virus attaches to a host cell and its nucleic acid is injected into (or enters) the cell.
• The infected host cell then replicates the virus particles, using its own ribosomes, enzymes and ATP.

Required practical 2: root tip squash and mitotic index

Aim: prepare a stained squash of plant root tip cells, find the stages of mitosis with an optical microscope, and calculate a mitotic index.

Method in outline

1. Cut off the last few millimetres of a growing root tip (for example from garlic or onion). This is where the dividing cells are.
2. Put the tip in warm dilute hydrochloric acid for a few minutes. The acid breaks down the material holding the cells together, so they can be spread out, and it stops further division.
3. Rinse the tip in water, then place it on a slide and add a stain that binds to chromosomes (for example toluidine blue or acetic orcein).
4. Break the tip up with a mounted needle, add a coverslip, and press down firmly and vertically (no sideways movement) to make a layer one cell thick.
5. Look at low power to find the dividing region, then switch to high power. Count cells in each stage.

Why each step matters

Step	Reason
Use only the tip	Cells divide in the region just behind the tip; older cells further back have stopped dividing
Acid	Separates the cells so they spread into a single layer
Stain	Chromosomes are colourless and would not be seen without it
Press straight down	A single layer lets light through; sliding the coverslip would break the cells and smear the chromosomes
Low power first	You find the right region before you lose the field of view

Mitotic index

Mitotic index = number of cells in mitosis ÷ total number of cells counted

• Count prophase, metaphase, anaphase and telophase cells as “in mitosis”. Interphase cells are not.
• Multiply by 100 if a percentage is asked for.
• A higher mitotic index means a higher proportion of cells dividing, so faster growth (or, in a tumour, faster cell division).

Worked reminder: 36 cells in mitosis out of 240 counted. Mitotic index = 36 ÷ 240 = 0.15, or 15%.

Using stage counts to estimate time. The proportion of cells in a stage reflects how long that stage lasts. If 12 out of 240 cells are in metaphase and the cycle takes 20 hours, metaphase lasts about (12 ÷ 240) × 20 = 1 hour.

Cell size in the practical

Actual size = size of image ÷ magnification

Measure cells with a calibrated eyepiece graticule. Worked reminder: a cell measures 6 eyepiece divisions, each worth 2.5 µm at this objective, so it is 6 × 2.5 = 15 µm long. A drawing of a 15 µm cell that is 60 mm long has a magnification of 60 000 ÷ 15 = ×4000.

Making counts valid and reliable

• Count many cells, from several fields of view, and from more than one root tip.
• Decide in advance how to score cells you are unsure about, and use the same rule throughout.
• Avoid counting the same cell twice; work across the slide in a set pattern.

Must-know distinctions

• Chromosome versus chromatid. After replication, one chromosome is made of two sister chromatids. In anaphase the chromatids separate, and each then counts as a chromosome.
• Mitosis versus cytokinesis. Mitosis divides the nucleus; cytokinesis divides the cytoplasm.
• Interphase is not a “resting” stage. DNA replication and growth happen then.
• Binary fission is not mitosis. Prokaryotes have no nucleus or spindle.
• Viruses are replicated by the host cell, not by dividing themselves.

Quick self-test

1. When in the cell cycle is DNA replicated?
2. Describe what happens to the chromosomes in metaphase.
3. What is the role of spindle fibres?
4. Why do chromatids appear V-shaped in anaphase?
5. Give two differences between binary fission and mitosis.
6. Why do viruses not undergo cell division?
7. Why is the root tip treated with hydrochloric acid in a squash?
8. Why must the coverslip be pressed vertically and not slid?
9. 18 cells are in mitosis out of 150 counted. Calculate the mitotic index.
10. Suggest why many cancer drugs cause hair loss.

Answers

1. In interphase.
2. Chromosomes line up on the equator of the cell, attached to spindle fibres by their centromeres.
3. They attach to the centromeres and shorten, pulling sister chromatids to opposite poles.
4. The spindle fibre pulls at the centromere, so the arms of the chromatid trail behind.
5. Binary fission has no spindle and no nucleus; it copies a circular DNA molecule and plasmids, and plasmid numbers in the daughter cells can differ.
6. They are non-living and are not made of cells; the host cell makes new virus particles.
7. To separate the cells so they spread out into a thin layer (and to stop division).
8. To get a single layer of cells without breaking them or smearing the chromosomes.
9. 18 ÷ 150 = 0.12 (12%).
10. The drugs act on rapidly dividing cells, and hair follicle cells divide rapidly too.

Where marks are usually lost

• Saying DNA replicates “in prophase”.
• Writing “chromosomes separate” in anaphase instead of “sister chromatids separate”.
• Calling binary fission a type of mitosis.
• Forgetting that cytokinesis is a separate step from nuclear division.
• Counting only metaphase cells as “in mitosis” when calculating a mitotic index.
• Describing the stain’s purpose as “to see the cells” rather than “to make chromosomes visible”.

Frequently asked questions

How do you calculate the mitotic index? Mitotic index = number of cells in mitosis ÷ total number of cells counted. Count cells in prophase, metaphase, anaphase and telophase as in mitosis, and multiply by 100 if a percentage is asked for.

Why is the root tip put in dilute hydrochloric acid in Required Practical 2? The acid breaks down the material holding the cells together, so the tip can be squashed into a thin layer for viewing under the microscope.

Is binary fission a type of mitosis? No. Prokaryotes have no nucleus and no spindle. In binary fission the circular DNA and any plasmids replicate and the cytoplasm divides.

Do viruses undergo cell division? No. Viruses are non-living. They inject their nucleic acid into a host cell, and the infected host cell replicates the virus particles.

When does DNA replicate in the cell cycle? DNA replicates during interphase, not during mitosis. Saying it replicates “in prophase” is a common error in AQA 7402 answers.

Official syllabus

AQA AS and A-level Biology specification (7401 and 7402), for first teaching 2015, exams June 2016 onwards, Version 1.5, published by AQA. Section 3.2.2 All cells arise from other cells, including Required practical 2.