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

Section 3.2.2 of the AQA A-level Biology (7402) specification, All cells arise from other cells, asks you to describe the eukaryotic cell cycle, explain what chromosomes are doing in each stage of mitosis, link uncontrolled division to cancer, and compare this with binary fission in prokaryotes and the replication of viruses. It also contains required practical 2: preparing a stained root tip squash, identifying the stages of mitosis and calculating a mitotic index and the actual size of cells.

This guide is aligned to the AQA AS and A-level Biology specification (A-level 7402), for first teaching 2015 with exams from June 2016, Version 1.5. Section 3.2.2 is AS content, so it can be examined on A-level Paper 1 and in synoptic questions on Papers 2 and 3. Use it with the cell cycle and mitosis revision notes and the cell cycle and mitosis practice questions. Microscope use and magnification are taught in the microscopy and cell fractionation study guide; meiosis is in section 3.4.3, covered by the mutation and meiosis study guide. Course hub: AQA A-level Biology.

What the specification asks for

Specification content (3.2.2)	What you must be able to do
Not all cells in multicellular organisms retain the ability to divide	State this and recognise that dividing cells show a cell cycle
DNA replication occurs during interphase	Place DNA replication in the cycle and explain why it must come before mitosis
Mitosis produces two daughter cells with identical copies of DNA	Explain why the daughter cells are genetically identical
Behaviour of chromosomes in interphase, prophase, metaphase, anaphase and telophase; role of spindle fibres attached to centromeres	Recognise each stage and explain the appearance of cells in each
Cytokinesis usually follows	Describe division of the cytoplasm
Mitosis is controlled; uncontrolled division leads to tumours and cancers; many cancer treatments target the rate of cell division	Explain the link and how treatments work in principle
Binary fission in prokaryotes	Describe replication of circular DNA and plasmids and division of the cytoplasm
Viruses do not undergo cell division	Explain how infected host cells replicate virus particles
Required practical 2 (AT d and e; MS 0.3 and 1.8)	Prepare and stain a root tip squash, identify stages, calculate a mitotic index and actual size

What is the cell cycle?

Within a multicellular organism, many specialised cells (for example most mature nerve cells) no longer divide. Eukaryotic cells that do keep the ability to divide go through a repeating cell cycle of two main parts:

• Interphase, the longest part, in which the cell grows, makes new organelles and replicates its DNA, so that each chromosome becomes two identical sister chromatids joined at the centromere.
• Mitosis, nuclear division, followed in most cases by cytokinesis, division of the cytoplasm.

Because DNA is copied by semi-conservative replication during interphase and each daughter cell receives one chromatid of every chromosome, the two daughter cells have identical copies of the DNA of the parent cell.

What happens in each stage of mitosis?

Stage	Behaviour of chromosomes	What you see under the microscope
Interphase	DNA replicates; chromosomes are long and uncoiled	Nucleus with a clear nuclear envelope; chromosomes not visible as separate threads; often a nucleolus
Prophase	Chromosomes condense (shorten and thicken), each visible as two chromatids joined at a centromere; the nuclear envelope breaks down; spindle fibres begin to form from the poles	Dense, thread-like chromosomes inside what was the nucleus
Metaphase	Chromosomes line up along the equator of the cell; spindle fibres are attached to each centromere	A neat line of chromosomes across the middle
Anaphase	Centromeres divide; spindle fibres shorten and pull sister chromatids apart to opposite poles, centromere first	Two groups of V-shaped chromatids moving apart
Telophase	Chromatids reach the poles and uncoil; a nuclear envelope reforms around each set	Two nuclei at opposite ends of one cell
Cytokinesis	The cytoplasm divides to form two cells	In plants, a new cell wall forms across the middle

The V shape in anaphase is a favourite exam point: the spindle fibre pulls on the centromere, so the arms of each chromatid trail behind. After separation, each chromatid is called a chromosome.

Why is uncontrolled mitosis linked to cancer?

Mitosis is a controlled process. If the controls fail, cells divide repeatedly and form a tumour. Tumours that invade other tissues are cancers. The specification links this to treatment: many cancer treatments are directed at controlling the rate of cell division. For example, some drugs stop DNA replication during interphase and others stop spindle fibres forming, so cells cannot complete mitosis. Because these treatments affect any rapidly dividing cell, they also harm normal dividing cells, such as those in hair follicles and the lining of the gut, which explains common side effects. Gene control of cancer (tumour suppressor genes and oncogenes) is A-level content in section 3.8.2.

How do prokaryotic cells divide?

Prokaryotic cells divide by binary fission:

1. The single circular DNA molecule replicates, and so do the plasmids.
2. The cytoplasm divides to produce two daughter cells.
3. Each daughter cell receives a single copy of the circular DNA and a variable number of copies of the plasmids.

Binary fission involves no spindle and no stages like those of mitosis, because there is no nucleus and only one chromosome.

How do viruses replicate if they do not divide?

Viruses are non-living and do not undergo cell division. A virus attaches to a host cell and injects its nucleic acid. The infected host cell then uses its own enzymes, ribosomes and ATP to replicate the virus particles, which are released to infect other cells. Linking this to section 3.2.4, HIV replicates inside helper T cells.

How do you carry out required practical 2?

The aim is to prepare stained squashes of cells from plant root tips, use an optical microscope to identify the stages of mitosis, and calculate a mitotic index. Root tips are used because the meristem just behind the tip is where cells are dividing.

1. Cut the last few millimetres of a growing root tip (garlic or onion roots are commonly used).
2. Place it in warm dilute hydrochloric acid for a few minutes. The acid breaks down the material holding the cells together, so they can be spread into a single layer, and it stops cell division.
3. Rinse the tip in water, then add a stain that binds to chromosomes (for example toluidine blue).
4. Place the tip on a slide, add a cover slip and press down gently but firmly (the squash) to spread the cells into one layer so light passes through and chromosomes can be seen. Do not twist or slide the cover slip.
5. Focus at low power, then high power, and count cells in each stage in several fields of view.

Safety and accuracy points. Wear eye protection with the acid; handle the stain with gloves. Count only cells that are clearly visible, decide in advance how to classify cells on the edge of the field of view, and count several fields of view so that the sample is representative.

How do you calculate a mitotic index?

mitotic index = number of cells in mitosis / total number of cells observed

Cells “in mitosis” are those in prophase, metaphase, anaphase or telophase; cells in interphase are not counted as dividing.

Worked example 1. In a root tip squash, a student counts 200 cells: 166 in interphase, 18 in prophase, 7 in metaphase, 5 in anaphase and 4 in telophase.

• Cells in mitosis = 18 + 7 + 5 + 4 = 34
• Mitotic index = 34 / 200 = 0.17 (or 17%)

Worked example 2 (time in a stage). The proportion of cells in a stage is proportional to the time that stage takes. If one complete cell cycle in this tissue takes 24 hours, the time spent in mitosis is 0.17 x 24 = 4.08, so 4.1 hours to 2 significant figures. Using the same sample, metaphase lasts 7 / 200 x 24 = 0.84 hours, about 50 minutes.

A higher mitotic index means more cells are dividing, so the tissue is growing faster. Tissue near the very tip of a root gives a higher index than tissue further back.

How do you calculate the actual size of a cell?

The specification gives the formula (MS 1.8):

actual size = size of image / magnification

Worked example 3. A cell in a photomicrograph measures 24 mm across, and the magnification is x 400.

• Actual size = 24 mm / 400 = 0.06 mm
• 0.06 mm x 1000 = 60 micrometres

Always convert to the unit the question asks for: 1 mm = 1000 micrometres.

Frequently asked questions

Why must DNA replicate before mitosis? So that each chromosome consists of two identical chromatids. When the chromatids are separated in anaphase, each daughter cell receives a complete, identical set of genetic information.

What is the difference between a chromatid and a chromosome? After DNA replication, one chromosome is made of two sister chromatids joined at the centromere. Once the chromatids separate in anaphase, each is counted as a separate chromosome.

Why is hydrochloric acid used in the root tip squash? It separates the cells so they can be squashed into a single layer, letting light pass through so that individual chromosomes can be seen. It also stops the cells dividing.

Do prokaryotes do mitosis? No. They divide by binary fission: the circular DNA and plasmids replicate and the cytoplasm divides. There is no nucleus and no spindle.

Why are viruses not said to divide? They are not cells and are non-living. Their nucleic acid is replicated by the host cell, which assembles new virus particles.

Common mistakes

• Saying DNA replicates during prophase. It replicates in interphase.
• Describing anaphase as chromosomes, rather than sister chromatids, being pulled apart.
• Forgetting to name centromeres when describing spindle fibre attachment.
• Including interphase cells in the “in mitosis” count for the mitotic index.
• Writing that the daughter cells of binary fission each get the same number of plasmids.
• Leaving the actual size in millimetres when the question asks for micrometres.

Official syllabus

AQA AS and A-level Biology (7401/7402) specification, Version 1.5 (26 November 2021), section 3.2 Cells, 3.2.2 All cells arise from other cells, including required practical 2 and the mathematical requirements MS 0.3 and MS 1.8.