OxfordAQA International AS and A-level Biology: HIV and plant virus transmission by aphids (9610)

This study guide teaches HIV and the spread of plant viruses by aphids from scratch. It follows the OxfordAQA International AS and A-level Biology (9610) specification, Version 5.1, and covers sections 3.2.4 (HIV as an example of a human disease caused by a virus) and 3.2.9 (the role of aphids in spreading plant viruses), plus the Unit 2 biological principle that viruses are acellular and non-living. The specification lists Units 1 and 2 under International AS and A-level, and Units 3 and 4 as International A-level only, so this material is in the first group.

Use it with the revision notes and the practice questions. Related pages: the causes of disease, the course hub and the printable checklist. Not sure where you stand? Try the free diagnostics.

What this guide covers

Section	What you must be able to do
Biological principle	State that viruses are acellular and non-living, and explain what that means
3.2.4.1	Describe HIV: RNA, capsid, reverse transcriptase, integrase, protease, envelope with glycoproteins
3.2.4.2	Describe the replication cycle in a helper T-cell; explain the effects of HIV drugs from information given
3.2.9.1	Describe plant virus diseases, their symptoms and why vectors are needed
3.2.9.2	Describe how aphids feed on phloem sap and how winged females spread colonies

Viruses are acellular and non-living

A virus is acellular. It has no cytoplasm, no ribosomes, no cell surface membrane of its own (an envelope, where present, is borrowed from a host) and no metabolism. It cannot respire, grow or divide. Outside a host it is a particle that does nothing. Inside a host cell, the host’s ribosomes, enzymes, amino acids and ATP are used to make new virus particles. That is why viruses are described as acellular and non-living.

Two consequences matter in exams. First, antibiotics that target bacterial cell walls or bacterial ribosomes do not work on viruses, because viruses have neither. Second, drugs against viruses must target a step that the virus needs, usually a viral enzyme, without harming the host cell.

3.2.4.1 The structure of HIV

HIV is the human immunodeficiency virus. Working from the inside out:

Component	What it is	Role
Genetic material	RNA (the particle carries two copies)	Carries the viral genes
Reverse transcriptase	Enzyme inside the capsid	Makes DNA from the RNA template
Integrase	Enzyme inside the capsid	Inserts viral DNA into a host chromosome
Protease	Enzyme inside the capsid	Cuts long viral proteins into working lengths
Capsid	Protein coat	Encloses the RNA and the three enzymes
Envelope	Lipid layer taken from the host cell surface membrane	Outer layer of the particle
Glycoproteins	Proteins with attached carbohydrate, embedded in the envelope	Bind to receptors on the host cell

The envelope originates from the host cell plasma membrane. It is not made by the virus. The glycoproteins are coded for by viral genes and are inserted into the membrane before the particle leaves.

Describe it as a sketch in words: a roughly spherical particle, an outer envelope studded with glycoproteins, a capsid inside it, and within the capsid the RNA and three enzymes.

Worked example 1: magnification

An electron micrograph shows one HIV particle with a diameter of 36 mm. The actual diameter is 120 nm. Calculate the magnification.

1. Convert to the same unit: 36 mm = 36 000 000 nm.
2. Magnification = image size / actual size = 36 000 000 / 120.
3. Magnification = ×300 000.

Check by working back: 120 nm × 300 000 = 36 000 000 nm = 36 mm.

3.2.4.2 The replication cycle of HIV

HIV infects CD4 helper T-cells, the cells that initiate the body’s response to infections. The cycle runs in this order.

1. Binding. Glycoproteins on the HIV envelope bind to CD4 receptors on the surface of the helper T-cell.
2. Release of contents. The capsid contents, including the RNA and the viral enzymes, are released into the cell. (The envelope fuses with the cell surface membrane to allow this.)
3. Reverse transcription. Reverse transcriptase uses the viral RNA as a template to make DNA. The specification describes the RNA genome as transcribed into double-strand DNA.
4. Integration. Integrase inserts the double-stranded viral DNA into a host cell chromosome. The cell now carries the viral genes and copies them as it divides.
5. Transcription and translation. The viral DNA may be transcribed, producing new viral RNA. Viral proteins are made using the host’s ribosomes. Protease cuts long protein chains into the separate proteins needed.
6. Assembly and release. New RNA, proteins and enzymes are packaged into capsids and released from the cell as new virus particles, which take a piece of host membrane as their envelope. This completes the replication cycle.

The word “may” matters: integrated viral DNA does not have to be transcribed straight away, so infected cells can carry the virus without producing it. As more helper T-cells are lost, the immune response is weakened. The patient becomes open to infections that a healthy immune system keeps in check. That is the stage called AIDS.

Explaining the effects of HIV drugs

The specification says you must use your knowledge of the structure and replication cycle to explain drug effects when given information. The question supplies the information, so the method matters more than any drug name. Use this method:

1. Read what the information says the drug blocks, or what is seen or missing in treated cells.
2. Match it to the step in the cycle and the enzyme or protein involved.
3. State what is then missing downstream, and the effect on new virus particles.

If the drug stops…	Then…
Binding to CD4 or release of contents	The virus cannot get its contents into the cell, so no DNA is made
Reverse transcriptase	No viral DNA is made from RNA, so nothing can be integrated
Integrase	Viral DNA is made but is not inserted into a chromosome, so no viral RNA is transcribed from it
Protease	Viral proteins are made but not cut to size, so particles that assemble are not infectious

Worked example 2: reading drug information

Cells are infected with HIV and treated with drug X. Viral RNA and viral proteins are found in the cell. Viral particles are released, but when tested they cannot infect new cells. The long protein chains inside them have not been cut. Suggest how drug X works.

1. RNA and proteins are made, so binding, entry, reverse transcription, integration and transcription must have worked.
2. The proteins are not cut into working lengths, and cutting is the job of protease.
3. Drug X therefore inhibits protease, so particles assemble but are not infectious.

Worked example 3: interpreting treatment data

A patient starts treatment. CD4 helper T-cell count rises from 180 to 410 cells per mm³ over 12 months. Viral load falls from 250 000 to 8 000 copies per mm³ of blood by month 3.

• Percentage change in CD4 count = (410 − 180) / 180 × 100 = +127.8% (to 1 decimal place).
• Fall in viral load = 250 000 / 8 000 = 31.25-fold.
• Explanation: less virus means fewer helper T-cells are infected and destroyed, so the count recovers.

Viral load values are often compared as powers of ten. log₁₀ of 250 000 is 5.398 and of 8 000 is 3.903, a fall of about 1.49 units, which is the same as a 31-fold fall.

3.2.9.1 Plant virus diseases

Many important plant diseases are caused by viruses. They reduce the amount and quality of crops in all parts of the world. Symptoms depend on the disease, but typically include:

• yellowing of leaves
• distortion of leaves, such as curling or crinkling
• abnormalities in other parts of the plant, such as stunted growth or poorly formed fruit.

Yellow leaves have less chlorophyll, so the leaf absorbs less light and photosynthesises more slowly. Less organic material goes into growth, so yield falls.

A plant cannot move, so a virus cannot be passed on by contact between individuals in the way it can between animals. Transmission usually involves a vector, a living carrier that moves the pathogen between hosts. Aphids are the standard example.

3.2.9.2 Aphids as feeders on phloem sap

Aphids are small insects with sucking mouthparts called stylets. Needle-like stylets are pushed into the plant and into a phloem vessel. Phloem sap is under high pressure. Once a vessel is punctured, the sap is forced into the aphid’s gut. The aphid does not need to pump hard to draw it in.

If an aphid feeds on an infected plant, virus particles in the sap can be taken up. When it moves to a healthy plant and feeds on its phloem, the virus can be passed into that plant. Feeding on the sap of a plant is therefore the route by which the virus enters the next plant’s transport system.

Some species produce winged females which migrate to start colonies on a new host plant. Wingless aphids stay on one plant or move short distances. Winged females can travel from field to field, carrying any virus they hold. A single founder can then start a new colony.

Worked example 4: colony growth and spread

A new colony starts with 8 females. Assume the population doubles every 4 days and none die.

• 20 days is 20 / 4 = 5 doublings.
• Population = 8 × 2⁵ = 256.

Suction-trap data over six weeks from a field of 400 monitored plants:

Week	1	2	3	4	5	6
Winged aphids caught	4	15	62	110	85	30
Total infected plants	2	3	9	24	52	78

• Infected plants at week 6 as a percentage = 78 / 400 × 100 = 19.5%.
• New infections each week: 1, 6, 15, 28, 26 (from week 2). These peak one week after the trap peak.
• The two sets of data rise together, but that is a correlation. It does not prove that the trapped aphids carried the virus. More evidence is needed, such as testing the aphids for the virus.

Common errors

• Describing a virus as a tiny cell, or saying that it respires or grows.
• Saying the HIV envelope is made by the virus. It comes from the host plasma membrane.
• Writing that HIV “attacks the immune system” without naming helper T-cells and the CD4 receptor.
• Saying integrase makes DNA, or that reverse transcriptase inserts it.
• Explaining a drug by naming it rather than linking the step blocked to what is missing.
• Saying aphids suck sap using muscle power. The pressure in the phloem forces the sap in.
• Saying aphids bite leaves. The stylets reach the phloem.

Next steps

Test yourself with the practice questions and condense with the revision notes.

Official syllabus

OxfordAQA International AS and A-level Biology (9610), Version 5.1, published by OxfordAQA: sections 3.2.4 and 3.2.9 of Unit 2, and the Unit 2 biological principle on viruses.