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2.6.3Cell Structure

HIV — AQA A Level Biology 7402 Revision Notes & Topical Questions

5 min read 8 parts Lesson 39 of 47 1 quiz

📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.6.3 HIV — AQA A Level Biology 7402 notes: Describe the structure of HIV, how it replicates in helper T cells, and how it leads to AIDS, including why antibiotics are ineffective.HIV (human immunodeficiency virus) is a retrovirus.

  • lipid envelope
  • glycoproteins
  • two molecules of single-stranded RNA
  • reverse transcriptase
  • T helper cells
  • opportunistic infections

Section 1 of 8The structure of HIV

HIV (human immunodeficiency virus) is a retrovirus. It is roughly spherical and has:

The structure of HIV
Lipid envelope
an outer lipid envelope taken from the host cell membrane when the virus buds out;
Glycoproteins
attachment proteins (glycoproteins) projecting from the envelope, which bind to receptors on host cells;
Capsid
a protein capsid inside, enclosing the genetic material;
two molecules of single-stranded RNA
two molecules of single-stranded RNA as the genetic material;
Reverse transcriptase
the enzyme reverse transcriptase, which makes DNA from RNA, and other enzymes such as integrase and protease.

Viruses are not cells, have no cytoplasm or ribosomes or metabolism, and can only replicate inside a host cell.

Section 2 of 8Replication in the host

HIV infects T helper cells (and some macrophages), because its attachment protein is complementary to the CD4 receptor on their surface. The steps are:

1
The attachment protein binds to the CD4 receptor on the T helper cell.
2
The viral envelope fuses with the cell-surface membrane and the capsid enters the cytoplasm.
3
The capsid uncoats and releases the RNA and enzymes.
4
Reverse transcriptase makes a single strand of complementary DNA (cDNA) from the viral RNA, then a double-stranded DNA copy.
5
The viral DNA is inserted into the host's DNA (it can lie dormant for years as a provirus).
6
When activated, the host's enzymes transcribe the viral genes into mRNA, and host ribosomes translate it into viral proteins.
7
New virus particles assemble and leave the cell by budding, taking part of the membrane as an envelope.

Section 3 of 8From HIV infection to AIDS

After infection there may be a brief flu-like illness, then a long symptom-free period, with the virus replicating slowly.

Over time the number of T helper cells falls because the virus destroys them or they are killed by cytotoxic T cells.
Without T helper cells, B cells and cytotoxic T cells are not properly activated, so the body cannot make a sufficient response to infections.
AIDS
When the T helper cell count is very low, the person has AIDS (acquired immunodeficiency syndrome) and suffers from opportunistic infections such as pneumonia, tuberculosis and certain cancers,
  • which a healthy immune system would normally defeat.
AIDS itself does not kill the person: the secondary infections do.
StageFeatures
Acute infectionHigh viral load, flu-like symptoms
Latent stageFew symptoms; T helper cell count slowly declines
AIDSVery low T helper count; opportunistic infections and cancers

Section 4 of 8Transmission, treatment and prevention

HIV is passed on by exchange of body fluids: unprotected sex, sharing needles, from mother to baby during pregnancy, birth or breastfeeding, and in contaminated blood products. It is not spread by casual contact. Prevention uses condoms, needle exchange programmes, screening of blood, and treatment of pregnant mothers.

Antibiotics are ineffective against viruses because they act on bacterial structures and processes, such as cell walls and bacterial enzymes or ribosomes, which viruses do not have.

Antiretroviral drugs
Treatment uses antiretroviral drugs that target viral processes: reverse transcriptase inhibitors, protease inhibitors and integrase inhibitors.
A combination of drugs reduces the viral load to very low levels and prevents the resistance that develops when single drugs are used,
  • because HIV mutates quickly.
There is no cure or fully effective vaccine, largely because of the high mutation rate and antigenic variation and because the virus hides in cells.

Section 5 of 8HIV in context

Compare HIV with other virusesa typical virus such as influenza has RNA but does not have reverse transcriptase, and does not integrate with the host's DNA.

1
The proviral DNA of HIV is hidden in the genome of the host cell,
  • so the immune system and drugs cannot easily reach it, and this makes a cure difficult.
2
The rate of mutation is high because reverse transcriptase does not proofread,
  • so many variants arise in a single patient and drug-resistant strains are selected,
  • which is an example of natural selection.
3
This is why combination therapy is used.
4
Globally, HIV is most common in sub-Saharan Africa, and transmission is reduced where education, testing and treatment are available.
5
Treatment with antiretrovirals can reduce the viral load to undetectable levels so that a person can live a near-normal life and the risk of passing on the virus falls dramatically.

Section 6 of 8Exam-style questions

  1. Describe how HIV replicates inside a T helper cell. [5]5 marks

  2. Explain why the symptoms of AIDS are caused by other infections, not by HIV directly. [3]3 marks

  3. Explain why antibiotics cannot be used to treat HIV. [2]2 marks

Section 7 of 8Key terms

TermMeaning
HIVHuman immunodeficiency virus, a retrovirus that infects T helper cells
AIDSSyndrome caused by low T helper count leading to opportunistic infections
RetrovirusVirus with RNA that is copied to DNA by reverse transcriptase
Reverse transcriptaseEnzyme that makes DNA from an RNA template
CapsidProtein coat surrounding the viral genetic material
Opportunistic infectionInfection that takes advantage of a weakened immune system
Antiretroviral drugDrug that inhibits viral enzymes to slow HIV replication

Section 8 of 8Exam technique

1
Saying that HIV is the same as AIDS - HIV is the virus, AIDS the syndrome.
2
Stating that HIV has DNA as genetic material - it has RNA and makes DNA using reverse transcriptase.
3
Writing that HIV kills the patient directly.
4
Stating that antibiotics kill viruses.
5
Forgetting that HIV specifically infects T helper cells because of the complementary receptor (CD4).

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