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2.3.8Cell Structure

Viral Particle Replication — AQA A Level Biology 7402 Revision Notes & Topical Questions

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📚 AQA A Level Biology 7402 Hub / Topical Revision
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2.3.8 Viral Particle Replication — AQA A Level Biology 7402 notes: Describe the structure of a virus and explain how viruses replicate inside host cells, with HIV as an example.Viruses are acellular, non-living infectious particles.A virus has genetic material (DNA or RNA, single…

  • lipid envelope
  • obligate intracellular parasites
  • Attachment
  • Replication and synthesis
  • retrovirus
  • reverse transcriptase

Section 1 of 8Viruses are not cells

Viruses are acellular, non-living infectious particles.

1Capsid
A virus has genetic material (DNA or RNA, single or double stranded) enclosed in a protein coat called a capsid, and some also have an outer lipid envelope taken from the host membrane with attachment proteins.
2
They have no cytoplasm, organelles or ribosomes, and cannot carry out metabolism or reproduce on their own.
3Obligate intracellular parasites
They are obligate intracellular parasites: they need a host cell's ribosomes, enzymes, amino acids, nucleotides and ATP to make new virus particles.
4
Because they do not divide, they do not replicate by mitosis or binary fission.

Section 2 of 8General steps of viral replication

1Attachment
Attachment attachment proteins on the virus bind to specific complementary receptor proteins on the host cell surface membrane. This specificity decides which cell types and species a virus can infect.
2Entry
Entry the virus enters by fusing its envelope with the membrane or by endocytosis, or injects its nucleic acid (bacteriophages).
3Uncoating
Uncoating the capsid breaks down, releasing the viral nucleic acid into the host cytoplasm or nucleus.
4Replication and synthesis
Replication and synthesis the host cell's enzymes and ribosomes replicate the viral genetic material and make viral proteins (capsid proteins and enzymes) using the host's amino acids and ATP.
5Assembly
Assembly new capsids form and are packed with nucleic acid.
6Release
Release new virus particles leave by lysis (host cell bursts) or by budding from the surface membrane, picking up an envelope.

Section 3 of 8HIV as a retrovirus

The human immunodeficiency virus (HIV) is a retrovirus. Its structure includes: two single strands of RNA, the enzyme reverse transcriptase, a capsid, a lipid envelope derived from the host membrane and attachment proteins (gp120) that bind to CD4 receptors on helper T cells. The replication cycle is:

Attachment proteins bind to CD4 receptors on a helper T cell; the envelope fuses with the cell membrane and the capsid enters the cell.
The capsid is removed (uncoating), releasing RNA and reverse transcriptase.
Reverse transcriptase makes a single-stranded DNA copy of the RNA, and then a double-stranded DNA (cDNA).
Integrase
The viral DNA enters the nucleus and is inserted into the host genome by integrase, forming a provirus, which may be latent for years.
The host's RNA polymerase transcribes the viral DNA into mRNA and genomic RNA; host ribosomes translate the mRNA into viral proteins.
New virus particles assemble and bud from the surface, taking an envelope with them.

Helper T cells are destroyed in large numbers, so the immune system is weakened and the patient develops AIDS, with increased susceptibility to opportunistic infections.

Section 4 of 8Control of viral infections

Antibiotics have no effect on viruses because they target bacterial structures (cell wall, 70S ribosomes) that viruses lack.

Control of viral infections
Antiviral drugs target viral processes: for HIV, reverse transcriptase inhibitors such as AZT, integrase inhibitors and protease inhibitors are used, usually in combination to reduce the chance of resistance developing.
Vaccination prepares the immune system for specific antigens, but HIV mutates rapidly, so its antigens vary and a vaccine is difficult to develop.
Other measures include safe sex, screening blood, and not sharing needles.

Section 5 of 8Links

Viral replication depends on nucleic acid synthesis (1.5), transcription and translation, host cell receptors (membranes) and the immune system.

Links
A virus does not appear in the cell theory because it is not a cell; it also shows how genetic information can pass from RNA to DNA, which is an exception to the usual direction of information flow.
In exam questions, link every step to the host cell's role: the host provides the machinery, the virus provides the instructions.

Bacteriophages show another life cycle.

1Lytic cycle
In the lytic cycle the phage injects its DNA into a bacterium, takes over the host machinery to make new phages and bursts the cell.
2Lysogenic cycle
In the lysogenic cycle the phage DNA is integrated into the bacterial chromosome and copied each time the bacterium divides, until an environmental trigger starts the lytic cycle.
3
This resembles the latent proviral stage of HIV.
4
The use of phages to treat bacterial infections (phage therapy) is attracting interest because of antibiotic resistance.

Also note that viral replication is error-prone, especially for RNA viruses, because reverse transcriptase and RNA polymerase have no proofreading ability.

Links
High mutation rates lead to rapid antigenic variation, as for influenza, so new vaccines are needed each year.
Compare virus replication with the cell cycle: viruses make many copies of their genome in the host and these are assembled, whereas cells copy their DNA once before each division.
This contrast, and the dependence on the host, are the central points for the exam.

Section 6 of 8Exam-style questions

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

  2. Explain why antibiotics do not kill viruses. [2]2 marks

  3. Suggest why HIV infects only certain cells in the body. [2]2 marks

Section 7 of 8Key terms

TermMeaning
VirusA non-cellular infectious particle that replicates only inside host cells.
CapsidThe protein coat of a virus.
RetrovirusA virus with RNA that uses reverse transcriptase to make DNA.
Reverse transcriptaseEnzyme that makes DNA using RNA as a template.
Attachment proteinViral protein that binds to a host receptor.
ProvirusViral DNA integrated into the host genome.
Obligate parasiteAn organism or particle that can only reproduce inside a host cell.
LysisBursting of a cell to release its contents.

Section 8 of 8Exam technique

1
Saying viruses are made of cells or have a nucleus.
2
Saying viruses reproduce by mitosis or binary fission.
3
Saying antibiotics can be used to treat viral infections.
4
Forgetting that reverse transcriptase makes DNA from RNA.
5
Saying that the virus uses its own ribosomes.

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