NDA (I) 2018 General Ability Test · Question 69 of 150

Correct — C, single stranded RNA. AIDS is caused by the human immunodeficiency virus, and HIV is a retrovirus. A retrovirus carries its genes as single-stranded RNA and brings with it an enzyme called reverse transcriptase, which copies that RNA into DNA once the virus is inside a host cell. The DNA copy is then inserted into the host's own chromosome, where it can sit for years before directing the manufacture of new virus particles. Reverse transcription is the defining feature of the group, and it works only because the genome starts as RNA.

• (a)single stranded circular DNA — This describes the genome of certain bacteriophages and small plant viruses, not HIV. If HIV already carried DNA it would have no need of reverse transcriptase, and the drugs that target that enzyme would not work.
• (b)double stranded DNA — Double-stranded DNA is what herpes viruses, the smallpox virus and the hepatitis B virus carry. Naming it here would put HIV in the wrong class of viruses altogether.
• (d)double stranded RNA — Double-stranded RNA is the genome of the rotaviruses, a common cause of severe diarrhoea in young children. It is RNA, so the option is half right, but HIV's RNA is single-stranded.

Viruses are classified partly by the kind of nucleic acid they carry — DNA or RNA, single-stranded or double-stranded. HIV attacks the helper T cells of the immune system, which are the cells that coordinate the body's defence; as their numbers fall the patient becomes vulnerable to infections that a healthy immune system would clear, and that collapse of immunity is acquired immunodeficiency syndrome. HIV was identified in the early 1980s, and its status as a retrovirus is what made reverse transcriptase the first great drug target against it.

The item is a straight recall question, but it becomes far easier to hold if the fact is attached to its consequence rather than memorised on its own. HIV is an RNA virus, therefore it needs reverse transcriptase, therefore the classic drugs against it are reverse transcriptase inhibitors. The same reasoning explains why HIV mutates so quickly — reverse transcriptase copies without the proofreading that DNA polymerases use, so errors accumulate, which is why treatment uses several drugs at once and why a vaccine has been so hard to make. As of the 2018 exam there was no cure, and that remains the position today, though antiretroviral therapy suppresses the virus well enough for people with HIV to live long and healthy lives.

• HIV is a retrovirus and carries single-stranded RNA as its genetic material.
• Reverse transcriptase copies that RNA into DNA, which is then integrated into the host cell's chromosome.
• HIV infects helper T cells, the coordinating cells of the immune system.
• AIDS is the condition that follows when the loss of those cells leaves the body open to other infections.
• Rotaviruses carry double-stranded RNA; herpes and hepatitis B viruses carry double-stranded DNA.

• Stopping at 'RNA' and missing whether the item wants single- or double-stranded.
• Assuming every virus carries DNA because DNA is the usual genetic material of living things.
• Confusing HIV, the virus, with AIDS, the condition it eventually produces.

NDA asks for the nucleic acid of a named virus, or for a general statement about viruses that is true or not true.

• practice — not a real PYQ

  The enzyme that allows a retrovirus to make DNA from its RNA genome is

  • (a)DNA ligase
  • (b)reverse transcriptase
  • (c)RNA polymerase
  • (d)restriction endonuclease

  Answer(b) reverse transcriptase — it reverses the usual flow from DNA to RNA, which is how the group gets its name.

• practice — not a real PYQ

  HIV mainly attacks which cells of the human body?

  • (a)Red blood cells
  • (b)Helper T cells
  • (c)Platelets
  • (d)Nerve cells

  Answer(b) Helper T cells — their loss disables the coordination of the immune response.