The haemolytic system of the complement fixation reaction consists of
Answer: D
Correct answer: 3 per cent suspension of sheep erythrocytes and hemolytic serum
The indicator system has two components: a 3% suspension of sheep red cells and the haemolytic serum (amboceptor) raised against them. Together they will lyse only if free complement remains - which is what makes them an indicator.
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Defective viruses are
Answer: A
Correct answer: Viruses that are genetically deficient and so incapable of producing infectious daughter virions
A defective virus is one missing part of its genome, so it can enter a cell but cannot complete replication on its own. It multiplies only in the presence of a helper virus that supplies what it lacks - hepatitis D, which needs hepatitis B, is the medical example.
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A toxoid contains
Answer: D
Correct answer: inactivated toxin
A toxoid is an exotoxin treated with formalin so that it loses its toxicity but keeps its antigenicity. The recipient makes antitoxin, which neutralises the real toxin on any later exposure. Diphtheria and tetanus vaccines are toxoids, which is why they protect against the toxin rather than against infection itself.
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The fourth phase of the viral multiplication cycle is
Answer: B
Correct answer: biosynthesis
The order is adsorption, penetration, uncoating, biosynthesis, maturation and release. The fourth step is where the viral genome is expressed and copied, using the host’s ribosomes and precursors.
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Which of these is a cocci occurring in single or pairs?
Answer: A
Correct answer: Diplococci
Diplococci divide in one plane and stay attached in pairs - Neisseria and Streptococcus pneumoniae are the familiar examples. Streptococci form chains, tetracocci groups of four, and staphylococci irregular clusters; the pattern follows from the planes of division and whether the cells separate.
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The first phase of the viral multiplication cycle is
Answer: D
Correct answer: adsorption or attachment
The cycle begins with adsorption: a protein on the virus binds a specific receptor on the host cell. Nothing can follow until that happens, and which receptor the virus recognises is what decides its host range and the tissues it infects.
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Prions are
Answer: C
Correct answer: Infectious protein particles, lacking detectable nucleic acid
A prion is an infectious protein with no nucleic acid whatever. It is a misfolded form of a normal cellular protein, and it propagates by inducing the normal form to adopt the same misfolding - the mechanism behind scrapie, BSE and Creutzfeldt-Jakob disease.
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Diagnostic serum contains
Answer: D
Correct answer: known antibody
A diagnostic serum is a reagent of known antibody, prepared by immunising an animal against a known organism. It is used to identify an unknown antigen - the organism isolated from the patient.
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Transcription is the process by which
Answer: B
Correct answer: genetic information carried in the bacterial DNA is transferred to m-RNA
Transcription is DNA to RNA: RNA polymerase reads one strand of the gene and builds a complementary messenger RNA. Making protein from that message is translation; copying DNA to DNA is replication; and RNA to DNA is reverse transcription.
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The interaction between somatic antigens and specific antibodies is
Answer: A
Correct answer: slow and fine granular sediment forms in 18-20 hrs. agglutinate do not break upon shaking
Somatic (O) antigens are lipopolysaccharide, lying on the cell surface itself, so the cells are drawn tightly together: agglutination is slow, the deposit is fine and granular, and it does not redisperse when shaken. Flagellar (H) agglutination is the opposite - rapid, loose and flocculent, and easily broken up.
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