What do the term dimorphic mean?
Answer: C
Some fungi are dimorphic, that is, they exist in two forms. Some pathogenic fungi have a unicellular and yeastlike form in their host, but when growing saprophytically in soil or in laboratory medium they have a filamentous mold form.
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Which of the following does not contain protein?
Answer: C
Correct answer: Lipoteichoic acid
Lipoteichoic acid is a polymer of glycerol phosphate with a lipid anchor - no protein at all. Pili and flagella are built from protein subunits (pilin and flagellin), and porins are proteins by definition.
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The last step in synthesis of peptidoglycan is
Answer: B
Correct answer: attaching two amino acids to form a cross-link
The final step is transpeptidation: the peptide side chains of neighbouring glycan strands are joined, turning the layers into one continuous mesh that can resist turgor pressure. It happens outside the membrane, and it is the step penicillin blocks - which is why the drug kills only growing cells.
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The covalent bond which links the cell walls of gram-positive bacteria containing two modified sugars - N - acetylmuramic acid (NAM) and N-acetyl glucosamine (NAG) is
Answer: B
Correct answer: 1,4-glycosidic bond
The glycan backbone is an alternating chain of NAM and NAG joined by β-1,4-glycosidic bonds - the same linkage as in cellulose and chitin. It is precisely this bond that lysozyme hydrolyses, which is why tears and saliva can lyse many bacteria.
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Bayer’s junctions are sites which help in joining which of the following?
Answer: A
Correct answer: cytoplasmic membrane and outer membrane
Bayer’s junctions are places where the cytoplasmic membrane comes into contact with the outer membrane, passing through the peptidoglycan. They are thought to be the routes by which lipopolysaccharide and other components made inside are delivered to the outer membrane, and the points at which some phages inject their DNA.
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Name the component of flagellum.
Answer: D
Correct answer: All of these
A bacterial flagellum has three parts: the filament, the long helical propeller of flagellin; the hook, a short curved coupling; and the basal body, the set of rings anchoring the motor in the membrane and wall.
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The cocci which form chains are
Answer: A
Correct answer: Streptococci
Streptococci divide repeatedly in one plane and remain attached, so the cells form chains. Dividing in one plane but separating after each division gives diplococci; dividing in several planes gives clusters.
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Which of these bacteria is resistant to penicillin as it lacks a cell wall?
Answer: C
Correct answer: Mycoplasmas
Penicillin works by blocking the cross-linking of peptidoglycan, so it can only kill an organism that is building a cell wall. Mycoplasmas have no cell wall at all - only a sterol-strengthened membrane - so there is nothing for the drug to act on. Spirochaetes, cyanobacteria and bdellovibrios all have walls.
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Peptidoglycan is made up of __________
Answer: D
Correct answer: N-acetylglucosamine, N-acetylmuramic acid and amino acids
Peptidoglycan has two parts. The backbone is an alternating chain of NAG and NAM, and short peptide chains of amino acids hang from each NAM and are cross-linked to those on neighbouring strands. Without the peptides there would be no cross-linking and no rigid mesh - which is exactly what penicillin prevents.
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In eukaryotic cells, ribosomes are
Answer: C
Correct answer: 80S
Eukaryotic cytoplasmic ribosomes are 80S, made of 60S and 40S subunits; prokaryotic ones are 70S, of 50S and 30S. The S is a Svedberg sedimentation coefficient, which is why the subunit values do not add up arithmetically. The difference is what lets antibiotics such as streptomycin and tetracycline attack bacterial ribosomes without harming ours - though mitochondria, having 70S ribosomes of their own, are not entirely spared.
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