Antibiotic such as penicillin blocks
- A.cell wall formation in bacteria
- B.RNA synthesis in bacteria
- C.DNA synthesis in bacteria
- D.division in bacteria
Correct answer
Explanation
The correct answer is A, cell wall formation in bacteria. Penicillin stops bacteria from building their peptidoglycan cell wall, so the weakened cells burst and die.
Penicillin binds to and blocks the transpeptidase enzymes, also called penicillin-binding proteins, that cross-link the peptidoglycan chains of the wall. Growing bacteria then cannot withstand the water pressure inside them. Because human cells have no cell wall, penicillin harms bacteria while largely sparing the patient. It was discovered by Alexander Fleming in 1928 from the mould Penicillium notatum and became the first widely used antibiotic in the 1940s.
Option B is wrong because blocking bacterial RNA synthesis is how rifampicin, a tuberculosis drug, works. Option C is wrong because DNA synthesis is the target of quinolones such as ciprofloxacin, which inhibit the enzyme DNA gyrase. Option D is wrong because penicillin's action is on wall formation; cell division fails only as a result of a damaged wall, not because division itself is the target.
Exam tip: penicillin = cell wall; rifampicin = RNA; ciprofloxacin = DNA gyrase; tetracycline and streptomycin = protein synthesis.
Practice Questions
View allThe fact that the planets move around the Sun, not in circles but in ellipses, was first demonstrated by
- A.Galileo
- B.Martin Luther
- C.Johannes Kepler
- D.Copernicus
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In 1609 Kepler showed that the planets move around the Sun in elliptical orbits, with the Sun at one focus of the ellipse.
Read the full explanation →Which one of the following statements is correct?
- A.Urea is produced in liver.
- B.Urea is produced in blood.
- C.Urea is produced from digestion of starch.
- D.Urea is produced in lung and kidney.
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Liver cells turn the toxic ammonia released from the breakdown of amino acids into urea through the ornithine cycle, also called the urea cycle.
Read the full explanation →Which one of the following is the correct sequence of passage of light in a compound microscope?
- A.Condenser-Objective lens-Eyepiece-Body tube
- B.Objective lens-Condenser-Body tube-Eyepiece
- C.Condenser-Objective lens-Body tube-Eyepiece
- D.Eyepiece-Objective lens-Body tube-Mirror
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Light passes through the condenser to the specimen, is picked up by the objective lens, travels up the body tube and reaches the eye through the eyepiece.
Read the full explanation →Which one of the following statements regarding haemoglobin is correct?
- A.Haemoglobin present in RBC can carry only oxygen but not carbon dioxide.
- B.Haemoglobin of RBC can carry both oxygen and carbon dioxide.
- C.Haemoglobin of RBC can carry only carbon dioxide.
- D.Haemoglobin is only used for blood clotting and not for carrying gases.
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It binds oxygen at its iron-containing haem groups and carries some carbon dioxide on its protein chains. Each haemoglobin molecule has four polypeptide chains and four haem groups, and each haem iron can hold one oxygen molecule, forming oxyhaemoglobin in the lungs.
Read the full explanation →Which one of the following statements regarding histone proteins is correct?
- A.Histones are proteins that are present in mitochondrial membrane.
- B.Histones are proteins that are present in nucleus in association with DNA.
- C.Histones are proteins associated with lipids in the cytosol.
- D.Histones are proteins associated with carbohydrates in the cytosol.
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They are the proteins around which DNA winds to form chromatin in the nucleus of eukaryotic cells. Histones are small basic proteins, rich in the positively charged amino acids lysine and arginine, which attract the negatively charged phosphate groups of DNA.
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