Human insulin molecule is composed of one α-chain having 21 amino acids and one β-chain having 30 amino acids. How many functional insulin genes occur in adult humans?
- A.One
- B.Two
- C.Three
- D.Four
Correct answer
Explanation
The correct answer is A, One. Humans have a single functional insulin gene, INS, on the short arm of chromosome 11, and both chains of insulin come from it.
The two chains are not made by two separate genes. The INS gene codes for one long precursor, preproinsulin. After the signal piece is removed it becomes proinsulin, a single chain in which the A and B chains are joined by a linking C-peptide. In the beta cells of the pancreas the C-peptide is cut out, leaving the A chain of 21 amino acids and the B chain of 30 amino acids held together by disulphide bridges. This is why, in making recombinant insulin, scientists prepared separate DNA sequences for the two chains.
B is wrong: two insulin genes are found in rodents such as mice and rats, not in humans.
C is wrong: there is no third insulin gene; the C-peptide is part of the same gene product.
D is wrong: humans do not carry four insulin genes.
Exam tip: one INS gene on chromosome 11 → preproinsulin → proinsulin → insulin (A chain 21, B chain 30 amino acids) plus C-peptide.
Practice Questions
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- A.haploid zygote
- B.diploid zygote
- C.non-motile male gametes
- D.motile male gametes
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- A.10⁻⁷ cm
- B.10⁻⁶ cm
- C.10⁻⁴ cm
- D.10⁻³ cm
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- A.CₙH₂ₙ₊₂
- B.CₙH₂ₙ₋₂
- C.CₙH₂ₙ₊₁
- D.CₙH₂ₙ₋₁
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- A.Cholera
- B.Malaria
- C.Smallpox
- D.Elephantiasis
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- A.N-m
- B.N/m
- C.N-m²
- D.N/m²
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The spring constant is force divided by extension, so its SI unit is newton per metre. Hooke's law says that, within the elastic limit, the restoring force of a spring is proportional to how far it is stretched or compressed: F = −kx.
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