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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance1 Mark
MCQ
For question, two statements are given $-$ one labelled Assertion and the other labelled Reason. Select the correct answer to these questions from the codes $(a), (b), (c)$ and $(d)$ as given below.
Assertion : $\text{UAA, UAG}$ and $\text{UGA}$ terminate protein synthesis.
Reason : They are not recognised by $\text{tRNA}$.
  • ✓
    Both assertion and reason are true, and reason is the correct explanation of assertion.
  • B
    Both assertion and reason are true, but reason is not the correct explanation of assertion.
  • C
    Assertion is true, but reason is false.
  • D
    Both assertion and reason are false
✓

Answer

Correct option: A.
Both assertion and reason are true, and reason is the correct explanation of assertion.
Synthesis of polypeptide terminates when a nonsense codon of $\text{mRNA}$ reaches the $A-$ site.
There are three nonsense codons $- \text{UAA, UAG}$ and $\text{UGA}$.
These codons are not recognised by any of the $\text{tRNAs}$.
Therefore, no more aminoacyl $\text{tRNA}$ reaches the $A-$ site. The $P-$ site $\text{tRNA}$ is hydrolysed and the completed polypeptide is released in the presence of release factor. Thus termination occurs.

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