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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance1 Mark
MCQ
What is not true for genetic code ?
  • A
    It is unambiguous.
  • ✓
    A codon in $\text{mRNA}$ is read in a non $-$ contiguous fashion.
  • C
    It is nearly universal.
  • D
    It is degenerate.
✓

Answer

Correct option: B.
A codon in $\text{mRNA}$ is read in a non $-$ contiguous fashion.
The genetic code is a triplet code called a codon. The information in $\text{DNA}$ must reside in the sequence of four nucleotides that constitute the $\text{DNA: A, T, G}$ and $C$.
Certain codons contain start and stop signals to initiate and terminate translation.
The initiation codon is usually $\text{AUG},$ which specifies methionine. In few $\text{mRNA, GUG}$ and $\text{UUG}$ also act as initiation codon. Out of $64$ codons three do not code for any amino acid and these are referred to as stop or termination codons $(\text{UAA, UAG}$ and $\text{UGA)}.$ There are some properties of genetic code and they are as follows :
Genetic code is unambiguous, i.e., each triplet specifies only a single amino acid.
No internal punctuation is used in the code.
Thus, the code is said to be commaless. Thus once the translation of $\text{mRNA}$ begins, the code is read in a non $-$ contiguous fashion i.e one after the other with no breaks between them.
The code is degenerate meaning that a given amino acid can be coded by more than one triplet codon. This is the case with $18$ out of $21$ amino acids.
These different codons that code for a single amino acid are said to be synonomous.
For example, $\text{UUC}$ and $\text{UUU}$ are synonomous codons for phenylalanine.
The code is non overlapping. After translation commences, any single ribonucleotide at a specific location within the $\text{mRNA}$ is part of only one triplet.
The genetic code is nearly universal meaning thereby that same code is used throughout all life forms. Exceptions exist though.
For example, $\text{UAG}$ is a stop codon in some species while in others it may code for pyrrolysine.

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