Q.Read carefully the sequence of codons in the mRNA unit and answer the questions.
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🔒 Start your 14-day free trial to unlock the full solution →Concept understanding — Properties of the Genetic Code
Properties of the Genetic Code
The genetic code is the set of rules by which the nucleotide sequence in mRNA is
translated into the amino-acid sequence of a protein — in effect, a dictionary that
tells the ribosome which amino acid to add for each triplet of bases it reads.
Why Scientists Felt the Need to Propose a Code
A protein is built from just 20 different amino acids, but a nucleic acid is built
from only 4 different nitrogenous bases. If one base coded for one amino acid, only
4 amino acids could ever be specified — far too few. George Gamow was among the first
to point out this mismatch and to argue mathematically that the code had to involve
more than one base per amino acid: even reading bases two at a time gives only
4×4 = 16 combinations, still short of 20, so scientists reasoned the code had to be
read in groups of at least three bases. This was later experimentally confirmed and the
code fully deciphered — largely through the work of Har Gobind Khorana and Marshall
Nirenberg, using synthetic RNA templates in cell-free translation systems.
Key Properties
- Triplet — each "word" of the code (a codon) is exactly three consecutive nucleotides, and 3 bases from a 4-letter alphabet give 43=64 possible codons — comfortably enough to specify 20 amino acids.
- Degenerate (redundant) — most amino acids are specified by more than one codon (of the 64 codons, 61 code for amino acids and 3 are stop signals, so several codons map to the same amino acid). This redundancy is thought to protect against the effect of some point mutations.
- Unambiguous — despite being degenerate, a given codon always codes for the same, single amino acid — it never specifies more than one.
- Commaless and non-overlapping — the code is read continuously from a fixed starting point, three bases at a time, with no punctuation between codons and no base shared between two consecutive codons. …
Translation of an mRNA can only begin once the ribosome encounters a start codon (AUG), which fixes the reading frame from that point onward, and it proceeds until a stop codon appears in that same frame.
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Translation can only begin at a start codon (AUG); changing the given sequence's first codon from AUA to AUG creates that start signal, and reading onward in this frame, translation would proceed until it reaches the stop codon UGA a little further along the strand.
Reading the given mRNA sequence in triplets (5' → 3'): AUA – UUU – UUC – UUC – UUU – UGA – UGU.
(a) Change needed to start translation: Translation of any mRNA can only begin at the start codon AUG (which codes for methionine and sets the reading frame). The first codon here is AUA (which codes for isoleucine, not a start signal). Changing just its third base from A to G converts it to AUG, giving the ribosome a start codon at that position and allowing translation to begin there.
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Showing the 12 most recent of 17 on this concept.
- CBSE 2026Set A1 markMCQQ.Which of the following statements is incorrect about the genetic code?(a) The codon is triplet(b) It is unambiguous and specific(c) It is degenerate(d) All 64 codons code for amino acids
›Reveal solutionSolution
The genetic code is triplet, unambiguous and degenerate, but only 61 of 64 codons code for amino acids — 3 are stop codons.
Of the 64 possible triplet codons, 61 specify amino acids and 3 (UAA, UAG, UGA) are stop/termination codons that do not code for any amino acid. So the statement that all 64 codons code for amino acids is incorrect. The code being triplet, unambiguous (each co …
- CBSE 2026Set BOTANY1 markMCQQ.When more than one codon codes for the same amino acid, the genetic code is called _____.(a) commaless(b) universal(c) degenerate(d) nonsense
›Reveal solutionSolution
The genetic code is called degenerate because a single amino acid can be coded for by more than one triplet codon.
The genetic code has several defining properties:
- It is a triplet code: three nucleotides (a codon) specify one amino acid.
- It is unambiguous: a given codon always codes for the same amino acid.
- It is commaless: codons are read continuously without gaps between them.
- It is (nearly) universal: the same codon specifies the same amino acid across almost all organisms. …
- CBSE 2026Set ANNUAL1 markQ.Fill in the blank: Some amino acids are coded by more than one codon, for this reason such codons are called ______ code.
›Reveal solutionSolution
Because most amino acids are specified by more than one codon, the genetic code is described as degenerate.
Of the 61 codons that code for amino acids, most amino acids are coded by more than one codon (e.g., leucine and serine each have six codons) - this redundancy is called the degener …
- CBSE 2026Set ANNUAL1 markMCQQ.Terminator codon is :(a) UAA(b) UAG(c) UGA(d) All of these
›Reveal solutionSolution
The genetic code has three stop (terminator/nonsense) codons that do not code for any amino acid: UAA, UAG and UGA.
Of the 64 possible codons in the genetic code, 61 are sense codons that specify amino acids, and 3 — UAA (ochre), UAG (amber) and UGA (opal) — are termination (stop/nonsense) codons. When the ribosome encounters any one of these three codons on the mRNA during translation, no aminoacyl-tRNA binds to …
- CBSE 2025Set F1 markMCQQ.Which of the following is an initial codon?(a) UGA(b) GUA(c) AUG(d) All of these
›Reveal solutionSolution
The initiation codon is AUG.
During translation, protein synthesis begins at the start (initiation) codon AUG, which codes for the amino acid methionine (formyl-methionine in bacteria). The codons UGA, UAA and UAG are stop (termination …
- CBSE 2025Set ANNUAL1 markMCQQ.Which of the following are the three codons which results in the termination of polypeptide chain synthesis?(a) UAA, UAG, UGA(b) UAA, UAG, GUA(c) UAA, UGA, UUA(d) UGU, UAG, UGA
›Reveal solutionSolution
Of the 64 codons in the genetic code, three - UAA, UAG, and UGA - do not code for any amino acid and instead act as termination (stop) signals for translation.
During translation, when the ribosome encounters one of these three codons in the mRNA reading frame, no tRNA with a matching anticodon is normally available (since they don't specify an amino acid); instead, release factors bind and trigger the release of the completed …
- CBSE 2025Set ANNUAL1 markMCQQ.Assertion (A): The genetic code is degenerate. Reason (R): Some amino acids are coded by more than one codon.(a) Both Assertion (A) and Reason (R) are true and Reason (R) is the correct explanation of Assertion (A).(b) Both Assertion (A) and Reason (R) are true, but Reason (R) is not the correct explanation of Assertion (A).(c) Assertion (A) is true but Reason (R) is false.(d) Both Assertion (A) and Reason (R) are false.
›Reveal solutionSolution
The genetic code IS degenerate, and the correct reason is exactly that several amino acids are specified by more than one codon.
The genetic code is described as degenerate because, out of the 64 possible codons (triplets of 4 bases), only 2 code for start/have special roles beyond amino acids and 61 code for the 20 standard amino acids, and 3 are stop codons. Since there are only 20 amino acids but 61 sense codons, most amino acids (all except methionine and tryptophan) are coded for by more than one codon — e.g., leucine, serine, and arginine each have 6 different codons. This redundancy is precisely what "degeneracy" of the genetic code means, and it is thought to minimise the deleterious effects of point mutations (many single-base changes, especially at the third/wobble position, still code for the same …
- CBSE 2024Set GG1 markQ.How many codons are there in genetic codes?
›Reveal solutionSolution
The genetic code has 43=64 codons in all.
Concept. The genetic code is a triplet code: each codon is a sequence of three nitrogenous bases on mRNA. Since there are 4 bases (A, U, G, C), the total number of possible triplet combinations is
4×4×4=43=64.
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- CBSE 2024Set E1 markMCQQ.Some amino acids are coded by more than one codon. Such code is known as(a) unambiguous(b) degenerate/redundant(c) universal(d) specific
›Reveal solutionSolution
A code in which one amino acid is specified by more than one codon is called degenerate (redundant).
The genetic code has several salient features: it is a triplet code, it is nearly universal, it is unambiguous and specific (one codon codes for only one amino acid), and it is degenerate. 'Degenerate' (or redundant) means that most amino acids are coded by more than one codon …
- CBSE 2024Set E1 markMCQQ.Which of the following is not a termination codon?(a) UAG(b) UGA(c) AUG(d) UAA
›Reveal solutionSolution
AUG is the start codon; UAA, UAG and UGA are the three stop (termination) codons.
The genetic code has three STOP codons that signal the end of translation and do not code for any amino acid:
- UAA, UAG and UGA are the termination (stop) codons — when the ribosome reaches one, the polypeptide is released. …
- CBSE 2023Set ANNUAL1 markMCQQ.How many effective codons are there for the synthesis of twenty amino acids?(a) 64(b) 61(c) 20(d) 3
›Reveal solutionSolution
Of the 64 possible triplet codons, 61 are 'sense' codons that specify amino acids; the remaining 3 are stop codons.
The genetic code is made of triplets of the four bases (A, U, G, C), giving 4³ = 64 possible codons.
- Of these 64, 3 codons (UAA, UAG, UGA) do not code for any amino acid — they are stop/nonsense codons that signal termination of translation. …
- CBSE 2022Set GO1 markMCQQ.For the initiation of protein synthesis, initiation codon is(a) AUG(b) UAG(c) UGA(d) UAA
›Reveal solutionSolution
The initiation codon is AUG.
Concept — where translation starts. During translation the mRNA is read in triplets (codons). Translation does not begin at the very first nucleotide but at a specific start codon, which fixes the reading frame and brings in the first amino acid.
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