Molecular Biology – Genetic Code: NEET MCQs with Answers

15 free questions on Molecular Biology – Genetic Code for NEET (UG), starting with previous-year NEET and AIPMT questions. Try each one, then tap Show answer for the answer and a short explanation.

Syllabus covered: Genetic Code

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Question 1

Previous-year question
If there are 999 bases in an RNA that codes for a protein with 333 amino acids, and the base at position 901 is deleted such that the length of the RNA becomes 998 bases, how many codons will be altered? [NEET 2017]
  1. 11
  2. 33
  3. 333
  4. 1
Show answer
Answer: (B) 33
Explanation: The deletion at base 901 falls in the 301st codon and shifts the reading frame from that point onwards. Codons 301 to 333 — a total of 33 codons — are therefore misread.

Question 2

Previous-year question
Which one of the following is the starter codon? [NEET 2016]
  1. AUG
  2. UGA
  3. UAA
  4. UAG
Show answer
Answer: (A) AUG
Explanation: AUG is the universal initiation codon and also codes for methionine. UAA, UAG and UGA are the three termination (nonsense) codons.

Question 3

Previous-year question
Genetic code consists of [AIPMT 1988]
  1. adenine and guanine
  2. cytosine and uracil
  3. cytosine and guanine
  4. All of the above
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Answer: (D) All of the above
Explanation: The genetic code is the sequence of nitrogen bases on mRNA that stores the information for linking amino acids in a definite order during protein synthesis. These bases are adenine, guanine, cytosine and uracil, so all of the given pairs are involved.

Question 4

Previous-year question
In the genetic dictionary, there are 64 codons as [AIPMT 1990]
  1. 64 amino acids are to be coded
  2. 64 types of tRNAs are present
  3. there are 44 non-sense codons and 20 sense codons
  4. genetic code is triplet
Show answer
Answer: (D) genetic code is triplet
Explanation: A sequence of three consecutive bases codes for one amino acid, so the code is a triplet code. With four kinds of nitrogenous bases, the number of possible triplets is 4 × 4 × 4 = 64.

Question 5

Previous-year question
Khorana first deciphered the triplet codons of [AIPMT 1992]
  1. serine and isoleucine
  2. threonine and histidine
  3. tyrosine and tryptophan
  4. phenylalanine and methionine
Show answer
Answer: (B) threonine and histidine
Explanation: Khorana synthesised an RNA with constantly repeating AC building blocks, which can be read as ACA CAC ACA CAC. The protein synthesised had two alternating amino acids in the sequence Thr–His–Thr–His, showing that ACA encodes threonine and CAC histidine (or vice versa).

Question 6

Previous-year question
The number of base substitution possible in amino acid codons is [AIPMT 1994]
  1. 261
  2. 264
  3. 535
  4. 549
Show answer
Answer: (D) 549
Explanation: There are 64 codons of which 61 code for amino acids. Each codon has three bases and each base can undergo transition or transversion in 3 ways, so the number of possible substitutions is 61 × 3² = 549.

Question 7

Previous-year question
Anticodon is an unpaired triplet of bases in an exposed position of [AIPMT 1995]
  1. mRNA
  2. rRNA
  3. tRNA
  4. nRNA
Show answer
Answer: (C) tRNA
Explanation: tRNA possesses an anticodon stem whose loop consists of seven unpaired bases; the third, fourth and fifth of these form the anticodon. This anticodon pairs temporarily with the complementary three bases of the codon on mRNA.
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Question 8

Previous-year question
The codons causing chain termination are [AIPMT 1997]
  1. TAG, TAA, TGA
  2. GAT, AAT, AGT
  3. AGT, TAG, UGA
  4. UAA, UAG, UGA
Show answer
Answer: (D) UAA, UAG, UGA
Explanation: UAA, UAG and UGA act as stop or terminator codons because they are not translated into any amino acid. They are also known respectively as ochre, amber and opal.

Question 9

Previous-year question
The transfer RNA molecule in 3D appears [AIPMT 2000]
  1. L-shaped
  2. E-shaped
  3. Y-shaped
  4. S-shaped
Show answer
Answer: (A) L-shaped
Explanation: Kim and coworkers (1973) suggested the L-shaped model of tRNA from X-ray diffraction studies of yeast phenylalanine tRNA. The three-dimensional structure of tRNA is thus an L of about 20 Å thickness.

Question 10

Previous-year question
Because most of the amino acids are represented by more than one codon, the genetic code is [AIPMT 2002, 1993]
  1. overlapping
  2. wobbling
  3. degenerate
  4. generate
Show answer
Answer: (C) degenerate
Explanation: Degeneracy means lack of specificity in the sense that more than one meaningful codon exists for an amino acid. Methionine and tryptophan are the exceptions, each having a single codon; leucine, serine and arginine have as many as six.

Question 11

Previous-year question
Out of 64 codons, 61 codons code for 20 types of amino acid. It is called [AIPMT 2002]
  1. degeneracy of genetic code
  2. overlapping of gene
  3. wobbling of codon
  4. university of codons
Show answer
Answer: (A) degeneracy of genetic code
Explanation: Since 61 sense codons specify only 20 amino acids, most amino acids are coded by more than one codon. This property is called the degeneracy of the genetic code, and it provides protection against the effects of mutation.

Question 12

Previous-year question
Which one of the following triplet codes, is correctly matched with its specificity for an amino acid in protein synthesis or as 'start' or 'stop' codon? [AIPMT 2003]
  1. UGU — Leucine
  2. UAC — Tyrosine
  3. UCG — Start
  4. UUU — Stop.
Show answer
Answer: (B) UAC — Tyrosine
Explanation: UAC correctly codes for tyrosine. UGU codes for cysteine, UCG for serine, and UUU for phenylalanine; the start codon is AUG and the stop codons are UAA, UAG and UGA.

Question 13

Previous-year question
Degeneration of a genetic code is attributed to the [AIPMT 2003]
  1. entire codon
  2. third member of a codon
  3. first member of a codon
  4. second member of a codon
Show answer
Answer: (B) third member of a codon
Explanation: The genetic code is degenerate largely because of the third base of the codon, as explained by the wobble hypothesis. For example GCU, GCC, GCA and GCG all code for alanine — the first two bases are fixed and only the third varies.

Question 14

Previous-year question
What would happen if in a gene encoding a polypeptide of 50 amino acids, 25th codon (UAU) is mutated to UAA? [AIPMT 2003]
  1. A polypeptide of 49 amino acids will be formed
  2. A polypeptide of 25 amino acids will be formed
  3. A polypeptide of 24 amino acids will be formed
  4. Two polypeptides of 24 and 25 amino acids will be formed
Show answer
Answer: (C) A polypeptide of 24 amino acids will be formed
Explanation: UAA is a stop codon, so translation terminates before the 25th amino acid is incorporated. Only the first 24 amino acids are joined, giving a truncated polypeptide of 24 amino acids.

Question 15

Previous-year question
In the genetic code dictionary, how many codons are used to code for all the 20 essential amino acids? [AIPMT 2003]
  1. 61
  2. 60
  3. 20
  4. 64
Show answer
Answer: (A) 61
Explanation: Of the 64 codons, three (UAA, UAG and UGA) are chain-terminating codons that are not read by tRNA. The remaining 61 codons are used to specify the 20 essential amino acids.

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