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Molecular Basis of Inheritance — BIOLOGY STD 12 Science — Question

Maharashtra BoardEnglish MediumSTD 12 ScienceBIOLOGYMolecular Basis of Inheritance4 Marks
Question
Read the following and answer any four questions from (i) to (v) given below:
Mutation explains the relationship between gene and DNA. The effects of large deletions and rearrangement in a segment of DNA results in loss or gain of gene and its function. Insertion or deletion of one or two bases changes the reading frame from the point of insertion or deletion. A classical example of point mutation is a change of single base pair in the gene for beta globin chain that results in change of amino acid residue glutamate to valine and results into a diseased condition called sickle cell anaemia.
  1. A mutation is a change produced by an alteration in the genetic mechanism and.
  1. May arise spontaneously.
  2. Is always induced by the environment.
  3. Is never advantageous.
  4. Is not inherited.
  1. The DNA code for glutamic acid is CTC or CTT. The code for valine is CAA or CAT. In sickle cell haemoglobin, valine is present instead of glutamic acid.
Assuming a single base pair substitution has occurred, what is the mRNA code in the affected mutant?
  1. CUU
  2. GAA
  3. GAG
  4. GUA
  1. A mutation involving the substitution of one nitrogenous base for another has altered the base sequence of a DNA molecule, coding for four amino acids, as shown below.
Normal A-G-C-A-T-G-G-A-T-C-C-T

Mutant A-G-C-A-T-G-C-A-T-C-C-T
The table shows six codons and the corresponding amino acids into which each is translated.
mRNA codon
Amino acid
AAG
Lysine
CUA
Leucine
GGA
Glycine
GUA
Valine
UAC
Tyrosine
UCG
Serine
The mutation has changed the amino acid.
  1. Leucine to valine.
  2. Lysine to glycine.
  3. Serine to leucine.
  4. Tyrosine to lysine.
  1. Assertion : Insertion or deletion of three or its multiple bases, insert or delete one or multiple codons and so one or multiple amino acids.
Reason: Reading frame remains unaltered with insertion or deletion of three or its multiple bases.
  1. Both assertion and reason are true, and reason is the correct explanation of assertion.
  2. Both assertion and reason are true, but reason is not the correct explanation of assertion.
  3. Assertion is true, but reason is false.
  4. Both assertion and reason are false.
  1. Part of the amino acid sequences in normal and sickle cell haemoglobin are shown.
Normal haemoglobin Sickle cell haemoglobin
Thr-Pro-Glu-Glu Thr-Pro-Val-Glu
mRNA codons for these amino acids are
Glutamine (Glu) GAA GAG
Praline (Pro) CCU CCC
Threonine (Thr) ACU ACC
Valine (Val) GUA GUG
Which transfer RNA molecule is involved in the formation of this part of the sickle cell haemoglobin?
✓

Answer

  1. (a) May arise spontaneously.
Explanation:

A mutation is a change in the DNA that changes the physiological effect of the DNA on the cell. Such phenomenon may be caused by radiation, chemical carcinogens, or may occur spontaneously.
  1. (d) GUA
Explanation:

Since a single base pair substitution has caused this mutation, the original codon for glutamic acid must have been CTT, and the mutant codon is CAT. The mRNA code for this mutant is hence GUA, i.e., complementary to CAT.
  1. (a) Leucine to valine.
Explanation:

The segment coding for GAT on the normal DNA molecule has been transcribed into CUA in the mRNA molecule, therefore the mutant DNA strand CAT will be transcribed into GUA on the mRNA molecule. This implies a change from the amino acid leucine to valine.
  1. (b) Both assertion and reason are true, but reason is not the correct explanation of assertion.
  2. (b)
Explanation:

CAU in tRNA is the only one that can compliment the valine in the sickle cell haemoglobin.

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