Question

Here is a sequence for part of a hypothetical gene: ACTTAGCT. Suppose this sequence was changed by a mutation so that it is now:

ACTTTGCT (note the change of an A to a T). Why might this change in DNA lead to a change in the shape of a protein? Answer in terms of the levels of protein structure, and how one level of structure --> effects on another level of structure. When this DNA is copied in a dividing cell, why is the mutation copied and not 'corrected' back to the original sequence?

Answer 1

Answer:

• As we know that sequence of a protein is coded in its DNA sequence which is then transcribed to RNA.
• Three bases together form a codon which codes for one amino acid.
• If the reading frame were to consider ACT-TAG-CT.. as the codons, so in the mutated sequence the TAG codon will be now TTG.
• TAG and TTG will be AUC and AAC in the corresponding RNA sequence respectively.
• AUC codes for isoleucine (Ile) while AAC codes for asparagine (Asn), Ile is a small aliphatic hydrophobic amino acid while Asn is polar because of its amide group.
• Change from an uncharged to a charged residue will lead to a drastic change in the shape of the protein.
• The charged residue may repel or interact with other neighbouring charged residues.
• When the mutated DNA is copied in the dividing cell, it is not 'corrected' because there is no way by which the cell can identify the mutant from original and decide what is the corrected sequence.
• It can just try to be accurate in further replication by copying now ACTTTGCT to ACTTTGCT and not ACTTAGCT, The cell doesn't have the machinery to identify mutant.
• It can onlyuse the enzymes responsible to maintain fidelity of replication at the dividing stage. It is another thing if during the life of the newly formed cell, it might mutate to ACTTAGCT!

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