A portion of the transcribed chain of a DNA molecule with a sequence of 180 nucleotides was examined. Before transcription, two sections of 14 and 26 nucleotides, respectively, were cut from the DNA molecule, and then a mutation was initiated – a deletion of 20 nucleotides. How many amino acid residues will a protein synthesized on the basis of mRNA obtained from a given DNA region contain? The presence of stop codons is not taken into account in the calculations.
The total number of nucleotides in the transcribed DNA fragment is 180. Before transcription, 40 nucleotides were cut out (14 + 26 = 40), 140 remained (180 – 40 = 140). After mutation (division – loss of 20 nucleotides from the chain), 120 nucleotides remain (140 – 20 = 120). Since one amino acid is encoded by three nucleotides (triplet), then 120 complementary nucleotides in the synthesized mRNA chain will encode 40 amino acids (120: 3 = 40).
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