9. In the universal genetic codon table, there are three stop codons TAA, TAG an
ID: 253744 • Letter: 9
Question
9. In the universal genetic codon table, there are three stop codons TAA, TAG and TGA. (A) Suppose a piece of DNA has even frequencies of the four nucleotides ACGT (that is, the probability of observing each nucleotide at any position is 0.25), calculate the probability of randomly selected codon is a stop codon in this piece of DNA. (B) If the GC content of a piece of DNA is 70%, where the frequencies of G and C are equal and frequencies of A and T are 0.12 and 0.18, respectively, what is the probability of a randomly selected codon is a stop codon in the DNA?Explanation / Answer
(A) Since the probability of observing any nucleotide is 0.25,
Probability of TAA = 0.25 * 0.25 * 0.25 = 0.015625
Probability of TAG = 0.25 * 0.25 * 0.25 = 0.015625
Probability of TGA = 0.25 * 0.25 * 0.25 = 0.015625
Probability of randomly selected codon is a stop codon = 0.015625 + 0.015625 + 0.015625 = 0.046875
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(B) GC content = 70%
Since G always base pairs with C,
G + C = 70/100 = 0.7
Probability of observing G or C = 0.7/2 = 0.35
Frequency of A = 0.12
Frequency of T = 0.18
Probability of TAA = 0.18 * 0.12 * 0.12 = 0.002592
Probability of TAG = 0.18 * 0.12 * 0.35 = 0.00756
Probability of TGA = 0.18 * 0.35 * 0.12 = 0.00756
Probability of randomly selected codon is a stop codon = 0.002592 + 0.00756 + 0.00756 = 0.017712
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