Problem 1
What is the minimum number of contiguous bases required to encode 20 amino acids? Explain your answer.
Problem 2
Why has the code remained nearly invariant through billions of years of evolution, from bacteria to human beings?
Problem 4
What are the key characteristics of the genetic code?
Problem 5
Usually degeneracy should be avoided, except perhaps in well-controlled circumstances. However, it is advantageous for the genetic code to be degenerate. What does it mean to say that the code is degenerate, and explain why a degenerate code is valuable?
Problem 6
Calculate the probability of synthesizing an error-free protein of 50 amino acids and one of 300 amino acids when the frequency of inserting an incorrect amino acid is \(10^{-2}\). Repeat the calculations with error frequencies of \(10^{-4}\) and \(10^{-6}\).
Problem 8
Explain how it is possible that some tRNA molecules recognize more than one codon.
Problem 9
(a) Write the sequence of the mRNA molecule synthesized from a DNA template strand having the following sequence. $$5^{\prime}-\text { ATCGTACCGTTA-3 }^{\prime}$$ (b) What amino acid sequence is encoded by the following base sequence of an mRNA molecule? Assume that the reading frame starts at the \(5^{\prime}\) end. $$5^{\prime}-U U G C C U A G U G A U U G G A U G-3^{\prime}$$ (c) What is the sequence of the polypeptide formed on addition of poly(UUAC) to a cell-free protein-synthesizing system that does not require a start codon?
Problem 11
The code word GGG cannot be deciphered in the same way as can UUU, \(\mathrm{CCC}\), and \(\mathrm{AAA}\), because poly(G) does not act as a template for protein synthesis. Poly(G) forms a triple-stranded helical structure. Why is it an ineffective template?
Problem 12
In a non-overlapping triplet code, each group of three bases in a sequence ABCDEF ... specifies only one amino acid-ABC specifies the first, DEF the second, and so forth-whereas, in a completely overlapping triplet code, ABC specifies the first amino acid, BCD the second, CDE the third, and so forth. Assume that you can mutate an individual nucleotide of a codon and detect the mutation in the amino acid sequence. Design an experiment that would establish whether the genetic code is overlapping or non-overlapping.
Problem 13
The RNA transcript of a region of T4 phage DNA contains the sequence \(5^{\prime}\) -AAAUGAGGA-3'. In theory, this sequence is capable of encoding three different polypeptides. What are they?
