11. Translation

The amino acid sequence of a portion of a polypeptide is

N...Cys-Pro-Ala-Met-Gly-His-Lys...C

Give the DNA template and coding strand sequences corresponding to the mRNA. Use the N, Pu, and Py symbols as placeholders.

The amino acid sequence of a portion of a polypeptide is

N...Cys-Pro-Ala-Met-Gly-His-Lys...C

What is the mRNA sequence encoding this polypeptide fragment? Use N to represent any nucleotide, Pu to represent a purine, and Py to represent a pyrimidine. Label the 5′ and 3′ ends of the mRNA.

Har Gobind Khorana and his colleagues performed numerous experiments translating synthetic mRNAs. In one experiment, an mRNA molecule with a repeating UG dinucleotide sequence was assembled and translated.

If the genetic code were overlapping rather than nonoverlapping, how would the result of this experiment be different?

Har Gobind Khorana and his colleagues performed numerous experiments translating synthetic mRNAs. In one experiment, an mRNA molecule with a repeating UG dinucleotide sequence was assembled and translated.

If the genetic code were a doublet code instead of a triplet code, how would the result of this experiment be different?

Har Gobind Khorana and his colleagues performed numerous experiments translating synthetic mRNAs. In one experiment, an mRNA molecule with a repeating UG dinucleotide sequence was assembled and translated.

How did the polypeptide composition help confirm the triplet nature of the genetic code?

Har Gobind Khorana and his colleagues performed numerous experiments translating synthetic mRNAs. In one experiment, an mRNA molecule with a repeating UG dinucleotide sequence was assembled and translated.

What is the sequence of the resulting polypeptide?

Har Gobind Khorana and his colleagues performed numerous experiments translating synthetic mRNAs. In one experiment, an mRNA molecule with a repeating UG dinucleotide sequence was assembled and translated.

Write the sequence of this mRNA and give its polarity.

An experiment by Khorana and his colleagues translated a synthetic mRNA containing repeats of the trinucelotide UUG.

How does the result of this experiment help confirm the triplet nature of the genetic code?

An experiment by Khorana and his colleagues translated a synthetic mRNA containing repeats of the trinucelotide UUG.

What is the result obtained from each reading frame?

The human β-globin polypeptide contains 146 amino acids. How many mRNA nucleotides are required to encode this polypeptide?

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

As determined by comparisons of ancient and recently evolved proteins, cysteine, tyrosine, and phenylalanine appear to be late-arriving amino acids. In addition, they are considered to have been absent in the abiotic Earth. All three of these amino acids have only two codons each, while many others, earlier in origin, have more. Is this mere coincidence, or might there be some underlying explanation?

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

The amino acids Ala, Val, Gly, Asp, and Glu are all early members of biosynthetic pathways and are more evolutionarily conserved than other amino acids. They therefore probably represent 'early' amino acids. Of what significance is this information in terms of the evolution of the genetic code? Also, which base, of the first two within a coding triplet, would likely have been the more significant in originally specifying these amino acids?

It has been suggested that the present-day triplet genetic code evolved from a doublet code when there were fewer amino acids available for primitive protein synthesis.

Can you find any support for the doublet code notion in the existing coding dictionary?

The mature mRNA transcribed from the human β-globin gene is considerably longer than the sequence needed to encode the 146–amino acid polypeptide. Give the names of three sequences located on the mature β-globin mRNA but not translated.

An early proposal by George Gamow in 1954 regarding the genetic code considered the possibility that DNA served directly as the template for polypeptide synthesis. In eukaryotes, what difficulties would such a system pose? What observations and theoretical considerations argue against such a proposal?