11. Translation

The Genetic Code

Open Question
In the experiments that deciphered the genetic code, many different synthetic mRNA sequences were tested.

Describe how the codon for phenylalanine was identified.
Open Question
Write a short essay that summarizes the key properties of the genetic code and the process by which RNA is transcribed on a DNA template.
Open Question
Several lines of experimental evidence pointed to a triplet genetic code. Identify three pieces of information that supported the triplet hypothesis of genetic code structure.
Open Question
Assuming the genetic code is a triplet, what effect would the addition or loss of two nucleotides have on the reading frame? The addition or loss of three, six, or nine nucleotides?
Open Question
The mRNA formed from the repeating tetranucleotide UUAC incorporates only three amino acids, but the use of UAUC incorporates four amino acids. Why?
Open Question
A portion of a DNA template strand has the base sequence 5′-...ACGCGATGCGTGATGTATAGAGCT...-3′

Which is the third amino acid added to the polypeptide chain?
Open Question
A portion of a DNA template strand has the base sequence 5′-...ACGCGATGCGTGATGTATAGAGCT...-3′

Identify the sequence and polarity of the mRNA transcribed from this fragmentary template-strand sequence.
Open Question
In studies using repeating copolymers, AC . . . incorporates threonine and histidine, and CAACAA . . . incorporates glutamine, asparagine, and threonine. What triplet code can definitely be assigned to threonine?
Open Question
What are the differences between the universal code and that found in the mitochondria of some species? Given that some changes (UGA =stop→Trp) have occurred multiple independent times in evolution, can you think of any selective advantage to the mitochondrial code?
Open Question
In a coding experiment using repeating copolymers (as demonstrated in Table 13.3), the following data were obtained:Copolymer Codons Produced Amino Acids in PolypeptideAG AGA, GAG Arg, GluAAG AGA, AAG, GAA Lys, Arg, GluAGG is known to code for arginine. Taking into account the wobble hypothesis, assign each of the four codons produced in the experiment to its correct amino acid.
Open Question
In the triplet binding technique, radioactivity remains on the filter when the amino acid corresponding to the codon is labeled. Explain the rationale for this technique.
Open Question
What is the role of codons UAA, UGA, and UAG in translation? What events occur when one of these codons appears at the A site of the ribosome?
Open Question
How many different proteins, each with a unique amino acid sequence, can be constructed that have a length of five amino acids?
Open Question
Predict the amino acid sequence produced during translation by the following short hypothetical mRNA sequences (note that the second sequence was formed from the first by a deletion of only one nucleotide):

Sequence 1: 5'-AUGCCGGAUUAUAGUUGA-3'
Sequence 2: 5'-AUGCCGGAUUAAGUUGA-3'

What type of mutation gave rise to sequence 2?
Open Question
Third-base wobble allows some tRNAs to recognize more than one mRNA codon. Based on this chapter's discussion of wobble, what is the minimal number of tRNA molecules necessary to recognize the following amino acids?

lysine