2004: The Genetic Code and Protein Synthesis

what are the possibilities for arranging/grouping three bases in an RNA chain?

Overlapping: 1 base overlap between 1 base in a nucleotide.

Punctuated: mRNA contains gaps

Unpunctuated: mRNA contains no gaps

What are the 3 characteristics of the genetic code?

Genetic experiments established that the code is a triplet (3 nucleotides), non-overlapping, unpunctuated code

Includes start (AUG) and stop codons

If you don’t have any information about where translation starts or stops, how many possible reading frames are possible for the ways DNA sequence can be decoded?

There are SIX reading frames in any DNA sequence.

What is the triplet code in mRNA? What does this mean for amino acids?

Groups of three consecutive nucleotides (= codons) that correspond to one amino acid.

The genetic code contains 64 different codons; with only 20 common amino acids, this leads to redundancy — some amino acids are specified by more than one codon

Deciphering the code: How did they establish which triplet coded for which amino acid?

They pioneered the use of in vitro translation systems

1. RNA template

2. Ribosomes (and other necessary “stuff”)

3. Individual radio-labelled amino acids

4. Suitable reaction buffer (pH, [salt])

*Proteins that are synthesized in the system can be precipitated with trichloroacetic acid (TCA) (usually) and analyzed.

Deciphering the code: What are 2 examples regarding how exactly did they use in vitro translation systems to establish which triplet coded for an amino acid?

1. When they used poly(U), which contains only uracils, a polypeptide containing only phenylalanine was synthesized

• Ergo, the codon UUU corresponds to the amino acid Phe

2. They also found that poly-C directed synthesis of polyproline CCC = Proline poly-A had apparently been tried earlier by Watson and Ochoa in a similar type of experiment without success. We now know that poly-A would direct synthesis of polylysine; however, polylysine (unlike most proteins) is soluble in trichloroacetic acid