Question

1. Summarize the main features or characteristics of the genetic code, including start & stop codons.

2. What is meant by base pair “wobble” and why might it be beneficial?

3. Describe the synthesis (and specificity) of aminoacyl-tRNA’s.

4. Name & describe the details in each of the three stages of protein synthesis, including the structure & functions of ribosomes and any factors involved.

5. Illustrate and/or describe the general structural, physical and/or chemical characteristics of amino acids.

6. Describe and/or illustrate the titration of an amino acid, including the pI’s or pK’s of ionizable groups.

7. Describe the basis for the occurrence and relationships of asymmetric carbons, stereoisomers, & enantiomers in amino acids.

8. Identify and/or describe the structures & properties of the 20 standard amino acids.

9. Distinguish between essential vs. non-essential amino acids vs. non-standard amino acids, including examples of the physiological roles.

Answer 1

The genetic code is the translation system between the nucleotides of DNA and RNA, and the amino acids that make up proteins. A codon is a triplet nucleotide sequence, with each sequence translating into different amino acids. With four nucleotides (adenine, cytosine, guanine, thymine or uracil), there are 64 possible combinations of three-nucleotide sequences. 61 of these code for 20 different amino acids. For example, if the triplet of nucleotides is UUA (uracil-uracil-adenine), the amino acid is leucine. The other three are called stop codons. These signal to the ribosome that protein synthesis has ended and to let go of the mRNA. There is also a start codon, but it codes for methionine in addition to start. The key features of genetic code are as follows :

• The genetic code is universal, i.e., codons translate into same amino acids across al the species.
• Each codon codes for only one amino acid, hence, it is unambiguous and specific.
• Some amino acids are coded for by more than one codon, hence the genetic code is degenerate. Most of this degeneracy involves the third nucleotide of a codon.
• The code is read in mRNA in a continuous fashion. There is no punctuation.
• The AUG codon has a dual function. It codes for methionine (met), and it also acts as a start (or initiator) codon to start protein synthesis.

2) The third base in a genetic codon is referred to as "wobble", since the 3^rd base of the codon can form non-Watson-Crick base pairing with the 1st base of a tRNA anticodon. The Wobble hypothesis is responsible for the degeneracy of genetic codons. The Wobble Hypothesis explains why multiple codons can code for a single amino acid. One tRNA molecule (with one amino acid attached) can recognise and bind to more than one codon, due to the less-precise base pairs that can arise between the 3rd base of the codon and the base at the 1st position on the anticodon. This hence explains why more codons exist than there are specific tRNA molecules.