The Genetic Code (SDS)

Question 1

Translation properties

Answer 1

Involves large macro molecular machines;
Accessory factors required;
Needs accuracy;
Polymerisation is endergonic

Question 2

How are sequences of 4 residues specifying 20 amino acids?

Answer 2

Codon: triplet code; 3 nucleotide bases;

Amino acid specified by more than one codon (degenerate)

Question 3

Why does nature use the triplet code?

Answer 3

2 bases per codon is insufficient (4^2=16 doublets)

3 bases per codon is sufficient (4^3=64 triplets)

Question 4

How do we know codons are read sequentially?

Answer 4

Using bacteriophage T4 crick and brenner discovered that a deletion of a nucleotide could abolish gene function and a 2nd mutation (insertion) could restore gene function. Thus, the genetic doe is read sequentially from a fixed point. Insertions/deletions = frameshift mutations as they shift the reading frame.

Question 5

How do we know the codon is a triplet?

Answer 5

2 insertions/deletions couldn’t restore gene function but 3 could.

Question 6

Describe the cell free system

Answer 6

E. Coli cells were broken and centrifuged - no cell walls;
Left with mRNA, DNA, ribosomes,enzymes (all needed for protein synthesis)
ATP, GTP and amino acids - protein synthesized;
Add DNase to remove DNA (halt synthesis);
Add purified/synthetic mRNA;
Recover resulting polypeptide

Question 7

Features of the genetic code

Answer 7

Degenerate (codons coding for the same amino acid are synonymous; synonyms only differ in 3rd nucleotide);
Stop (nonsense) codons (UGA, UAG, UAA);
Start codons (AUG, GUG)

Question 8

Is the genetic code universal?

Answer 8

No;
Genetic codes of certain mitochondria were variants of the standard genetic code;
Alternate genetic code in ciliated protozoa;

Question 9

Features of tRNA

Answer 9

Clover leaf structure;
Similar structure;
5’ terminal phosphate group;
Acceptor;
D-arm;
Anticodon arm;
3’ CCA sequence with free 3-OH;
T _I_ C arm;
Complex tertiary structures;
Maintained by complex stacking interactions and base pairing within and between helical stems;

Question 10

How many bases of tRNA can be post-transciptionally modified?

Answer 10

25%

Question 11

Aminoacyl-tRNA synthetases attach amino acids to tRNAs. They require 2 equally important recognition steps -

Answer 11

1. Amino acid is activated;
2. Aminoacyl reacts with tRNA to form aa-tRNA

(Synthetase contacts tRNA in the acceptor stem and anticodon loop; must be able to bind to both)

Question 12

Isoaccepting tRNAs

Answer 12

Due to degenerate genetic code, more than 1 tRNA may carry a specific amino acid

Question 13

2 classes of aminoacyl-tRNA synthetases which differ in

Answer 13

Structural motif;
Anticodon recognition;
Site of aminoacylation;
Amino acid specificity;

Question 14

How many tRNAs are required to translate 61 triplets?

Answer 14

31

Question 15

Wobble

Answer 15

Each of the 61 codons are to read by different tRNAs;
Many tRNAs bind to 2 or 3 codons;
Non Watson crick base pairing occurs at the 3rd anticodon position