Translation - The Genetic Code

Question 1

How is mRNA read?

Answer 1

In blocks of 3
- 3 nucleotides => 1codon
- 4 bases => 64 codon combination

Question 2

How genetic code was deciphered - In vitro translation system from E.coli

Answer 2

• When RNA was added, proteins encoded by the RNA were synthesised
• DNA of a defined sequence was made chemically, used as template for RNA polymerase + RNA used in in vitro
• Then, amino acids joined to its tRNA molecule are bound to the ribosome in presence of each triplet
• Codon binding studies found all 64 codons

Question 3

How genetic code was deciphered - Polynucleotide phosphorylase

Answer 3

Polynucleotide phosphorylase was used to synthesise homopolymers of RNA
- There were defined ratios of bases

• PolyU coded for polyphenylalanine
• PolyC coded for polyproline
• PolyA coded for polylysine

Question 4

What does UUU code for?

Answer 4

Most common codon
Synthesises phenylalanine

Question 5

2Us and 1Gs (UUG, UGU, GUU)

Answer 5

Next common after UUU
Synthesises valine, leucine and cysteine

Question 6

1Us and 2Gs (UGG, GUG, GGU)

Answer 6

Next common after 2Us and 1Gs
Synthesises tryptophan and glycine

Question 7

GGG

Answer 7

Least common codon
Synthesise glycine

Question 8

The genetic code

Answer 8

• Is a triplet code that is read in the 5’ to 3’ direction
• Most amino acids are coded for by more than one codon
• Is universal (exc. mitochondria + chloroplasts)
• Is non-overlapping + degenerate