Question

The table below shows some of the 64 available codons and their associated amino acids.

Codon	Amino acid
AGG	arginine
CAG	glutamine
GGG	glycine
GGU	glycine
GUU	valine
UUA	leucine
UCA	serine
UUU	phenylalanime

The diagram below shows the coding strand of a length of DNA with its bases indicated.

T A C A A T C C C A A A A T C

a) Write down the base sequence of a length of the mature RNA that would be transcribed from this DNA. 

b) In a eukaryotic cell, the base sequence of the mRNA might be different from the sequence of the HnRNA. Explain why. 

c) ‘Genetic code is nearly universal’. Explain this statement. 

d) Explain why glycine has two codons in the above table. 

e) Give reasons why RNA is less stable than DNA

Answer 1

a) 3’ RNA- AUG UUA GGG UUU UAG 5’ 
Polypeptide chain - methionine- leucine -glycine –phenylalanine 

b) because introns are spliced off during the processing of HnRNA. 

c) From bacteria to human a genetic code will code for same amino acid e.g. UUU will code for Phenylalanine in all organisms. Some exceptions to this rule have been found in mitochondrial codons and in some protozoans. 

d) Glycine has two codons because the genetic code is degenerate, that means a single amino acid can be coded by more than one codon. 

e) Presence of 2’ – OH groups at every nucleotide and Uracil instead of thymine makes RNA less stable.