Question

3. A small peptide has the amino acid sequence Phe-Leu-Tyr-Ala-Leu-Gly-Glu. A shorter variant of this peptide was discovered that had the sequence Phe-Leu-Tyr-Ala; it was demonstrated that this shorter peptide was due to a single base substitution in the second Leu codon. What type of mutation (missense, nonsense, frameshift) gave rise to this shorter peptide? Which of the six possible Leu codons was used in the synthesis of the original (long) peptide (can you eliminate some as possibilities)?

Answer 1

ANSWER)

Phe-Leu-Tyr-Ala-Leu-Gly-Glu

PHE= UUU,UUC

LEU= UUA UUG, CUU, CUC, CUA, CUG

TRY= UAU,UAC

ALA=GCU.GCC,GCA,GCG

GLY=GGU,GGC,GGA,GGG

STOP CODON = UAA, UAG,AGA

SUBSTITUTION CONDITION THAT LEAD TO SHORT PEPTIDE FORMATION

• IF LEU CODON = UUA CAN BE CONVERTED IN UAA DUE TO SUBSTITUTION OF MIDDLE U INTO A
• IF THEIR IS CODON OF LEU= UUG THEN HERE ALSO CONVERSION OF MIDDLE U INTO A COULD LEAD TO STOP CODON

THE POSSIBILITY CAN BE ELIMINATED IF REMAINING CODON THAT IS CUU, CUC, CUA, CUG CAN BE USED FOR LEU

IT IS A KIND OF NONSENSE MUTATION : A nonsense mutation is the substitution of a single base pair that leads to the appearance of a stop codon where previously there was a codon specifying an amino acid.