Question

1. How many different mRNA sequences can code for a polypeptide chain with the amino acid sequence Met - Leu - Arg? Be sure to include a stop codon. Explain your answer!

1. 5′ ...GGAGCUCGUUGUAUU... 3′ Is this sequence RNA or DNA? How can you tell? Which amino acids are encoded, if the reading frame is as shown, starting from the correct end? What would be the effect on the amino acid sequence if the sequence were changed to 5′ GGAGACUCGUUGUAUU 3′? Explain why the amino acid sequence changes.

Answer 1

a. Given the polypeptide sequence, Met-Leu-Arg

Methionine has a single codon AUG

Leucine has 6 potential codons UUA,UUG,CUU,CUC,CUA and CUG

Arginine also has 6 potential codons CGU,CGC,CGA,CGG,AGA and AGG

Including the 3 potential stop codons UAA,UAG and UGA

The product of the potential codons of the given polypeptide along with the stop codon gives the total number of possible mRNA sequences.

ie., mRNA sequences.

b. The given sequence is an mRNA sequence.

DNA sequence comprises of four nucleotides Adenine,Guanine, Thymine and Cytosine represented by A,G,T and C respectively

RNA sequence comprises of four nucleotides Adenine,Guanine, Uracil (replaces thymine and base pairs with adenine) and Cytosine represented by A,G,U and C respectively.

The given sequence has Uracil therefore a RNA sequence.

c. 5' GGA GCU CGU UGU AUU 3'

Encoding polypeptide for above sequence is Glycine-Alanine-Arginine-Cystine-Isoleucine.

Changed amino acid sequence is

5'GGA GAC UCG UUG UAU U 3' (the nucleotide A in bold is the mutation)

Encoding polypeptide changes to Glycine-Aspartic acid- Serine- Leucine-Tyrosine

The change in amino acid sequence is due to the insertional mutation (addition of one or more base pair to the sequence). Here base pair A is inserted.