Question

The terminal portion of an RNA transcript of a region of T4 phage DNA contains the sequence 5′-AAAUGAGGA-3′. The entire sequence encodes three different polypeptides. What amino acid(s) will terminate the three different polypeptides?

I get how you get a peptide with Lys and the stop codon. I don’t know how you get the other two peptides. Please, explain.

Answer 1

The given sequence of DNA is 5' - AAAUGAGGA- 3'. Three different polypeptides are being coded using this fragment of DNA.

This can be achieved by changing the reading frame. A reading frame refers to the frame in which the codons are being read. There are three possible reading frames here and each of them can code for a different polypeptide.

The three reading frames and their corresponding polypeptides are:

1. 5' - AAA UGA GGA -3'

Using a codon table, we see that AAA codes for Lys and UGA codes for a Stop codon. Hence, GGA will not even be read.

Therefore, this frame will code for Lys-Stop.

2. 5' - AA AUG AGG A -3'

Here, we see that AUG is one codon and AGG is another codon. AUG codes for Met and AGG codes for Arg. Hence, this frame will code for Met-Arg.

3. 5' - A AAU GAG GA -3'

Here, we see that AAU is one codon, and GAG is another codon. AAU codes for Asn and GAG codes for Glu. Hence, this frame will code for Asn-Glu.