Question

Assume that the given mRNA (start site is not depicted) is theoretically translated in two reading frames.
Translation starting from the first nucleotide (Reading frame 1)
Translation starting from the second nucleotide (Reading frame 2)

Frame 1: 5'--CUCGCUUUGCCGAUCAAGGGUUA--3'
Frame 2: 5'--GUGGCACUCAGUCCUUAAUGGCG--3'

Answer the following question: How many amino acids will be specified in case (a) and case (b) on translation? Justify your answer.

Hint:

Translation occurs in triplets (codons), and stops when a stop codon (e.g., UAA, UAG, UGA) is encountered.

Answer 1

Case (a) — Frame 1:
The sequence is: CUC GCU UUG CCG AUC AAG GGU UA
Grouping codons from the first nucleotide: \[ \text{CUC} \to \text{Leu},\quad \text{GCU} \to \text{Ala},\quad \text{UUG} \to \text{Leu},\quad \text{CCG} \to \text{Pro},\quad \text{AUC} \to \text{Ile},\quad \text{AAG} \to \text{Lys},\quad \text{GGU} \to \text{Gly} \] The last codon fragment “UA” is incomplete (only 2 bases), so it will not be translated. 
Total amino acids in case (a): 7 Case (b) — Frame 2: 
The sequence is: GUG GCA CUC AGU CCU UAA UGG CG
Grouping codons from the second nucleotide: \[ \text{GUG} \to \text{Val},\quad \text{GCA} \to \text{Ala},\quad \text{CUC} \to \text{Leu},\quad \text{AGU} \to \text{Ser},\quad \text{CCU} \to \text{Pro},\quad \text{UAA} \to \text{Stop} \] Translation terminates at the stop codon UAA. So, codons after UAA will not be translated. 
Total amino acids in case (b): 5