Biology (translation) extremely confused, final is tomorrow

[SA32]

Homework Statement

8) Suppose the following DNA sequence was mutated from 5'-AGAGAGAGAGAGAGAGAGA to

5'-AGAAGAGAGATCGAGAGA. What amino acid sequence will be generated based on this mutated DNA?

A) glu-arg-glu-leu-leu-leu

B) ser-leu-ser-leu-ser-leu

C) ser-ser-leu

D) leu-phe-arg-glu-glu-glu

E) arg-glu-arg-glu-arg-glu

The Attempt at a Solution

DNA template strand: 5' AGA - AGA - GAG - ATC - GAG - AGA 3'
mRNA strand: 3' UCU - UCU - CUC - UAG - CUC - UCU 5'

But ribosomes read mRNA in the 5' to 3' direction.

mRNA strand: 5' UCU - CUC - GAU - CUC - UCU - UCU 3'
protein: ser - leu - asp - leu - ser - ser

But this isn't one of the choices.

I get C (ser ser leu) if I read from 3' to 5', because UAG is a stop codon, but I'm staring at a video that shows a ribosome reading the mRNA strand 5' to 3'. Where did I go wrong?

 

[iansmith]

Based on wording on the question, you have to assume that the sequence they give you is the sequence that will be translated and not it's complementary sequence.

Essentially the question gives you the coding strand and not the anti-coding/non-coding strand. Transcription from DNA to RNA is made using the non-coding strand.

C is the right answer.

For future reference, look at the wording of the sentence, if anti-coding/non-coding sequence is ever used, the question is asking you to create the complementary strand before starting translation. If coding strand present or non-coding strand is absent, you don't need to create the complementary strand.

This is not fast and steady rule but it hold true most of the time.

 

[Blueshift5]

it is important to understand the directionality of DNA and RNA and how it affects translation. In this case, it seems that there was a misunderstanding in the direction of the mRNA strand. Ribosomes read mRNA in the 5' to 3' direction, but the mRNA strand given in the question is already in the 5' to 3' direction. Therefore, the correct mRNA strand would be 5' UCU - CUC - GAU - CUC - UCU - UCU 3'. This would result in the amino acid sequence ser-leu-asp-leu-ser-ser, which matches with choice B. It is also important to note that the stop codon, UAG, should be excluded from the amino acid sequence as it does not code for an amino acid. I hope this helps clarify your confusion and good luck on your final tomorrow!