Transcription and RNA Processing

ID #2255

In one of our homework questions, I had the following: The figure below represents a portion of a protein-coding gene. Only one strand is shown, and the strand shown is the template strand. If each codon of the resulting mRNA is read in-frame from the appropriate end of the transcript, which one of the following most accurately represents the nature of the mutation illustrated in the second template strand? 5'…ATAAGCCGTATAGCGGGA…3' wt to 5'…ATAGGCCGTATAGCGGGA…3' mutant Incorrect: Deletion Incorrect: Missense mutation Incorrect: Nonsense mutation Incorrect: Insertion Correct: Point mutation I put missense mutation, which I thought was the answer because the amino acid specified does indeed change, and it's not a stop codon. I thought it could be point mutation as well, but isn't missense MORE SPECIFIC than point mutation because a missense is a type of base substitution mutation which in turn is a type of point mutation? And another related question, frameshift mutations are also point mutations, correct? Could you clarify these two issues?


I'm guessing that you read the template in the orientation it was written (from 5'-->3') instead of reading it the way the polymerase always reads the template strand of DNA (from 3'-->5'). 
If you read the template from 3'-->5' you get: 
WT template DNA: 
3' AGGGCGATATGCCGAATA 5' 
WT coding DNA (reverse complement of WT template DNA): 
5' TCCCGCTATACGGCTTAT 3' 
WT mRNA (changing WT coding DNA T's to U's and separated into codons because that's what the problem stem told you the mRNA is read in frame): 
5' UCC CGC UAU ACG GCU UAU 3' 
WT protein: 
N- Ser Arg Tyr Thr Ala Tyr -C 
Doing the same steps with the mutation: 
mutant template DNA: 
3' AGGGCGATATGCCGGATA 5' 
mutant coding DNA: 
5' TCCCGCTATACGGCCTAT 3' 
mutant mRNA: 
5' UCC CGC UAU ACG GCC UAU 3' 
mutant protein: 
N- Ser Arg Tyr Thr Ala Tyr -C 
In other words, changing the template from 3'-CGA-5' to 3'-CGG-5', gives you an mRNA change from 5'-GCU-3' to 5'-GCC-3', and you get an amino acid change from Ala to Ala. So, it is not a missense mutation, but it is instead a samesense mutation which stems from a point mutation. 
Like you said in your question, a point mutation is not the most specific term you could use since base substitutions (missense, nonsense, and samesense aka silent) and frameshift mutations all fall under the category of point mutations.

Categories for this entry

Print this record Print this record
Send to a friend Send to a friend
Show this as PDF file Show this as PDF file
Export as XML-File Export as XML-File