You have the following gene sequence that is transcribed into a complete mRNA th

Question

You have the following gene sequence that is transcribed into a complete mRNA that encodes a small protein: 5'- TTTGATGTACCACCGCATCAGGTTTACATCATGAGTAGTTTT -3' 3*. ACTACATGGTGGCGTAGTCCAAATGTAGTACTCATCAAAA -5' The bottom strand of this sequence is the template strand. Write the mRNA sequence of the gene that is transcribed. Be sure to label the 5' and 3' ends. This region of the mRNA you wrote out in part a) contains the codon that encodes for the N terminus and the C terminus of the protein. Write out the amino acid sequence (using the single letter code above) encoded by the mRNA from part b). Be sure to label the N- and C- termini. Can this peptide be modified by a kinase? Explain why or why not? If C-G base pair #10 from left to right is deleted by a frame-shift mutation, will this have an effect on the length of the peptide? Explain why or why not.

Explanation / Answer

a)

The term template strand refers to the sequence of DNA that is copied during the synthesis of mRNA and hence has same DNA sequence as in coding strand except that instead of thymine mRNA has uracil. You should always remember synthesis of mRNA occurs in 5' ------- 3' direction.

mRNA sequence: 5' - UUU GAU GUA CCA CCG CAU CAG GUU UAC AUC AUG AGU AGU UUU - 3'

b)

To find the amino acid sequence divide the sequence after every 3 bases (which represents one codon) as shown below and see the triplets are in frame

5' - UUU GAU GUA CCA CCG CAU CAG GUU UAC AUC AUG AGU AGU UUU - 3'

Locate the start codon in messenger RNA (mRNA) transcript from 5' - end. The most common start codon in eukaryotes is AUG and the location of start codon in our sequence is shown as AUG. The region before it is the untranslated region found in almost every mRNA.

Now that you have start conon read the successive base triplets, each representing one codon coding for a amino acid.

Start codon represents the N-terminus of polepeptide while other end is C-terminus.

So, we have in our case following protein sequence : N-terminus----M - S - S - F---- C-terminus.

c)

Yes the peptide can be modified by kinase.

The chemical activity of a kinase involves transferring a phosphate group from a nucleoside triphosphate (usually ATP) and covalently attaching it to specific amino acids with a free hydroxyl group.

In our peptide, we have two amino acids i.e. serine which contains one -OH group and thus can be a target of serine protein kinase.

d)

If C - G base pair at position 10 is deleted this will shift the reading frame as shown below

It doesn't matter whether you write sequence for template or coding strand. Here, i am showing for coding strand

5' - TTT GAT GTA CCA CCG CAT CAG GTT TAC ATC ATG AGT AGT TTT - 3'

when C is deleted the codon frame in the sequence will be read as

5' - TTT GAT GTA CAC CGC ATC AGG TTT ACA TCA TGA GTA GTT TT - 3'

Note a major change i.e we don't find ATG as a start codon in sequence. Thus, there will not be any translation of protein or sequence will not be translated into a protein.

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