Question

The following is a sequence of nucleotides in a DNA double helix that codes for a short polypeptide. The messenger RNA encoded by this DNA has both translational initiation and termination codons.

                                    STRAND A: T T T A G T T A T C A A T C T T G G G T A G A A C

                                    STRAND B: A A A T C A A T A G T T A G A A C C C A T C T T G

1. Which strand is the template strand (2.5 pts)?
2. What is the polarity (5’ and 3’ ends) of the individual strands of the above DNA double helix? (type underlined and in bold at each end where the underlines are placed) (2.5 pts)
3. What is the base sequence of the mRNA encoded by this DNA?
4. Indicate the sequence and 5’ and 3’ ends of the resulting mRNA (IOWs, type out the sequence with the ends indicated, i.e., 5’ – AUGCCCGAU...-3’)

Answer 1

a.

Answer – Out of the following strand only strand A while reading from the other hand has a start codon (ATG) and a stop codon (TGA) shown in bold as follows –

STRAND A: T T T A G T T A T C A A T C T T G G G T A G A A C

STRAND B: A A A T C A A T A G T T A G A A C C C A T C T T G

Strand A can also be written as

Strand A: CAAG ATG GGT TCT AAC TAT TGA TTT

Since, strand A consist start and stop codon, thus, strand B will serve as template strand for transcription generating mRNA with start and stop codon corresponding to strand A.

Therefore, Strand B is the template strand.

b.

Answer – The following will be the polarity of each strands –

STRAND A: 3’ - T T T A G T T A T C A A T C T T G G G T A G A A C - 5’

STRAND B: 5’ - A A A T C A A T A G T T A G A A C C C A T C T T G - 3’

c.

Answer – Since strand B serve as template, the following will be the base sequence of mRNA complementary to the template strand –

U U U A G U U A U C A A U C U U G G G U A G A A C

d.

Answer – sequence and 5’ and 3’ ends of the resulting mRNA would be

5’ - CAAG AUG GGU UCU AAC UAU UGA UUU – 3’