Question

4. Given the following information:

One strand of a section of DNA isolated from E. coli reads

                                    5’-GTAGCCTACCCATAGG-3’

                                    3’ CATCGGATGGGTACC’5

1. Suppose that an mRNA is transcribed from this DNA using the complementary strand as a template. What will the sequence of the mRNA in this region be (please include orientation)?

2. How many different polypeptides chains could potentially be made from this sequence of RNA? Would the same polypeptide chains be made if the other strand of the DNA served as the template for transcription?

3. What peptide would be made if translation started exactly at the 5’ end of the mRNA in part A? In other words, what is the first amino acid made from the 1^st reading frame.

4. Assuming you are using the first reading frame, when tRNA^Ala leaves the ribosome, what tRNA will be bound next into the ribosome?

5. Suppose this stretch of DNA is transcribed as indicated in part A, but do you not know which reading frame is used. Could this DNA segment originate from the beginning of the coding region of a gene? The middle? The end? How can you tell? Hint, you must translate all three reading frames and analysis them all to gain insight if it could be the beginning, middle, or end or some combination. You will have to do that for all three reading frames.

Answer 1

1A. 5’-GUAGCCUACCCAUAGG-3

2A. This strand contains no AUG code in the directino of replication. As a result this section of mRNA would only code for a peptide if the was an AUG codon upstream. Assuming there is a start codon before this segment, there are three possible peptides it could code for. Depending on where the reading froam started the first codon in this strand could be GUA, or if it was one base to the right could be UAG, and finally, if the start codon was two bases over than the first full codon in this section would be AGC.

                    IF the other strand of DNA served as the template, the mRNA from that strand would code for a differnet peptide because of the anti-parrellility. However, if the other strand of DNA was trascribed it would code for a peptide because there is a start codon in this segment. If this was the beginning of the sequence, the reading fram would be set and there would be one possible peptide. IF the start codon were further upstream, there would be three different reading frames possible and therefore, the possibility of three differentsequences.

3A. IF translated exactly, this sequence would code for Val-Ala-Tyr-Pro-Stop.  When (ala) is ejected from the ribosome, (tyr) will be in the P site and (pro) will be in the A site. When alanine forms a peptide, the bond between tRNA and the amino acid would be broken.

4A. This sequence could be any part except for the beginning because the lack of AUG. It could be anything else because the stop codon could be immediatly after the segment or far from the end of the segment.