Each of the following DNA fragments has exactly one thing wrong with it. Indicat

Question

Each of the following DNA fragments has exactly one thing wrong with it. Indicate a) what is wrong with the fragment, b) a possible source of the problem, c) which DNA repair mechanism(s) are necessary to completely repair the problem, and d) what the correct DNA sequence should be. For all examples, assume the top strand to be the template strand. In the last example, assume T=T is a thymine dimer, and A-A is a normal continuation of the DNA sequence.

5’-ATCATACTGCATGCAT=TACTGACGAGTCATCTAC-3’

3’-TAGTATGACGTACGTA-ATGACTGCTCAGTAGATG-5’

Explanation / Answer

a. The fragment contains a thymine dimer (pyrimidine dimer) which is a molecular lesion. This dimerization destroyes the normal base pairing between two strands of DNA in that particular area.

b. The possible source of this problem is photochemical reaction. UV-light induces the formation of dimer by the formation of covalent bond between two consecutive pyrimidine base. Here thymine contains c=c double bond which take part in this dimerization reaction.

c. There are two mechanism for repairing of thymine dimer. I. Photoreactivation repair: The photoreactivating enzyme (PRE) is activated by blue light which breaks the dimer and restoration of normal base pairing occurs. II. Excision repair: The excision of dimer occurs by uvr system (endonucleases - multienzyme complex) and the gap created is filled by the proof reading activity of DNA polymerase I.

d. The correct DNA sequence after repair will be same. The only difference will be the absence of covalent bond between Thymine. Either of the two repair mechanism brings normal base pairing. Hence the correct sequence will be as follows:

5’-ATCATACTGCATGCATTACTGACGAGTCATCTAC-3’ 3’-TAGTATGACGTACGTAATGACTGCTCAGTAGATG-5’

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