Exposure to ultraviolet radiation causes thymine dimers in the promoter region f
ID: 195516 • Letter: E
Question
Exposure to ultraviolet radiation causes thymine dimers in the promoter region for the abcX gene. Following nucleotide excision repair of the dimers, mutations are introduced into the promoter. Which of the following would be an expected outcome? You isolate ABCX protein from normal cells and from cells carrying the mutation and run them on native agarose gel electrophoresis, and the protein from cells carrying the mutation migrates faster that the protein from normal cells You perform a standard cDNA microarray using cDNA from normal cells as the control sample and cDNA from cells carrying the mutation as the experimental sample, and you obtain a green spot for abc)x O You perform a Southern blot using a probe complementary to a portion of one of the introns of abcX, and you get no band. You perform PCR using primers that detect a portion of the second exon of abeX and you get no bandon the gelExplanation / Answer
Answer
The change in the nucleotide sequence of the genome or genetic elements is known as mutation. Mutations occur when DNA is exposed to ultra violet radiation. The exposure of DNA to UV rays result in the formation of covalent bonds between two pyrimidine residues at the carbon-carbon double bonds. This forms the pyrimidine dimers. If these covalent bonds are formed between two thymine residues, it is thymine dimer.
Formation of thymine dimers in DNA forms a lesion. This is done repair by photoreactivation or excision repair. Photoreactivation just restores the normal base pair by breaking the dimer.
Excision repair may be nucleotide or base excision repair. In nucleotide excision repair, the effected DNA strand is repaired by replacing the damaged template with the normal template. During repair of these dimers there is a chance of mutation.
These repaired genes are screened by DNA microarray test. In this the sample gene binds to labelled DNA and hybrid DNA is formed. The hybridized DNA appears red and least hybridized appears green color. The given abcx gene is already repaired, it shows green spot in this test.
As the mutation occurs at promoter region, it reduces the mRNA and so the protein of the cell. The second exon may not be detected as mutation occurs at the promoter region. In southern blotting the probe used may be complementary to the intron sequence of the DNA strand, this results in band formation.
From the above explanation, the option b is correct. A green spot may be detected in the cDNA microarray test.
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