Imagine doing an experiment in which a culture of non-dividing mammalian cells i
ID: 262685 • Letter: I
Question
Imagine doing an experiment in which a culture of non-dividing mammalian cells isexposed a single, brief dose of UV light. These cells are then cultured in medium withthymidine, a DNA precursor, that is radioactively labeled. Aliquots of these cells areharvested at different times after uv light exposure and their DNA is analyzed for theincorporation of thymidine. What’s observed is an increase within minutes in the amountof labeled thymidine followed by a plateau (i.e., no more increase) after about 30minutes in the incorporation of labeled thymidine into DNA. Explain these results.
Explanation / Answer
Cells are exposed to a brief dose of UV. These cells are undividing mammalian cells. Aliquots are taken out at different time points(under 30 minutes). First major concept to understand is that, UV affects thymine in the DNA. DNA when exposed to UV, forms thymine dimers and other sorts of DNA damage also arise such as double/ single strand breaks etc. Hence when the aliquots are taken at the start of the experiment, thymine is rapidly taken into the cell from the surrounding environment containing labelled thymidine. These tagged- thymidines get intergrated into the genome where damge has occured. This shows as a rising peak followed by a pleateu. This can be explained as the cells might have recoverd from the DNA damage and hence no thymidine was being taken up. Another explaination might be that the cells have suffered a great amount of damage by the UV radiation and there has been a lot of damage to the DNA. This might have triggered the cell to go into programmed cell death (apoptosis). Thus no more thymidine was being taken up from the media.