Molecular Biology Of The Cell 6th Edition by Bruce Alberts, Alexander Johnson, Julian Lewis, David Morgan, Martin Raff, Keith Roberts, Peter Walter

Exercise 7

You are investigating DNA synthesis in tissue-cul- ture cells, using to radioactively label the replication forks. By breaking open the cells in a way that allows some of the DNA strands to be stretched out, very long DNA strands can be isolated intact and examined. You overlay the DNA with a photographic emulsion, and expose it for 3 to 6 months, a procedure known as auto- radiography. Because the emulsion is sensitive to radioac- tive emissions, the 3H-labeled DNA shows up as tracks of silver grains. Because the stretching collapses replication bubbles, the daughter duplexes lie side by side and cannot be distinguished from each other. You pretreat the cells to synchronize them at the beginning of S phase. In the first experiment, you release the synchronizing block and add immedi- ately. After 30 minutes, you wash the cells and change the medium so that the total concentration of thymidine is the same as it was, but only one-third of it is radioactive. After an additional 15 minutes, you prepare DNA for autoradi- ography. The results of this experiment are shown in Fig- ure Q5-2 a. In the second experiment, you release the syn- chronizing block and then wait 30 minutes before adding After 30 minutes in the presence of midine, you once again change the medium to reduce the concentration of radioactive thymidine and incubate the cells for an additional 15 minutes. The results of the second experiment are shown in Figure Q5-2B.
E. coli and significantly lower than expected by chance. Why do you suppose that CG dinucleotides are underrepresented in the human genome?
E. coli and human genomes, there are no striking differences except for one dinucleotide, he frequency of CG dinucleotides in the human genome is significantly lower than in