Abstract
Differential scanning calorimetry (DSC) of double stranded DNA was carried out using plasmid pJL3-TB5 DNA (5277 base pairs), whose entire nucleotide sequence has been determined. The DSC curve for the linear DNA of the plasmid used in the present study gave seven separate peaks including two main peaks which range from 82° to 98°, suggesting that DNA melts independently at several cooperative blocks along the chain. The DSC curve obtained agreed with the histogram of the frequency versus GC content which was constructed from the GC distribution map along the DNA chain of pJL3-TB5. These results are obvious evidence that the thermal transition of DNA occurs blockwise along the molecular chain. The two main peaks were estimated to be due to the heat transition of the two distinct regions, containing 46-50% and 54-58% GC, respectively. DSC of covalently closed circular (ccc-) DNA of the same plasmid did not show any heat transition peak, indicating that ccc-form is stabilized compared with linear form.
