(1) GC-content is the percentage of the number of G (guanine) bases and the number of C (cytosine) bases with respect to the total number of bases in an oligonucleotide.
(2) G-C and A-T base pairs are formed in DNA. The G-C base pair is more stable than the A-T base pair, since the G-C base pair has three hydrogen bonds, while the A-T base pair has only two (see Fig. 1).
(3) It is thus important from a stability standpoint how many G-C base pairs are present in ds-DNA (double strand DNA). In other words, the amount of G-C base pairs affects Tm. Although the Tm depends not exclusively on the G-C content, the Tm tends to increase as the G-C content increases. Fig. 2 shows a rough correlation between G-C content and Tm. Values of Tm in a range enclosed by a dashed line are suitable for the PCR. The G-C content may be adjusted in accordance with the data in Fig. 2.