Cytosine (C) and Guanine (G) are more steady under expanding heat since C and G have three hydrogen bonds while Adenine (An) and Thymine (T) have just two. The more hydrogen bonds there are, the more steady the nucleotides are. More bond dependability requires more warmth vitality to separate the securities, and since G≡C have more hydrogen securities than A=T, they are thusly more steady under expanding heat.
