There are 16 ways for 4 kinds of bases to combine as a pair within DNA primary sequence (for example, ct, ag, cg, tc and so on.), and the frequencies of occurrences of different pairs in sequence are not the same. Especially, the frequency of cg is very low. In this article, we take the frequency of cg as parameter and calculate the molecular connectivity indices. Then, we compare the sequences of 10 species based on the indices and group them into different classes. The results agree with the phylogenetic tree satisfactorily.

Key words: DNA primary sequence, cg pair of bases, molecular connectivity indices, similarity analysis