Читать книгу Snyder and Champness Molecular Genetics of Bacteria - Tina M. Henkin - Страница 53

The sugar and phosphate groups of DNA form what is often called a backbone to support the bases, which jut out from the chain. This structure allows the bases from one single strand of DNA to form hydrogen bonds with another strand of DNA, thereby holding together two separate nucleotide chains (Figure 1.3B). The first clue that pairing between specific bases could form the basis for the structure of DNA came from Erwin Chargaff's observation about the ratios of the bases; no matter the source of the DNA, the concentration of guanine (G) always equals the concentration of cytosine (C) and the concentration of adenine (A) always equals the concentration of thymine (T). These ratios, named Chargaff's rules, gave Watson and Crick one of the essential clues to the structure of DNA. They proposed that the two strands of the DNA are held together by specific hydrogen bonding between the bases in opposite strands, as shown in Figure 1.4. Thus, the amounts of A and T and of C and G are always the same because A's pair only with T's and G's pair only with C's to hold the DNA strands together. Each A-and-T pair and each G-and-C pair in DNA is called a complementary base pair, and the sequences of two strands of DNA are said to be complementary if one strand always has a T where there is an A in the other strand and a G where there is a C in the other strand.

It did not escape the attention of Watson and Crick that the complementary base-pairing rules essentially explain heredity. If A pairs only with T and G pairs only with C, then each strand of DNA can replicate to make a complementary copy, so that the two replicated DNAs will be exact copies of each other. Offspring containing the new DNAs would have the same sequence of nucleotides in their DNAs as their parents and thus would be exact copies of their parents.