<Ul> <Li> Deoxyarchaeosine </Li> <Li> 2, 6 - Diaminopurine </Li> </Ul> <Li> 2, 6 - Diaminopurine </Li> <P> Twin helical strands form the DNA backbone . Another double helix may be found tracing the spaces, or grooves, between the strands . These voids are adjacent to the base pairs and may provide a binding site . As the strands are not symmetrically located with respect to each other, the grooves are unequally sized . One groove, the major groove, is 22 Å wide and the other, the minor groove, is 12 Å wide . The width of the major groove means that the edges of the bases are more accessible in the major groove than in the minor groove . As a result, proteins such as transcription factors that can bind to specific sequences in double - stranded DNA usually make contact with the sides of the bases exposed in the major groove . This situation varies in unusual conformations of DNA within the cell (see below), but the major and minor grooves are always named to reflect the differences in size that would be seen if the DNA is twisted back into the ordinary B form . </P> <P> In a DNA double helix, each type of nucleobase on one strand bonds with just one type of nucleobase on the other strand . This is called complementary base pairing . Here, purines form hydrogen bonds to pyrimidines, with adenine bonding only to thymine in two hydrogen bonds, and cytosine bonding only to guanine in three hydrogen bonds . This arrangement of two nucleotides binding together across the double helix is called a Watson - Crick base pair . Another type of base pairing is Hoogsteen base pairing where two hydrogen bonds form between guanine and cytosine . As hydrogen bonds are not covalent, they can be broken and rejoined relatively easily . The two strands of DNA in a double helix can thus be pulled apart like a zipper, either by a mechanical force or high temperature . As a result of this base pair complementarity, all the information in the double - stranded sequence of a DNA helix is duplicated on each strand, which is vital in DNA replication . This reversible and specific interaction between complementary base pairs is critical for all the functions of DNA in living organisms . </P>

Which two molecules make up the backbone of dna