Structure

In double-stranded DNA, the two DNA strands are held together by exact A: T and G: C hydrogen bonding between the bases of the two strands, in which case the two strands are said to be complementary. The two strands are oriented in opposite 5' to 3' directions, such that one strand is oriented 5' to 3' and the complementary strand is oriented 3' to 5' (T) in an antiparallel fashion (see Figure 1-2). In this case, "anti-" refers to the head (or 5' end) of one DNA strand being adjacent to the tail (or 3' end) of the opposite strand.

The molecular curves of the two DNA strands form antiparallel helices known as the DNA double helix. This double helix form (the B form) has ten nucleotide pairs (base pairs) per turn, occupying 3.4nm. Because the bonds between the sugar and the base are not perfectly symmet-rical,the strands curve slightly. The slight curve of the offset glycosidic bonds results in major and minor grooves characteristic of the B form of the double helix.5 Many molecular diagnostic assays target the minor groove of DNA with sequence-specific probes known as minor groove binding (MGB) probes. Two other forms of DNA exist as the Z and A forms. The Z form acquires a zigzag shape, while the A form has a very shallow and very deep groove.

Figure 1-2. Double-stranded DNA. The two DNA strands are oriented in an antiparallel relationship, with asymmetric base pairing of two DNA strands that generates the minor and major grooves of the DNA double helix. (Reprinted from Leonard D. Diagnostic Molecular Pathology, copyright 2003, with permission from Elsevier.)

Figure 1-2. Double-stranded DNA. The two DNA strands are oriented in an antiparallel relationship, with asymmetric base pairing of two DNA strands that generates the minor and major grooves of the DNA double helix. (Reprinted from Leonard D. Diagnostic Molecular Pathology, copyright 2003, with permission from Elsevier.)

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