Thymine (T)
Guanine (G)

Figure 1.7 DNA strands. Two DNA strands are held together by relatively weak hydrogen bonds between base pairs (bp); A specifically binds to T, and C to G to give rise to a DNA duplex or double helix structure. Most eukaryotic DNA is in a B-DNA right-handed helical structure of 10 bp per turn. The so called 5' end of the DNA strand has a terminal carbon atom number 5 without a phosphodiester bond while the at the 3' end of the strand there is carbon atom number 3. The two DNA strands always anneal together such that the 5' to 3' direction of one strand is opposite to, and complimentary in sequence to, the other strand. The sequence is normally quoted only for one strand in the 5' to 3' direction, either as 5'pGpApG-OH 3' (p is the phosphodiester bond, -OH is the terminal OH group at the 3' end of the strand) or in abbreviated form as 5' GAG 3'.

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