Figure 3-B-1.  Computer model of base pairing in DNA.  In a normal DNA molecule, adenine (A) is paired with thymine (T), guanine (G) is paired with cytosine (C).  The uracil (U) of RNA can also pair with adenine (A), since U differs from T by only a methyl group located on the other side of hydrogen bonding.


A DNA molecule has two strands, held together by the hydrogen bonding between their bases.  As shown in the above figure, adenine can form two hydrogen bonds with thymine; cytosine can formthree hydrogen bonds with guanine.  Although other base pairs [e.g., (G:T) and (C:T) ] may also form hydrogen bonds, their strengths are not as strong as (C:G) and (A:T) found in natural DNA molecules.

The following figure shows an example of base pairing between DNA’s two strands.

Figure 3-B-2.  Schematic drawing of DNA’s two strands.


Due to the specific base pairing, DNA’s two strands are complementary to each other.  Hence, the nucleotide sequence of one strand determines the sequence of another strand.  For example, in Figure 3-B-2, the sequence of the two strands can be written as

5′ -ACT- 3′

3′ -TGA- 5′

Note that they obey the (A:T) and (C:G) pairing rule.  If we know the sequence of one strand, we can deduce the sequence of another strand.  For this reason, a DNA database needs to store only the sequence of one strand.  By convention, the sequence in a DNA database refers to the sequence of the 5′ to 3′ strand (left to right).