Base Complementarity
Definition
The specific pairing rules that govern double-stranded DNA: adenine pairs with thymine (2 hydrogen bonds) and guanine pairs with cytosine (3 hydrogen bonds). Any other combination is geometrically incompatible with the double helix. This rule is why knowing one strand's sequence immediately specifies the other strand's sequence.
- A pairs with
- Thymine, via 2 hydrogen bonds
- G pairs with
- Cytosine, via 3 hydrogen bonds
- Basis of
- Double helix geometry
- Practical consequence
- One strand's sequence fixes the other
Common questions
Why does base complementarity matter for STR profiling specifically?+
PCR amplification and probe or primer design both depend on knowing the exact complementary sequence flanking a repeat region, so primers can bind and copy only the target locus. Without reliable complementary pairing, the targeted amplification that STR typing depends on would not work.
What happens when a base pairs with a non-complementary partner?+
A mismatch like that is geometrically strained and destabilises the helix at that position, which is why DNA polymerases have proofreading mechanisms to detect and correct such errors during replication. Occasional uncorrected mismatches are one source of natural mutation.
Related terms
- Antiparallel Orientation
- The two strands of the DNA double helix run in opposite directions: one strand reads 5' to 3', its complement reads 3'...
- Denaturation
- The separation of double-stranded DNA into two single strands by disrupting the hydrogen bonds between base pairs. In PCR, denaturation is achieved...
- Deoxyribonucleotide
- The monomer unit of DNA, consisting of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (adenine, thymine, guanine,...
- GC Content
- The proportion of base pairs in a DNA molecule that are guanine-cytosine. Because G-C pairs form three hydrogen bonds (versus two for...
- Phosphodiester Backbone
- The repeating sugar-phosphate chain that forms the structural scaffold of each DNA strand. The backbone is highly charged and hydrophilic, faces outward...