Each strand of DNA is made of chemicals called bases.
Note that these are different to bases in relation to acids and alkalis in chemistry.
There are four different bases in DNA:
- thymine, T
- adenine, A
- guanine, G
- cytosine, C 
Complementary base pairing
There are chemical cross-links between the two strands in DNA, formed by pairs of bases.
They always pair up in a particular way, called Complementary base pairing
Two examples are:
- thymine pairs with adenine (T–A)
- guanine pairs with cytosine (G–C) 
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