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The nucleotides are identical except for the base, which can be one of four bases:

• adenine
• thymine
• guanine
• cytosine

There are chemical cross-links between the two strands in DNA, formed by pairs of bases held together by hydrogen bonds. They always pair up in a particular way, called complementary base pairing:

• thymine pairs with adenine (T-A)
• guanine pairs with cytosine (G-C)

These basic units are linked together to form strands by strong bonds between the deoxyribose sugar of one nucleotide and the phosphate of the next nucleotide. These strong bonds form a sugar-phosphate backbone.

The ends of the DNA strand are called:

• the 5' end (said as "5 prime end") at the phosphate end
• the 3' end (said as "3 prime end") at the deoxyribose end

The two strands of DNA are antiparallel which means that one strand runs in a 5’ to 3’ direction and the other runs in a 3’ to 5’ direction. This creates the twisting double helix structure of DNA.

All cells store their genetic information in the base sequence of DNA.

It is this base sequence which forms the genetic code.

The genotype is determined by the sequence of bases.