The nucleotides are identical except for the base, which can be an adenine, thymine, guanine or 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:
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, and the 3' 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, and it is this base sequence which forms the genetic code. The genotype is determined by the sequence of bases.