DNA: bases and double helix

DNA consists of two antiparallel strands in a double helix. Its bases pair complementarily: A with T and G with C. This pairing helps copy the sequence accurately during replication; enzymes check and correct many errors.

Key points

Four bases form the language of DNA: adenine, cytosine, guanine and thymine, A, C, G, T for short.
Pairing is complementary: A with T via two, G with C via three hydrogen bonds.
Because each strand is the template for the other, DNA can be duplicated with few errors.

#The four letters

Each strand is a long chain of building blocks, the nucleotides. Every nucleotide consists of a sugar, a phosphate and one of four bases. Sugar and phosphate form the load-bearing backbone, the bases point inward. The order of the bases along the strand is the actual message.

#Why there are two strands

The two strands run in opposite directions and are held together by hydrogen bonds between the bases. A pairs with T, and G with C. If one strand reads A C G T, its complementary sequence is T G C A. This structure allows one strand to serve as a template during DNA replication; enzymes and repair mechanisms also help maintain accuracy.

#From the helix to the sequence

For analysis the spatial helix is reduced to a plain sequence of letters. At the vast majority of positions all humans carry the same base, that is the reference. At some positions people differ by a single letter, and exactly such sites are the markers that Genome reads.

What Genome measures. What Genome reads at a position is ultimately the question of which base sits there: A, C, G or T. From these letters genotypes and markers arise.

Sources

1Watson & Crick, 1953 Molecular structure of nucleic acids: a structure for deoxyribose nucleic acid. Nature 171:737-738. doi.org/10.1038/171737a0
2Franklin & Gosling, 1953 Molecular configuration in sodium thymonucleate. Nature 171:740-741. doi.org/10.1038/171740a0