DNA: bases and double helix

DNA is a double helix of two strands that wind around each other like a twisted ladder. The rungs are base pairs: A always pairs with T, G always with C. The order of the bases is the information, and the fixed pairing makes exact copying possible.

DNA · DOUBLE HELIX AND BASE PAIRING AT GC TA BASE PAIRING A T A always binds T two hydrogen bonds G C G always binds C three hydrogen bonds Sugar and phosphate form the backbone of each strand. Each strand is the exact complement of the other, which is what allows copying.

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. Only A fits to T and G to C. If one strand reads A C G T, the opposite strand necessarily carries T G C A. This complementarity is no accident but the principle that makes copying and repair possible at all.

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.

Related topics

What is a gene?Gene structure: exon, intron, promoterFrom gene to proteinMutation and variantrsIDs, microarray and SNP lookup

Sources

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