1.1 The structure of DNA

Our genome is composed of DNA (deoxyribonucleic acid). DNA consists of two chains of nucleotides coiled around each other in a characteristic double helix structure. Each nucleotide consists of the following components:

• five-sided sugar

• phosphate group

• nitrogenous base

There are four main types of bases that form nucleotides:

• “A” (adenine)

• “G” (guanine)

• “C” (cytosine)

• “T” (thymine)

(Note, we tend to use the words nucleotide and base interchangeably. Technically, a base is just one component of an entire nucleotide.)

The two strands of DNA contain complementary base pairs. “A” pairs with “T” via two hydrogen bonds. “C” pairs with “G” via three hydrogen bonds. We often use “bp” to refer to base pairs. See the diagram below for more details.

Source: NIH Educational Resources

DNA has directionality. We always read DNA sequences in the “5 prime” to “3 prime” direction. (These are named after the positions of the carbon atoms in the sugars of the individual nucleotides that are joined by phosphate groups, which are annotated in the image below). Below is an example fragment of DNA showing the sequence of the two complementary strands:

5' - ACTGCTAGCT - 3'
3' - TGACGATCGA - 5'

The strand on the top would be read as “5’-ACTGCTAGCT-3’”. The complementary strand would be read as “5’-AGCTAGCAGT-3’”. Both of these sequences describe the same double stranded DNA molecule. We call the second sequence the reverse complement of the first sequence, since it is obtained by taking the reverse of the first sequence, and replacing each nucleotide with its complement.

The next section describes how this DNA is organized into the 46 chromosome copies of your genome.