Chemical Structure of DNA Subunits
DNA is a polymer made of nucleotide subunits. A nucleotide consists of 3 chemical groups; a sugar, a phosphate and a nitrogenous base (Fig. 1). In the case of DNA, the sugar is deoxyribose.
Figure 1. Chemical structure of a nucleotide. A nucleotide consists of three chemical groups – a phosphate group (red), deoxyribose sugar (blue) and a nitrogenous base (green). Adapted from Chemical structure of deoxyadenosine monophosphate by Hbf878, 2018. CC0 1.0 Universal Public Domain.
All DNA nucleotides have the same phosphate (orange circle) and sugar (blue circle) shown in Figure 1. In the case of DNA, the sugar is deoxyribose. RNA nucleotides (an alternative form of nucleotide used in various cell processes) has the same structure as DNA nucleotides, though it’ll have a ribose sugar instead of a deoxyribose sugar. The nitrogenous base in DNA varies, which we’ll talk about in a moment.
Note that the carbons on the deoxyribose sugar are numbered in Figure 1. Numbering carbons allows a biochemist to describe the bonding and the structure. Let us see if you can look at a DNA nucleotide the way a biochemist will.
There are 4 different nucleotides in DNA that are a result of 4 different bases. These bases are named adenosine (A), cytosine (C), guanine (G) and thymine (T). The purine nitrogenous bases (A and G) are bigger because they are made of two rings of atoms. The pyrimidines bases (T and C) are one ring and smaller. Figure 2 shows the structure of these four nitrogenous bases and how the structure allows them to form hydrogen bonds and pair in the DNA molecules.
Figure 2. The nitrogenous bases of DNA and their H-bonding. Notice there are two hydrogen bonds between adenine and thiamine and there are three between guanine and cytosine. L. Johnson & M. Sutter, 2018, CC BY-NC-SA 2.0.