The Building Blocks of DNA: What Is DNA Made Of?

The building blocks of nucleic acids are called nucleotides. There are only four types of nucleotides. This is one of the reasons why nucleic acids seem relatively simple compared to proteins. Each nucleotide has a sugar that forms a ring. Bonded to one part of this sugar is something called a phosphate group. This phosphate group is just a phosphorus atom with a bunch of oxygen around it. Bonded to another part of the sugar is a nitrogenous base. It is the nitrogenous base that differs from nucleotide to nucleotide. There are four different kinds of nitrogenous bases.

Organic chemists like to number carbons. They actually love carbons because organic chemistry is all about carbons, so they don’t even write them down when they are drawing structures of molecules. If there is a line between two elements, you must assume that there is a carbon there. Instead of writing them, they number the carbons in the sugar around the ring counter-clockwise from one to six. Like this:

There are four kinds of nitrogenous bases. These can be classed into two different groups which differ in size. The pyrimidines have a single six-element ring that’s made of carbons and nitrogens. There are two kinds of pyrimidines, we call them cytosine and thymine. The details aren’t important but these are two kinds of pyrimidines. Here we have drawings of them in order: cytosine and thymine.

The purines, the second kind of nitrogenous bases, are bigger. They are composed of two rings. In addition to the six-element ring that the pyrimidines are made of there is an additional five-element ring that is attached on the side. There are two kinds of purines: adenine and guanine.

We usually refer to the different nucleotides that we find in nucleic acids by the single letter which designates the type of base that it has. We have:

A: adenine.
G: guanine.
C: cytosine.
T: thymine.

Next time we’ll begin to see how this relatively simple polymer, with phosphates and sugars, could serve as a code.

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