DNA and polymerisation

Chemistry higher tier

Nucleic acids such as deoxyribonucleic acid (DNA) and ribonucleic acid (RNA) are polymers. These polymers have special significance in living cells as they carry the genetic information of the organism. Coded in the DNA is information used to control cell growth and development as well as the information needed to carry out all the processes which keep the cell and ultimately the organism alive. DNA is a very large molecule and like many large molecules it is a polymer composed of many smaller units linked together. In the case of DNA the smaller monomer units that link together are called nucleotides.

Nucleotides are the momomers from which the DNA polymer is built. The nucleotides consist of a base (sometimes called an amine base), a sugar molecule and a phosphate group all linked together. To work out the structure of the DNA polymer chemists will hydrolyse (break down) the polymer into smaller parts in an attempt to see how it is actually built up. The structure of DNA is outlined below:

dna structure

The sugar molecule in DNA

Ribose is a simple sugar molecule that is made in our body. Many people, mainly athletes, take ribose supplements as it is claimed to give you extra energy and reduce fatigue. Its chemical formula is C5H10O4. Its structure is shown below.

structure of ribose sugar and deoxyribose

You can see that ribose sugar has a ring structure in the shape of a pentagon. The carbon atoms in the ring are mostly joined to a H atom and an O-H group. The second sugar molecule shown is deoxyribose, this is very similar to the structure of ribose sugar except there is one atom of oxygen missing on the second carbon atom in the ring (deoxy indicates that oxygen is missing). This sugar deoxyribose is the one found in DNA.

DNA structure

Nucleotides and nucleosides

As mentioned above the monomers for DNA, the nucleotide molecules consist of a phosphate group, a sugar and a base all linked together. The sugar found in DNA is deoxyribose and there are four different bases found in the nucleotides. These bases are adenine, guanine, cytosine and thymine. These names are often shortened to A, G, C and T. The nucleotide molecules can be hydrolysed to give simpler molecules called nucleoside. Nucleosides consist only of a sugar molecule, deoxyribose and a base, one of adenine, thymine, cytosine or guanine, this is shown below: structure of nucleotides and nucleosides

The nucleotide monomers link up through the phosphate groups to form a strand of the DNA polymer as shown in the image on the side of the page. This gives the basic structure of DNA as shown where there is a backbone of phosphate-sugar molecules linked up with the bases sticking out from the sugar molecule. Remember the bases are either one of four molecules, A,T,G or C. The order or sequence of these bases are different in different people and this is one of the factors that makes us all different from one and other.
In 1953, James Watson and Francis Crick suggested the structure of DNA consisted of two of these polymer strands shown opposite twisted together in a double helix structure. The two strands of nucleotides run in opposite directions to each other. The two strands are attracted to each other through weak intermolecular bonding between the bases on each strand. However Watson and Crick found that the intermolecular bonding ONLY occurs between adenine (A) and thymine (T) on different strands and between guanine (G) and cytosine (C) bases on different strands.
The reason for this A......T and G.....C base pairing on different strands is simply due to the shapes of the base molecule which allows them to fit together easily and so allows strong intermolecular attraction between the two DNA strands. The image below below the twisted helix shape of the DNA molecule.
The strands consist of the phosphate- sugar molecules with the bases paired up across the strands.

DNA double helix structure

Key Points