DNA and polymerisation

Chemistry higher tier

The structure of DNA

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 monomers 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. An outline of how the structure of the DNA polymer is shown below. The nucleotide molecules all contain the same sugar and phosphate group but there are four possible amine bases; so there are four different nucleotides.

Model of the dna structure showing nucleotides and nucleosides.

The sugar molecule in DNA

Ribose is a simple sugar molecule that is made in the human 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 showing two strands of DNA and showing how base pairs link across the strands.

Nucleotides and nucleosides

As mentioned above the monomers for the DNA polymer; the nucleotide molecules consist of a phosphate group, a sugar and a base all linked together. The sugar molecule found in DNA is called deoxyribose and there are four different bases found in the nucleotides. These bases are adenine, guanine, cytosine and thymine. These names are often simply shortened to the letters 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: Model of the structure of nucleotides and nucleosides found in DNA.

The structure of the DNA polymer resembles a coiled spring or to give its structure the proper name; it is referred to as a double helix (shown at the bottom of the page). The phosphate groups and the sugar molecules form the strands of the helix while the bases link together the two strands of DNA which make up the double helix. 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 is 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 between the two strands of DNA ONLY occurs between the bases 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 shows the twisted double helix shape of the DNA molecule and the image on the side of the page shows the base pairings across the two strands of DNA in the double helix structure.
The strands consist of the phosphate- sugar molecules with the bases paired up across the strands.

DNA double helix structure showing the base pairs linking across the strands.

Key Points

Practice questions

Check your understanding - Questions on the structure of DNA

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