Introduction to DNA and RNA

Introduction to DNA and RNA

Many observations contributed to the evidence from which the structure of DNA was eventually deduced by Watson and Crick:

1. Chemical analysis.

Analytical techniques of pure DNA revealed the basic constituent molecules, but did not show how they were joined together.

2. Chargaff's work on base equivalence.

Chargaff analysed the DNA from a wide variety of organisms and found that the ratio between Purines and Pyrimidines was constant. He also showed that the ratios of Adenine to Thymine and of Guanine to Cytosine were consistent.

Later this indicated the A-T and G-C bonding in DNA.

3. Franklin and Wilkins' work on X-ray crystallography.

DNA is a very ordered molecule and has a consistent symmetry. The technique of X-ray crystallography can reveal the pattern of such regular molecules. A beam of X-rays passing through the molecule is scattered by the atoms to give a distinctive X-ray diffraction pattern, which may be photographed and measured.


Ribonucleic acid (RNA) is also a polynucleotide. The chain of nucleotides is formed in exactly the same way as in DNA, but the molecule has some very important differences:

  1. It is a single stranded molecule.
  2. The pyrimidine Thymine never occurs but is always replaced by Uracil, another pyrimidine. (Think "No cup of T for U!")
  3. It is much smaller than DNA.
  4. It comes in three different forms, ribosomal, transfer and messenger.

Ribosomal RNA is 80% of the total RNA in a cell. It is involved with the formation of ribosomes and is therefore important as the site of protein synthesis in a cell.

Messenger RNA is 3-5% of the total RNA in a cell, depending on the protein synthesis activity at the time. It forms in the nucleus and is used to communicate the genetic code in the allele to the ribosome during protein synthesis.

Transfer RNA is a clover leaf shaped molecule and is up to 15% of the total RNA in the cell. It is involved in carrying the amino acids through the cytoplasm to their correct places in a growing polypeptide chain.

The DNA X-ray diffraction photographs showed the molecule was a double helix and also revealed the exact pitch of the helix, together with the layer distances. Each layer is of course one base pair.

James Watson and Francis Crick

In 1953, James Watson and Francis Crick, working at the Cavendish Institute in Cambridge put forward their model for the structure of DNA. This work was based on the previous research carried out by Rosalind Franklin and Maurice Wilkins into the X-ray diffraction patterns of crystalline DNA.

DNA is a polymer of nucleotides.

Nucleotides are made up of:

  • a phosphate.
  • a sugar - deoxyribose.
  • a base - either adenine, guanine, thymine or cytosine.

In DNA the sugar is always the same but each nucleotide will have only of the four nitrogenous bases. The phosphate sugar and base are linked together:

DNA nucleotide

DNA is a macromolecule polymer made of subunits called nucleotides. The nucleotides are arranged in two chains which are coiled into a spiral shape called a double helix.

nucleotides are linked together

The nucleotides are linked together, the sugar of one with the phosphate of the next:

DNA is the molecule from which the gene alleles on the chromosomes are made.

As with all nucleotides, those in DNA have three parts. These are a pentose sugar called deoxyribose, a phosphate group and a nitrogenous base.

The sugar and the phosphate are exactly the same in every nucleotide, but the base varies. There are four bases in DNA and each nucleotide contains one of them. The bases are called Adenine, Guanine, Thymine and Cytosine. (A,G,T and C for short).

The nucleotides are joined in a specific order. The order of the nucleotides means that the bases they contain are in a certain order, it is this order which forms the genetic code.

Look at these diagrams showing the sugar and the phosphate. Note that the sugar has its carbon atoms numbered according to their positions in the molecule.

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The sugar and phosphate join together to make the backbone of the DNA molecule. The 3' carbon on one sugar is joined to the 5' carbon on the next by means of a phosphate bridge, like this.

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Diagrammatically shown as:

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Each time the sugar joins to a phosphate, a molecule of water is eliminated in a condensation reaction.

This sugar-phosphate-sugar bond is called a phosphodiester bond.

The process repeats so that a very long chain of nucleotides is made, a polynucleotide. Note that the bases protrude from the side of the chain.

There will be a spare 5' sugar atom at one end of the chain and a spare 3' atom at the other. The chain thus has a 3' to 5' direction reading up the page.

In DNA a second polynucleotide chain forms next to the first, but this runs in the opposite direction. The chains are therefore described as antiparallel.


The bases now find themselves opposite one another and bond together with weak hydrogen bonds. When this occurs Adenine always pairs with Thymine (A-T) and Guanine with Cytosine (G-C). There is a good reason for this complementary pairing.

Adenine and Guanine both have a double ring structure and are classified chemically as Purine bases.

A purine molecule looks like this:

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Thymine, Uracil and Cytosine all have a single ring structure and are classified as Pyrimidines.

Their molecules look like this:

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When the base pairs form, a consistent spacing is obtained between the polynucleotide chains.

The whole double chained molecule is formed into a double helix spiral, caused by the bond angles between each base pair. Each complete turn of the spiral includes ten base pairs. This takes up a distance of 34 Angstrom units.


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