For an organism to grow it must make new pieces of itself and then assemble these pieces into something functional. These simple units can then form larger and larger structures. In humans this process starts with proteins then moves on to form cells, then tissues, then organs and eventually whole bodies! In viruses this process is much shorter and simpler beginning with proteins which are then assembled into the final product, a virion (a single virus particle).

So what tells the machinery of an organism to make proteins? Simply, the information is stored by a blueprint or template within deoxyribonucleic acid or DNA whihc forms the organism's genes. But, even though DNA has all the information needed to make proteins, it needs to be "decoded" into something that enzymes can recognise. This process is carried out by ribonucleic acid or RNA. RNA is a middle-man or mediator for the early steps required to produce an organism - be it a human or a virus. RNA is also needed as a template to make more copies of the DNA. So you can see that there are two processes going on - making proteins from the DNA, and making new DNA copies from the original DNA.

What we have just described is a general plan. There are some exceptions to these rules and one big exception is found among viruses. Some viruses don't have genes made of DNA, they are made of RNA instead. Becauseof these, some viruses have developed special ways of making proteins and copies of their genes. More on that later.

DNA was discovered in 1869 by Friedrich Miescher, a Swiss physician. He foudn an acidic substance in the nuclei fo cells that he named nucleic acid. It was not until 1930 that Albrecht Kossel and Phoebus Levene established that nucleic acids were in fact DNA which consisted of repeating molecules containing sugar, nitrogenous bases and phosphate.

Nucleic acids are one of several macromolecules found in the body (others include proteins and carbohydrates) which are formed by lots of individual molecules (nucleotides) strung together to form a polynucleotide. Each nucleotide consists of a sugar, a nitrogen base and a phosphate group. In RNA the sugar is called ribose (how the name ribonucleic acid comes about), and in DNA it is called deoxyribose which means that it is missing ("deoxy") a carbon atom compared to ribose.

The combination of a sugar with a any one of five different nitrogen bases creates a nucleoside. The five bases are divided into two categories based on the structure of their molecules; purines have two ring structures (adenine and guanine) while pyrimidines have one (thymine, cytosine and uracil). Adenine, guanine, thymine and cytosine are found in DNA whereas RNA replaces thymine with uracil.

If a phosphate molecule is added to a nucleoside it becomes known as a nucleotide. Nucleotides with a ribose sugar are therefore ribonucleotides, and nucleotides with a deoxyribose sugar are deoxyrobonucleotides. Each nucleotide's name can be shortened to a single letter, A for adenine, C for cytosine, G for guanine, T for thymine and U for uracil.

The chemical bond that links one nucleotide to another is formed between the phosphate group of one nucleotide and the sugar group of the next nucleotide via an ester bond between a carbon atom and an oxygen atom. Two things remain the same no matter how many nucleotides are added to the growing polynucleotide chain; one end of the chain has a free phosphate group and the other end has a free hydroxyl (-OH) group. These ends are called 5' ("five prime") and 3' ("three prime") respectively.

This naming system comes from the way we present a sugar structure when we draw it on paper. We start at the "top, right-hand" carbon and count in a clock-wise manner. The phosphate group of the previous nucleotide is linked to carbon number 5, and the phosphate group of the next nucleotide is linked to carbon number 3. The naming system, 5' to 3' is used to describe the order of the nucleotides in the DNA strand. Think of the system as being similar to the way European people are taught to read and write - from the left side of a page to right side.