DNA: The Information Molecule
The Double Helix Molecule, Watson and Crick and the Human Genome
The history and structure of the most important molecule in living organisms, Deoxyribonucleic Acid.
One of the most important molecules in biochemistry is Deoxyribonucleic Acid (DNA). This article will attempt to describe the structure of the molecule and some of the chemical features which make it so important.
Nucleic Acids
The nucleic acids were first isolated from a cell in 1868. The name comes from the fact that they were acidic and were found in the nucleus of a cell. There are two types of nucleic acid: deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Molecular biologists spent many years puzzling over the way information was stored and communicated within a living organism until experiments by Oswald Avery in the 1940s led researchers to deduce that the information was carried in DNA.
Double Helix
The question of the structure of DNA was puzzled over throughout the first half of the twentieth century, culminating in the famous double-helix model produced by James Watson and Francis Crick on February 28th 1953. In the next fifty years scientists successfully determined the code in the human genome.
Natural Polymer
DNA is so amazing because it has so few variations available and yet it is used in living organisms to make every single type of protein used in every process. DNA is basically a polymer, a long chain molecule based on repeating units (monomers). The monomers in synthetic plastics are usually exactly the same which would be useless for carrying information. In living organisms, the monomers are usually of the same type, allowing them to be joined together, but they have groups added on which are often different.
Amino Acids
Proteins, which play an important part in the workings of cells, are made up of twenty different monomers called amino acids. DNA, on the other hand, uses only four different types of monomers, which are actually always paired together. The fact that all the information needed by a cell is coded on four units is amazing.
Organic Bases
The backbone of the DNA molecule is a chain of phosphorylated sugar groups. The sugar is a ring made up of four carbon atoms and an oxygen atom called deoxyribose. These are then joined together by a phosphate group. On one of the carbon atoms of the deoxyribose an organic nitrogen base is attached. It is these bases that are different. Two bases are of the type “Purine”, called Adenine and Guanine, and two are of the type “Pyrimidine”, called Cytosine and Thymine.
Watson and Crick determined that DNA takes a double helical structure. Imagine a long chain of phosphorylated ribose units with bases sticking out at one side. Then imagine another chain with its bases pointing towards the bases on the first chain. The bases pair up: Adenine to Thymine and Cytosine to Guanine. The pairs are connected with two or three hydrogen bonds and the chains twist together to make a double helix.
Unzipping
This structure is elegant for several important reasons. The most important is that it can be replicated by “unzipping” the hydrogen bonds, forming two chains again, and then adding a new chain to each of the original chains. This forms two identical DNA helices.
Transcription
The code in DNA comes from groups of three bases which are translated into one amino acid for a protein. Again the DNA is unzipped, and this time RNA is used to take the information to the place where amino acids are joined together in the exact sequence necessary for the purpose of the protein.
A beautiful video of DNA Replication can be found here.
A demonstration of how proteins are made from RNA can be seen here.
