In the early 1990s the sequencing of human DNA was announced as the key to understanding how the human body works and, importantly, how it can be fixed when it breaks down.
The prospect of being able to cure all diseases was touted. Alas, unlocking the information contained in human DNA has proved far more difficult than originally thought. Here's why.
A single strand of DNA can be thought of as a train of molecules. Each carriage of this train is called a nucleotide and carries one of four different types of passengers, called chemical bases. The four types of base are denoted A, G, C, T for short. DNA comprises another train whose nucleotide 'carriages' sit directly opposite the first train. Its chemical base passengers face the first train's chemical base passengers.
Two rules apply to which passenger type is allowed in the second train. If a nucleotide carriage in the first train carries an A base, then the carriage opposite must carry a T base, and if the first train carriage has a G base the second must have a C base.
The twin strands of the DNA molecule are chemically bonded and spiral to form a double helix. In human DNA this is 3300 million base pairs long. The exact sequence of bases in a DNA molecule is unique to each person (identical twins are nearly, but not quite, the same). The total information content of DNA, including the base sequence, is called the genome. The goal of the Human Genome Project, which started in 1990, was to determine the sequence of the chemical base pairs that make up the DNA of a selected human. The base sequence of the first human was published in 2003 although the identity of that human was kept secret for privacy reasons.
DNA contains genes. A gene is a molecular unit of heredity and is the section of DNA that contains enough information to make polypeptide molecules which contain amino acids.The machinery of the cell links up the correct sequence of polypeptide molecules to make a particular type of protein. Human DNA contains about 23,000 genes which make about 100,000 different types of protein. Proteins have many functions, with some forming the building blocks of the body, while others are involved in chemical control and signalling and a third type are called enzymes (molecules that speed up chemical reactions).
These days human DNA can be sequenced with relative ease, so it has been possible to link variations and faults in genes to some diseases. Knowing the identity of the faulty genes would be the first step in prevention and cure. Although the genetic roots of many diseases have been traced, applying this technique to a huge number of diseases has turned out to be far more complex than originally thought, for the following reasons.
First, it was wrongly thought the information to make a given protein was contained in a single section of the DNA molecule. Many different sections provide the information to make a single protein and the sequence in which those sections of DNA are activated is critical.
Second, it was thought that only 3 per cent of the DNA molecule coded for proteins, the other 97 per cent being junk. It turns out that a substantial portion of this DNA is not junk but provides information to construct a molecule called RNA. There are many different types of RNA, each has a vital role to play, including regulating the information extraction of some genes and protecting cells from viral attack. There must be diseases caused by errors in the decoding of these sections of DNA.
Third, some diseases such as haemophilia have a clear genetic root and can be inherited from a parent. In many diseases the link is hazy. For example, there is a tendency for heart disease in some families.
Environmental factors play a role but there is evidence to show that many small mutations in DNA, individually having little effect, can come together in a person to increase the risk. This makes the process of disease prediction and eradication much more problematic.
It may be many, many years before the information in DNA can be fully deciphered and faults linked to most diseases. However, biochemical techniques and modern computing are playing a huge role in advancing the science.
- © Fairfax NZ News
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