The Human Genome Project – discovering the human blueprint
Box 1 | Genes – the basic facts
Genes are made of DNA
To understand genes, we must first understand DNA (deoxyribonucleic acid) – the stuff of which genes are made. DNA is an enormously long, thin molecule, composed of compounds called bases strung together like beads in a necklace.
Each DNA molecule is made of two strands of DNA intertwined together. Within the DNA are four different types of bases (adenine, thymine, guanine and cytosine – often referred to by their first letters: A, T, G and C), each base is paired with another base on the opposing strand. The order in which the base pairs are arranged can be used to send a message – usually an instruction to other biological molecules about what to do and what to make. That is why bases are sometimes said to comprise the genetic alphabet.
Some stretches of DNA have no obvious function
Just as the letters in a whole book are not all stuck together, so DNA bases are organised into the equivalent of individual words, paragraphs or chapters. There is also punctuation and, it appears, plenty of gibberish that conveys nothing – probably the result of past errors in the copying process that takes place during reproduction. Stretches of DNA with no obvious function are sometimes referred to as junk DNA or non-coding DNA. However, the discovery of microRNA and its role in development has led to a new appreciation of parts of the genome that were once considered ‘junk DNA’. Some scientists believe that it is this 'junk' that is the key to the complexity possesed by some organisims that have a similar number of genes to other, much more simple organisms.
Genes contain information to make proteins
A gene is a sequence of DNA bases that typically codes for a protein. Proteins in turn manufacture and control all the living processes. For example, the gene for brown eyes is an instruction that gets the cells in the iris of the eye to make a dark pigment. A different sequence of bases would spell a different message with different consequences – rather like spelling out a different sentence using the same letters of the alphabet.
DNA molecules are organised into chromosomes
Each normal cell in an individual’s body (apart from egg or sperm cells) contains identical DNA molecules, organised into manageable units called chromosomes. In humans each cell contains 22 pairs of autosomes plus two sex chromosomes. Males have an X and a Y; females have two Xs. But not every gene is necessarily switched on in every cell. For example, the cells that make pigment in the eye also contain the genes for making tooth enamel or liver tissue but, fortunately, don’t do so because those genes are inactive.
The genome refers to all the genes within a species
Taken together, all the genes within a living thing or within a species are called the genome. Naturally, there are slight differences in the sequence of DNA bases between individuals of a species, but it’s obvious that all humans have more characteristics in common than they do differences.
The amount of information needed to make a living thing is staggering
If you’ve ever tried to follow do-it-yourself instructions to assemble a piece of furniture, you’ll realise how nightmarishly complicated would be the task of putting together a living thing. Although scientists are not yet in the business of much organism-building, they are already at the stage of trying to read and puzzle out all the instructions.
Very simple organisms, such as tiny viruses, can be made with only a few genes. Even though they are simple and small, each virus gene is still hundreds of thousands of DNA bases long. The first complete viral genome was worked out in 1977 in the United Kingdom by Dr Fred Sanger, a double Nobel laureate in science. Since then, molecular biologists have succeeded in reading the entire message of several viral genomes. Recently it was announced that the genome of a common yeast species has also been sequenced (ie, all the bases have been read). The information to make a yeast cell is a staggering 15 million bases.
But when we move to organisms larger than a single cell, the amount of information we are faced with makes a multi-volume encyclopaedia look like a comic book.
Consider this: a tiny worm has 80 million letters in its genetic library; the fruit fly has 155 million. And humans? Well, we don’t have the most by any means, but we do weigh in with a hefty 2800 million. That’s why the Human Genome Project is such a massive undertaking.
Box
Box 2. Gene mapping and DNA sequencing
Related sites
What is a genome? (Wellcome Trust, Sanger Institute, UK)
Genes and Chromosomes (NSW Health Centre for Genetics Education)
Page updated October 2005.