It’s a Snip!
To understand a little of the science of genomics, it helps to have a quick biology lesson first. Most of us can visualise the familiar twisted double helix model of DNA. In genetic terms, DNA is the recipe book containing the genes, the ingredients for the proteins that build us and regulate and maintain our bodily functions. Our genetic ingredients are packaged in chromosomes, of which we have 23 pairs in the nucleus of each of our cells. Each chromosome is like a separate pantry, containing hundreds or thousands of genes. Our genome is the body’s complete set of DNA.
Each of the two strands of DNA is made of four bases – chemical units known as A, C, G and T, the letters that comprise our genetic alphabet. We inherit 3 billion bases from our mother and 3 billion from our father. That’s a mind-boggling number. As Nessa Carey writes in her book Junk DNA: “If DNA were a ladder, with each base representing a rung and each rung was 25cm from the next, the ladder would stretch 75 million km, roughly from Earth to Mars. The complete works of Shakespeare are reported to contain 3,695,990 letters. This means we inherit the equivalent of just over 811 books the length of the Bard’s canon from Mum and the same number from Dad. That’s a lot of information.”
But wait – there’s more! The genetic variations we are discussing in this story are not the ones in the protein-coding genes themselves, but are so-called single-nucleotide polymorphisms, or SNPs (pronounced Snips), which are differences in the bases of genes and 90% of which are found in what used to be called our “junk” DNA.
We’ve got about 10 million SNPs, and through studying variations or clusters of variations in these, scientists are discovering just how important their role is in predisposing us to hundreds of diseases. The differences are subtle – if we think back to the genetic alphabet, the variation can be in just one letter of the four in each chemical unit. So a defect in a single protein-coding gene will have a dramatic effect – it can cause a devastating disease such as Huntington’s, for example, while variations in SNPs will operate more like dimmer switches on lights and can subtly affect the expression of other genes in proximity to them.
We inherit 3 billion bases from our mother and 3 billion from our father. That’s a mindboggling number.