Do mistakes matter?Every time one of your cells divides into two, it must copy the DNA code in its nucleus, one copy for each of the new cells. You have around 6000 million chemical 'letters' - base pairs - of DNA code in nearly every cell. Although they are not always copied perfectly, your cells correct almost all mistakes immediately. A few go undetected, but most of these changes, known as mutations, have no effect because they occur in the DNA that is not in a gene - non-coding DNA, sometimes called 'junk' DNA.
How are mistakes corrected?
Most DNA mutations occur when your cells divide. But your DNA is also under constant attack from other chemicals in your cells. Fortunately, your cells have a very efficient DNA repair system which is constantly scanning your DNA. Without these DNA repair enzymes, your cells would soon accumulate too much damage to survive. There are two main repair systems: one for removing and replacing a single base-pair mutation, and another for removing long stretches of unwanted, extra base pairs.
How can mutations affect genes?
Very occasionally, a DNA mutation occurs in a gene, changing its RNA molecule and sometimes affecting the protein it codes for. The RNA code uses four chemical 'letters' (bases), known as A, C, U, and G, in groups of three. Adding, removing or changing just one base can have a drastic effect on a gene, just as changing one letter in a sentence of three-letter words can alter its meaning.
What's the effect of changing a DNA base?
The DNA code CCA GCA CAC CAA tells the cell to add the amino acids proline, alanine, histidine and glutamine to the protein it is making. If just one of the DNA bases is changed, a different amino acid may be substituted. Adding or removing a base can have an even more drastic effect, as all the code that follows is disrupted.
Imagine changing just one letter in the sentence 'THE CAR WAS RED'. This can change its whole meaning. Adding or removing a letter from the sentence can have an even more drastic effect since all the words that follow are disrupted, making nonsense of the sentence. Similarly, changing, adding or removing a single letter in the DNA code for a gene can affect the protein it makes.