If all the DNA could be pulled out of the 46 chromosomes of one cell and stretched end to end it would measure about two metres in length - yet is so thin that it is barely visible even with the most powerful microscope. If all the DNA in a human body were stretched end to end it would reach to the moon and back about 10,000 times. The DNA strand is divided up into genes. Every cell of a person's body has an identical set of about 100,000 genes. These genes carry the coded instructions for the manufacture of the basic building blocks of the human cell, known as proteins.
Different genes issue different instructions.
Let's use an imaginary instruction to illustrate how this works. We'll pretend that we are a gene that is instructing a protein to eat some pie. Now, if even just one letter is missing, the whole instruction can become garbled and the action will not take place. This is called a point mutation and clearly the protein will not understand what we are telling it to do. Sometimes matters become worse, and a whole chunk of the instruction goes missing.