 So let's spend a few minutes thinking about DNA. Just living today it's hard to listen to the media and not learn something about DNA or hear about DNA for a criminal case. Recently this has been very much in the media. DNA is really the material that allows for heredity to take place. In other words, parents pass certain traits onto their offspring and they're able to do this because of DNA. All living organisms use DNA of one form or another. So DNA is a very important molecule or macro molecule for us to understand. A gene is really the unit of heredity. When parents pass genetic information onto their offspring they do that in the form of genes. A gene may code for yellow pigment to be laid down in a feather of a bird or a gene may code for the hormone insulin and so it will cause cells or allow cells to produce insulin. So genes are very important to life as we know it. Now DNA is arranged into long threads in eukaryotic cells and these long threads of DNA are called chromosomes and genes are housed on chromosomes. There may be many genes on one particular chromosome. The way that genes work is according to what we call the genetic code. DNA is made up of four basic building blocks and these building blocks are represented in this diagram. There is a building block that we represent with an A because of its structure, one that we represent with a T, one that we represent with a G and one that we represent with a C. And so A, T, C and G work very much like letters in our alphabet do. So if you look at these five letters you see I've arranged them so they spell trees but if we put it in another arrangement it spells terse, two words that have very different meanings but have the same letters and the same number of letters. So the way the genetic code works is a sequence of the building blocks of DNA can represent one particular type of protein. So when humans and all organisms pass genetic information on to their offspring they do so by using DNA.