Genes - are segments of DNA that represent a unique product that is made inside the cell that allows the cell to function normally. Most genes code for proteins and some genes code for functional molecules like transfer RNA (tRNA) and ribosomal RNA (rRNA). The code in a gene is written with the letters (bases) A, T, G and C in a very specific order and that specific order determines the product being formed in the cell. For example, human red blood cells have the hemoglobin protein (red color of blood) and this protein is called hemoglobin A. This protein is made up of 4 subunits, 2 alpha subunits (alpha 1 and alpha 2) and 2 beta subunits. In other words, the subunit proteins are very similar to each other and 4 of them clump together to form the functional hemoglobin protein. The alpha subunits are coded by two genes called HBA1 (alpha 1) and HBA2 (alpha 2). The beta subunits are coded by a gene called HBB. When the HBA1, HBA2 and HBB genes are turned on, they will produce a message to the blood cells to produce hemoglobin. This message is called a messenger RNA (mRNA). This RNA is then used as a code template to make the protein subunits (alpha 1, 2 and beta) which when made by the cell will form a tetramer (4 subunit) protein which becomes hemoglobin A. This is how most proteins are made in the cells. Hemoglobin was used as an example here because it is one of the most studied of all proteins and we all have it (we all know - we have blood).

Similar to what was described above, there are other genes that code for proteins that influence everything from our eye color to our ability to hear properly. The arrangement of the letters (bases) in a gene is called the DNA sequence of that gene. It is set for each gene and if even one letter (base) is changed, it could affect the function of that gene. It could create a lot of problems and that is one way we get diseases. See Mutations in the Genetics Section and also the section on DNA. By the way, there are machines that can figure out the order of the letters in a gene or in any piece of DNA. You got it – that is called DNA sequencing.