In this lesson, we will define the term "gene". The nuclei in your cells contain DNA, which is the genetic material that you got from both of your biological parents - half from mom and half from dad. Your DNA is arranged in 23 pairs of chromosomes. Each of these chromosomes contains genes. There are over 20,000 of them in total for humans. And these genes determine who you are and what goes on in your body. Let's do some review. Amino acids are the basic building blocks of proteins, and amino acids join together by peptide bonds to form polypeptides. Proteins, in turn, are made of one or more polypeptides. Which amino acids are linked together and the specific sequence in which they are linked together determines the protein. For example, actin, a major protein in your muscles, has specific amino acids linked together in a certain order. Myosin, another muscle protein, has a different combination of amino acids. Now let's get back to genes. A gene is a segment of DNA that contains the instructions for making a single polypeptide. Put more simply, a gene determines which amino acids link together and in what order to make a specific polypeptide. And a gene is a molecular unit of heredity. It's what is passed on, generation to generation. Recall the basic structure of DNA, which is similar to a ladder. The two upright parts are made of alternating sugars and phosphates and each step on the ladder is one of four bases: adenine, guanine, cytosine, or thymine. Now imagine the ladder twisting, more like a spiral staircase. This is the form of DNA and it is referred to as a double helix. Only one strand of DNA is used to make a polypeptide and the information needed is stored in the DNA base sequence of the gene. Each three DNA based form the code for a specific amino acid and thus the order of the DNA bases, from the beginning of the gene to its end, determine which amino acids will be linked together and their order, and that determines what polypeptide is made. To make a polypeptide, the DNA strands unwind only where the gene to be used in located and a strand of messenger RNA, or mRNA, is built off of the DNA template. This process is called transcription and the information from the gene's DNA base sequence is coded into the base sequence of the mRNA. The mRNA then leaves the nucleus and goes to a ribosome where the coded information is now used to build the polypeptide, one amino acid at a time, in the process called translation. It may seem from this that genes have a very narrow function, which in a way is true. They carry the instructions for making polypeptides, but they also have regulatory functions determining which genes are expressed in which cells, which in turn determines what your cells can do. And proteins are made of the polypeptides that are built according to your genes, which means your genes determine the production and development of things such as your muscles, your hair and nails, the hemoglobin that carries oxygen to your cells, the antibodies that keep you healthy, many of your hormones, pigments that color your hair, your eyes and your skin, and the enzymes that promote all of those chemical reactions that occur in your body, to name just a few. To a great extent, you are who and what your genes build.