DNA looks like a long double-stranded helix. The two strands are said to be complementary - the "sequence" on one strand determines sequence on the other. One reason for this is for protection against mutation. If something happens to one of the strands of DNA, it is possible to read the opposite strand to make a new copy of the damaged strand. To simplify things, think of DNA as a long piece of ticker-tape. Written on this ticker tape are all sorts of instructions, or genes. Genes are discrete units of information that tell cells what to do. For example, there might be a gene for brown hair or a gene for green eyes. Each one of our cells contains the exact same DNA sequences, or genes (with some exceptions). There are controls in place to make sure that the correct genes get expressed in the correct tissues - to ensure that you grow fingernails only on your fingers and hair only on specific parts on your skin, etc.

There are systems in our cells that read the DNA sequence and "translate" it into protein. Proteins are made by linking amino acids together. When you eat protein, it gets broken down into amino acids in your stomach - that's why it is important to eat enough protein! DNA contains the instructions, proteins carry out the instructions.

Common proteins
Name	Function
Collagen	Structural in skin
Keratin	Waterproofing
Myosin	Structural in muscle
Hemoglobin	Carry oxygen in blood

So if DNA makes proteins and proteins perform functions, then the ultimate goal of genetic engineering is to alter proteins. Since proteins don't last very long, they are poor targets for genetic engineering. DNA is copied from one generation to the next, from parents to children. Changes in DNA can be carried over generations, making it a good target for lasting changes. There are two basic types of modifications - addition or deletion of function. To add a function to a cell all you have to do is introduce a new gene that codes for the given function. Deletion of function can be performed by either "knocking out" a gene or introducing an "antisense" gene to interfere with the cell's ability to express a given gene.

Next we will go over regulation of gene expression. Regulation is necessary to make sure that genes are only expressed in appropriate tissues.