D.N.A.
D.N.A. stands for deoxyribonucleic acid
(dee-AHK-si-righ-boh-noo-clee-ik-
acid).
DNA is a thread like substance that can be found in almost all animal cells and plant cells. DNA is located inside the chromosomes, which are thread like bodies found in the nucleus, of every cell in the human body except red blood cells and ova and sperm (sex cells). The same DNA is in your foot, your finger nail, and your brain. The reason that your foot doesn't chew and your hair doesn't walk is because parts of your DNA can be turned on and off. If our DNA couldn't do that, we'd all look pretty weird. The sections of DNA that make each of us different are called genes. You inherit your genes from your parent.
DNA is shaped in a double helix which makes it look a lot like a twisted ladder. The outside of the ladder is the sugar backbone. The rung-like pieces are nucleic acids. The four nucleic acids are (T) thymine, (A) adenine, (C) cytosine, and (G) guanine. A will always pair up with T, and C will always pair up with G. Chemically those are the only bonds that fit together correctly, giving DNA its specific structure. We also have RNA (ribonucleic acid). RNA is chemically very similar to DNA and plays important roles in making proteins.
When you were created, the sex cells from your parent's had only 23 chromosomes each. When the two cells bonded, it made your first cell and you then had 46 chromosomes. Your first cell is called a zygote. Your first cell will then start mitosis and after thousands and thousands of cell divisions you will finally become you.
To learn a lot more about DNA, RNA, and other interesting topics like cloning and genetic engineering, you'll want to visit the I Can Do That Web site.