DNA, or Deoxyribonucleic acid, serves as instructions for living organisms. It resides in the nucleus of every cell. A DNA molecule looks like a twisted ladder shape known as a "double helix." The "rungs" of this ladder are built with four letters, A, C, T and G. These letters join together in specific ways to make each body part function as designed. In order to do this, DNA must continually copy itself so it can be passed on to other cells.
Humans are created by cell divisions. A baby begins as one cell inside its mother's body. The cell then goes through millions of divisions, as the DNA in that first cell is continually copied. Cells vary in the rate they are replaced by other cells. After a human is a few months of age, brain and nerve cells are rarely replaced. Yet cells that make up fingernails, hair and the linings of our stomach are copied constantly throughout our lifetime.
The letters on the ladder mentioned above, A, C, T and G, are nucleobases. They stand for Adenine, Cytosine, Thymine and Guanine. Nucleobases are nitrogen-based molecules. Every rung in the DNA ladder is made up of two of these bases. The bases are joined together to form base pairs. They can bond together only in specific ways. Cytosine always bonds with Guanine, and Thymine always bonds with Adenine. The base pairs can also unlink.
When the strands unlink, or separate, the bases on each strand then move toward free bases, which are extra bases that are contained in the nucleus of each cell. Again, only Cs can bond with Gs, and only As can bond with Ts. Each newly formed strand is just like the strand it has replaced. Once copied, the strands twist back into the double helix shape. Each cell ends up with the specific DNA it needs for its body function. Sex cells are duplicated by a process called meiosis. Other cells, called somatic cells, are duplicated by a process called mitosis.
Reading the Letters
Adenine, Cytosine, Thymine and Guanine are often compared to letters that make up words. These words relay the instructions for building proteins. The long sections, or words, of DNA that produce the codes for the proteins are called genes. A human cell contains 46 chromosomes, and each chromosome contains between 1,000 to 2,000 genes. These blueprints are what makes up each person's unique genetic code. No two people, with the exception of identical twins, have exactly the same set of genes.