No, really, what is DNA?

Friday, August 15, 2008
by Conrad Storad

DNA is very important stuff. But what is it, exactly?

The definition can take many forms. A science dictionary will tell you that DNA is short for deoxyribonucleic acid. DNA is a major component of chromosomes, the structures that carry genetic information inside a living organism.

DNA can reproduce itself. It is the substance that cells and other living things use to pass hereditary traits from one generation to the next. That helps. But you probably want to know more.

DNADNA is a molecule. To be exact, it is a nucleic acid. A biochemist will tell you that the DNA molecule is made up of two sugar-phosphate strips. Each strip holds chains of four chemical bases. These bases are adenine (A), thymine (T), cytosine (C), and guanine (G).

The A, T, C, and G bases are called nucleotides. They are like chemical building blocks. The nucleotides bond in a certain order. A will bond only with T. C bonds only with G. The nucleotides bond to join the strips into a long twisted double strand. Scientists call this shape a double helix.

A simple gene is made of about 1,000 base pairs. Humans have anywhere from 20,000 to 25,000 genes arranged into 23 chromosome pairs. A huge number of the bases — more than 97 percent — have no known function. Scientists call them “junk DNA.”

In the human body, the chromosomes contain a person’s unique DNA. The chromosomes are found in the nucleus of nearly every cell. Cells are like tiny factories. They need energy and food to survive and do their specific jobs. DNA is genetic information. It holds the plan for making new, exact copies of each cell.

Now consider that each twist of DNA is made of more than 3 billion nucleotides. These are the bits of code. A normal human being has about 100 trillion cells of many kinds. Multiply 3 billion by 100 trillion and you get the number of chemical building blocks present in an average human body at any one time. The number is huge: 300,000,000,000,000,000,000,000. But think of each double stranded twist of DNA in another way.

Science writer Roger Martin likes to describe it as a giant master recipe book. Contained in this huge volume is a recipe for making every kind of protein that each and every one of those 100 trillion cells needs to live. DNA is life’s master molecule.