As long as a cell's membrane is intact and it is making all of the enzymes it needs to function properly, the cell is alive. The enzymes it needs to function properly allow the cell to create energy from glucose, construct the pieces that make up its cell wall, reproduce and, of course, produce new enzymes.
So where do all of these enzymes come from? And how does the cell produce them when it needs them? If a cell is just a collection of enzymes causing chemical reactions that make the cell do what it does, then how can a set of chemical reactions create the enzymes it needs, and how can the cell reproduce? Where does the miracle of life come from?
The answer to these questions lies in the DNA, or deoxyribonucleic acid. You have certainly heard of DNA, chromosomes and genes. DNA guides the cell in its production of new enzymes.
The DNA in a cell is really just a pattern made up of four different parts, called nucleotides or bases. Imagine a set of blocks that has only four different shapes, or an alphabet that has only four different letters. DNA is a long string of blocks or letters. In an E. coli cell, the DNA pattern is about 4 million blocks long. If you were to stretch out this single stand of DNA, it would be 1.36 mm long -- pretty long considering the bacteria itself is 1,000 times smaller. In bacteria, the DNA strand is like a wadded-up ball of string. Imagine taking 1,000 feet (300 meters) of incredibly thin thread and wadding it up -- you could easily hold it in your hand. [A human's DNA is about 3 billion blocks long, or almost 1,000 times longer than an E. coli's. Human DNA is so long that the wadded-up approach does not work. Instead, human DNA is tightly wrapped into 23 structures called chromosomes to pack it more tightly and fit it inside a cell.]
The amazing thing about DNA is this: DNA is nothing more than a pattern that tells the cell how to make its proteins! That is all that DNA does. The 4 million bases in an E. coli cell's DNA tell the cell how to make the 1,000 or so enzymes that an E. coli cell needs to live its life. A gene is simply a section of DNA that acts as a template to form an enzyme.
Let's look at the entire process of how DNA is turned into an enzyme so you can understand how it works.