The Cell's Secret Code

All the proteins in your body are made from protein building blocks called amino acids. There are twenty different amino acids used to make proteins, but there are only 4 different nucleotides in DNA and RNA. How can a 4-letter code specify 20 different amino acids?

Actually, the DNA code is designed to be read as triplets. Each "word" in the code, called a codon, is three letters long. There are also special "start" and "stop" codons that mark the beginning and end of a gene. As you can see, the code is redundant, that is, most of the amino acids have at least two different codons.

Just about every living thing uses this exact code to make proteins from DNA.

How Do Genes Work?

Genes are often called the blueprint for life, because they tell each of your cells what to do and when to do it: be a muscle, make bone, carry nerve signals, and so on. And how do genes orchestrate all this? They make proteins. In fact, each gene is really just a recipe for a making a certain protein.

Here are several examples of how RNA codes for amino acids. These codons are for tryptophan, the amino acid that contributes to the sleepy feeling you may have after eating turkey; phenylalanine, an amino acid used in the aspartame sweetener in diet soda; tyrosine, an important amino acid in intracellular signaling processes; and cystine.

And why are proteins important? Well, for starters, you are made of proteins. 50% of the dry weight of a cell is protein of one form or another. Meanwhile, proteins also do all of the heavy lifting in your body: digestion, circulation, immunity, communication between cells, motion-all are made possible by one or more of the estimated 100,000 different proteins that your body makes.

But the genes in your DNA don't make protein directly. Instead, special proteins called enzymes read and copy (or "transcribe") the DNA code. The segment of DNA to be transcribed gets "unzipped" by an enzyme, which uses the DNA as a template to build a single-stranded molecule of RNA. Like DNA, RNA is a long strand of nucleotides.

This transcribed RNA is called messenger RNA, or mRNA for short, because it leaves the nucleus and travels out into the cytoplasm of the cell. There, protein factories called ribosomes translate the mRNA code and use it to make the protein specified in the DNA recipe.

If all this sounds confusing, just remember: DNA is used to make RNA, then RNA is used to make proteins-and proteins run the show.

"Junk" DNA

Scientists first studying DNA sequences were surprised to find that less than 2% of human DNA codes for proteins. If 98% of our genetic information (or "genome") isn't coding for protein, what is it for?

At first it wasn't clear, and some termed this non-coding DNA "junk DNA." But as more research is done, we are beginning to learn more about the DNA between the genes-intergenic DNA.

Intergenic DNA seems to play a key role in regulation, that is, controlling which genes are turned "on" or "off" at any given time.

For example, some intergenic sequences code for RNA that directly causes and controls reactions in a cell, a job that scientists originally thought only proteins could do.

Intergenic DNA is also thought to be responsible for "alternative splicing," a kind of mix-and-match process whereby several different proteins can be made from one gene.

In short, it now seems that much of the interest and complexity in the human genome lies in the stuff between the genes... so don't call it junk.