When I talked about what a gene is, I mentioned that a gene can be transcribed from DNA to RNA and then translated from RNA into a protein. A very important part of that process is the genetic code. The genetic code is like the secret language of DNA, it’s how DNA actually stores information.

You may remember playing with hidden messages as a child. One variation was writing a secret message hidden within lots of extra words. Using a decoder, like a punched-out paper laid over the nonsensical words, a clear message was revealed by reading the uncovered words.

For a gene that makes a protein, the jumble of words is the RNA that is transcribed from the DNA. This is the messenger RNA, or mRNA, and is a series of three letter words, called codons. The decoders in this case are the ribosomes, small molecular machines in the cell. The mRNA is fed into the ribosome and codon by codon, is read. That’s just like how only a small fraction of the word jumble is looked at to get the right code, except here, the way to read the code is by matching up the codon of the mRNA with an anti-codon of transfer RNA, or tRNA for short. The anti codon is the complimentary sequence of the codon, remember that DNA is always present in two strands, but RNA is usually only found as a single strand. That’s convenient, because it allows complimentary tRNA molecules to pair with individual codons. The ribosome isolates a single 3-nucleotide codon, allowing the proper tRNA to pair with it. The tRNA is necessary because it also has an amino acid attached to it. Amino acids are the building blocks of proteins, so that’s how the sequence of DNA is translated into to a protein. The tRNA is like a decoder dictionary. It matches the codon of the mRNA with the right amino acid, then the next codon is read, another tRNA is found and the next amino acid is placed onto the growing string of the protein. This occurs until the full protein is built.

Proteins do the bulk of work in a cell, but they don’t replicate themselves, they are all made by ribosomes, which are just the machines which match up tRNA with mRNA, which comes directly from the DNA.

So back to the genetic code: Since there are four nucleotides, we can have 64 three-letter combinations, or 64 codons, but there are only 20 amino acids. That means that there is redundancy in the code: different codons will match with the same amino acid. That’s just fine because it makes the system more robust. If a mutation occurs, changing the sequence of a codon, it may not necessarily change the amino acid at the end.

So the genetic code is remarkably similar to that childhood code of ours, it requires a special reader that can read many different messages, no matter what the mRNA looks like.