2.2. From DNA to Proteins. The Genetic Code

What is DNA’s alphabet?

We use codes everyday; alphabets are also codes. Let’s take the word “koala”. In English, the letters ‘k’, ‘o’, ‘a’, ‘l’ and ‘a’ in that particular order mean an animal that lives in Australia and eats eucalyptus leaves. If you didn’t know any English, you wouldn’t be able to guess what the word means from the letters that are in it. The letters ‘k’, ‘o’, ‘a’, and ‘l’ appear in lots of other words where they don’t mean anything to do with koalas. Different languages use different alphabets to convey meaning.

DNA’s code is written in only four ‘letters’, called A, C, T and G. The meaning of this code lies in the sequence of the letters A, T, C and G in the same way that the meaning of a word lies in the sequence of alphabet letters. Your cells read the DNA sequence to make chemicals that your body needs to survive.

What does DNA code for?

A gene is a length of DNA that contains the instructions to make a chemical in your body. The DNA in a gene usually codes for a protein.

In our cells, proteins are the workforce; they get everything done. Proteins break down our food to release energy. Proteins organise the transport of useful chemicals between cells. Often, these useful chemicals are themselves proteins.

As well as doing things, proteins are the building blocks for most of your body. In the same way that a wall is made mostly of bricks, your body is made mostly of protein.

We talk about genes having different characteristics. For instance, if you hear about ‘genes for eye colour’, it means that these genes code forprotein pigments in the iris of each of our eyes. Genes can come in different versions. Some people’s versions code for proteins that make their eyes look blue while other people’s versions make proteins that make their eyes look brown.

Making proteins

What are proteins made of?

The ingredients of a protein are amino acids. To build a protein we need to build a long chain of amino acids. There are 20 different types of amino acids, so there are lots of different protein chains we can build. Biologists give amino acids a code letter, as for DNA. This is much easier than writing out the whole name each time. For example, M is methionine, L is leucine, F is phenylalanine (because P is proline).

Three DNA letters, one amino acid?

The DNA code uses groups of three ‘letters’ to make meaning. This means that when the cell reads the instructions encoded in the DNA sequence to make a protein, it reads it three letters at a time. Most groups of three letters – known as triplets or codons-code for an amino acid.

Since there are four different DNA letters (A, G, C and T), there are 4 x 4 x 4 = 64 different combinations that can be used. However, as there are only 20 different types of amino acid, some of these 64 codons code for the same amino acid. Some of the 64 codons don’t code for any of the amino acids. Instead they provide the punctuation and grammar, like where the cell should start and stop.

An Example (with codon wheel)

An example of a DNA sequence might be

CCC TGT GGA GCC ACA CCC TAG

If you use our DNA decoder (codon wheel*), you can decode this triplet by triplet. Start from the inside of the wheel: find the first letter of your codonin the centre of the wheel and work outwards, through the second ring (with the next letter) and so on, to find the corresponding amino acid.This would make the amino acid chain:

P - C - G - A - T - STOP = Proline-Cysteine-Glycine-Alanine-Threonine-STOP

Check your answers from the codon wheel with our new codon cracker at right. Click on the coloured letter buttons to enter the triplet then click on translate to decode the triplet.

*Try decoding a longer sequence and looking up the protein it comes from with our Codon Cracker with Function Finder below.

Codon cracker with Function Finders: Simple activity to decode DNA into an amino acid chain with link to live protein database.
Most proteins are actually much longer than this. Many of the proteins that make your body makes contain hundreds of amino acids. So if we know what the DNA sequence is, we can work out which amino acids the protein must contain and in what order.
Advertisements

Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Google photo

You are commenting using your Google account. Log Out /  Change )

Twitter picture

You are commenting using your Twitter account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s