4. DNA & Protein Production

What you need to know...

    • Structure of DNA: double-stranded helix held by complementary base pairs.

    • DNA carries the genetic information for making proteins. The four bases A, T, C and G make up the genetic code. The base sequence determines amino acid sequence in protein.

    • Messenger RNA (mRNA) is a molecule which carries a copy of the code from the DNA, in the nucleus, to a ribosome, where the protein is assembled from amino acids.

Source: SQA

Notes

We've already learnt that DNA is located in the nucleus in the form of chromosomes and these replicate during mitosis to ensure daughter cells have the same quantity of DNA as the mother cell. But what is DNA, and what does it do? In this topic we'll discuss the structure of DNA and how it codes for the manufacture of proteins by cells. In the next topic we'll learn how these proteins go on to perform many crucial functions in cells.

DNA Structure

DNA stands for Deoxyribonucleic Acid. It is a chemical molecule found in all cells which consists of very long chains of repeating components. The repeating unit which makes up a molecule of DNA consists of three structures. Two of these are always the same, but one of these - the base - can come in four different forms. So, a short section of a molecule of DNA could be represented in the diagram below.

As you can see from the diagram, the DNA molecule consists of a long chain of repeating units attached to a sequence of bases. There are four possible bases in a DNA molecule: A, T, C & G. The diagram only shows a short length of DNA, but one chromosome would be many millions of bases long. As we'll see later, the sequence of these bases is crucial for the functions of the cell, and therefore life itself.

If you're particularly observant, you will have noticed something about the shapes of the bases in the diagram above. They have been drawn in such a way as to represent the fact that the bases are complementary pairs. DNA in cells is actually found as double stranded molecules with the two strands joining at the bases. When the bases bond they can only do so in certain pairings. From the diagram above, can you predict which base pairs with which?

As you can see the base A always pairs with T, and C pairs with G. This results in two strands of DNA which are mirror images of each other. However, this double stranded DNA molecule doesn't naturally exist as a straight ladder as shown in the diagram above, it naturally coils to form a double stranded helix instead.

There are many resources online to explore the structure of DNA further such as this one, you can also find out how this structure was discovered on YouTube.

Making Proteins

So that's what DNA looks like, but what does it actually do? We've already mentioned that DNA codes for the production of proteins but how does this actually work? Before starting to explain this you need to know a little bit about the structure of proteins. We'll learn a lot about proteins in our next topic, but you already know something about proteins. You'll know that there are different types of proteins for example, such as the protein haemoglobin in your red blood cells which not only makes your blood red, but more importantly binds to oxygen in your lungs and carries it to all the tissues in your body. Another protein you might have heard of is keratin. Keratin is an important structural protein in your skin and is also the key protein in your hair and nails. Clearly these are two very different proteins with very different functions which arise from their very different structures. Amazingly, like all proteins, these two proteins are made in exactly the same way using the same twenty ingredients.

Proteins consist of long chains of a repeating chemical unit called amino acids. These chains can be hundreds or thousands of amino acids long. There are only twenty naturally occurring amino acids and the order the amino acids are joined together will determine which protein is produced. So the following two sequences of amino acids would ultimately result in proteins which have very different structures, and therefore very different functions also [amino acids have quite complicated names so I've just used numbers instead to represent the different amino acids].

An accidental change in one of the bases in the DNA code can have a dramatic effect on the protein produced if it changes the sequence of the amino acids. If you look at the protein Haemoglobin on Proteopedia you'll notice in the last paragraph of the introduction it says

"Perhaps the most well-known disease caused by a mutation in the hemoglobin protein is sickle-cell anemia. It results from a mutation of the sixth residue in the β hemoglobin monomer from glutamic acid to a valine. This hemoglobin variant is termed 'hemoglobin S' (2hbs)."

Glutamic acid and valine are amino acids. So, what this means is that if just one amino acid is changed to another it can change the protein which can result in a disease; sickle-cell anaemia in this case. If you click on the the green "glutamic acid to a valine" link on Proteopedia it'll zoom in and show you the site of the mutation.

But what decides the order of the amino acids in the protein molecule? Well, this is when we come back to DNA. The sequence of basis on the DNA molecule is what directs the sequence of amino acids in the protein molecule - that's how it all links together!

mRNA

So, the sequence of bases in DNA codes for the sequence of amino acids of a protein. But, there's a problem. In order to produce a protein you need ribosomes. Ribosomes catalyse the reactions of protein synthesis and if you can remember back to the first topic you'll know that ribosomes are found in the cytoplasm. And what's wrong with that? Well, the DNA remember is in the form of large chromosomes inside the nucleus. Because the DNA code is in a different part of the cell from the ribosomes, a messenger molecule is required to carry the code from the nucleus to the cytoplasm. This molecule is called messenger RNA, or mRNA for short. RNA is a bit like a smaller version of DNA. RNA stands for Ribonucleic Acid so you'll see that it's quite a similar molecule. RNA molecules are single stranded instead of double stranded and are much much shorter than the DNA in the chromosome.

In order to produce a protein then your cells first make a copy of the code from the DNA into an mRNA molecule in the nucleus. This mRNA molecules then leaves the nucleus and enters the cytoplasm where it comes together with ribosomes and uses the code to join amino acids together in a specific order to produce a particular protein.

This fantastic video includes some animations of protein synthesis, definitely worth a watch. Which of the structures discussed in this topic can you identify in these animations? You can learn loads more about DNA on this fantastic website.