3 DNA: Spot the difference

Here we look at DNA, the molecule which contains the instructions for making each living creature. It is contained within the genes of every individual living thing on Earth. Closely related creatures have DNA that is very similar, and distantly related creatures have DNA that is very different. By looking at how similar or different their DNA molecules are, we can see how closely related two species are.

Activity 2

We'll start by looking at the models of two short DNA molecules shown below. Each model resembles a twisted ladder or ‘double helix’ and the rungs of the ladder are made of pairs of bases, here shown in colours green, yellow, red and blue, and you will see that the green one always pairs with the yellow one and that the red one always pairs with the blue one. It is the order, or ‘sequence’ of these coloured bases which is crucial. There is one difference between the molecules represented here. Can you spot it?

Figure 3

Click on the link below for a printable version of the sequence of coloured bases in the model.

Spot the difference [Tip: hold Ctrl and click a link to open it in a new tab. (Hide tip)]

In the diagram it is as if the model had been untwisted, so that the rungs of the ladder can be seen more clearly. Here the bases are shown not only in colour but also using the letters by which they are commonly known.

You will see that (adenine) always pairs with (thymine) and that (cytosine) always pairs with (guanine).

A difference like this represents a mutation. Such differences accumulate over time, so creatures which are on different branches of an evolutionary tree have more differences in their DNA than creatures on the same branch, which are more closely related.

This means that if we look at the similarities and differences between the DNA of different creatures, we can tell which ones are most closely related.

Now look back at the evolutionary tree of mammals. You will see that the whale, the mammals in the Order Artiodactyla (hippo, camel, cow, giraffe, etc.) and the zebra and tapir are all in the ‘blue’ group.

The mouse is a rodent, in the Order Rodentia, the rabbit is in the Order Lagomorpha, and the human is in the Order of Primates. All these are in the green group, and have a more distant evolutionary origin from those in the blue group. Therefore it is not surprising that their DNA is more different from the whale than the others.

Here we have seen that whales are closely related to four-legged land mammals such as the hippo, camel, cow and giraffe. The evidence for this comes from both DNA sequences and from fossils. These are two very different sources of evidence, and both point to the same conclusions, which means that the evidence for these conclusions is overwhelmingly strong.