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.
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?
Click on the link below for a printable version of the sequence of coloured bases in the model.
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).
Question 5: Can you spot the difference between the two DNA molecules?
Fourth base pair from the top, is - in the left hand molecule and - in the right hand molecule.
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.
Activity 3: DNA sequence evidence
Here we look at some short DNA sequences from different mammals. The sequences are from a very small section of just one gene, and it is the gene for a protein called casein which is one of the constituents of mammalian milk.
Click on the following link to see the DNA sequences.
There are sequences from two different species of whale, and from a variety of other mammal species. The sequences are colour coded as in Activity 2 but here we have simplified the task and you are only looking at one of the halves of the double helix of the DNA.
You will see that some of these sequences look very similar.
To compare two of these sequences drag one of them so that it is next to the one you want to compare it with. Alternatively, you could print out the sequences in colour, and cut them into strips to compare them.
Note to teachers
This activity is more effective if the sequences are presented as strings of coloured beads, as it is then easy to lay different sequences next to each other to make comparisons. It would not be necessary to present the sequence of every species listed here, and a suitable subset would be whale, hippo, cow, camel, sheep, pig, zebra, human, rabbit. It would be advisable to choose the same colour scheme used in Activity 2, with four colours, one for each of the four bases A, C, G and T.
Question 6: Can you find two mammals which have the same DNA sequence? Write down the names of the mammals with the same sequence.
Any of the pairs of mammals listed below is a correct answer here
Sheep and goat
Toothed whale and baleen whale
Zebra and tapir
Question 7: What does this tell us about this pair of mammals?
They are closely related.
Question 8: How many pairs of mammals have the same sequence for this stretch of DNA? Which mammals are they?
Three pairs: sheep and goat; toothed whale and baleen whale; zebra and tapir.
Question 9: Which mammals have a sequence similar to the whale?
Any of the following is correct – hippo, cow, camel, giraffe, sheep, goat, deer.
Question 10: What does this tell us about the closest relatives to the whales?
Whales are closely related to the mammals in the Order Artiodactyla, e.g. hippo, cow, camel, giraffe, sheep, goat, deer.
Question 11: This evidence here comes from DNA sequences, and shows us that whales have origins similar to the four-legged land mammals in the Order Artiodactyla. What other source of evidence suggests that whales are related to land mammals which have legs?
Fossil evidence, as shown in Figure 2 on the previous screen.
If you would like to take a more detailed look at what the DNA evidence can tell us, try counting the differences between the whale sequence and each of the other sequences.
Click on the link below to find a table you can print out to enter your answers on.
Click on the link below to find a completed table.
Question 12: Which three of the mammals listed here have DNA which is most different from the whale DNA?
Human, rabbit and mouse.
Question 13: What does this tell us?
They are more distantly related to the whale than the other mammals.
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.
Question 14: Which group on the evolutionary tree are the human, rabbit and mouse in?
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.