Comparing DNA Sequences to Understand Evolutionary Relationships with BLAST: Biology Lab

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  • 0:02 Genes and BLAST
  • 1:38 Cladograms
  • 3:18 Using BLAST to Make a…
  • 5:49 Lesson Summary
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Instructor: Jennifer Szymanski

Jen has taught biology and related fields to students from Kindergarten to University. She has a Master's Degree in Physiology.

About 90% of human DNA is the same as mouse DNA, and that's a good thing. In this lesson, you'll explore how scientists use genetic similarities to determine how living things are related.

Genes and BLAST

You might have heard someone use the phrase, 'Are you a man, or are you a mouse'? The answer to that is easy - you're all human. Or are you? Genetically, you're actually about 90% the same as a mouse. This is one reason we use mice as model organisms, which are creatures we use to study genetics and disease in the hopes that the results will be applicable to humans.

Scientists know we're so closely related to mice because they've been able to study the genome, or the sum total of genetic material, in both species. Using technology, they've been able to determine the sequence of the bases that make up each organism's genetic code, the number and order of the bases G, A, C, and T.

There are a lot of them. In fact, it would take about ten years to find a specific gene sequence in human DNA. So geneticists have created a database called Basic Local Alignment Search Tool, or BLAST, to help find sequences quickly and to compare sequences to one another.

To use BLAST, you'll have to access a National Institutes of Health website; you'll be able to locate it by using an Internet search engine to look up Basic Local Alignment Search Tool. You'll then input a genetic sequence from one organism and compare it to that of other organisms. Before we go over exactly how to do that, though, let's see how you might be able to use the results of your search.


One tool that scientists use to show relationships between organisms is called a cladogram, which is a visual representation of the evolutionary relationship between species. You might remember that species evolve from a common ancestor. Different species, such as frogs and salamanders, arise because of different environmental factors. These factors place pressure on a species and often enable organisms that have adaptations most suitable for the environment to survive and reproduce most successfully.

You can think of cladograms as evolutionary trees. One species is found on the end of each branch. The more closely the organisms are related by evolution, the more closely they are found on the cladogram. For example, in the cladogram of insects shown, butterflies and flies are more closely related than butterflies and beetles.

Insect Cladogram
insect cladogram

A well-made cladogram will also show what are called derived characters, which are features that set a group of organisms apart from the others on the cladogram. In this cladogram, for example, only the gorilla lacks a tail.

Derived Characters
derived characters

Shared characters are features that are present in all organisms after that point on the cladogram. For example, salamanders, lizards, tigers, and gorillas all 'share' the presence of lungs.

Shared Characters
shared characters

Morphology is the word for 'forms' and physical structures like tails and lungs, and that's a pretty good way to make a cladogram. But we can make better ones using DNA sequences. And that's where BLAST comes in.

Using BLAST to Make a Cladogram

Scientists have determined that certain gene sequences are conserved, which just means shared between species. In this illustration, each letter represents an amino acid, a subunit of a protein.

Conserved Sequence
conserved sequence

Each of the existing 20 amino acids is created by a specific sequence of DNA. Therefore, this sequence of amino acids is reflective of a specific DNA sequence. If we compare this sequence in different animals, we can see that there are various numbers of differences in the amino acids. For example, the human and cow sequences differ in 4 places, while the human and chimp sequences show no differences. We interpret this to mean that the chimp and human are more closely related than the cow and human.

Cow vs. Human
cow vs human
Human vs. Chimp
human vs chimp

So, let's dig into BLAST.

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