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.
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.
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.
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.
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
Human vs. Chimp
So, let's dig into BLAST.
First, use an Internet search engine to find AP Biology Investigative Labs page. You'll find that at AP Central. Scroll down until you get to Lab 3 and click on the link that says, 'Gene files for Investigation 3: Comparing DNA Sequences to Understand Evolutionary Relationships with BLAST.' Scroll down again until you see links for gene 1, gene 2, gene 3, and gene 4. Download one of these (they will not open on your computer).
Next, return to the BLAST homepage that you opened earlier and click on 'Saved Strategies' at the top of the page. Click on 'Upload Search Strategy,' and choose one of the files you've just downloaded. Then click 'View.' Without changing anything else, scroll down and click 'BLAST.'
You should get a two-part screen in response. You can scroll past the top part of the screen to 'Sequences Producing Significant Alignments.' This list is made of the names of species that have genetic sequences quite similar to the sequence you just uploaded. Those with greater similarity come first. You can click on the species name to find out more about it, or you can choose 'Distance Tree of Results' to see a cladogram of how your species is related to other species based on DNA sequences.
The higher the BLAST score, the more closely sequences are related. Scientists can then use these results to create a cladogram or to revise one that already exists by adding a newly-discovered organism.
Let's briefly recap this lesson.
Scientists are able to learn how organisms are related by studying their genomes, the sum total of genetic material in each species. Because the human genome is so extensive, geneticists have created a database called Basic Local Alignment Search Tool, or BLAST, to help isolate and compare sequences quickly. They can use the results to make a cladogram, which is a visual representation of the evolutionary relationship between species.
Cladograms show two types of characters: derived characters, which are features that set a group of organisms apart from the others on the cladogram, and shared characters, which are features that are present in all organisms after that point on the cladogram. Cladograms can be based on physical features, but are more accurate when made based on genetic sequences. The more conserved, or identical, DNA sequences that organisms share, the more closely they are related.
Work through this video lesson and transcript to ensure that you can:
- Provide the meanings of genome and cladogram
- Identify the purpose of BLAST
- Point out the two types of characters shown on cladograms
- Use BLAST