Our example tells us that below are the DNA and amino acid sequences for 3 different but related species. We want to use the sequences to answer the following questions. So here are the sequences. We have the DNA sequences on the top. So we see the sequences there for species 1, 2, and 3, and those nucleotides look to be broken up into codons.

And then below that, we have the amino acid sequence. So we have 3 amino acid sequences, again for the same species, 1, 2, and 3. We see we're using the 3-letter abbreviations here, and you can see that they're sort of lined up underneath the codons. Alright. So our first question is based on the DNA sequence, which two organisms do you expect to be more closely related?

Alright. So take a look. How would you know and well, which ones do you think are more closely related? Well, we're using this idea of homology, and the homology is the idea that, well, things that are inherited from a common ancestor, those are the same and those show that things are more closely related to each other. So as I look at the DNA sequences, the sequences that are more like each other that show more homology, they're probably that way because they were inherited from the same common ancestor.

So I'm going to just start by going through and highlighting differences I see. So in species 2 here, that nucleotide's unique. Here's a difference. Here's a difference. Here's a difference in species 1.

Here's a difference in species 2, and another one there. Alright. So as I look at that, species 2 seems to have a lot of places that have a unique nucleotide different from the other ones. That means that species 1 and 3 show more homology. If they show more homology, I assume that's because they inherited that from the same common ancestor.

So I believe that species 1 and 3 are more closely related for that reason. Alright. Now, B, based on only these data, do you expect to see differences in the traits of these organisms? Why or why not? Right?

The traits, the phenotypes of these organisms. Well, look here. How would you know, and what do you expect to see? Well, for the traits, the phenotypes, I'm going to look at the amino acid sequence. Right?

Because the proteins are things that actually go out and do things in the cell. And as I look at the amino acid sequences here, well, they're identical in all three organisms. So I just based on these data, I don't expect to see any difference in the phenotypes. So I'm going to say no difference in the traits. Why? Well, the amino acid sequence is the same in all three organisms.

Next, it asks us to circle the regions of the DNA and amino acid sequence that show homology in all three organisms. Alright. What are we going to circle?

Well, we're going to circle the places that are the same because we're going to assume that they're the same because they were the same in that common ancestor. So in all three organisms, well, it's going to be these two nucleotides, these two nucleotides. Really, the first two nucleotides in every codon here. There's a difference in the third one, and then as I look at the amino acid sequences, well, they're all identical. So I'm going to circle the whole thing.

Our next question then is, did you circle the same regions for both sets of sequences? Well, obviously, no. Now but then we want to explain why or why not. So I'll write no.

So how is it that the entire amino acid sequence could show homology even when the DNA sequence that codes for that amino acid sequence doesn't show complete homology? Well, I'm going to note that those nucleotides that don't show homology are in the 3rd position. Remember the 3rd position in a codon is often a silent site. So I'm going to say differences in DNA are in silent sites. Alright.

So DNA holds a lot of information when we're trying to make evolutionary relationships and using homology to do it. And that's why we use DNA so much in modern biology today. You know, every protein has typically thousands of nucleotides that code for it. And so there's just a ton of data that you can use to look for homologies. Every nucleotide could be a possible homology that you can use to establish evolutionary relationships, and that's a lot of data.

So that's why today molecular homology is what scientists try to use if they can to build phylogenetic relationships. Alright. With that, we have more practice problems after this and I'll see you there.