15 February 2006

Molecules and Evolution.

I spend a lot of time on this blog writing about evolution, and about Intelligent Design, and about why one is bad and the other good. This time, I'm going to try to do something a little different, and discuss one of the many lines of evidence for evolution: the similarities that we see when we compare DNA sequences from different animals.

The example that I'm going to use is hemoglobin from humans and chimps. Hemoglobin is a protein that is found in red blood cells. The hemoglobin picks up oxygen in the lungs and carries it to the different tissues. Hemoglobin is not a single molecule. It is a complex protein that is made of several smaller proteins. Each of those proteins is produced by a gene. We're going to look at one of those genes: alpha-globin.

The National Institutes of Health maintains a database known as GenBank. GenBank's basically Google for hard-core biology geeks. It's an international database where scientists store DNA and protein sequences. I rummaged around there, and came up with a alpha-globin DNA sequences from a human and a chimp. I'll put these sequences in pairs of lines, with the human sequence on top and the chimp sequence on the bottom.
Human: ACT CTT CTG GTC CCC ACA GAC TCA GAg AGA
Chimp: ACT CTT CTG GTC CCC ACA GAC TCA GAa AGA

Human: ACC CAC CAT GGT GCT GTC TCC TGC CGA CAA
Chimp: ACC CAC CAT GGT GCT GTC TCC TGC CGA CAA

Human: GAC CAA CGT AAG GCC GCC TGG GGT AAG GTC
Chimp: GAC CAA CGT AAG GCC GCC TGG GGT AAG GTC

Human: GGC GCG CAC GCT GGC GAG TAT GGT GCG GAG
Chimp: GGC GCG CAC GCT GGC GAG TAT GGT GCG GAG

Human: GCC CTG GAG AGG ATG TTC CTG TCC TTC CCC
Chimp: GCC CTG GAG AGG ATG TTC CTG TCC TTC CCC
That's only part of the sequence, but it's pretty representative. I showed the first 150 "letters" of the DNA sequence. There's one difference in these 150 bases, and the full sequence shows 99% similarity.

I've shown you that humans and chimps have very similar alpha-globin gene sequences, but so far that's all I've shown you. I haven't explained why I think that this similarity indicates evolution instead of something else. After all, it might just be that the gene does the same things, and that explains the similarity.

The gene does, in fact, do exactly the same thing in chimps that it does in humans. The gene makes a protein that is part of hemoglobin in both species. In fact, even though the genes are slightly different, the portion of the gene that I showed the sequence for makes exactly the same protein in both species. There is a difference in the DNA sequence, but not in the protein sequence.

Our proteins are strings of amino acids joined together in a line. There are twenty amino acids that we use in our proteins. The "genetic code" that tells our body the order that the amino acids should go in uses three letter DNA "words" to indicate each amino acid. There are four DNA letters, so there are 64 possible words. That's a lot more words than we need, so most of the amino acids have more than one "word" assigned in the code.

This means that there are literally hundreds of possible DNA sequences that could make exactly the same protein sequence that we see in human and chimp alpha-globin. The sequences that humans and chimps use are almost identical. They aren't exactly the same, but they are really close. The sequence for an orang-utan is a bit more different.

These slight differences, more than the similarities, are what makes us view this as evidence for evolution.
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