Instructions: Have students read in pairs.
Mabel: I saw a neat TV show the other day.
Abel: Me too, CSI had this thing . . .
Mabel: (Interrupting) No, this was on the Learning Channel.
Abel: (Rolls eyes) Oh. I guess you're going to tell me about it.
Mabel: Of course. It was about fruit flies and Human poly Q diseases.
Abel: Flying fruit? Polly Cue? Is this a new musical group?
Mabel: No . . . Researchers are using Drosophila. . .
Abel: Oh - fruit flies.
Mabel: . . . to research diseases caused by a tri-nucleotide repeat.
Abel: Tri-nucleotide. Three nucleotides. That makes a codon, right?
Mabel: Yes . . .
Abel: And a codon codes for 1 amino acid. See, I was paying attention in Biology class.
Mabel: Exactly. Poly-Q means that the amino acid being repeated is glutamine.
Abel: I understand that poly means many, but how do you get Q from glutamine?
Mabel: Because G was already taken for glycine. But, Let's get back to the TV program.
Abel: Oh, yeah. What did fruit flies have to do with it?
Mabel: Well, to start with, fruit flies have compound eyes. Each facet of the eye has 6 cells arranged around the outside, and two cells stacked in the middle. All together, that makes up the omatidia (oh-ma-TID-ee-ah).
Abel: Don't genes make the eyes develop that way?
Mabel: Exactly. There's a gene called "glass" that is important for the development of the eye. Researchers are using the glass gene to see if errors affect eye development.
Abel: Let me guess - there are putting a whole bunch of extra glutamines in the glass gene.
Mabel: Yep. In the DNA, the tri-nucleotide CAG, which codes for glutamine, repeats. That interrupts the gene and disrupts the development of the eye.
Abel: I get it, but why does the CAG repeat happen?
Mabel: Slippage. A hairpin loop forms in the DNA, and as it is replicated, it gets added in.
Abel: Didn't you say something about human disease?
Mabel: Yes, there are several human diseases caused by poly-Q repeats.
Abel: Like what?
Mabel: Spinocerebellar Ataxia, Myotonic Dystrophy, Fragile X Syndrome, Machado-Joseph Disease, and Huntington's Disease.
Abel: I've heard of Huntington's Disease. I did a project on it in Biology. People who have Huntington's have jerky movements, slurred speech, seizures, mood swings, impaired judgment and personality changes. It's like getting really old, only you're kinda young, like 50.
Mabel: The number of glutamine repeats determines how early the onset of the disease is.
Abel: You mean the more repeats there are, the earlier they get the disease?
Mabel: 40-59 repeats means you'll get the disease as an adult. 60 or more repeats means you can get it as a child!
Abel: Isn't Huntington's a dominant disease? That means if your mom or dad has it, you'll get it too.
Mabel: Not exactly, you only have a 50% chance of getting the disease, but if you do get it, it's likely to have an earlier onset.
Abel: Because of slippage?
Mabel: Exactly. I bet you would have enjoyed the program.
Abel: Maybe I'll catch it on a rerun. Let me tell you about CSI . . .