Genomes & Junk DNA
Jonathan McLatchie takes on PZ Myers in a spirited attack on junk DNA [Treasure in the Genetic Goldmine: PZ Myers Fails on "Junk DNA"]. The Intelligent Design Creationists are convinced that most of our genome is functional because that's what a good designer would create. They claim that junk DNA is a myth and their "evidence" is selective quotations from the scientific literature. They ignore the big picture, as they so often due.
I discussed most of the creationist arguments in my review of The Myth of Junk DNA.
Jonathan McLatchie analyzes three argument made by PZ Myers in his presentation at Skepticon IV. In that talk PZ said that introns are junk, telomeres are junk, and transposons are junk. I have already stated that I diasgree with PZ on these points [see PZ Myers Talks About Junk DNA]. Now I want to be clear on why Jonathan McLatchie is wrong.
- Introns are mostly junk. I think PZ exaggerated a bit when he dismissed all introns as junk. My position is that we should treat introns as functional elements of a gene even though many (but not all) of them could probably be deleted without affecting the survival of the species. Each intron has about 50-80 bp of essential information that's required for proper splicing [Junk in Your Genome: Protein-Encoding Genes]. The rest of the intron, which can be thousand of base pairs in length, is mostly junk [Junk in Your Genome: Intron Size and Distribution]. Some introns contain essential gene regulatory regions and some contain essential genes. That does not mean that all intron sequences are functional.
- Telomeres are not junk. I don't think telomeres are junk [Telomeres]. They are absolutely required for proper DNA replication. PZ Myers agrees that telomeres (and centromeres) are functional DNA (28 minutes into the talk). Jonathan McLatchie claims that PZ describes telomeres as junk DNA, "Myers departs from the facts, however, when he asserts that these telomeric repetitive elements are non-functional." McLatchie is not telling the truth.
- Defective Transposons are Junk. PZ Myers talks about transposons as mobile genetic elements and states that transposons make up more than half of our genome. That's all junk according to PZ Myers. My position is that the small number of active transposons are functional selfish genes and the real junk is the defective transposon sequences that make up most of the genome [Transposon Insertions in the Human Genome]. Thus, I differ a bit from PZ's position. Jonathan McLatchie, like Jonathan Wells, argues that because the occasional defective transposon in the odd species has acquired a function, this means that most of the defective transposon sequences (~50% of the genome) are functional. This is nonsense.
[Image Credit: The image shows human chromosomes labelled with a telomere probe (yellow), from Christoher Counter at Duke University.]