The segments of our DNA that are commonly called "genes" consist of protein-coding exons and non-protein-coding introns. Initially, the entire DNA segment is transcribed into RNA, but between ninety and ninety-five percent of the initial RNAs are "alternatively spliced."It's up to you, dear readers, to figure out all the things wrong with this explanation. You can start with the math. Arithmetic isn't one of their strong points. Or maybe it's an understanding of biology that's the real weak point?
What is alternative splicing? Imagine that the initial RNA derived from its DNA template has the organization A—B—C—D—E—F, where the letters represent blocks that specify amino acid sequences and the dashes in between the letters stand for introns. Alternative splicing enables multiple proteins to be constructed given the same RNA precursor, say, ABCDF, ACDEF, BCDEF, and so forth. In this way, hundreds or thousands of proteins can be derived from a single gene.
There’s more. The messenger RNAs that are produced by this process—and therefore the proteins that are made in a cell—are generated in a way that depends on the stage of development as well as the cell and tissue type. In the above example, a nerve cell may express the ACDEF version of a messenger RNA whereas a pancreatic cell may produce only the BCDE version. The differences are biologically essential.
What does this have to with introns? Everything. It is the presence of introns that makes this permutative expansion of messenger RNAs possible in the first place.
So let’s do the math. At least ninety percent of gene transcripts undergo alternative splicing, and there are at least 190,000 introns in the human genome. That means we have at least 0.90 x 190,000 = 171,000 introns that participate in the alternative-splicing pathway(s) available to a cell.
Thursday, June 03, 2010
Creationists, Introns, and Fairly Tales
Richard Sternberg thinks that introns are important. He has to think that way because he's an Intelligent Design Creationist and the idea that introns could be mostly junk and not have a function isn't part of his faith [Matheson’s Intron Fairy Tale].