A sampling of new research uncovers more function for junk DNA, undermines one of the best arguments for biological evolution
My wife enjoys shopping at large club stores. I, however, don’t care for them much. Still, from time to time I’ll join Amy on her shopping excursions, usually enticed by the prospects of free food samples. Nothing’s better than walking up and down store aisles greedily ingesting morsels of tasty food on a late Saturday morning, right before lunch.
Club stores offer free bites of food to let the customers know about all the wonderful products that are available. I’m going to follow suit in this article by offering up a sampling of recent discoveries that ascribe function to junk DNA with the hope that you will have an idea of the remarkable advances happening in molecular biology—advances that are eroding support for one of evolution’s best arguments.
Junk DNA and the Case for Biological Evolution
Evolutionary biologists consider the existence of junk DNA as one of the most potent pieces of evidence for biological evolution. According to this view, junk DNA results when biochemical processes and chemical and physical events transform a functional DNA segment into a useless molecular artifact. Such pieces of DNA remain part of an organism’s genome solely because of its attachment to functional DNA. In this way, junk DNA persists from generation to generation.
Evolutionists also highlight the fact that in many instances identical (or nearly identical) segments of junk DNA appear in a wide range of related organisms. Frequently, the identical junk DNA segments reside in corresponding locations in these genomes. Evolutionists take this to indicate that these organisms shared a common ancestor, suggesting that the junk DNA segment arose prior to the time that the organisms diverged from their shared evolutionary ancestor.
The challenge represented by junk DNA takes on a similar logical form to the problem of evil:
- God is all-good.
- God is all-powerful.
- God is all-knowing.
- Junk DNA exists.
For skeptics and atheists, the last statement is incompatible with the first three. Evolutionists ask, “Why would a Creator purposely introduce nonfunctional, junk DNA at the exact location in the genomes of different, but seemingly related, organisms?”
Responding to the Junk DNA Challenge
Proponents of intelligent design and creationism respond to this valid objection by highlighting the many recent findings that attribute function to junk DNA. Here is a sample of findings that have been reported during the last few months—just to give you a taste.
This class of junk DNA belongs to a category of sequences known as transposable elements—pieces of DNA that jump around the genome, or transpose. In the process of moving around the genome, some transposable elements make additional copies of themselves, and therefore increase in number when they transpose. SINES belong to a subclass of transposable elements, called retrotransposons. Molecular biologists believe that these DNA elements duplicate and move around the genome through an RNA intermediate and the activity of reverse transcriptase.
SINES range in size from 100 to 300 base pairs (genetic letters). In primates, the most common SINES are the so-called Alu sequences. In fact, there are about 1.1 million Alu copies in the human genome (roughly 12% of the human genome). Alu sequences contain a segment that the cell’s machinery can use to produce an RNA message. In this way, SINES can duplicate and move around the genome as reverse transcriptase back-converts SINE RNA into DNA.
Previous work has identified a functional role for SINE DNA in gene regulation and stress response. (For discussions about SINE DNA function see my books Who Was Adam? and The Cell’s Design.)
New work has now uncovered a role for a newly discovered subclass of SINE DNA in regulating gene expression during brain development in mammals.
This class of junk DNA consists of DNA sequences that interrupt the coding region of a gene. The DNA sequences that make up genes in eukaryotes consist of stretches of nucleotides that specify the amino acid sequence of a protein (called exons) interrupted by nucleotide sequences that don’t code for anything (called introns). After the gene is copied into a messenger RNA molecule, the intron sequences are excised and the exons spliced together by a protein-RNA complex known as a spliceosome. Because introns interrupt coding sequences of DNA and are excised by the cell’s machinery, many scientists view these elements as junk DNA.
A recent study, however, indicates that introns do serve a function. It appears that some of these sequences help direct messenger RNA to specific locations in the cell.
Once assembled and processed, messenger RNA migrates from the nucleus of the cell into the cytoplasm. At ribosomes, messenger RNA directs the synthesis of proteins. Once produced, the proteins diffuse away from the ribosomes and begin their work for the cell.
Up until now biochemists thought that before exiting the nucleus, where the splicing and other processing reactions take place, all the introns were removed from the RNA message. But new work indicates this is not the case. Researchers discovered a messenger RNA molecule in the cytoplasm of neurons with an intronic sequence.
This messenger RNA molecule harbors a copy of the information needed to produce a protein component of a channel complex that permits the flux of calcium ions across the membranes of the dendrites. This process is essential for nerve transmission. It appears as if this intron helps direct the messenger RNA to the appropriate location in the dendrites where it then directs the production of the proteins that will eventually form the calcium ion channel.
This newly discovered regulatory mechanism may well be a general strategy that helps dictate gene expression in neurons and maybe other cell types as well.
These two advances give a sampling of flavors for the numerous discoveries that have been published in the last few months assigning function to so-called junk DNA. I could continue, but I don’t want to overdo it. After all, consuming too many samples can ruin Saturday’s lunch out.
Next week, I’ll resume the sampling by describing another newly recognized function for a class of junk DNA known as pseudogenes. I hope you’re hungry for more.