Study of Fossil Remains Identifies another Function for Junk DNA
An exceptional paper published a few months ago in Nature provides the first-ever estimates of genome sizes for dinosaurs (even though there is no DNA left behind in their remains). As a by-product, this work highlights another important function for junk DNA.
The research team used an ingenious approach to estimate dinosaur genome size. These workers took advantage of the relationship between cell size and genome size. It turns out that larger cells tend to have more DNA in the nucleus than smaller cells.
Cell biologists believe that, in addition to housing genetic information, DNA plays a skeletal role in the nucleus; the more DNA in the nucleus, the larger the volume of this organelle because of the space taken up by DNA.
The ratio of the nuclear volume to cell volume is a critical parameter. To accommodate reasonable growth rates, large cells need to have a large nucleus. A large nucleus has a large surface area. This characteristic allows for rapid transport of mRNA from inside the nucleus to the cytoplasm, where it can direct protein synthesis. Transport of mRNA occurs through nuclear pores. A large nucleus has more pores than a small one because of its greater surface area. Large cells require a much higher rate of protein synthesis than do smaller cells in order to have enough subcellular and biochemical components to sustain their activity. This requirement necessitates a sufficiently rapid transport of mRNA to the cytoplasm.
Researchers have noted that when dinosaur bones are sliced, the outlines of bone cells, or osteoclasts, are evident. The research team used this observation to estimate osteoclast size for a number of dinosaurs from fossilized bone slices. They then established a calibration for genome size by correlating DNA content to osteoclast size for a number of living vertebrates (amphibians, reptiles, birds, and mammals).
The team discovered that dinosaur genome sizes cluster into two groupings that correspond to theropod and nontheropod dinosaurs. They found that theropod dinosaurs had a much smaller genome size, on average, than did their nontheropod counterparts.
The scientists rationalized this find by noting that other evidence suggests theropods had a much higher metabolic activity than nontheropod dinosaurs. Smaller cell sizes would benefit creatures with high metabolic activity. Smaller red-blood cells have a higher surface-to-volume ratio and, consequently, a more efficient exchange of gases (like oxygen and carbon dioxide) with surrounding tissues.
The researchers speculated that the amount of noncoding DNA found in their genomes dictated the difference in cell sizes for these two groups of dinosaurs. The larger genomes of nontheropods contained more noncoding DNA than theropod genomes.
In broad terms, the genomes of organisms consist of coding DNA (which harbors genes) and noncoding DNA. For most organisms, noncoding DNA comprises a vast majority of their genomes. Over 95 percent of the human genome, for example, consists of noncoding DNA.
Evolutionary biologists have traditionally thought of noncoding DNA as junk—the product of random biochemical events. They 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 undirected biochemical processes and random chemical and physical events transform a functional DNA segment into a useless molecular artifact. Junk 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. Skeptics ask, “Why would a Creator purposely introduce nonfunctional, junk DNA into the genomes of organisms?”
However, the estimates of dinosaur genome sizes indicate that noncoding DNA is not junk at all. Rather, it plays a critical role as a nucleoskeleton.
Studies like these help explain why the Creator purposely introduced noncoding DNA into the genomes of organisms. Junk DNA is a veritable lagerstatte of design.