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Wednesday, August 24, 2022

Junk DNA vs noncoding DNA

The Wikipedia article on the Human genome contained a reference that I had not seen before.

"Finally DNA that is deleterious to the organism and is under negative selective pressure is called garbage DNA.[43]"

Reference 43 is a chapter in a book.

Pena S.D. (2021) "An Overview of the Human Genome: Coding DNA and Non-Coding DNA". In Haddad LA (ed.). Human Genome Structure, Function and Clinical Considerations. Cham: Springer Nature. pp. 5–7. ISBN 978-3-03-073151-9.

Sérgio Danilo Junho Pena is a human geneticist and professor in the Dept. of Biochemistry and Immunology at the Federal University of Minas Gerais in Belo Horizonte, Brazil. He is a member of the Human Genome Organization council. If you click on the Wikipedia link, it takes you to an excerpt from the book where S.D.J. Pena discusses "Coding and Non-coding DNA."

There are two quotations from that chapter that caught my eye. The first one is,

"Less than 2% of the human genome corresponds to protein-coding genes. The functional role of the remaining 98%, apart from repetitive sequences (constitutive heterochromatin) that appear to have a structural role in the chromosome, is a matter of controversy. Evolutionary evidence suggests that this noncoding DNA has no function—hence the common name of 'junk DNA.'"

Professor Pena then goes on to discuss the ENCODE results pointing out that there are many scientists who disagree with the conclusion that 80% of our genome is functional. He then says,

"Many evolutionary biologists have stuck to their guns in defense of the traditional and evolutionary view that non-coding DNA is 'junk DNA.'"

This is immediately followed by a quote from Dan Graur, implying that he (Graur) is one of the evolutionary biologists who defend the evolutionary view that noncoding DNA is junk.

I'm very interested in tracking down the reason for equating noncoding DNA and junk DNA, especially in contexts where the claim is obviously wrong. So I wrote to Professor Pena—he got his Ph.D. in Canada—and asked him for a primary source that supports the claim that "evolutionary science suggests that this noncoding DNA has no function."

He was kind enough to reply saying that there are multiple sources and he sent me links to two of them. Here's the first one.

I explained that this was somewhat ironic since I had written most of the Wikipedia article on Non-coding DNA and my goal was to refute the idea than noncoding DNA and junk DNA were synonyms. I explained that under the section on 'junk DNA' he would see the following statement that I inserted after writing sections on all those functional noncoding DNA elements.

"Junk DNA is often confused with non-coding DNA[48] but, as documented above, there are substantial fractions of non-coding DNA that have well-defined functions such as regulation, non-coding genes, origins of replication, telomeres, centromeres, and chromatin organizing sites (SARs)."

That's intended to dispel the notion that proponents of junk DNA ever equated noncoding DNA and junk DNA. I suggested that he couldn't use that source as support for his statement.

Here's my response to his second source.

The second reference is to a 2007 article by Wojciech Makalowski,1 a prominent opponent of junk DNA. He says, "In 1972 the late geneticist Susumu Ohno coined the term "junk DNA" to describe all noncoding sections of a genome" but that is a demonstrably false statement in two respects.

First, Ohno did not coin the term "junk DNA" - it was commonly used in discussions about genomes and even appeared in print many years before Ohno's paper. Second, Ohno specifically addresses regulatory sequences in his paper so it's clear that he knew about functional noncoding DNA that was not junk. He also mentions centromeres and I think it's safe to assume that he knew about ribosomal RNA genes and tRNA genes.

The only possible conclusion is that Makalowski is wrong on two counts.

I then asked about the second statement in Professor Pena's article and suggested that it might have been much better to say, "Many evolutionary biologists have stuck to their guns and defend the view that most of human genome is junk." He agreed.

So, what have we learned? Professor Pena is a well-respected scientist and an expert on the human genome. He is on the council of the Human Genome Organization. Yet, he propagated the common myth that noncoding DNA is junk and saw nothing wrong with Makalowski's false reference to Susumu Ohno. Professor Pena himself must be well aware of functional noncoding elements such as regulatory sequences and noncoding genes so it's difficult explain why he would imagine that prominant defenders of junk DNA don't know this.

I think the explanation is that this connection between noncoding DNA and junk DNA is so entrenched in the popular and scientific literature that it is just repeated as a meme without ever considering whether it makes sense.

1. The pdf appears to be a response to a query in Scientific American on February 12, 2007. It may be connected to a Scientific American paper by Khajavinia and Makalowski (2007).

Khajavinia, A., and Makalowski, W. (2007) What is" junk" DNA, and what is it worth? Scientific American, 296:104. [PubMed]

1 comment :

Mikkel Rumraket Rasmussen said...

The idea that all non-coding DNA was once considered nonfunctional junk definitely a meme that survives because it just gets uncritically repeated in pop-sci articles, and in the introductions of papers where it's routinely asserted but never backed up with a reference.