"By the late 1960s, knowledgeable scientists were used to the idea that genes occupied only a small part of the genome, and in 1974 the editor of the journal Cell, Benjamin Lewin, was expressing the consensus view of the experts when he wrote that the C-value Paradox could be resolved by assuming that much of the genome is composed of nonfunctional repetitive DNA (junk DNA)." (Chapter 2 of Laurence Moran (2023) What's in your genome.)
It may be that 'knowledgeable scientists' in the late 1960s knew that much of the genome is composed of junk DNA, but the 'consensus view' was not widely known in all sub-disciplines of the biological research community. Maybe the journal Cell was not read in the evolutionary biologist community. Probably, those experts were experts in a different field with its own journals and conferences.
|Eli C. Minkoff (1984) Evolutionary Biology.|
I checked the oldest textbook I have, Minkoff (1984) Evolutionary biology. There is no 'junk DNA' and no 'non-coding' DNA in the index, despite the 'consensus view'. Yes, there are tRNA and rRNA (p.16), but these RNAs are not labeled as 'non-coding RNA' or 'non-coding DNA'. They are in the business of producing proteins. They are the very embodiment of the genetic code. Therefore, it would be somewhat counterintuitive to call them 'non-coding'. Yes, there is 'genetic drift', neutral mutations, 'neutralism versus selectionism', 'genetic load', 'mutational load' in his book, but Minkoff did not connect these concepts with 'non-coding DNA'. The concept is absent anyway. 'Centromere' is mentioned once casually (p.19), I could not find 'telomere'. Anyway, 'centromere' is not labeled as 'non-coding DNA'. Why is non-coding DNA absent from the book?
I think I found part of the answer in the following passage:
"One of the fundamental tenets of modern synthetic theory of evolution is that natural selection operates on the phenotype rather than the genotype. No genetic change can be influenced by natural selection unless it first produces some phenotypic change. It is largely for this reason that modern evolutionary biologists must be aware of the manner in which phenotypes are controlled." (p.114)This was an eye-opener for me. The phenotype is the most important, the genotype is important only in so far it has an effect on the phenotype. Who cares about DNA that does nothing? Evolutionary biology has the task of explaining the organism.
A second foundational paradigm I found here:
"Proteins are among the most important of all biological molecules. (...) The great intricacies of living systems are all the result of enzyme-controlled activities (...) enzymes are therefore the chemical basis of life". (p.17).
Taken together these two principles explain the mindset of evolutionary biologists in those days. If they did know about non-coding DNA, it simply had no relevance to the goals of their daily research. On page 37 there is a table labeled as 'The Genetic Code for Translation of mRNA Codons into Amino Acid sequences'. The famous table. It makes sense in this context, because the Genetic Code is the link between DNA and proteins. The reason for the existence of the Genetic Code is to produce proteins. The Watson-Crick structure of DNA plus the chemical structure of the four bases is present in the book. Minkoff knows the necessary biochemistry. Unfortunate exception: introns and splicing are absent! Introns were discovered in 1977.
There is one isolated and thus mysterious remark which vaguely suggests something like 'non-coding DNA':
"Not all of the genotype is transcribed and translated into a portion of the epigenotype, nor are all the transcribable genes ever transcribed at the same time." (p.114) ['epigenotype' = "the polypeptides that result from the immediate transcription and translation of the genotype"]
That's all. Probably, Minkoff was vaguely aware of non-coding DNA. But why include it in his textbook? He did not elaborate the concept because in his opinion it was simply not relevant or nothing was known about it. DNA which is not transcribed and translated has nothing to contribute to the phenotype of the organism, consequently nothing to biology and evolution. It doesn't fit in the evolutionary biology paradigm of that time .
So, that is the 'true history' of non-coding DNA based upon Minkoff (1984) and that was taught to biology students at that time. He did not say that non-coding equals junk, but by omitting non-coding DNA, he implied that non-coding DNA is unimportant. If one makes statements about the history of junk DNA, one has to investigate the evolutionary biology textbooks, especially older ones. Minkoff was an eye-opener for me. I checked more evolutionary biology textbooks: 8 out of 17 do not have 'non-coding DNA' in the index.
"As Sandwalk readers know, there was never a time when knowledgeable scientists said that all non-coding DNA was junk. They always knew that there was functional DNA outside of coding regions." (Sandwalk)
I think one has to take into account that there are different scientific disciplines with their own paradigms, leaders, journals, conferences, and networks.
Thanks for reading. Have a nice day!
- On page 18 he writes: "there
are other sequences in each DNA molecule that do not appear to
determine the amino acid sequence of any polypeptide. Some of these may function as "spacers", and others are
believed to function as regulatory genes, which control the transcription of other genes." (page 18, chapter 2: Basic Principles of Genetics). Here he describes non-coding regulatory genes! He doesn't realize that these non-protein-coding DNA sequences must have indirect effects on the phenotype, and consequently are important for evolutionary biology! In the subsequent development of evolutionary biology, the evolutionary importance of regulatory genes became evident. Added: 21 Aug 2023
- Junk DNA in the Evolution textbooks (2) from 1996 to 2023 26 Jul 23
- Junk DNA in the evolution textbooks. Bergstrom and Dugatkin 2023 12 Jul 23
- Periannan Senapathy (1994) claimed that the human genome consists of more than 90% junk DNA. 4 Jul 2023
- Scientists say: 90% of your genome is junk. Have a nice day! Biochemist Laurence Moran defends junk DNA theory 26 Jun 23