The main issue in this field concerns the number of non-coding genes in the human genome. I cover the available data in my book and conclude that there are fewer than 1000 (p.214). Those scientists who promote the importance of RNA (e.g. Tom Cech) would like you to believe that there are many more non-coding genes; indeed, most of those scientists believe that there are more non-coding genes than coding genes (i.e. > 20,000). They rarely present evidence for such a claim beyond noting that much of our genome is transcribed.
Tom Cech is wise enough to avoid publishing an estimate of the number of non-coding genes but his bias is evident in the following paragraph from near the end of his article.Although most scientists now agree on RNA's bright promise, we are still only beginning to unlock its potential. Consider, for instance, that some 75 percent of the human genome consists of dark matter that is copied into RNAs of unknown function. While some researchers have dismissed this dark matter as junk or noise, I expect it will be the source of even more exciting breakthroughs.
Let's dissect this to see where the bias lies. The first thing you note is the use of the term "dark matter" to make it sound like there's a lot of mysterious DNA in our genome. This is not true. We know a heck of a lot about our genome, including the fact that it's full of junk DNA. Only 10% of the genome is under purifying selection and assumed to be functional. The rest is full of introns, pseudogenes, and various classes of repetitive sequences made up mostly of degraded transposons and viruses. The entire genome has been sequenced—there's not much mystery there. I don't know why anyone refers to this as "dark matter" unless they have a hidden agenda.
The second thing you notice is the statement that 75% of the genome is transcribed at some time or another and, according to Tom Cech, these transcripts have an unknown function. That's strange since protein-coding genes take up roughly 40% of our genome and we know a great deal about coding DNA, UTRs, and introns. If you add in the known examples of non-coding genes, this accounts for an additional 2-3% of the genome.1
Almost all the rest of the transcripts come from non-conserved DNA and those transcripts are present at less than one copy per cell. As the ENCODE researchers noted in 2014, they are likely to be junk RNA resulting from spurious transcription. I'd say we know a great deal about the fraction of the genome that's transcribed and there's not much indication that it's hiding a plethora of undiscovered functional RNAs.
Photo credit: University of Colorado, Boulder.
1. In my book I make a generous estimate of 5,000 non-coding genes in order to avoid quibbling over a smaller number and in order to demonstrate that even with such a obvious over-estimate the genome is still 90% junk.