Many have written how the term ‘junk DNA’ is an imperfect one, and how junk DNA may have a tangential role in evolutionary changes. A new study published in Science visits this topic, specifically focusing on repetitive non-coding sequences in and around promoter regions of the human genome. The authors of this study have published their findings under the title, “Unstable Tandem Repeats in Promoters Confer Transcriptional Evolvability.”
The findings concluded that the repeats affect the activity of neighboring genes by way of how tightly the downstream DNA is wrapped around a complex of proteins collectively called a nucleosomes. A nucleosome is one of the half dozen packaging features of the eukaryote genome which allows a genome that is 3 billion base pairs long or 6 feet in length to be squeezed into a tiny little nucleus. About 167 basepairs wrap around one nucelosome. DNA that is more wrapped around a nucleosome is harder to be activated, and thus otherwise non-coding/junk tandem repeats of sequences determine how tightly the local DNA is wrapped around these protein complexes.
The extra cool finding about this paper is that the tandem repeats are very unstable, as you possibly could tell from the title. The authors found out that the number of repeats changes a lot during DNA replication, as if the DNA pol III exonucleases don’t bother proof-reading these areas! These changes affect the local DNA packaging, which in turn alters gene activity. In this way, unstable junk DNA is one of the faster acting mechanisms in altering gene activity with each cellular division.
As an extra step, the researchers conducted a experiment investigating the impact of these tandem repeats on yeast cells. They found out that when a repeat is present near a gene, it is possible to select yeast mutants that show vastly increased activity of this gene. But, when the repeat sequences were removed, this fast evolution was impossible.
So what does this all mean for human evolution? Well, unstable pieces tandem repeats of ‘junk’ non-coding DNA are one of the many ways of regulating gene expression and honing on when a gene’s activity can enable organisms like humans to quickly adapt to changes in their environments.
- Vinces, M., Legendre, M., Caldara, M., Hagihara, M., & Verstrepen, K. (2009). Unstable Tandem Repeats in Promoters Confer Transcriptional Evolvability Science, 324 (5931), 1213-1216 DOI: 10.1126/science.1170097