Why is ‘junk DNA’ not deleted from the original genome over millions of years of evolution?
A bold new hypothesis developed and substantiated by Tel Aviv University scientists offers a possible solution to the question of why neutral sequences, also known as “junk DNA,” continue to exist within the human genome for millions of years.
According to the researchers, it’s a matter of proximity to the stuff that really matters: junk DNA is often located in the vicinity of functional DNA. Deletion of cells on the edges between junk and functional DNA is likely to damage the functional regions, which is why the evolution opted against it. The researchers dubbed this dynamic “border induced selection.”
The model was developed under the leadership of the PhD student Gil Loewenthal in the laboratory of Prof. Tal Pupko from the Shmunis School of Biomedicine and Cancer Research, Faculty of Life Sciences and in collaboration with Prof. Itay Mayrose (Faculty of Life Sciences, Tel Aviv University). It furthers our understanding of the huge variety of genome sizes observed in nature.
“In previous studies… it was found that the rate of deletions is greater than the rate of additions in a variety of creatures including bacteria, insects, and even mammals such as humans,” Prof. Pupko explains. “The question we tried to answer is how the genomes are not deleted when the probability of DNA deletion events is significantly greater than DNA addition events.”
“We have provided a different view to the dynamics of evolution at the DNA level,” says Loewenthal, the project leader. “We claim that in shorter neutral segments, deletions are likely to delete adjacent functional segments which are essential for the functioning of the organism, and therefore will be rejected. We call this phenomenon ‘border-induced selection.’ If so, when the segment is short, there will be a reverse bias so that there will be more insertions than deletions, and therefore short neutral segments usually are retained.”
“A good match was obtained between the results of the simulations and the distribution of lengths observed in nature,” according to Loewenthal.
Last year, another group of researchers from Tel Aviv University published an important research on the risks involved in DNA sequencing.