Ancient mystery of the genome solved: what our DNA has been hiding for so many years

Scientists at the University of California, Santa Cruz have uncovered an ancient genetic mystery. She is

is the origin of introns.

What are introns?

Introns are segments of non-coding DNA thatmust be removed from the genetic code before it can be translated into proteins. This ancient feature is found in all eukaryotic life - a wide range of organisms, including all animals, plants, fungi and protozoa. However, it is absent in the genomes of prokaryotes, such as bacteria. There is a huge difference in the number of introns found in the genomes of different species, even between closely related ones.

Why are they important?

Introns are important because they providepossibility of alternative splicing. It is the process of cutting certain nucleotide sequences from RNA molecules and joining the sequences that remain in the "mature" molecule during RNA processing.

In turn, this allows one geneencode multiple transcripts and therefore perform complex cellular functions. Introns can also influence gene expression, the rate at which genes are turned on to make proteins and other non-coding RNA. Ultimately, they have a neutral or even slightly negative effect on the species in which they exist. And that's why.

Binary code of the helical DNA molecule. Source: en.freepik.com

When introns are spliced incorrectly, the gene they live in is sometimes damaged and even killed. These "missed cases" of splicing are the cause of some cancers.

What is the mystery of biology?

One of the fundamental mysteries of modernbiology is associated precisely with the origin of introns. What puzzles scientists is that there is a huge difference in the number of introns that are found in the genomes of different, even closely related species. In addition, usually the length of the intron sequence is 10–100 times the length of the coding DNA sequences. The cause of this intron explosion was also unknown.

What have the scientists done?

In a new study carried outby the University of California at Santa Cruz and published in the journal Proceedings of the National Academy of Sciences (PNAS), scientists have understood where introns come from. Scientists suggest that their source is introners.

And what is it?

Introners are a newly discovered typemobile elements of the genome that are able to multiply and move along the DNA. According to the authors, they are also the only plausible explanation for events where thousands of introns appeared in the genome of one species, seemingly at the same time.

What did the scientists find out?

Biologists have studied the genomes of 3,325 eukaryoticspecies to find out how common introns are and in which groups of species they are most common. In total, they found 27,563 introners in the genomes of 175 species. This is 5.2% of the studied samples. Scientists believe that this proves the involvement of introners in the origin of introns, since the first have been identified in all species from animals to unicellular organisms, whose common ancestor lived more than 1.7 billion years ago.

In addition, it turned out that introners meetin marine organisms 6.5 times more often than in terrestrial species. According to a preliminary assessment, this is due to horizontal gene transfer. During this process, genes are passed from one species to another. But in the course of vertical transfer, genes are passed from parent to offspring.

Also, more introners were found in speciesfungi, which are also known to have higher rates of horizontal gene transfer. This further supports the idea that this phenomenon contributes to an increase in the number of itroners.

Overall, the diversity of species in which they are found suggests that introners are the fundamental and most common source of introns on the tree of life.

Why is it so important?

A new study challenges one of thecomprehensive theories of genome evolution: namely, what drives genome complexity in eukaryotes. Many species are thought to have had low effective population size at some point in their evolution. This means that very few organisms of the species produced offspring to create the next generation. This is what allowed elements that were known to have little negative impact on the population to accumulate in the genome.

Polygonal DNA concept. Source: en.freepik.com

Following this theory, itroners, from neutral toslightly harmful are more common in populations with lower "efficacy rates". But the researchers found something else. For example, that Symbiodinium receives the most introns among the "interrogated". But this is a protist, which has a higher effective population size than humans, land plants and other invertebrates.

What's the bottom line?

The new study also points to the difficultyarising not from an adaptation created by the genome itself, but as a response to the conflict caused by an invading mobile element - an introner - when it tries to reproduce. As introners and other elements struggle to survive and persist, this conflict results in the complexity of the genome.

Ultimately, the researchers concluded thatintroners do not necessarily directly lead to lower expression. However, genes that are less expressed are more tolerant of an element that can negatively affect them. This is because they mean less to the survival of the species.

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