Genes can jump? Oh yes, they can. Some DNA sequences (genes) can “jump” or, move around from one position of the genome to other positions. These mobile genetic elements are called Transposable Elements (TEs), or, popularly “jumping genes”. These elements can cause mutations leading to diseases or, other abnormalities. If they are “jumping around” in the genome of gametes (developing sperm or egg cells), the genetic changes are passed on to the descendants, thus playing an important role in the process of evolution. TEs are widespread and abundant in all eukaryotic genomes, varying from 50% (in humans) to 70% (in maize) of the genome.
How were they discovered?
Since the discovery of genetic material and their understanding, genes were considered to be fixed and linearly positioned on the chromosomes of an organism. But Barbara McClintock suggested that the genetic elements of an organism are far more mobile and dynamic than they were thought to be. She performed cross-breeding studies with maize and observed genetic variations of corn kernel phenotypes (colour patterns). Her observations revealed the presence of “jumping genes” or, transposons, which moved from their positions in the genome and may differ from one generation to another. She found them responsible for many gene mutations, mostly insertions, deletions and translocations. She suggested that the genome of an organism is not a static entity, but is prone to alteration and rearrangements. She faced a lot of criticism from the contemporary scientific community for her revolutionary ideas and her work. Gradually, with more discoveries and advanced studies on cytogenetics, the presence and importance of TEs was widely confirmed in many eukaryotes, from bacteria to humans. In 1983, Barbara McClintock finally received the much-deserved Nobel Prize for the discovery of “jumping genes”.
Types Of Transposons
There are many ways to categorize TEs, but the most common distinction is based on their requirement for reverse transcription (transcription of RNA to DNA) to transpose.
- Class I transposons or, Retrotransposons – Class I TEs primarily produce RNA transcripts. These RNA transcripts are then reverse transcribed back into DNA with the help of the reverse transcriptase enzyme. This reverse-transcribed DNA is inserted into new locations of the genome. The major part of human “jumping genes” belongs to Class I TEs.
- Class II transposons or, DNA transposons – Class II TEs do not use RNA transcripts for their movement in the genome. They encode protein transposase, which excises and insert the DNA transposons into new locations of the genome.
What do they do? Are they important?
“Jumping genes” can cause mutations. Depending on their new locations, these new mutations can cause functional changes, abnormalities, diseases, novel genetic products, or they can also be silent. If a transposon “jumps around” in functional genes, in exons, introns or, even in the flanking regions of the gene (may contain gene regulators), it can probably damage or alter its genetic expression. Some of the transposons have been related to haemophilia and cancer in humans. Mutations caused by TEs are not always destructive. Transposons can pave way for genomic evolution by shuffling exons and altering gene regulatory regions.
As per the general scientific consensus, most of the TEs seem to be silent. Some of them can´t “jump around” because of their new mutations, some of them can move but, are unable to produce any phenotypic effect because of DNA methylation or, chromatin remodelling. Some TEs have known to be epigenetically silenced and, Barbara McClintock was the first scientist to suggest that transposons may regulate gene expressions. Studies have shown that “jumping genes” are mostly prohibited from “jumping around” by various endogenous gene regulatory mechanisms, to maintain the relative stability of the genome. For example, small non-coding interfering RNAs (siRNAs) can prevent transposition in human cells.
Depending on their new locations, TEs may cause damage or influence evolution. They have been observed to make a lot of copies of themselves. Interestingly, combined with the capability of the genome to intrinsically inhibit the “jumping around” of most of the TEs, the only thing these “jumping genes” have done for sure is making numerous and repetitive copies of themselves. They are also called “selfish DNA”, “junk DNA” or, “repetitive DNA” for this reason. However, a lot of recent studies have shown that 75 % of our repetitive DNA sequences controls and regulates gene expression. However, when and how these “jumping genes” move to develop reproductive cells and propagate to future generations, have not been fully studied yet. Current studies are trying to track the “jumping around” of TEs so that their movements can be better understood and predicted, if possible.
1. Slotkin, R. K., & Martienssen, R. Transposable elements and the epigenetic regulation of the genome. Nature Reviews Genetics 8, 272–285 (2007)