Transposons: 'The Jumping Genes'

by Maddy Ross

I’m sure you are aware of genetic mutations and the effects they can have, but did you know that your DNA has the ability to literally copy and paste itself? There are sections of your genetic code called transposons that have the ability to essentially change their position and jump around in your DNA, hence their nickname ‘jumping genes’. 

In fact studies show that almost 46% of your genome is made of transposons that at some point or another over the course of evolution changed the sequence of your genes. In reality the mutations happening in your body right now often have very little or no discernible effect on you, in extreme cases however as I will discuss, mutations can have a significant impact on the way your body functions. 

There are two main types or classes of transposable elements: class 1 and class 2. Class 1 transposons are firstly copied into RNA through transcription and then that RNA is turned back into DNA through reverse transcription using reverse transcriptase usually encoded by the transposon. This DNA is then integrated back into the gene sequence in a different place meaning the original transposon never actually leaves the genome, it essentially copies and pastes itself somewhere else. Class 2 transposons work a little differently. There is no RNA stage and the transposon is cut out of the gene sequence using transposase, then moved by the enzyme and reinserted somewhere else in the genome. This would be a cut and paste method of transposition. 

Transposable elements were first discovered by Barbra McClintock in the mid 20th century when experimenting with maize which have a very high proportion of transposons. The significance of her findings was not appreciated at the time but she eventually won a Nobel Prize for physiology or medicine in 1983. 

During her time at Cornell University, McClintock developed staining techniques to aid her in visualizing the maize chromosomes that eventually helped in her finding of transposition. By 1932 she had published many articles on maize genetics including a major one published with Harriet Creighton detailing for the first time the physical crossing over of chromosomes during meiosis. 

At the Carnegie institution McClintock continued her research on chromosome breakage and fusion on maize and on a particular breakage that always occurred at locus “D” but after more research she discovered that this locus actually could change position. Further experiments found that this breakage required another dominant locus “A” which could also initiate its own transposition. She summarised all her research in an article as well as a talk which was apparently received with dead silence showing how little people understood the significance of her work. 

Her findings were not truly appreciated until years later as our knowledge of genetics and protein synthesis improved. After further research into transposons, scientists have discovered that they are present in all organisms. 

So how and why did evolution select for these jumping genes? The theory is that transposons have been moving around in our DNA for millions for years, changing the gene sequence and causing mutations. The effects of these mutations would determine whether the insertion would remain present in our DNA. If the insertion provided an evolutionary advantage to the individual then the mutation would be more likely to be passed down to the next generations. Any insertions that were not useful or caused illness would be less likely to be passed down. Hence natural selection would cause useful insertions to stay in the genome. Eventually the likelihood of the transposons to move without doing more bad than good decreased and over 99% of the transposons in the human genome lost their ability to move. However the transposable elements currently in our genome can sometimes cause disease and problems. 

LINE-1 is a very active transposon in the human genome but rarely causes harmful mutations, for example it has inserted itself in our genome so often that it makes up around 18% of our genome. However if the transpon inserts itself into an important part in the genome and disrupts the the gene sequence enough then it can have negative effects. For example an insertion of LINE-1 into the gene responsible for clotting blood can cause haemophilia A. Research has even found that abnormally high levels of LINE-1 can be linked to various forms of cancer. In fact an insertion of LINE-1 into the APC gene has been found to cause colon cancer. 

Despite their potential to wreck havoc in our genome, transposons can be incredible tools when carrying out genetic research. They can beused in transgenesis, introducing a foreign gene from one organism into the genome of another, and insertional mutagenesis, when genetic information is changed by a mutation. This means transposons can be used for analysis of genomes, studies of embryonic development, identifying genes and pathways in disease and even play a part in gene therapy. So while they may not play a massive role in changing the way our bodies function anymore, they can definitely change the way geneticists work and experiment with DNA.

Sources: 

https://sitn.hms.harvard.edu/flash/2018/transposons-your-dna-thats-on-t he-go/ https://www.youtube.com/watch?v=eSD_tbjfSlA https://www.pnas.org/content/109/50/20198 https://en.wikipedia.org/wiki/Transposable_element#Diseases https://www.ncbi.nlm.nih.gov/pmc/articles/PMC2874221