In 1983 Barbara McClintock received the Nobel prize, only the second woman to be awarded an unshared Nobel prize, the other being Marie Curie. McClintock’s award came some 30 years after she made her discoveries about ‘jumping genes’.
What was it about ‘jumping genes’ that led one writer to suggest that Barbara McClintock would go down in history as one of the greatest biologists of the twentieth century? She found evidence that genes are not fixed in their positions along the chromosomes as had previously been thought. They can move and even control their movements along the length of a chromosome. Her evidence came from maize kernels that showed streaks and spots of colour caused by mutation.
McClintock found that there were two genes involved: a gene actually causing the mutation and a regulator gene controlling the activity of the mutated gene. Through carefully observing sets of chromosomes she located the positions of the genes.
However, these genes did not always appear in the same place. This only made sense if the genes were mobile units of inheritance that ‘jumped’ about and turned colour genes on and off. At the time her discovery was largely disregarded, partly because her ideas were far ahead of the general knowledge on other aspects of inheritance. It was only in the late 1970s and early ’80s that improved techniques enabled other scientists to confirm her discoveries.
Today Barbara McClintock’s ‘jumping genes’ are an accepted biological mechanism and an area of research in molecular biology and genetics. It is estimated that approximately 1 in 700 human mutations are caused by jumping genes. These are now known to be short sections of DNA that are able to move (transpose) themselves from one site in the DNA to another – sometimes on a different chromosome. These movable sections are more formally called transposons. They can insert
themselves at any point on the chromosomes and their effects can be disastrous.
When a transposon moves to areas of DNA which contain functioning genes it can cause mutations. For example, haemophilia is caused by a transposon being inserted into a clotting factor gene. If a transposon inserts itself into the section of the DNA that regulates transcription of a gene it can either increase or decrease transcription. Such changes can result in cancers.
All organisms, from bacteria to humans, have these transposons and in some cases they make up large amounts of the genome. In maize over 50% of the genome is made up of transposons. In humans the figure is 10%. Many of the transposons in our genome are retrotransposons. RNA is made from the retrotransposon DNA. This RNA sequence is converted into DNA before it is inserted in its new site. This is done using two enzymes, both of which are coded for by sequences on the original
retrotransposon. The first enzyme, reverse transcriptase, makes a DNA strand from the RNA template and then the second enzyme inserts the newly made piece of DNA into the chromosome. This process is so similar to the way that retroviruses, such as HIV, work (see Topic 6) that there is speculation as to which came first. Are retroviruses escaped retrotransposons, or are retrotransposons retroviruses that no longer leave the cell?
Transposons are also responsible for shuffling the order of the genes on a chromosome. This is because when a transposon moves it often drags a neighbouring piece of DNA with it. One recent research finding is that gene shuffling caused by transposons could play a part in the evolution of some species. In a study of rock wallabies in North West Australia it was shown that adjacent rock wallaby species that look very similar have very different arrangements of chromosomes. This is remarkable when other related species like, for example, the camel and llama, separated by an ocean, have exactly the same number and type of chromosomes. It seems that the wallabies have been badly affected by retrotransposons and have failed to inactivate them. This has resulted in such a drastic reshuffling of chromosomes that it has been likened to playing Lego® with chromosomes. Even though
any two such rock wallabies look similar and may mate, their different chromosomes do not match up and so they cannot produce fertile offspring – there is a reproductive barrier between them. This has resulted in several different species of rock wallaby living in essentially the same ecological niche.
Salters-Nuffield Advanced Biology Resources
Extension 4.3 Student SheetQuestions
Q1 What is a transposon?
Q2 How do transposons cause mutations?
Q3 Given that such a large proportion of our DNA consists of transposons, how is it that there are not many more catastrophic mutations?
Q4 Use the information in the passage above to draw a diagram to show how a retrotransposon moves from one side of the genome to another.
Q5 Does the rock wallaby study make us rethink the theory of evolution?
Visit the Nobel Laureates section of the Nobel website to find out more about Barbara McClintock and her research. The website is in the weblinks that accompany this extension.