Arxiu d'etiquetes: corn

Transponable elements: the jumping genes of our genome

In the same way that grasshoppers are jumping and moving through the field, there is a type of genes that jump through our genome and change its position. Our genome is not static, so read on to know everything about these kinds of genes.


Barbara McClintock discovered transposable elements, or also called mobile genetic elements because of their ability to move around the genome. The “jumping genes,” as this American geneticist christened them, changed the knowledge about genetics so far, since at first the scientific community did not believe in the idea that a DNA sequence could move on its own.

She had a special relationship with corn, a plant domesticated by man for 10,000 years and has become one of the three most cultivated cereals in the world. In addition, it is one of the most important staple foods since from it many derived products are made, such as flours and oils. Its great industrial value has made it have been studied in depth and its genome has been sequenced.

McClintock began studying the DNA of corn and observed that there were a number of genetic sequences that, without knowing how, changed position within the genome. Somehow, these sequences turned on or off the expression of other corn genes and this was observed with the naked eye; the grains of a corn cob could be of different colours (Figure 1), even within the same grain there were areas of various colours. Then McClintock sought the answer of how this was possible if the genes responsible for colour were inherited from the parents. The result was the discovery of the transposable elements, which led her to win the Nobel Prize in Medicine in 1983.

elemento transponible maiz
Figure 1. (A) P gene gives a purple grain. (B) A transponable element is inserted in the middle of the P gen and the grain has no pigmentation. (C) Corn cob wit some grains with P gene intact and others with P gene interrupted by a mobile genetic element. (Source: Porque biotecnología, adaptation)


When the transposable elements jump and change position they produce a loss of bases when leaving the place where they rested. This loss of some bases does not have “much” importance. But if the transposable element is inserted into a gene, there is an addition of a large number of bases that will cause the loose of gene’s function. For this reason, mobile genetic elements produce mutations because by jumping and changing their location, they alter the DNA sequence and prevent genes from encoding proteins through the genetic code. However, when they jump again, the gene regains its functionality and expresses itself as if nothing had happened.

Often, these jumping genes are considered parasites, because the cell cannot get rid of them. Although they can also bring benefits to the cell, such as transporting advantageous genes. The best known example is not found in humans, but in bacteria and their resistance to antibiotics such as penicillin, discovered by Alexander Fleming. The spread of antibiotic resistance is due to genes that encode enzymes that inactivate them, and that are located in mobile genetic elements. It is usually related to the horizontal transfer of genes, in which they can move from one cell to another as if they were bees that go from flower to flower. When this happens, the transposable element is introduced into a new cell and inserted into the genome of this new cell. That is when it will be faithfully transmitted to its progeny through the normal process of DNA replication and cell division.


It is estimated that in the human genome there are 44% transposable elements, which can amount to 66% taking into account repeated fragments and short sequences derived from them. The consequence is that we have more than 1000 genes regulated, directly or indirectly, by sequences from transposable elements.

So far, two types of transposable elements are known: class I transposable elements or retrotransposons and class II transposable elements or DNA transposons. They are classified according to whether they require reverse transcription to jump and transpose or not.

Reverse transcription is similar to the transcription process, but with the difference that it occurs in reverse. That is, if in the classical transcription process a single strand of RNA is obtained from a double strand of DNA, in reverse transcription of an RNA molecule a DNA molecule is obtained. This is common in viruses such as HIV virus (AIDS) or hepatitis virus, but also in some class I transposable elements. These are very abundant and represent 90% of the transposable elements of our genome.

Instead, the others are class II transposable elements or DNA transposons. These are the elements that McClintock discovered in corn, with a 10% representation in our genome and responsible for the spread of antibiotic resistance in bacterial strains.

It should be noted that DNA transposons never use intermediaries, but are autonomous. They jump from one place of the genome to another by themselves, without any help. The mechanism they use is called “cut and paste” and is similar to the cut and paste we use on the computer. The DNA transposon cuts the DNA sequence that has end and look for another place to settle. Then there it also cuts the DNA sequence and is “hooked” (Figure 2).

Figure 2. Mechanism of cutting and pasting (Source: SITN: science in the news)

It is currently known that the activity of transposable elements is a source of evolutionary innovation due to the generation of mutations, which could have been key both in the development of organisms and in different evolutionary phenomena such as speciation; the process by which a population of a given species gives rise to another or other species.

The vast majority of these mutations are deleterious to organisms, but some of them will lead to adaptive improvement and tend to spread throughout the population. We could put our hand in the fire and we probably wouldn’t burn to ensure that much of the variability that life shows around us originally comes from the displacement of mobile genetic elements or transposable elements.

(Main picture: ABC Canada)

Corn as example of genetic improvement in plants

Nowadays, genetic improvement is into the spotlight. However, it is not a new topic because we have done genetic improvement during years, in plants or in animals. In this article, I will discuss genetic improvement in plants, giving the corn as example, which is a plant domesticated by humans for 10,000 years.


Genetic improvement in plants is the process based on theoretical principles and methods for obtaining varieties of crop plants, which guarantee under high environmental conditions and production, high and stable yields of the products grown with the required quality.


The aims of genetic improvement are:

  • Increase performance:
    • Improvement of productivity: increasing the potential productive capacity of individuals.
    • Resistance improvement: obtaining genotypes resistant to pests, diseases and adverse environmental conditions.
    • Improvement of agronomic characteristics: obtaining new genotypes that are better adapted to the demands and application of the mechanization of agriculture.
  • Increase the quality: improvement of the nutritive value of the vegetal products obtained.
  • Extend the area of exploitation: adapting the varieties of the species already cultivated to new geographical areas with climatic characteristics or extreme soil types.
  • Taming new species: transforming wild species into crops with utility and profitability for man.


Before starting the process, you have to define the objectives you want to achieve and, therefore, define those characters that you want to improve in order to obtain a specific phenotype.

The steps that follow in the process of genetic improvement are:

  1. First step: to find within the genetic variability of the collected species, or of the species that can hybridize, individuals that have these characters.
  2. Second step: these individuals hybridize with each other and with plants with good general agronomic characteristics. The result will be a base population that will segregate for a large character name, from which individuals will be selected that are closest to the desired variety.
  3. Third step: to verify that these individuals are better in one or more aspects than the varieties that are in the market, a fact that normally forces to carry out comparative tests.


The maize plant (Zea Mays) has been domesticated by man for 10,000 years. At this time it has become one of the three most grained cereals in the world and this increase of the crop is linked to the development of varieties that are better adapted to the needs of each place.

Maize is one of the most important staple foods since it makes many derived products (flours, oils). As it has a great value in the industry, it is a much studied plant and its genome has been sequenced.


Maize is affected by the European borer (Figure 1), Ostrinia nubilalis. It is a plague of cereals, particularly of corn. It is a native lepidopteran of Europe that infested the millet, before the arrival of the corn.

Its butterfly is about 2.5cm long. The female is yellowish brown with irregular bands on the wings, and the male is smaller and darker. The female lays eggs under the leaves.

Figure 1. Picture of European corn borer female (Source: Discover Life)

The borer makes tunnels inside the corn (Figure 2) that cause the plant to break and fall to the ground. It has to be taken into account that when the maize is still immature it is not affected by the borer, thanks to the natural defenses of these plants in the growing stage.

Figure 2. European corn borer larva in maize (Source: Iowa State University)


We can find two types of modified corn:

  • Crops producing their own herbicide (Bt)
  • Herbicide-tolerant crops (Monsanto)

Bt maize is a plant genetically modified by modern biotechnology to defend itself against the attack of lepidopteran insects. Using recombinant DNA technology, maize was modified by inserting a bacterium gene of Bacillus thuringensis (Bt), such that its leaves, stem and pollen express the Bt protein of bacteria. Bt maize is the importation and the new tool for the control of damages and losses caused by insect pests.

Herbicide-tolerant maize is maize that has been improved by the use of recombinant DNA technology to tolerate the use of certain types of herbicides. With the use of these technologies for the possible state deactivate or replace the sequence of susceptibility by another that confer resistance and that allow a crop plant to tolerate the use of the herbicide.


To transform a normal plant to a transgenic plant, the gene that produces a characteristic of interest is identified and separated from the rest of the gene material of a donor organism.

A donor organism can be a bacterium, fungus or any other plant. In the case of Bt maize, the donor organism is a naturally occurring soil bacterium, Bacillus thuringiensis, and the gene of interest produces a protein that kills lepidopteran larvae. This protein is called Bt delta endotoxin.

The Bt delta endotoxin was selected for the fact that it is highly effective for controlling larvae of caterpillars. It is during the larval stage when most of the damage occurs from the European corn borer. The protein is very selective, in general, it does not harm the insects in other orders (like beetles, flies, bees and wasps). Therefore, transgenics that have the Bt gene are compatible with biological control programs, since they harm predators and parasitoids less than insecticides with a broad spectrum of insects. Bt endotoxin is considered safe for humans, other mammals, fish, birds and the environment due to its selectivity.