What Are Jumping Genes?

Introduction

The rearrangement of genetic information within and among DNA molecules encompasses a variety of processes, collectively placed under the heading of genetic recombination. Genetic events fall into three classes-

• Homologous Genetic Recombination- Involves genetic exchanges between two DNA molecules.

• Site-specific recombination- Exchange occurs only at a particular DNA sequence.

• DNA transposition- It is distinct from both other classes in that it usually involves a short segment of the DNA molecule with the remarkable capacity to move from one location in a chromosome to another.

Jumping Genes or Transposons

Jumping genes are also called transposons. They are named so because these DNA segments are capable of jumping from one site to the target site. These jumping genes were first observed in maize in the 1940s by Barbara McClintock. These segments of DNA found in virtually all cells move or jump from one place on a chromosome (the donor site) to another on the same or different chromosome (the target site).

Fig. Altered pigmentation in maize kernels due to transposons

DNA sequence homology is not usually required for this movement called transposition; the new location is more or less determined randomly.

Insertion of transposition in an essential gene could kill the cell that is why the process of transposition is very tightly regulated to avoid this and it does not occur frequently. They are simplest type of molecular parasite that are adapted to replicate passively within the chromosome of the host cell. In some cases, they carry genes that are useful to the host cell and thus exist in a kind of symbiosis with the host cell.

Structure of Transposons

Transposons in bacteria differ in structure, but most have short repeated sequences at each end that act as the binding site for the transposase. During transposition, a short sequence at the target site usually 5 to 10 base pairs long is duplicated to form an additional short repeat sequence that flanks each end of the inserted transposon. These duplicated segments result from the cutting mechanism used to insert a transposon into the DNA at the new location.

Classes of Bacterial Jumping Genes

Bacteria contain two classes of jumping genes-

Insertion Sequences

These are also called simple transposons. They contain only the sequences required for transposition and the genes for proteins (transposase) that promote the process.

Complex Transposons

They contain one or more genes required for transposition. These extra genes may help in conferring resistance to antibiotics and thus enhances the chances of host survival. The spread of antibiotic resistance elements among disease-causing bacterial populations that is rendering some antibiotics pose no effect because of complex transposition.

Pathways of Transposition

There are two ways by which transposition occurs in bacteria. They are as follows-

Direct or Simple Transposition

In this mechanism, the transposon is cut on either side and excised this forces the movement of the transposon to the new location. This results in the formation of a donor DNA left with a double-stranded break that must be repaired. A staggered cut is made at the target site and transposon is inserted into it. Later the gaps are filled by DNA replication to duplicate the target site sequence.

Replicative Transposition

In this, there is a replication of the entire transposon, which causes the formation of a copy at the donor location. The peculiarity of this process is the formation of an intermediate known as cointegrate that contains a DNA at the target site which is covalently linked to the donor site.

So, we can say that at the integration one can find two complete copies of transposons that have the same relative orientation in the DNA. In some transposons which are well characterized, the cointegrate intermediate is converted into a product by site-specific recombination, in which an enzyme called recombinase mediates the deletion reaction.

Eukaryotes also have transposons that are structurally similar to bacterial transposons, and they show similar mechanisms of transposition too. In certain organisms, however, an RNA intermediate is involved in the mechanism of transposition.

Mechanism of Transposition

• Cuts are made in the target sequences by an enzyme called transposase that is present in the transposon.

• The target sequences are copied, and both the copies are pulled apart to accommodate transposon in the center.

• The transposon ligates itself at the free ends thus generated. The nicks are sealed by ligases and the strand becomes continuous.

Functions of Jumping Genes

• Jumping genes or transposons may cause a mutation by inserting them into the regulatory sequences, thus causing a change in their expression. Sometimes transposons insert themselves in between the stop codon which results in the formation of truncated proteins.

• The gene that codes for the red pigment of the eye in Drosophila was mutated which gave rise to a white-coloured eye in Drosophila. The main cause of which is the spontaneous mutation caused by transposons.

• They are known to cause several genetic diseases in Humans and the development of the functional immune system is also attributed to transposons.

• In bacteria they help in nullifying the effect of antibacterial drugs and toxins.

• New genes are formed as a result of exon duplication and divergence.

Conclusion

Earlier it was thought that genes have a fixed locus and they do not tend to move. But studies conducted by Barbara McClintock on corn kernels suggested that segments of DNA tend to jump from one location to another within the chromosomes that caused the different pigmentation of corn kernels.

This led to the discovery of transposons which are a type of mobile genetic element. They are found to cause not only spontaneous mutations but also confer antibiotic resistance to bacteria. Deeper studies can open the doors for more extensive research and their possible outcomes in the field of molecular biology.