In this article, we will discuss about the transposons, also called transposable elements. These elements are short repeated sequences found in the jumping genes. Transposons excise from the donor site and insert at the target site. It disable the function of the gene temporarily. We will discuss several types of transposons and mechanism of these elements. We will describe, how these element effect the functions of genes. Some elements directly insert at the donor site without copying and some insert after formation of complement strand.
In the intricate world of genetics, transposable elements stand out as fascinating elements that have shaped the evolution of life on Earth. Also known as “jumping genes,” transposons are segments of DNA capable of moving within a genome, thereby altering the genetic landscape of an organism. This article explores the discovery, classification, mechanisms, and implications of transposons, shedding light on their significance in genetic diversity and evolution. https://profiles.nlm.nih.gov/spotlight/ll
DISCOVERY OF TRANSPOSONS:
Transposable elements were first discovered by American geneticist Barbara McClintock in the 1940s while studying maize (corn) genetics. Her groundbreaking research revealed that certain genes seemed to “jump” in and out of chromosomes, leading to variations in the phenotypes of maize plants. McClintock’s work, which was initially met with skepticism, eventually earned her the Nobel Prize in Physiology or Medicine in 1983.
CLASSIFICATION OF TRANSPOSONS:
Transposable elements can be classified into two major types based on their mechanisms of movement: DNA transposons and retrotransposons.
a. DNA TRANSPOSONS:
These transposons move directly from one genomic location to another through a “cut-and-paste” mechanism. They encode a transposase enzyme that recognizes specific DNA sequences, excises the transposon, and inserts it elsewhere in the genome. Examples of DNA transposons include the Ac/Ds elements in maize and the Tn5 transposon in bacteria.
Retrotransposons, on the other hand, transpose via an RNA intermediate. They are first transcribed into RNA molecules, which are then reverse transcribed into DNA by a reverse transcriptase enzyme. This DNA copy is subsequently inserted into a new genomic location. Retrotransposons can further be classified into two subgroups: long terminal repeat (LTR) retrotransposons and non-LTR retrotransposons. LTR retrotransposons, such as the Ty elements in yeast and the human endogenous retroviruses (HERVs), possess long terminal repeats resembling viral sequences. Non-LTR retrotransposons include the LINE-1 (L1) elements, which are abundant in mammalian genomes.
MECHANISM OF TRANSPOSITION:
The movement of transposable elements within a genome can have profound effects on the genetic makeup of an organism. While the exact mechanisms vary between different transposon types, the general steps involved in transposition are as follows:
Transposase enzymes or other transposition-related proteins recognize specific DNA sequences at the transposon ends or elsewhere in the genome.
The transposon is excised from its original genomic location, with the help of enzymes called transposases, leaving behind a double-strand break. In eukaryotic transposons, two enzymes needed for excision: transposases cut at the donor site and resolvases cut at the target site.
The excised transposon is either directly inserted into a new genomic location (DNA transposons) or reverse transcribed into DNA and inserted (retrotransposons).
IMPLICATIONS OF TRANSPOSABLE ELEMENTS:
Transposons have had a profound impact on the evolution of genomes. They can disrupt genes, leading to mutations, but can also introduce new genetic material into a genome, potentially contributing to genetic diversity and adaptation. Transposons have been implicated in various genetic phenomena, including gene regulation, genome rearrangements, and the formation of new genes. Furthermore, their activity has been linked to diseases, such as cancer, where transposons can disrupt tumor suppressor genes or cause genomic instability.
Transposons, the “jumping genes” discovered by Barbara McClintock, have revolutionized our understanding of genetics and evolution. Their ability to move within genomes has shaped the genetic diversity and complexity of organisms across the tree of life. Further research into transposons promises to unveil additional insights into the mechanisms of genetic variation, adaptation, and disease.
McClintock, B. (1950). The origin and behavior of mutable loci in maize. Proceedings of the National Academy of Sciences, 36(6), 344–355.
Wicker, T., Sabot, F., Hua-Van, A., Bennetzen, J. L., Capy, P., Chalhoub, B., Flavell, A., Leroy, P., Morgante, M., Panaud, O., Paux, E., SanMiguel, P., & Schulman, A. H. (2007). A unified classification system for eukaryotic transposable elements. Nature Reviews Genetics, 8(12), 973–982.
Hancks, D. C., & Kazazian, H. H. (2016). Roles for retrotransposon insertions in human disease. Mobile DNA, 7(1), 9. doi: 10.1186/s13100-016-0065-9.
Bailey, J. A., et al. Molecular evidence for a relationship between LINE-1 elements and X chromosome inactivation: The Lyon repeat hypothesis. Proceedings of the National Academy of Sciences 97, 6634–6639 (2000).
Feschotte, C., et al. Plant transposable elements: Where genetics meets genomics. Nature Reviews Genetics 3, 329–341 (2002). https://www.nature.com/scitable/content/Plant-transposable-elements-where-genetics-meets-genomics-28336/