Understanding Transposable Elements
Transposons are only one of many different movable genetic elements found within our genome. Movable genetic elements allow for genetic material to be shared and transferred between cells and within cells giving rise to mutations and variation in a population.
Barbra McClintock, an outstanding cytogeneticist, who was curious as to why the spots on corn kernels, known as maize or Indian corn, kept moving around, first discovered transposons. Transposons are sequences of DNA that can relocate themselves within a genome of a single cell. Transposition can occur in two different mechanisms. The first being a “cut and paste”, where a gene is removed from a location in the genome and inserted somewhere else in the genome, and the second being a “copy and paste”, where a replica of a gene is inserted somewhere else in the genome. Transposons can be classified into two classes: Class I and Class II, according to which method of transposition they use.
Class I transposons use the “copy and paste” method of transposiion. The first step in a Class I transposition is complete transcription of a specific gene to RNA. The RNA is then revered transcripted to form a double stranded DNA transposon consisting of just the gene. An enzyme called reverse transcriptase, often coded for by the transposable element, is what synthesizes reverse transcription. The DNA containing the gene is then inserted into the host DNA. This allows for the gene to be transcribed at its original gene location as well as at another site in the cell. These transposons are often called retrotransposons and behave very similarly to retroviruses.
Class II transposons use the “cut and paste” method of transposition. This process is much simpler than the “copy and paste” method. The fist step in Class II transposition is a cut of the DNA surrounding a gene. The gene is then moved to another location in the cell and inserted into the host. These are often called DNA transposons because this method does not contain an RNA intermediate and stays as a doubled stranded gene throughout the whole process.
Transposons can occur in both eukaryotes and prokaryotes and are very common processes in both cell classes. Their function can sometimes be harmful or beneficial to the cell. Often times transposable genes are over expressed in a cell creating a harmful environment for the cell and blocking the processes of vital functions required for cell survival. Such genes are called Selfish genetic elements and are usually removed by the cell, but can sometimes still slip through. Transposons however are generally beneficial to the evolutionary process by creating variation within a population and inducing mutations, which in turn also create variation. Their discovery was a major breakthrough in science and brought further insight into how genomes evolve over time.