DNA transposon
There are three main classifications for movement for DNA transposons: "cut and paste,"[6] "rolling circle" (Helitrons),[7] and "self-synthesizing" (Polintons).
These three main classes are then further broken down into 23 different superfamilies characterized by their structure, sequence, and mechanism of action.
As newly inserted DNA into active coding sequences, they can disrupt normal protein functions and cause mutations.
[10] Host systems repair these gaps resulting in the target sequence duplication (TSD) that are characteristic of transposition.
[12] Helitrons encode an unknown protein which is thought to have HUH endonuclease function as well as 5' to 3' helicase activity.
This enzyme would make a single stranded cut in the DNA which explains the lack of Target Site Duplications found in Helitrons.
Helitrons were also the first class of transposable elements to be discovered computationally and marked a paradigm shift in the way that whole genomes were studied.
As one of the most complex known DNA transposons in eukaryotes, they make up the genomes of protists, fungi, and animals, such as the entamoeba, soybean rust, and chicken, respectively.
They contain genes with homology to viral proteins and which are often found in eukaryotic genomes, like polymerase and retroviral integrase.
They share their many structural characteristics with linear plasmids, bacteriophages and adenoviruses, which replicate using protein-primed DNA polymerases.
For replication, they utilize a protein-primed DNA polymerase B, retroviral integrase, cysteine protease, and ATPase.
Second, the Polinton undergoes replication using the DNA polymerase B, with initiation started by a terminal protein, which may encoded in some linear plasmids.
Polintons exhibit high variability between difference species and may tightly regulated, resulting in a low frequency rate in many genomes.
This can happen in three distinct ways: 1. alteration of function, 2. chromosomal rearrangement, and 3. a source of novel genetic material.
Barbara McClintock first discovered and described DNA transposons in Zea mays,[19] during the 1940s; this is an achievement that would earn her the Nobel Prize in 1983.
[22] The insertion and subsequent expression of hobo-like sequences results in the loss of germ cells in the gonads of developing flies.
[26] These examples show that transposons can greatly influence the process of evolution by rapidly inducing changes in the genome.
For example, chemical modifications of DNA can constrict certain areas of the genome such that transcription enzymes are unable to reach them.
RNAi, specifically siRNA and miRNA silencing, is a naturally occurring mechanisms that, in addition to regulating eukaryotic gene expression, prevents transcription of DNA transposons.
[34] There is evidence suggesting that at least 40 human DNA transposon families were active during mammalian radiation and early primate lineage.
