Transposons are DNA sequences that may shift their location within a genome, occasionally causing or reversing mutations and affecting the genetic identity and genome size of the cell. Transposons are also known as selfish genetic elements because they have the ability to boost their own transmission at the expense of other genes in the genome. Transposable elements constitute a significant portion of the genome and account for the majority of the DNA mass in a eukaryotic cell. Transposition frequently causes structural chromosomal abnormalities. Class-I TEs or retrotransposons, function primarily through reverse transcriptase, whereas Class-II TEs, or DNA transposons, encode the protein transposase, which they require for insertion and excision (IS elements in bacteria, Ac/Ds elements in corn, and P elements in drosophila), and some of these TEs also encode proteins unrelated to the transposition process (IS elements in bacteria, Ac/Ds elements in corn, and P elements (Composite transposons and Tn3 elements in bacteria). Non-autonomous TEs require the presence of another TE to move, whereas autonomous TEs can move on their own. Copy and paste is the mechanism of Class-I TEs, which requires the creation of RNA from DNA and reverse transcription of the RNA to DNA in the presence of reverse transcriptase (Retrovirus like elements and LINEs in Humans). The majority of Class-II/DNA transposons are transposed in the presence of transposase via cut-and-paste mechanism that does not need the production of an RNA intermediary.