Themobilome is the entire set ofmobile genetic elements in agenome. Mobilomes are found ineukaryotes,^[1]prokaryotes,^[2] andviruses.^[3] The compositions of mobilomes differ among lineages of life, withtransposable elements being the major mobile elements in eukaryotes, andplasmids andprophages being the major types in prokaryotes.^[4]Virophages contribute to the viral mobilome.^[5]

Transposable elements are elements that can move about or propagate within the genome, and are the major constituents of theeukaryotic mobilome.^[4] Transposable elements can be regarded as geneticparasites because they exploit the hostcell'stranscription andtranslation mechanisms to extract and insert themselves in different parts of the genome, regardless of thephenotypic effect on the host.^[6]

Eukaryotic transposable elements were first discovered inmaize (Zea mays) in whichkernels showed a dotted color pattern.^[7]Barbara McClintock described the maizeAc/Ds system in which the Aclocus promotes the excision of the Ds locus from the genome, and excised Ds elements canmutategenes responsible forpigment production byinserting into theircoding regions.^[8]

Other examples of transposable elements include:yeast (Saccharomyces cerevisiae)Ty elements, aretrotransposon which encodes areverse transcriptase to convert itsmRNA transcript into DNA which can then insert into other parts of the genome;^[9][10] andfruit fly (Drosophila melanogaster)P-elements, which randomly inserts into the genome to cause mutations ingerm line cells, but not insomatic cells.^[11]

Plasmids were discovered in the 1940s as genetic materials outside ofbacterialchromosomes.^[12] Prophages are genomes ofbacteriophages (a type of virus) that are inserted into bacterial chromosomes; prophages can then be spread to other bacteria through thelytic cycle andlysogenic cycle ofviral replication.^[13]

While transposable elements are also found in prokaryotic genomes,^[14] the most common mobile genetic elements in the prokaryotic genome areplasmids andprophages.^[4]

Plasmids and prophages can move between genomes throughbacterial conjugation, allowinghorizontal gene transfer.^[15] Plasmids often carry genes that are responsible for bacterialantibiotic resistance; as these plasmids replicate and pass from one genome to another, the whole bacterialpopulation can quicklyadapt to theantibiotic.^[16][17] Prophages can loop out of bacterial chromosomes to produce bacteriophages that go on to infect other bacteria with the prophages; this allows prophages to propagate quickly among the bacterial population, to the harm of the bacterial host.^[13]

Discovered in 2008 in a strain ofAcanthamoeba castellaniimimivirus,^[18]virophages are an element of the virus mobilome.^[5] Virophages are viruses thatreplicate only when host cells are co-infected withhelper viruses.^[19] Following co-infection, helper viruses exploit the host cell's transcription/translation machinery to produce their own machinery; virophages replicate through the machinery of either the host cell or the viruses.^[19] The replication of virophages can negatively impact the replication of helper viruses.^[18][20]

Sputnik^[18][21] andmavirus^[22] are examples of virophages.

• Genetics