Editing Transcription (biology) (section)

== Reverse transcription ==
[[File:RetroTranscription.jpg|thumb|right|300px|Scheme of [[reverse transcription]]]]
{{main|Reverse transcription}}

Some [[viruses]] (such as [[HIV]], the cause of [[AIDS]]), have the ability to transcribe RNA into DNA. HIV has an RNA genome that is ''reverse transcribed'' into DNA. The resulting DNA can be merged with the DNA genome of the host cell. The main enzyme responsible for synthesis of DNA from an RNA template is called [[reverse transcriptase]].<ref>{{Cite book |last=Clark |first=David P. |url=https://books.google.com/books?id=9Hw-mstILcIC |title=Molecular Biology |date=2005-06-24 |publisher=Elsevier |isbn=978-0-08-045421-4 |pages=63 |language=en}}</ref>

In the case of HIV, reverse transcriptase is responsible for synthesizing a [[complementary DNA]] strand (cDNA) to the viral RNA genome. The enzyme [[ribonuclease H]] then digests the RNA strand, and reverse transcriptase synthesises a complementary strand of DNA to form a double helix DNA structure (cDNA). The cDNA is integrated into the host cell's genome by the enzyme [[integrase]], which causes the host cell to generate viral proteins that reassemble into new viral particles. In HIV, subsequent to this, the host cell undergoes programmed cell death, or [[apoptosis]], of [[T cell]]s.<ref>{{cite web | url =http://www.dissercat.com/content/nekotorye-osobennosti-mekhanizmov-apoptoza-pri-vich-infektsii | title =Some patterns of apoptosis mechanism during HIV-infection | vauthors =Kolesnikova IN | date =2000 | work =Dissertation | access-date =February 20, 2011 | language =ru | url-status =live | archive-url =https://web.archive.org/web/20110710133716/http://www.dissercat.com/content/nekotorye-osobennosti-mekhanizmov-apoptoza-pri-vich-infektsii | archive-date =July 10, 2011 }}</ref> However, in other retroviruses, the host cell remains intact as the virus buds out of the cell.{{citation needed|date=April 2023}}

Some eukaryotic cells contain an enzyme with reverse transcription activity called [[telomerase]]. Telomerase carries an RNA template from which it synthesizes a [[telomere]], a repeating sequence of DNA, to the end of linear chromosomes. It is important because every time a linear chromosome is duplicated, it is shortened. With the telomere at the ends of chromosomes, the shortening eliminates some of the non-essential, repeated sequence, rather than the protein-encoding DNA sequence farther away from the chromosome end.

Telomerase is often activated in cancer cells to enable cancer cells to duplicate their genomes indefinitely without losing important protein-coding DNA sequence. Activation of telomerase could be part of the process that allows cancer cells to become immortal. The immortalizing factor of cancer via telomere lengthening due to telomerase has been proven to occur in 90% of all carcinogenic tumors ''[[in vivo]]'' with the remaining 10% using an alternative telomere maintenance route called ALT or Alternative Lengthening of Telomeres.<ref name="pmid20351727">{{cite journal | vauthors = Cesare AJ, Reddel RR | title = Alternative lengthening of telomeres: models, mechanisms and implications | journal = Nature Reviews Genetics | volume = 11 | issue = 5 | pages = 319–30 | date = May 2010 | pmid = 20351727 | doi = 10.1038/nrg2763 | s2cid = 19224032 }}</ref>
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