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illustrating the growing Primary Transcript s. "Begin" indicates the 3' End of the DNA, where new RNA synthesis begins; "end" indicates the 5' End , where the primary transcripts are almost complete.]] Transcription is the process through which a DNA sequence is Enzymatically copied by an RNA Polymerase to produce a complementary RNA . So to say, it is the transfer of genetic information from DNA into RNA. In the case of protein-encoding DNA, transcription is the beginning of the process that ultimately leads to the Translation of the genetic code (via the MRNA intermediate) into a functional Peptide or Protein . The stretch of DNA that is transcribed into an RNA molecule is called a ''transcription unit''. Transcription has some proofreading mechanisms, but they are fewer and less effective than the controls for copying DNA; therefore, transcription has a lower copying fidelity than DNA Replication .1 As in DNA Replication , transcription proceeds in the 5' → 3' direction (i.e. the old polymer is read in the 3' → 5' direction and the new, complementary fragments are generated in the 5' → 3' direction). Transcription is divided into 3 stages: ''initiation'', ''elongation'' and ''termination''. INITIATION In transcription, one strand of DNA, the ''non-coding strand'', is used as a template for RNA synthesis. As transcription proceeds in the 5' → 3' direction, and uses base pairing complimentarity with the DNA template to specify the correct copying, the DNA template strand is that oriented in the 3' → 5' direction. The strand that is not used as the template is called the ''coding strand'', and has the DNA sequence that reflects that of the RNA produced. Transcription begins with the binding of of promoter sequences. Transcription initiation is far more complex in Eukaryote s, the main difference being that eukaryotic polymerases do not recognize directly their core promoter sequences. Unlike DNA Replication , transcription does not need a Primer to start. The DNA unwinds and produces a small open complex and synthesis begins on only the template strand. ELONGATION Unlike DNA replication, mRNA transcription can involve multiple RNA polymerases, so many mRNA molecules can be produced from a single copy of the gene. This step also involves a proofreading mechanism that can replace an incorrectly added RNA molecule. TERMINATION Bacteria use two different strategies for transcription termination: in Rho-independent Transcription Termination , RNA transcription stops when the newly synthesized RNA molecule forms a Hairpin Loop , followed by a run of ''U''s, which makes it detach from the DNA template. In the "Rho-dependent" type of termination, a protein factor called "Rho" destabilizes the interaction between the template and the mRNA, thus releasing the newly synthesized mRNA from the elongation complex. Transcription termination in eukaryotes is less well understood. It involves cleavage of the nascent transcript, followed by template-independent addition of ''A''s at its new 3' end, in a process called Polyadenylation . PROKARYOTIC VS. EUKARYOTIC TRANSCRIPTION
MEASURING AND DETECTING TRANSCRIPTION Transcription can be measured and detected in a variety of ways:
TRANSCRIPTION FACTORIES Active transcription units are clustered in the nucleus, in discrete sites called ‘transcription factories’. Such sites could be visualized after allowing engaged polymerases to extend their transcripts in tagged precursors (Br-UTP or Br-U), and immuno-labeling the tagged nascent RNA. Transcription factories can also be localized using fluorescence in situ hybridization, or marked by antibodies directed against polymerases. There are ~10,000 factories in the nucleoplasm of a HeLa Cell , among which are ~8,000 polymerase II factories and ~2,000 polymerase III factories. Each polymerase II factory contains ~8 polymerases. As most active transcription units are associated with only one polymerase, each factory will be associated with ~8 different transcription units. These units might be associated through promoters and/or enhancers, with loops forming a ‘cloud’ around the factory. TRANSCRIPTION INITIATION COMPLEX Transcription Factors mediate the binding of RNA polymerase and the initiation of transcription. The RNA polymerase only binds to the promoter after certain Transcription Factors are assembled. The completed assembly of transcription factors and RNA polymerase bound to the promoter is called the ''transcription initiation complex''. HISTORY A molecule which allows the genetic material to be realized as a protein was first hypothesized by Jacob and Monod . RNA synthesis by RNA Polymerase was established '' In Vitro '' by several laboratories by 1965; however, the RNA synthesized by these enzymes had properties that suggested the existence of an additional factor needed to terminate transcription correctly. Recently, Roger D. Kornberg won the 2006 Nobel Prize In Chemistry "for his studies of the molecular basis of eukaryotic transcription".2 TERMINOLOGY
5'- promoter - +1 transcription initiation site - RNA-coding sequence - terminator - transcription termination site - 3' <-- upstream downstream --> REVERSE TRANSCRIPTION Some viruses (such as HIV , the cause of AIDS ), have the ability to transcribe RNA into DNA in order to see a cell's genome. The main enzyme responsible for this type of transcription is called Reverse Transcriptase . In the case of HIV, reverse transcriptase is responsible for synthesising a Complementary DNA strand (cDNA) to the viral RNA genome. An associated enzyme, ribonuclease H, digests the RNA strand and reverse transcriptase synthesises a complementary strand of DNA to form a double helix DNA structure. This cDNA is integrated into the host cell's genome via another enzyme (integrase) causing the host cell to generate viral proteins which reassemble into new viral particles. Subsequently, the host cell undergoes programmed cell death (apoptosis). REFERENCES SEE ALSO
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