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Transcription of RNA from DNA

Central Dogma


How does the sequence information from DNA get transferred to mRNA so that it can be carried to the ribosomes in the cytoplasm? This process, called transcription is highly analogous to DNA replication. Of course, there are different effectors, or proteins, that direct transcription. Primary among these is the RNA polymerase holoenzyme, an agglomeration of many different factors that together direct the synthesis of mRNA on a DNA template.

Initiation of Transcription

RNA polymerase must be able to recognize the beginning of a gene so that it knows where to start synthesizing an mRNA. It is directed to the start site of transcription by one of its subunits' affinity to a particular DNA sequence that appears at the beginning of genes. This sequence is called a promoter. It is a unidirectional sequence on one strand of the DNA that tells the RNA polymerase both where to start and in which direction (that is, on which strand) to continue synthesis. The bacterial promoter almost always contains some version of the following elements:

The RNA polymerase then stretches open the double helix at that point in the DNA and begins synthesis of an RNA strand complementary to one of the strands of DNA. We call the strand from which it copies the antisense or template strand, and the other strand, to which it is identical, the sense or coding strand.

The RNA polymerase recruits rNTPs (ribonucleic nucleotides triphosphates) in the same way that DNA polymerase recruits dNTPs. However, since synthesis is single stranded and only proceeds in the 5' to 3' direction, there is no need for Okazaki fragments.

It is important to note that synthesis once again proceeds in a unidirectional fashion, because of the reasons outlined in the previous section.

Termination of Transcription

How does RNA polymerase know when to stop transcribing a gene? This system has been elucidated in prokaryotes. It is important to know that since there is no nucleus in prokaryotes , ribosomes can begin making protein from an mRNA immediately upon its synthesis. At the end of a gene, the sequence of the mRNA allows it to form a hairpin loop, which blocks the ribosome. The ribosome falls off the mRNA, and that is the termination signal recognized by the RNA polymerase. As soon as the ribosome falls off the mRNA, the RNA polymerase falls off the DNA and transcription ceases.

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