written by: Balachandar Radhakrishnan•edited by: Paul Arnold•updated: 9/11/2009
The genetic code is written in the language of DNA, but is not translated directly into proteins. It goes through a process of conversion into an intermediate stage where it exists as RNA. This RNA is known as messenger RNA.
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DNA to RNA to Protein
The genetic code is written in the language of DNA, the four chemical bases - A, C, G and T. However, when it is time to make a protein, the DNA code is converted into RNA before it is translated into protein. This article is an in depth look at the first stage in this process, how RNA is formed from DNA. Before we look at how this happens, let me introduce some terminology;
Transcription - The process by which DNA is converted into RNA.
Translation - The conversion of RNA into protein.
Central Dogma of Molecular Biology - The name given for the whole process, the idea that information coded in the DNA is converted into RNA and then into protein.
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RNA transcription involves a large subset of proteins. The chief enzyme that is involved is called RNA polymerase. Now, although the DNA is double stranded only one strand codes for a gene product and hence only this strand is transcribed - and it's called the coding strand.
RNA transcription involves the faithful copying of the DNA message, however this process is not as stringently controlled as the process of DNA replication. In order for a DNA strand to be transcribed, there's a special sequence at each end of a gene. At the beginning this region is known as the promoter, and the stop signal at the end is called the terminator.
The enzyme that operates transcription, RNA polymerase, recognizes the promoter sequence as the location for starting transcription and the terminator as the point to end transcription. Once RNA polymerase identifies the promoter sequence it operates along with other entities called transcription factors to unwind the DNA strand and create a transcription bubble, which is the ground zero at which the enzyme operates and releases the nascent messenger RNA transcript. The transcription bubble moves along the coding strand until it reaches the terminator sequence. The process of transcription is split into the following stages.
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Stages of Transcription
Initiation - RNA polymerase along with its co-factors binds to the DNA strand and unwinds the coding strand. Once the strand has been unwound, the RNA polymerase starts moving along. Sometimes it falls away after reading a few bases. But the process starts over again until it progresses without any enzyme fall off. Once the RNA polymerase has traversed around 15 to 20 bases the process becomes more stable and moves along steadily. However, the initial stagger releases short stretches of RNA transcript called abortive transcripts and the process is called abortive transcription
Elongation - This is the stage when the RNA transcript is extended as the coding strand is copied to make a complementary sequence that will be known as messenger RNA. It uses the other strand of DNA, now known as the template strand for RNA synthesis.
Termination - As the name suggest this involves the termination of the transcription process. Termination occurs when the RNA polymerase transcribes a GC rich sequence of DNA which causes a hairpin like structure of the RNA to destabilize the enzyme complex and thereby abort the reaction. There is also yet another method of termination called “Rho" dependent termination where a co-factor called Rho binds to the transcription enzyme complex and destabilizes the reaction.
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The next step of making a protein occurs when messenger RNA is transported into the cytoplasm. Before that happens there is an intermediate stage where “RNA editing" occurs. There is also a plethora of reactions that happen after transcription that modify the transcript, these are called post transcriptional modifications. Then the messenger RNA is ready to be shuttled out of the nucleus and into the cytoplasm ready to be translated into a protein.