Transcription
The genetic information that is passed on from parent to offspring is carried by the DNA of a cell. The genes on the DNA code for specific proteins that determines our appearance, different facets of our personality, our health etc. In order for the genes to produce the proteins, it must first be transcribed from DNA to RNA in a process known as transcription. Thus, transcription is defined as the transfer of genetic information from the DNA to the RNA.
The process of transcription occurs in the nucleus of the cell. There are three different phases involved: initiation, elongation, and termination.
To initiate the process of information transfer, one of the strands of the double stranded DNA serves as a template for the synthesis of a single strand of RNA that is complementary to the DNA strand. The enzyme RNA polymerase binds to a particular region of the DNA that is termed as the "promoter." The promoter is a particular unidirectional sequence that appears at the beginning of the genes, and tells the enzyme where to start the synthesis and which strand to synthesize. Once the enzyme is bound to the promoter, it unwinds the DNA and starts to make a strand of RNA with a base sequence complementary to the DNA template that is downstream of the RNA polymerase binding site. The strand from which it copies is known as the template or the antisense strand, while the other strand to which it is identical is called the sense or the coding strand.
After initiation, is the process of elongation. The substrates for RNA polymerase are nucleoside triphosphates. The RNA polymerase matches a base on the DNA to an RNA nucleotide (by complementary base pair binding) and then adds that nucleotide to the elongating RNA strand. As a new ribonucleotide triphosphate forms a bond with the 3'- hydroxyl end of the growing strand, a pyrophosphate is given off. The energy that is needed for synthesizing RNA is derived from splitting up of the triphosphate into a monophosphate and releasing the other two inorganic phosphates.
The next phase is called "termination." Termination occurs when the RNA polymerase reaches a signal on the DNA template strand that tells it to stop. Once this termination signal is recognized by the RNA polymerase, it releases the DNA and transcription ceases. The newly synthesized RNA strand now undergoes "post-transcriptional processing."
Eucaryotic genes are not continuous. A typical gene consists of both coding sequences (exons) and non-coding sequences (introns). The primary transcript that is formed at the end of the transcription is actually known as hnRNA and is an exact copy of the gene with both introns and exons. A process called "RNA splicing" occurs and the introns are removed. The remaining exons are joined together to form the final mRNA product which codes for a single protein. This post-transcriptional RNA processing take place in the nucleus. Besides splicing, the hnRNA strand also has to be capped and poly-adenylated before being transported to the cytoplasm for translation into proteins. The 5' capping of the hnRNA occurs soon after the beginning of transcription. A methylated G nucleotide that is believed to play an important role in the initiation of protein synthesis is added to the 5' end. For the addition of a poly A tail, one hundred to two hundred residues of adenylic acid are added by an enzyme known as poly-A polymerase. This tail helps to guard the RNA transcript against degradation and enables the transcript to exit from the nucleus to the cytoplasm where it can be translated into proteins.
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