The process of copying genetic information from one strand of the DNA into RNA is termed as transcription. Unlike replication, only one strand is transcript. A transcription unit in DNA is defined by three regions in the DNA- a promoter, the structural gene, a terminator. The promoter and terminator flank the structural gene in a transcription unit. The promoter is said to be located towards 5’-end of the structural gene. The terminator is located towards 3’-endof the coding strand and it usually defines the end of transcription. The process of transcription involves initiation in which the enzyme RNA polymerase binds to promoter and initiates transcription. It uses nucleoside triphosphates as substrate. Opening of the helix is elongation, as the polymerases reaches the terminator region, the RNA falls off, so also the RNA polymerase which results in termination of transcription.
However in eukaryotes three types of RNA polymerase is used for the transcription of the three RNAs (mRNA, rRNA, tRNA). The coding sequences (the exons) of many eukaryotic genes are interrupted by a number of non coding sequences called introns. When a gene containing introns is transcribed, the RNA initially retains those introns. These are then removed to produce the mature mRNA. The process of introns removal is called splicing.
RNA editing is another mechanism that allows an RNA to be changed after transcription so as to encode a different protein from that encoded by the gene. Two method for editing are: enzymatic modification of bases (generating forms that alter how they are read by tRNAs) and the insertion or deletion of multiple U nucleotides within the message.
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