Sythensis and replication of the DNA link
Synthesis of RNA exhibits several features that are synonymous with DNA replication. RNA synthesis requires accurate and efficient initiation, elongation proceeds in the 5' - 3' direction (i.e. the polymerase moves along the template strand of DNA in the 3' - 5' direction), and RNA synthesis requires distinct and accurate termination. Transcription exhibits several features that are distinct from replicationSynthesis of RNA exhibits several features that are synonymous with DNA replication. RNA synthesis requires accurate and efficient initiation, elongation proceeds in the 5' - 3' direction (i.e. the polymerase moves along the template strand of DNA in the 3' - 5' direction), and RNA synthesis requires distinct and accurate termination. Transcription exhibits several features that are distinct from replication.
RNA Replication Explanation
DNA Synthesis Process
DNA of a Gemstone
Signals are present within the DNA template that act in cis to stimulate the initiation of transcription. These sequence elements are termed promoters. Promoter sequences promote the ability of RNA polymerases to recognize the nucleotide at which initiation begins. Additional sequence elements are present within genes that act in cis to enhance polymerase activity even further. These sequence elements are termed enhancers. Transcriptional promoter and enhancer elements are important sequences used in the control of gene expression.
Heading with infoE. coli RNA polymerase is composed of 5 distinct polypeptide chains. Association of several of these generates the RNA polymerase holoenzyme. The sigma subunit is only transiently associated with the holoenzyme. This subunit is required for accurate initiation of transcription by providing polymerase with the proper cues that a start site has been encountered.
In both prokaryotic and eukaryotic transcription the first incorporated ribonucleotide is a purine and it is incorporated as a triphosphate. In E. coli several additional nucleotides are added before the sigma subunit dissociates.
Elongation involves the addition of the 5'-phosphate of ribonucleotides to the 3'-OH of the elongating RNA with the concomitant release of pyrophosphate. Nucleotide addition continues until specific termination signals are encountered. Following termination the core polymerase dissociates from the template. The core and sigma subunit can then reassociate forming the holoenzyme again ready to initiate another round of transcription.
In E. coli transcriptional termination occurs by both factor-dependent and factor-independent means. Two structural features of all E. coli factor-independently terminating genes have been identified. One feature is the presence of 2 symmetrical GC-rich segments that are capable of forming a stem-loop structure in the RNA and the second is a downstream A rich sequence in the template. The formation of the stem-loop in the RNA destabilizes the association between polymerase and the DNA template. This is further destabilized by the weaker nature of the AU base pairs that are formed, between the template and the RNA, following the stem-loop. This leads to dissociation of polymerase and termination of transcription. Most genes in E. coli terminate by this method.
Factor-dependent termination requires the recognition of termination sequences by the termination protein, rho. The rho factor recognizes and binds to sequences in the 3' portion of the RNA. This binding destabilizes the polymerase-template interaction leading to dissociation of the polymerase and termination of transcription.