where does rna polymerase initiate synthesis of mrna
RNA polymerase Eukaryotes have multiple types of nuclear RNAP, each responsible for synthesis of a distinct subset of RNA. All are structurally and mechanistically related to each other and to bacterial RNAP. RNA polymerase I synthesizes a pre-rRNA 45S (35S in yeast). which matures and will form the major RNA sections of the ribosome. RNA polymerase II synthesizes precursors of mRNAs and most snRNA and microRNAs.RNA polymerase III synthesizes tRNAs, rRNA 5S and other small RNAs found in the nucleus and cytosol. RNA polymerase IV synthesizes siRNA in plants. RNA polymerase V synthesizes RNAs involved in siRNA-directed heterochromatin formation in plants. Eukaryotic chloroplasts contain an RNAP very highly structurally and mechanistically similar to bacterial RNAP ("plastid-encoded polymerase"). They also contain a second, structurally and mechanistically unrelated, RNAP ("nucleus-encoded polymerase"; member of the "single-subunit RNAP" protein family). Eukaryotic mitochondria contain a structurally and mechanistically unrelated RNAP (member of the "single-subunit RNAP" protein family).
Translation (biology) Even though the ribosomes are usually considered accurate and processive machines, the translation process is subject to errors that can lead either to the synthesis of erroneous proteins or to the premature abandonment of translation. The rate of error in synthesizing proteins has been estimated to be between 1/105 and 1/103 misincorporated amino acids, depending on the experimental conditions.[7] The rate of premature translation abandonment, instead, has been estimated to be of the order of magnitude of 10âˆ’4 events per translated codon.[8] The correct amino acid is covalently bonded to the correct transfer RNA (tRNA) by amino acyl transferases. The amino acid is joined by its carboxyl group to the 3' OH of the tRNA by an ester bond. When the tRNA has an amino acid linked to it, the tRNA is termed "charged". Initiation involves the small subunit of the ribosome binding to the 5' end of mRNA with the help of initiation factors (IF). Termination of the polypeptide happens when the A site of the ribosome faces a stop codon (UAA, UAG, or UGA) on the mRNA. tRNA usually cannot recognize or bind to stop codons. Instead, the stop codon induces the binding of a release factor protein that prompts the disassembly of the entire ribosome/mRNA complex and the hydrolysis and the release of the polypeptide chain from the ribosome. Drugs or special sequence motifs on the mRNA can change the ribosomal structure so that near-cognate tRNAs are bound to the stop codon instead of the release factors. In such cases of 'translational readthrough', translation continues until the ribosome encounters the next stop codon.[9]