Ribosomal RNAs (rRNAs) – structural and catalytic components of ribosomes o Small Nuclear RNAs (snRNAs and snoRNAs) – spliceosomes and rRNA, tRNA modification, respectively o Micro RNAs (miRNAs, siRNA and Crispr RNA) – short RNAs that block expression of complementary mRNAs o Long Noncoding RNAs – long RNAs that regulate gene transcription o *NOTE: Many RNAs don’t encode protein RNA is synthesized in the 5’to 3’ direction using the 3’ to 5’ DNA template strand. The RNA and DNA strands are anti-parallel Transcription can utilize as the 3’ to 5’ template, either single strand of the double-stranded DNA (dsDNA), but always occurs in the 5’ to 3’ direction Just like DNA replication, transcription is a chemical reaction o DNA template o 4 ribonucleoside triphosphates (rNTPs) A, U, C, G RNA n + rNTP RNA n+1 + PP i o DNA dependant RNA polymerase o *NOTE: The use of ribonucleoside triphosphates 5’
in transcription rather than deoxynucleoside triphosphates that are used in DNA replication General features of RNA synthesis o Similar to DNA synthesis except: The precursors are ribonucleoside triphosphates (rNTPs) Only one strand of DNA is used as a template RNA chains can be initiated de novo (no primer required) o The RNA molecule will be complementary to the DNA template (anti- sense) strand and identical (except that uridine replaces thymidine) to the DNA non-template (sense) strand o RNA synthesis is catalyzed by RNA polymerases and proceeds in the 5’ 3’ direction Transcription in Prokaryotes: Generalized structure of a prokaryotic gene: Steps in transcription 1. Initiation – RNA polymerase binds, unwinds, and joins first 2 nucleotides o Initiation of RNA synthesis DOES NOT require a primer 2. Elongation – complementary nucleotides continue to be added during the elongation process o Localized DNA unwinding ahead of RNA polymerase generates a transcription bubble 3’
o Transcription bubble moves with the RNA polymerase and the unwound DNA rewinds behind it 3. Termination – transcription stops when RNA polymerase reaches the terminator region of the gene o The newly synthesized RNA together with RNA polymerase are released E. coli RNA polymerase o Core: 2 ’ Transcribes any DNA o Holoenzyme: 2 ’ Transcribe specific genes Functions of the subunits: o : assembly of the tetrametric core o : ribonucleoside triphosphate binding site o ’ : DNA template binding region o : not essential for transcription, but helps stabilize core o : initiation of transcription specifically at a promotor Model of transcription based on x-ray diffraction data Details of steps in transcription 1. Initiation of Transcription o Promotor recognition requires the RNA polymerase holoenzyme o Sigma factor recognizes and binds to the -35 element, thus positioning the RNA polymerase at the promotor to begin transcription o Two important sequence elements: The -35 element to which the factor binds; -35 sequence: 5’ TTGACA 3’ Catalytic site σ (sigma) factor
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- Fall '13