Lecture 2.26 Notes

Lecture 2.26 Notes - 401 Notes. 2/26/10 Transcription...

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401 Notes. 2/26/10 Transcription – expression of genetic information as protein requires transcription of an RNA intermediate called messenger RNA (mRNA). Additionally, the process of translation of the nucleotide code into the protein code requires a variety of RNAs that serve structural and functional roles in the translation machinery known as ribosomal RNAs (rRNA). Transcription of DNA is selective – limited regions of DNA are transcribed into RNA at any given time •Control of transcriptional initiation is a key step in the regulation of gene expression •selective transcription involves ability of proteins to recognize particular DNA sequences •Transcription is mediated by an enzyme called RNA polymerase (RNAP) •Core RNAP - large complex enzyme consisting of multiple subunits that has full catalytic activity of the enzyme ( i.e. - can synthesize RNA chains from NTP precursors in a DNA template-directed manner) •RNAP core cannot recognize signals in DNA that instruct it in where to initiate an RNA chain (a.k.a. – promoters) and requires the activity of an additional polypeptide known as the σ (specificity) factor for promoter recognition. σ only associates with core polymerase for the step of promoter recognition and is released shortly after the initiation of RNA synthesis •Following release of the σ subunit, transcriptional elongation continues until signals in the DNA template causes the termination of transcription. •Elongation of RNA chain involves mechanisms that are very similar to chain elongation during DNA synthesis: •Chain growth occurs with a 5’ 3’ directionality •Nucleophilic attack of 3’-hydroxyl group at 3’ end of nascent transcript on the α-phosphate of the incoming NTP •Nascent RNA chain is antiparallel to template strand •Identity of incoming NTP is determined by complementary base pairing rules Two important differences; •Unlike DNAPs, RNAP is capable of de novo polynucleotide synthesis (no primer required) •Only one strand of DNA is usually used as template for RNA synthesis (the template strand)
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Structure of bacterial promoters: •Promoter sequences tend to be variations on the theme of a consensus sequence •Recognition of promoters requires binding of σ subunit •Multiple σ subunits allow RNAP to recognize different classes of promoters transcription can be regulated by use of alternative σ factors. Transcription of termination in Bacteria can occur by at least 2 distinct mechanisms •Intrinsic termination: •if the template strand encodes an RNA transcript that contains a G+C-rich stem-loop structure followed by a string of U residues, transcription is terminated by RNAP alone. •rho-dependent termination
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Lecture 2.26 Notes - 401 Notes. 2/26/10 Transcription...

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