Transcript Document
Transcription 3 main types of RNA • All are made from a DNA Template • mRNA: encode AA sequence of one of more polypeptides from a gene or set of genes • tRNA: reads info. In mRNA and transfers appropriate AA to growing peptide chain • rRNA: part of ribosome, cellular machines synthesize proteins 1 Synthesis • Resembles DNA replication in its chemical mechanism, polarity, use of template • Has initiation, elongation, termination phase • Does not need a primer, only involves limited segment of DNA molecule 2 RNA Polymerase • • • • DNA directed RNA polymerase DNA template Nucleotides (ATP, CTP, UTP, GTP) Builds in the 5’ to 3’ direction 3 4 RNA Polymerase 5 Binds to initiation site through sequences called promoter elements that are recognized by factor 6 7 8 Once initiation occurs factor dissociates 9 Recognized by 70 Recognized by 32 10 RNA Polymerase has helicase activity Gyrases/topoisomerases are needed to prevent positive supercoiling ahead of the transcription bubble 11 An electron micrograph of three contiguous ribosomal genes from oocytes of the salamander Pleurodeles waltl undergoing transcription. 12 Transcription is tightly regulated Not all genes are transcribed equally 13 Transcription Factor • Upstream of RNAP II initiation site, different combinations of specific DNA sequences each is recognized by corresponding site specific DNA binding protein • Each combination of DNA sequence and cognate DNA binding protein represent a control module 14 15 16 17 18 19 The lac operon 20 Eukaryotic transcription Heterochromatin: transcriptionally inactive Euchromatin: may be transcriptionally active Eukaryotic RNAP have almost no affinity for promoters on their own 21 Eukaryotic RNA Polymerases • RNAP I: synthesis of transcripts called preribosomal RNA – precursor 18s, 5.8s, 28s RNA • RNAP II: synthesis of mRNA • RNAP III: synthesize tRNA 22 23 RNA Polymerase II • Has 12 subunits • Needs transcription factors that stimulate the enzyme to bind to a nearby promoter • General TF required at every RNAP II promoter 24 Functionally equivalent to factor 25 Gaining access to the DNA template Histones must be modified to access DNA Histone acetyltransferases: decrease + charge Histone deacetylases: increase + charge 26 Lysine NH3+ O Acetyllysine N H CH3 27 Gene Silencing 5% of cytidine residues are methylated at the 5 position NH2 N N H O These are most often found at CG sequences Hypermethylation is associated with heterochromatin 28 Recruiting RNA polymerase 29 Regulated Transcription 30 Regulated Transcription 31 RNA Processing • Most RNA is processed after synthesis • Ribozymes: catalyze post-transcriptional processing • Primary Transcript: newly synthesized RNA molecule – occurs in mRNA and tRNA –contains sequence encoding one gene but encoded polypeptide not continuous • Noncoding Regions: introns • Coding Region: exons 32 33 34 35 36 Post-transcriptional control 37 38 39 RNA Editing Changes the Meaning 40 RNA Editing Changes the Meaning C to U A to I 41 There are many ways to regulate the meaning of genes after transcription Prevent translation 42 There are many ways to regulate the meaning of genes after transcription Degrade the message 43 44 45 Target the mRNA 46 What about mistakes? Nonsense Mediated Decay 47