IB Transcription Review PDF

Transcription (review) BIO/MBB 201 Transcription: Passing Genetic Information from DNA to RNA Recall: The Central Dogma of Molecular Biology Transcription Translation DN RNA protein Agene (genotype) g...

Transcription (review) BIO/MBB 201 Transcription: Passing Genetic Information from DNA to RNA Recall: The Central Dogma of Molecular Biology Transcription Translation DN RNA protein Agene (genotype) gene expression biological function (phenotype) 1. Some Features a. involve only certain regions of DNA ⇒ coding regions – DNA sequences that code for RNA (mRNA, tRNA, rRNA) vs. noncoding regions – do not code for a gene product b. coding regions differ in terms of the time they are switched on ⇒ gene expression is regulated c. major point of control of gene expression ⇒ transcription control especially of initiation d. RNA synthesis occurs in a 5' to 3' direction e. consists of three events: i. initiation ii. elongation iii. termination  promoter terminator 5' 3' DNA RNA coding sequence 3' 5' +1 transcription transcription termination site initation site (-) upstream (+) downstream of the gene of the gene 2. RNA polymerase Multi-functional: unwinds DNA, binds rNTPs, binds DNA template (antisense), complements DNA with RNA, rewinds DNA no proofreading ability; error made every 104 – 105 ribonucleotides incorporated able to initiate chain growth; no primer needed Prokaryotic – 1 kind vs. Eukaryotic – 3 kinds  Eukaryotic RNA pol: 3 types I II III Location nucleolus nucleoplasm nucleoplasm No./cell 40,000 40,000 20,000 genes rRNA hnRNA (mRNA tRNA; transcribed precursor); 5S rRNA snRNPs; (small, stable, microRNA RNAs) % activity 50-70 50-70 10 3. Some transcription elements and transcription factors a. Promoters i. prokaryotes (E. coli) -35 region Pribnow box; -10 TTGACA 15-20 bp TATAAT 5-8 bp initiation site ii. eukaryotes Goldberg/Hogness box; -25 -75 -25 +1 A GGCCAATCT TATA T A b. Enhancers - DNA sequences that could activate transcription - located thousands of bases away from the promoter - specific binding sites for gene regulatory proteins that activate transcription - found in eukaryotes and less commonly in prokaryotes c. Terminator - sequence that signals RNA polymerase activity to stop - releases both the DNA template and the RNA transcript; polymerase detach d. Transcription factors in eukaryotes - had to assemble in a complex on the DNA at the promoter in order to recruit the RNA polymerase to this site © 2002 by Bruce Alberts, Alexander Johnson, Julian Lewis, Martin Raff, Keith Roberts, and Peter Walter. Stages of Transcription mRNA processing a. Capping - addition of a methylated G nucleotide to 5' end of the RNA molecule - occurs after about 30 nucleotides of RNA are added - functions: protects the growing RNA transcript from degradation ribosome recognition b. Tailing - addition of 100-200 residues of adenylic acid to the 3' end of transcript - poly A polymerase (nuclear enzyme) - signal for cleavage : AAUAAA located 10-30 nucleotides upstream from the site of cleavage - functions: ~ aids in the export of mature mRNA from the nucleus ~ RNA stability ~ serves as a recognition signal for the ribosome  c. Splicing hn RNA ≠ mature RNA (faithful copy (directly codes of the gene) for a protein) - process of cutting introns out of immature RNAs and stitching together the exons to form the mature RNA - splicing signals tells the splicing machinery where to “cut and paste” 3' exon intron sequence 5' exon C A UUUUUUUUUUU G 5' N AG A 3' GAG GU G AGU A CCCCCCCCCCC darwin.nmsu.edu/.../images/SplicingMechanism.gif rRNA Processing large RNA precursor gene in E. coli, 7 RNA precursor genes encode different types and number of tRNA ribonuclease III (RNase III) large RNA precursor gene 30S precursor RNA final product 16S tRNA 23S 5S tRNA eukaryotic RNA precursor genes in cluster rRNA synthesis and processing rRNA genes – located in the nucleolar part of the nucleus. highly repetitious (mammalian cells: 100 -2000 per cell – transcription units separated by NTS – transcribed by RNA polymerase I into giant primary transcripts (18S, 5.8S and 28S rRNA + ETS + ITS) – many polymerases operate on the same transcription unit rRNA synthesis and processing rRNA processing nucleotide modification by a snoRNA-dependent mechanism – pseudouridylation – methylation – for proper folding of rRNA and binding of riboproteins exo- and endo-nucleolytic cleavages guided by snoRNA (small nucleolar RNAs) in complex with proteins tRNA synthesis and processing prokaryote cluster of tRNA genes cleavage from a large tRNA precursor eukaryote cluster of tRNA genes that are transcribed individually RNA polymerase III