Lecture 6 Transcription PDF

Biol1110 Genes to Organisms Getting the message: DNA transcription Lecture 6 – Transcription  Lecture outline DNA, chromosomes, genes & RNA How information flows from genes to proteins Transcription in bacteria and eukaryotes RNA processing in eukaryotes...

Biol1110 Genes to Organisms Getting the message: DNA transcription Lecture 6 – Transcription  Lecture outline DNA, chromosomes, genes & RNA How information flows from genes to proteins Transcription in bacteria and eukaryotes RNA processing in eukaryotes Lecture 6 – Transcription  DNA Deoxyribonucleic acid Lecture 6 – Transcription DNA replication Lecture 6 – Transcription  Genes Genes produce – RNAs – Proteins Control regions Termination region Protein coding region (ORF) Control regions Termination region exon intron exon intron Lecture 6 – Transcription  Genomes Genome – The entire genetic information of an organism Lecture 6 – Transcription  RNA Ribonucleic acid Uracil replaces Thymine Mostly single- stranded (can fold on itself) Less stable than DNA Lecture 6 – Transcription DNA & RNA Lecture 6 – Transcription  Flow of information DNA stores hereditary information – how an organism looks and functions Proteins are structural components of cells and are involved in how an organism functions Gene expression is the link between DNA and the synthesis of proteins (or RNA) the Central dogma  The cookbook analogy http://recipecurio.com/recipe- http://www.bfeedme.com/wp copies/collection3/sour-cherry- http://img.timeinc.net/re - pudding-recipe.jpg cipes/i/recipes/sl/02/11/ content/uploads/2007/01/co chocolate-cake-sl- okbook-clipart.gif 366569-l.jpg transcription translation DNA mRNA polypeptide/prot ein Lecture 6 – Transcription  mRNA is a mobile intermediate mRNA is the intermediate between genes and the proteins for which they code transcription translation DNA mRNA polypeptide/prot ein Lecture 6 – Transcription  One gene – one? Hypotheses on what a gene produces – One gene – one enzyme – One gene - one protein – One gene – one polypeptide – But that’s not even correct (alternative splicing) Lecture 6 – Transcription  Transcription Synthesis of RNA using DNA Similar process to DNA replication DNA replication – DNA provides template for synthesis of complementary DNA strand Transcription – DNA provides template for synthesis of complementary RNA Lecture 6 – Transcription strand  Types of RNA mRNA – messenger RNA, carries the message from DNA rRNA – ribosomal RNA, structural RNA that forms part of the ribosome tRNA – transfer RNA, http://sevendaysperweek.blogspot.com/2015/10/stpm- structural RNA biology-biological-molecules-part_13.html involved in the synthesis of amino acid chains Lecture 6 – Transcription  Localisation in the cell Bacteria - no nuclei, no RNA processing Eukaryotes - transcription & RNA processing occurs in nucleus, translation occurs in the cytoplasm Lecture 6 – Transcription  Template strand Each gene is transcribed from one of the two DNA strands – the template strand The opposite strand can also act as the template strand Template strand is 3¢ to 5¢ Lecture 6 – Transcription  Complementary base pairing Transcription, similar to DNA replication, involves complementary base pairing but thymine is replaced by uracil Like DNA, RNA synthesized in an antiparallel direction to the template strand DNA (template strand) 3¢ A A T C G A T G C 5¢ RNA 5¢ U U A G C U A C G 3¢ Transcription Lecture 6 – Transcription  Coding strand The coding strand is identical to RNA (except for U), and is complementary to the template strand – coding because nucleotide triplets (codons) code for an amino acid DNA (template strand) 3¢ A A T C G A T G C 5¢ DNA (coding strand) 5¢ T T A G C T A C G 3¢ RNA 5¢ U U A G C U A C G 3¢ Coding strand = non-template strand Non-coding strand = template Lecture 6 – Transcription Strand terminology DNA 3¢ - 5¢ = template strand = non-coding strand DNA 5¢ - 3¢ = non-template strand = coding strand Lecture 6 – Transcription  Transcription RNA polymerase – separates the DNA double helix and adds RNA nucleotides Like DNA polymerase and DNA replication, RNA pol can only add nucleotides in a 5¢ to 3¢ direction Unlike DNA pol, do not need a primer for RNA polymerase to attach and add nucleotides Only one RNA pol in bacteria Multiple in eukaryotes (at least three) – RNA pol II used for making mRNA Lecture 6 – Transcription  Promoters & terminators Promoter - where RNA pol attaches and where the start of transcription occurs Terminator - in bacteria, where transcription stops Upstream and downstream Lecture 6 – Transcription  Stages of transcription Prokaryotes & Eukaryotes have three stages of transcription, but some details of these stages are different The three stages of transcription: – Initiation – Elongation – Termination Lecture 6 – Transcription  Stages of transcription 1. Initiation In eukaryotes, transcription factors mediate binding of RNA pol II TATA Box is an important sequence for initiation of transcription Lecture 6 – Transcription  Stages of transcription 2. Elongation RNA pol untwists the double helix 10-20 nucleotides at a time Adds nucleotide at the 3¢ end Adds 40 nucleotides per second in eukaryotes Lecture 6 – Transcription  Stages of transcription 2. Elongation Multiple RNA pols can transcribe the same gene in a convoy – can make lots of the protein quickly Lecture 6 – Transcription  Stages of transcription 3. Termination In bacteria, the transcribed terminator signal causes RNA pol to detach In eukaryotes, polyadenylation signal AAUAAA, signals proteins to bind RNA molecule, cut it, which releases RNA pol from the RNA transcript Lecture 6 – Transcription  Stages of transcription 3. Termination Proteins catch up and cleave off the RNA pol II Lecture 6 – Transcription  RNA processing Pre-mRNA is produced in eukaryotes RNA processing occurs in the nucleus Lecture 6 – Transcription  RNA processing 5¢ CAP (modified G) after 20-40 nucleotides into the transcript 50-250 A ’s are added at the 3¢ end forming a poly A tail Lecture 6 – Transcription  RNA processing Modifications (CAP) and poly A tail are: – Help facilitate export of mRNA out of nucleus and into cytoplasm (where ribosomes are and where translation occurs) – Protect mRNA from enzymatic degradation – Help ribosomes attach to the 5¢ end of mRNA Lecture 6 – Transcription  mRNA splicing Non-coding regions of primary transcript (pre-mRNA) removed Exons (expressed) are coding Introns (intervening sections) are non-coding Hence, coding regions are split into sections Lecture 6 – Transcription  Example of mRNA splicing pre-mRNA with exons & introns E = exon; I = intron E1 I1 E2 I2 E3 I3 E4 I4 E5 I5 E6 DNAASDFASISASDFTHECGHDTHREADARETVTOFSRVTLIFE mature mRNA DNAISTHETHREADOFLIFE Lecture 6 – Transcription  mRNA splicing Proteins and small RNAs complex called the spliceosome recognizes and cuts out introns After RNA splicing, mature RNA moves to cytoplasm mRNA for the egg protein, ovalbumin, being Lecture 6 – Transcription spliced to remove introns  Alternative RNA splicing Why is it important to have introns? Different exons spliced give rise to different proteins In many cases, different exons code for the different domains in a protein Exon shuffling may result in the evolution of new proteins https://sites.google.com/site/bio1040genbio2/chapter-18-regulation-of-gene- expression/alternative-rna-splicing-and-mrna-degradation Lecture 6 – Transcription  Universal information flow Almost universal information flow from bacteria to eukaryotes More support for common ancestry of all life Utility in biotechnology – can put a gene like insulin from humans into https://www.eduhk.hk/biotech/eng/classrm/class_health5.html bacteria to produce it in mass amounts Lecture 6 – Transcription  Lecture summary Ribonucleic acid (RNA) similar to DNA but has a ribose sugar, uracil replaces thymine, and is generally single stranded Flow of information DNA ➔ RNA ➔ protein Transcription is the synthesis of RNA using the information in DNA RNA is synthesized by RNA polymerase using the 3¢ to 5¢ template (non-coding strand) Initiation of transcription occurs at the promoter and ends at the terminator sequence or downstream of the polyadenylation signal Lecture 6 – Transcription  Lecture summary RNA processing of the pre-mRNA in eukaryotes involves adding a 5¢ CAP and poly A tail RNA processing also involves removing introns via the spliceosome Alternative mRNA splicing can give rise to different proteins The near-universality of the flow of genetic material can be harvested (biotechnology) Lecture 6 – Transcription