Chapter Transcription Translation short PDF

What do we know at this point? Information of DNA in form of specific sequences of nucleotides – Genes Inherited genes lead to specific traits by controlling protein synthesis – Proteins link genotype (genome) to phenotype (proteome)  Gene expression – process by which DNA di...

What do we know at this point? Information of DNA in form of specific sequences of nucleotides – Genes Inherited genes lead to specific traits by controlling protein synthesis – Proteins link genotype (genome) to phenotype (proteome)  Gene expression – process by which DNA directs protein synthesis – includes two stages: transcription and translation Central Dogma of Molecular Biology DNA RNA transcribed Proteins translated RNA is intermediate between genes and proteins Loading…  The Genetic Code Genetic code – sequence of bases in an mRNA molecule – triplet code Read in groups of three nucleotide bases – codons – Specify a particular amino acid  Characteristics of the genetic code Linear triplet code – 64 possible codons – 61 sense and 3 nonsense (stop) codons UnambiguousLoading… but Redundant No overlap of codons – Specific reading frame Universal  mRNA – Codon – set of 3 RNA nucleotides – T of DNA substituted for U of RNA tRNA – Anticodon – 3 RNA nucleotide part of tRNA molecule – Allows complimentary binding of tRNA to mRNA codon 10  mRNA has 5’ ribosomal-binding site Start codon usually AUG Typical polypeptide is a few hundred amino acids in length Stop codons (nonsense codons) – UAA, UAG or UGA 12  Three stages of transcription Initiation Elongation Termination 1. Initiation – Promoters - Upstream from gene where RNA polymerase binds – TATA box in eukaryotes – Transcription factors required for RNA polymerase to recognize and bind to promoter  Loading… Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings  Elongation – RNA polymerase synthesizes RNA – Template strand used for mRNA transcription Coding strand strand is not used – Synthesized 5’ to 3’ – Uracil substituted for thymine  Termination – RNA polymerase reaches termination sequence – Causes both the polymerase and newly- made RNA transcript to dissociate from DNA  In eukaryotes this is TFIIB 3’ 5’ In bacteria 17  RNA Processing in Eukaryotes Bacterial mRNAs can be translated immediately Eukaryotes create pre-mRNA that requires processing into mRNA – Introns – non-coding sequences – Exons – coding sequences  Splicing – removal of introns Spliceosome – removes introns – Composed of snRNPs (small nuclear RNA and proteins) – Alternative splicing – results in single gene coding for multiple proteins – Exon shuffling – two or more exons from different genes brought together  Additional RNA processing Capping – Modified guanosine attached to 5’ end – Needed for mRNA to exit nucleus and bind ribosome Poly A tail – 100-200 adenine nucleotides added to 3’ end – Increases stability and lifespan in cytosol 21  The Machinery of Translation Requires many components – mRNA – tRNA – ribosomes – translation factors Most cells use a substantial amount of energy on translation 23  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. tRNA Amino acid attachment site at the 3′ single-stranded region 3 ′ 5 ′ Different tRNA molecules encoded by Stem- different genes Hydrogen bonds G G C loop structure – tRNASer carries serine Anticodo n (a) Two-dimensional structure of tRNA Common features 5 ′ – Folded single strand 3′ single strande d – Anticodon region – Amino acid attachment site Anticodo n (b) Three-dimensional structure of tRNA BIOLOGY PRINCIPLE Structure determines function Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. The structure of tRNA has two functional sites: a 3’ single-stranded region where an amino acid is attached and an anticodon that binds to a codon in mRNA. Ribosomes two subunits (large and small) made of proteins and ribosomal RNA (rRNA) Three binding sites for tRNA: – P site holds tRNA that carries the growing polypeptide chain – A site holds tRNA that carries the next amino acid to be added to the chain – E site is exit site, where discharged tRNAs leave the ribosome  The Stages of Translation Initiation – mRNA, first tRNA and ribosomal subunits assemble Elongation – Synthesis from start codon to stop codon Termination – Complex disassembles at stop codon releasing the completed polypeptide 27  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. amino acid methionine met 5' mRNA E site P site A site 3' met small ribosomal subunit large ribosomal subunit 5' start codon 3' A small ribosomal subunit binds to mRNA; an initiator tRNA with the anticodon UAC The large ribosomal subunit pairs with the mRNA start completes the ribosome. codon AUG. Initiator tRNA occupies the P sit e. The A site is ready for the next tRNA. Initiatio n  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. met met asp met peptide met ser ser thr tRNA ser bond ser ala ala ala C U G ala tryp tryp U G tryp anticodon tryp val G val val val asp asp asp A U C C A U C A U C U G G U A G A C C A U C U G C U G G U A G A C G U A G A C G U A G AC A C C 3' 5' 3' 5' 5' 3' 5' 3' 4. The ribosome moves forward; 2. Two tRNAs can be at 3. Peptide bond formation 1. A tRNA–amino acid the “empty” tRNA exits from the a attaches the peptide approaches the ribosome E site; the next amino acid–tRNA ribosome at one time; chain to the newly and binds at the A site. complex is approaching the the anticodons are arrived amino acid. ribosome. paired to the codons. Elongation  Copyright © The McGraw-Hill Companies, Inc. Permission required for reproduction or display. asp ala tryp asp release factor val ala glu tryp val U U A A U G A glu 5' stop codon 3' U The ribosome comes to a stop U C codon on the mRNA. A release factor binds to the site. A A U G G A 3' 5' The release factor hydrolyzes the bond between the last tRNA at the P site and the polypeptide, releasing them. The ribosomal subunits dissociate. Termination