Chapter 17b_translation_upload.pptx

Biological Science Seventh Edition Chapter 17 Transcription RNA Processing, and Translation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Chapter 17 Opening Roadmap Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Transcription, RN A...

Biological Science Seventh Edition Chapter 17 Transcription RNA Processing, and Translation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Chapter 17 Opening Roadmap Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Introduction to Transcription, RN A Processing, and Translation Proteins are the stuff of life and serve various roles in cells A cell builds the proteins it needs from instructions encoded in its genome: – Transcribe DNA into RNA – Translate mRNA into protein Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Both transcription and translation are regulated- can be highly amplified or shut down completely © 2017 Pearson Education, Inc. Figure 17.7 In Eukaryotes, a Cap and a Tail Are Added to mRNAs Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 17.3 An Introduction to Translation The sequence of mRNA bases is converted to an amino acid sequence A complex set of steps are involved Translation is carried out with the use of ribosomes, mRNA, and tRNAs Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.8 Transcription and Translation Can Be Coupled, or Occur Simultaneously, in Bacteria Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved An Overview of Translation In bacteria, ribosomes begin translating an m RNA before transcription is complete: – Multiple ribosomes attached to an mRNA form a polyribosome – Many copies of a protein are produced from one mRNA In eukaryotes, transcription and translation are separated: – mRNAs are synthesized and processed in the nucleus – Mature mRNAs are transported to the cytoplasm for translation by ribosomes and polyribosomes Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved form 17.4 the Structure and Function of Transfer RNA The adapter molecule used in translation is called transfer RNA (tRNA): – An aminoacyl tRNA is a tRNA linked to its amino acid – Amino acids are transferred from tRNAs to a growing polypeptide Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved What is the Structure of tRNA s? tRNAs are relatively short: 75–95 nucleotides long They can form secondary structures by folding into a stem-and-loop A CCA sequence at the 3′ end is the amino acid binding site The loop at the opposite end contains the anticodon: – Has a sequence of three nucleotides – Can base-pair with the mRNA codon Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.11 The Structure of an Aminoacyl Transfer RNA Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved How Are Amino Acids Attached to tRNAs? ATP is required to attach tRNA to an amino acid Aminoacyl-tRNA synthetases “charge” the tRNA: – Catalyze the addition of amino acids to tRNAs There are 20 amino acids: – Each has a different aminoacyl tRNA synthetase – For each amino acid, there is one or more tRNAs Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved How Many tRNAs Are There? There are 61 different codons but only about 40 tRNAs in most cells Francis Crick proposed wobble pairing as a solution: – The anticodon’s third position can form a nonstandard base pair One tRNA is able to read more than one codon mRNA codons Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved 17.5 Ribosome Structure and Function in Translation Ribosomes contain many proteins and ribosomal RNA (rRNA)- about equal amounts by mass Ribosomes can be separated into two subunits: 1. The small subunit holds the mRNA in place 2. The large subunit is where peptide bonds form Large subunit from an archaeon. Proteins are blue, two RNA chains are tan and yellow. Peptidyltransferase site is in green. GIF by David S. Goodsell, CC BY 4.0, Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved https://commons.wikimedia.org/w/index.php?curid=2837739 Ribosome Structure and Function in Translation (1 of 3) During translation, three tRNAs line up within the ribosome The tRNAs fit into three sites in the ribosome, bound to corresponding mRNA codons: 1. A site (acceptor or aminoacyl)—tRNA carries an amino acid 2. P site (peptidyl)—holds growing peptide chain 3. E site (exit)—tRNAs without amino acids exit the ribosome Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.13 Ribosomes Contain Three tRNA Binding Sites Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Ribosome Structure and Function in Translation (2 of 3) The ribosome synthesizes proteins in a three-step sequence: 1. An aminoacyl tRNA enters the A site; remains if there is a codon-anticodon match 2. A peptide bond forms between the amino acid on the A-site tRNA and the polypeptide on the P-site tR NA 3. The ribosome moves down the mRNA by one codon and all three tRNAs move down one ▪ The tRNA in the E site exits position: ▪ The A site is available for another tRNA to bind Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Ribosome Structure and Function in Translation (3 of 3) The protein grows by one amino acid with each repeat of the three steps Amino acids are always added to the carboxyl end (C-terminus) of the polypeptide Translation has three phases: 1. Initiation 2. Elongation 3. Termination Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Initiating Translation (1 of 2) The initiation phase of translation begins near the AUG start codon In bacteria, the small ribosomal subunit binds to the ribosome binding site, or Shine–Dalgarno sequence, of the mRNA: – About 6 bases upstream from the start codon – Mediated by initiation factors The first tRNA is called the initiator tRNA: – It carries a modified methionine (f-Met) in bacteria Khan Academy (link in Canvas) Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Initiating Translation (2 of 2) Translation initiation is a three-step process in bacteria: 1. The mRNA binds to a small ribosomal subunit 2. The initiator tRNA bearing f-Met binds to the start codon 3. The large ribosomal subunit binds so that the initiator tRNA is in the P site Translation is now ready to begin Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Initiating translation in eukaryotes Key initiation factors and small ribosomal subunit bind to 5’ cap (not to Shine-Dalgarno sequences) Small ribosomal subunit + initiator tRNA slides along mRNA (scanning) to find AUG This difference might be related to the lack of polycistronic mRNAs in eukaryotes Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Comparison of prokaryotic and eukaryotic initiation Figure 17.14 Initiation Requires Binding of Initiator tRNA to mRNA, and Assembly of the Ribosome Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Elongation: Extending the Polypeptide At the start of elongation: – The initiator tRNA is in the P site – The E and A sites are empty An aminoacyl tRNA binds to the codon in the A site Amino acids at the P and A sites are in the ribosome’s active site, where peptide bond formation occurs The amino acid on the P-site tRNA is transferred to the amino acid on the A-site tRNA Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Is the Ribosome an Enzyme or a Ribozyme? The active site of the ribosome is entirely ribosomal RNA Ribosomal RNA catalyzes peptide bond formation Therefore, the ribosome is a ribozyme Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Moving Down the mRNA (1 of 2) Translocation occurs when the ribosome slides one codon toward the 3′ end of the mRNA: – Elongation factors help move the ribosome Translocation accomplishes three things: 1. The uncharged tRNA from the P site moves into the E site and is ejected from the ribosome 2. The tRNA attached to the growing protein moves into the P site 3. Opens the A site to expose a new codon, which is available to accept a new aminoacyl tRNA Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Moving Down the mRNA (2 of 2) The three steps of translocation repeat at each codon along the mRNA: 1. Arrival of the aminoacyl tRNA 2. Peptide bond formation 3. Translocation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.15 Elongation Extends the Polypeptide Chain—Part 1 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.15 Elongation Extends the Polypeptide Chain—Part 2 Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Terminating Translation Termination occurs when the A site encounters a stop codon A protein called a release factor enters the A site: – Resembles tRNAs in size and shape – Hydrolyzes the bond linking the P-site t RNA to the polypeptide chain The newly synthesized polypeptide, tRNAs, and ribosomal subunits separate from the mRNA Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.16 Termination Occurs When a Release Factor Binds to a Stop Codon Encountered by the Ribosome Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Web Activity: Synthesizing Proteins Start at 1’15” Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Translation Overview Prokaryotes Eukaryotes  Polycistronic messages possible.  Only monocistronic messages.  Ribosome structure.  Ribosome structure.  The ribosome is a ribozyme!  The ribosome is a ribozyme!  Initiator tRNA = fmethionine.*  Initiator tRNA (special) contains methionine.  Ribosome Binding Site (Shine-Dalgarno sequence)  Ribosome scans 5’ UTR to find AUG codon. © 2017 Pearson Education, Inc. Polypeptides Are Modified after Translation Most proteins go through an extensive series of processing steps before they are functional: – Called post-translational modification These steps take place in various locations in a cell Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Figure 17.17 The Major Steps of Gene Expression in a Eukaryotic Cell Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved