Chapter 17 Transcription, RNA Processing, and Translation PDF

Chapter 17 Transcription RNA Processing, and Translation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Learning Objectives Describe the steps of transcription. Describe t...

Chapter 17 Transcription RNA Processing, and Translation Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved  Learning Objectives Describe the steps of transcription. Describe the steps needed to process primary transcripts. Describe the roles of ribosomes, mRNA, and tRNAs in translation. Loading… Analyze the structure and function of transfer RNA. Explain the events of translation initiation, elongation, termination, and post-translational modification. Copyright © 2020, 2017, 2014 Pearson Education, Inc. All Rights Reserved Part I: Transcription  Review: Differences between DNA Loading… and RNA  1. Stores genetic information organized in the form of genes which can come in different versions (ex. purple or white flower). when you turn a gene on = gene expression process which regulates when and where a gene will be turned on = gene regulation 2. It transmits genetic information from one generation to the next. Review: In order to do this it must replicate. Replication must be it is repaired. exact, if not, What are If it escapes reparation, then you have a mutation. Mutations can have no effect, be harmful or beneficial. DNA’s functions? What are genes at the molecular level? “Genes are sequences of nucleotides that direct primarily the synthesis of polypeptides”. Gene Expression: process by which DNA directs the synthesis of proteins (or RNAs) Transcription versus Translation Functions of DNA and RNA DNA RNA Genetic material transferred from Key player in protein synthesis parent to offspring Role in regulation of gene Contains the instruction to expression make proteins which serve many functions in the body CENTRAL DOGMA OF MOLECULAR BIOLOGY: INFORMATION FLOWS FROM DNA TO RNA TO POLYPEPTIDE Transcription: DNA RNA Translation: RNA protein Ribosome = site of translation The Genetic Code For each gene, one DNA strand is the template strand (non- coding), the other is the coding strand TEMPLA TE mRNA (5’ 3’) complementary to CODING template mRNA triplets (codons) code for amino acids in polypeptide chain Differentiate between DNA’s template (non-coding strand) and non-template (coding) strands. Template vs. Non-template Strands Template strand (non-coding strand) Loading…RNA transcript Non-template strand (coding strand) RNA POLYMERASE IS THE ENZYME RESPONSIBLE FOR CREATING THE mRNA TRANSCRIPT IT CAN ONLY MOVE IN THE 5’ TO 3’ DIRECTION LOOK AT THE PICTURE. Identify the significance of the genetic code. The Genetic Code 64 different codon combinations 1 amino acid is coded by multiple codons Reading frame: groups of 3 must be read in correct groupings This code is universal: all life forms use the same code.  Describe the usual flow of genetic information in a cell (central dogma) and the steps of the transcription process (Key words: promoter, transcription factors, RNA polymerase). What do we need for Transcription? Transcription unit: stretch of DNA that codes for a polypeptide or RNA (ex. tRNA, rRNA) RNA polymerase: Separates DNA strands and synthesizes mRNA in 5’ 3’ direction by adding RNA nucleotides (Uracil instead of Thymine) in a Transcription complimentary fashion http://hyperphysics.phy-astr.gsu.edu/hbase/Organic/transcription.html Attaches to promoter (start of gene) and stops at terminator (end of gene) What is a promoter? A sequence of DNA that tells RNA polymerase where to start transcription and which strand to transcribe. DNA Is Transcribed to Form RNA (A) INITIATION RNA polymerase binds to the promoter and starts to unwind the DNA strands.  (B) ELONGATION RNA polymerase unwinds the DNA template strand as it moves from 3′ to 5′ and produces the mRNA transcript by adding nucleotides to the 3′ end of the growing mRNA strand pairing A-U and G-C. As RNA polymerase moves, it untwists DNA, then rewinds it after mRNA is made.  When RNA polymerase reaches the termination site, the RNA transcript is set free from the template. (C) TERMINATION 3′ 5′ RN A The RNA has the same base sequence as the DNA coding strand, but with U instead of T. In more detail, again…. 1. Initiation Eukaryotes: TATA box = DNA sequence (TATAAAA) within the promoter Transcription factors recognize TATA box before RNA polymerase can bind to DNA promoter  2. Elongation 3. Termination Elongation: RNA polymerase continues adding bases to the growing mRNA strand in a complimentary fashion to the template strand Termination: RNA polymerase transcribes a terminator sequence in DNA, then mRNA and polymerase detach. It is now called pre-mRNA for eukaryotes (or primary transcript). Prokaryotes = mRNA ready for use Briefly explain mRNA processing in eukaryotes and differentiate between the flow of genetic information between prokaryotes and eukaryotes. Flow of Genetic Information in Prokaryotes vs. Eukaryotes Notice: Where does translation take place? Do you see anything done to the mRNA right after transcription in prokaryotes?  mRNA processing in eukaryotes Eukaryotic cells modify RNA after transcription Pre-mRNA has introns (noncoding sequences) and exons (codes for amino acids) MORE DIFFERENCES: Prokaryotic cells’ genome do not contain introns. Prokaryotic genes can share more than one promoter. Each eukaryotic gene has its own promoter. Before editing the pre-mRNA: 5’ cap (modified guanine) and 3’ poly-A tail (50-520 A’s) are added Help export from nucleus, protect from enzyme degradation, attach to ribosomes Guanine modified cap looks like this (FYI): RNA Splicing Splicing = introns cut out, exons joined together  Think-Pair-Share All of the following events or modifications to mRNA take place in a eukaryotic cell’s nucleus, except: a. Addition of a “cap” on the 5ʹ end b. Removal of introns c. Translation of the mRNA to a polypeptide d. Addition of a “tail” of adenine nucleotides on the 3ʹ end e. Transcription of the pre-mRNA from a gene Application Part II: Translation  List and identify all the key components of translation. Components of Translation 1. mRNA = message 2. tRNA = interpreter 3. Ribosome = site of translation tRNA: links information in mRNA codons to specific amino acids NOTICE 3’ END ATTACHES AMINO ACID Specific to each amino acid Transfer AA to ribosomes Anticodon: pairs with complementary mRNA codon  Ribosomes Ribosome = rRNA + proteins made in nucleolus LET’S LOOK Nontemplate strand AT BASE PAIRING A DNA MINUTE THIS IS WHEN Template strand THE GENETIC CODE TABLE Transcription COMES INTO PLAY Note that the first Codon LOOK AT THE base in the codon Loading… mRNA CODON (5' end) pairs with mRNA Translation AUG the last base (3' end) in the Amino acid FIND IT IN THE TABLE anticodon. Charged tRNA Codon–anticodon pairing It CODES FOR THE AMINO ACID METHIONINE  The Genetic Code 64 different codon combinations 1 amino acid is coded by multiple codons Reading frame: groups of 3 must be read in correct groupings This code is universal: all life forms use the same code. The small ribosomal subunit binds to its recognition sequence on mRNA. Methionine-charged tRNA binds to the AUG start codon, completing the initiation complex. The large ribosomal subunit joins the initiation complex, with methionine-charged tRNA now occupying the P site.  Figures 10.15–10.17 Translation Takes Place in Three Steps The process repeats. ELONGATION Anticodon Incoming N terminus tRNA Codon recognition: The anticodon of an incoming tRNA binds to the codon at the A site. Elongation: Free tRNA is moved to the E site, and then released, as the ribosome shifts by one Peptide bond formation: Pro is codon, so that the growing polypeptide chain linked to Met by peptidyl transferase moves to the P site. activity of the large subunit. Figures 10.15–10.17 Translation Takes Place in Three Steps TERMINATION A release The release remaining factor factor components binds to the complex disconnects (mRNA and ribosomal the whenpolypeptide a stop codonthe from subunits) enters tRNA separate. the in the A site. P site. Stop codon Release factor N terminus  Compare gene expression in prokaryotes vs. eukaryotes Characteristic Prokaryotes Eukaryotes Transcription and translation In the cytoplasm Transcription – nucleus occurs in _____ Translation – cytoplasm Gene structure No introns Coding regions (exons) interrupted by non coding introns mRNA modification None 5’cap and 3’ poly A tail added Introns cut out and exons spliced together Signals that start Initiation and Termination in Transcription and Translation Transcription Translation Initiation Promoter in AUG start DNA codon in mRNA Termination Terminator Stop codon in sequence in mRNA DNA  Describe the different types of mutations. Possible effects of mutations on gene expression A Missense Mutation causes Sickle-Cell Anemia Missense mutation DNA mRNA A nucleotide Protein substitution that changes the amino acid is a missense mutation.