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Chapter 16

Chapter 16 Textbook Notes - Transcription and Translation

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Fiona Rawle

Notes From Reading CHAPTER 16:T RANSCRIPTION AND T RANSLATION (PGS .342-365) Key concepts - After the enzyme RNA polymerase binds to a specific site in DNA with the help of other proteins, it catalyzes the production of an RNA molecule. The base sequence of the RNA produced is complementary to the base sequence of the DNA template strand. - Some sections of an RNA are encoded by gene regions called exons, while others are encoded by gene regions called introns. During RNA processing, introns are removed and the ends of the RNA receive a cap and tail. - Inside ribosomes, mRNAs are translated to proteins via intermediary molecules called transfer RNAs. Transfer RNAs carry an amino acid and have a three-base-pair anticodon, which binds to a three-base-long mRNA codon. The amino acid carried by the transfer RNA is then added to the growing protein via formation of a peptide bond. - Mutations are random changes in the DNA that may or may not produce changes in the phenotype. Introduction - A cell builds the proteins it needs from instructions encoded in its genome. - The flow of information in the cell is as follows: DNA ↓(information storage) ↓ Transcription mRNA ↓ (information carrier) ↓ Translation Proteins (product) 16.1 Transcription in Bacteria - The first step in converting genetic information into proteins is transcription, the synthesis of an mRNA version of the instructions stored in DNA - Like the DNA polymerase, an RNA polymerase performs a template-directed synthesis in the 5’  3’ direction - But unlike DNA polymerases, RNA polymerases do not require a primer to begin transcription - Transcription occurs when RNA polymerase matches the base in a ribonucleotide triphosphate with the complementary base in a gene - Once a matching ribonucleotide is in place, RNA polymerase catalyzes the formation of a phosphodiester bond between the 3’ end of the growing mRNA chain and the new ribonucleotide Notes From Reading CHAPTER 16:T RANSCRIPTION AND T RANSLATION (PGS .342-365) - RNA polymerase performs this synthesis by transcribing only one strand of DNA, called the template strand (Figure 16.1) - The other DNA strand is called the non-template strand - The sequence of the non-template strand matches the sequence of the RNA, except that RNA has uracil (U) in place of thymine (T). - Transcription consists of three phases: initiation, elongation, and termination. RNA Polymerase Structure and Function - Bacterial RNA polymerase is a large, globular enzyme with several interior channels - The enzyme’s active site, where phosphodiester bonds are formed, is located where these channels intersect Initiation: How Does Transcription Begin in Bacteria? - A holoenzyme is an enzyme made up of a core enzyme and other required proteins. - Prokaryotic RNA polymerase is a holoenzyme made up of the core enzyme, which has the ability to synthesize RNA, and a sigma subunit. - Sigma acts as a regulatory factor, telling RNA polymerase where and when to start synthesizing RNA. The sigma subunit is required for the initiation phase of transcription. - Transcription of bacterial genes is initiated at specific sections of DNA called promoters. - Promoters have two key regions: the 10 box and the 35 box - The names come from their locations: for example, the 10 box is found ten bases upstream from the transcription start site. (Downstream is in the direction RNA polymerase moves during transcription; upstream is in the opposite direction.) - To initiate transcription, sigma identifies and binds to the –10 and –35 boxes, properly orienting the RNA polymerase holoenzyme for transcription at the start site - Next, sigma opens the DNA double helix and the template strand is threaded through the RNA polymerase active site. - An incoming ribonucleoside triphosphate (NTP) pairs with a complementary base on the DNA template strand, and RNA polymerization begins. - Sigma dissociates from the core enzyme once the initiation phase of transcription is completed. Elongation and Termination - During the elongation phase of transcription, RNA polymerase moves along the DNA template and synthesizes RNA in the 5'  3' direction. - Transcription ends with a termination phase - In this phase, RNA polymerase encounters a transcription termination signal in the DNA template - This signal codes for RNA forming a hairpin structure. - The hairpin causes the RNA polymerase to separate from the RNA transcript, ending transcription. Notes From Reading CHAPTER 16:T RANSCRIPTION AND TRANSLATION (PGS.342-365) - To summarize, transcription begins with sigma, as part of the holoenzyme complex, binds to the promoter at the start of a gene - Once binding occurs, RNA polymerase begins to synthesize mRNA by adding ribonucleotides that are complementary to the template strand in DNA - Transcription ends when a termination signal at the end of the gene leads to the formation of a hairpin in the mRNA, disrupting the transcription complex 16.2 Transcription and RNA processing in Eukaryotes - Eukaryotic transcription shares many fundamental characteristics with prokaryotic transcription. - Transcription in eukaryotes is initiated by proteins called basal transcription factors - These factors begin transcription by matching RNA polymerase with the appropriate promoter region in DNA, a function analogous to that of sigma in bacteria - Eukaryotic cells contain three types of RNA polymerase, named I, II, and III - Each of these enzymes transcribes different classes of RNA - Only RNA polymerase II transcribes genes that code for proteins, producing mRNA - The promoters in eukaryotic RNA are more diverse and complex than are bacterial promoters. - The promoters recognized by each type of RNA polymerase differ. - Many promoters recognized by RNA polymerase II include a sequence called a TATA box analogous in function to the prokaryotic 10 and 35 boxes. - In eukaryotes, transcription is followed by several important RNA processing steps. The Startling Discovery of Eukaryotic Genes in Pieces - The protein-coding regions of eukaryotic genes are interrupted by noncoding regions. To make a functional mRNA, these noncoding regions must be removed. - Exons are the coding regions of eukaryotic genes that will be part of the final mRNA product. - The intervening noncoding sequences are called introns, and are not in the final mRNA. - Eukaryotic genes are much larger than their corresponding mature mRNA. - Introns are sections of genes that are not represented in the final mRNA product - As a result, eukaryotic genes are much larger than their corresponding mature RNA transcripts Exon, Introns, and RNA Splicing - Generates a primary RNA transcript that contains exons and introns. - As transcription proceeds, the introns are removed from the growing RNA strand by a process known as splicing - Introns are removed by splicing. - Small nuclear ribonucleoproteins (snRNPs, pronounced “snurps”) form a complex called a spliceosome. This spliceosome catalyzes the splicing reaction. - The transcription of eukaryotic genes by RNA polymerase - Current data suggest that both the cutting and rejoining reactions that occur during splicing are catalyzed by RNA molecules in the spliceosomes Notes From Reading CHAPTER 16:T RANSCRIPTION AND T RANSLATION (PGS .342-365) Adding Caps and Tails to Transcripts - Primary RNA transcripts are also processed by the addition of a 5' cap and a poly (A) tail. - With the addition of cap and tail, processing is complete; the product is a mature mRNA. - The 5' cap serves as a recognition signal for the translation machinery. - The poly (A) tail extends the life of an mRNA by protecting it from degradation. - With the addition of the cap and tail, processing of the primary RNA transcript is complete - The product is a mature mRNA 16.3 An Introduction to Translation - To synthesize a protein, the sequence of bases in a messenger RNA molecule is translated into a sequence of amino acids in a polypeptide - The genetic code specifies the relationship between the bases of a triplet codon in mRNA and the amino acid it codes for - In translation, the sequence of bases in the mRNA is converted to an amino acid sequence in a protein Ribosomes are the Site of Protein Synthesis - Ribosomes catalyze translation of the mRNA sequence into protein. - In bacteria, transcri
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