Transcription and Translation PDF

Anu Singh-Cundy Gary Shin Discover Biology SIXTH EDITION CHAPTER 12 From Gene to Protein © 2015 W. W. Norton & Company, Inc.  Coding Genes Store Information Needed to Build RNA and Proteins, Which in Turn Produce the Phenotypes of Most Genetic Traits...

Anu Singh-Cundy Gary Shin Discover Biology SIXTH EDITION CHAPTER 12 From Gene to Protein © 2015 W. W. Norton & Company, Inc.  Coding Genes Store Information Needed to Build RNA and Proteins, Which in Turn Produce the Phenotypes of Most Genetic Traits A gene is any DNA sequence that is copied (transcribed) into RNA. Proteins, including enzymes, are the key determinants of an individual’s phenotype. Transcription  RNA Molecules Are Single- Stranded Polynucleotides Like DNA, RNA is a polymer of nucleotides. RNA is single- stranded, whereas DNA forms a double- stranded molecule twisted into a spiral shape (double helix). DNA and RNA also differ in the type of sugar used (ribose in RNA, deoxyribose in DNA). RNA uses the base uracil (U) in place of thymine (T) in DNA molecules.  DNA Stores Information in the Nucleus, RNA Carries Information from the Nucleus to the Cytoplasm DNA RNA Structur Double-stranded; two polynucle­- Single-stranded polynucleotide; may fold e otide strands wound into a helix back on itself Sugar Deoxyribose Ribose Nucleoti A, G, C, and T A, G, C, and U des Function Stores genetic information Expresses genetic information—for example, by direct­ing the manufacture of a specific protein Stability Highly stable in most cells Generally much less stable Location Nucleus, chloroplasts, and Nucleus, chloroplasts, mitochondria, and mitochondria in eukaryotes; cytosol in eukaryotes; cytosol in prokaryotes cytosol in prokaryotes Three Types of RNA Assist in the Manufacture of Proteins TYPE OF RNA FUNCTION Messenger RNA (mRNA) Specifies the order of amino acids in a protein using a series of three-base codons, where different amino acids are specified by particular codons. Ribosomal RNA (rRNA) As a major component of ribosomes, assists in making the covalent bonds that link amino acids together to make a protein. Transfer RNA (tRNA) Transports the correct amino acid to the ribosome, using the information encoded in the mRNA; contains a three-base anticodon that pairs with a complementary codon revealed in the mRNA.  Information Flows from DNA to RNA to Proteins A complementary mRNA sequence is made using the information in the DNA sequence of protein-coding genes during the process of transcription. During translation, amino acids are covalently linked in the sequence dictated by the base sequence of the mRNA; translation is carried out by ribosomes in the cytoplasm.  RNA Polymerase Synthesizes RNA Using One Strand of the DNA as a Template Transcription occurs in the nucleus. An enzyme called RNA polymerase synthesizes RNA using one strand of the DNA as template.  Transcription Synthesis of a complementary mRNA strand from a DNA template Transcription begins when RNA polymerase binds to the promoter sequence on DNA Transcription proceeds in the 5' 3' direction; only one of the two DNA strands is transcribed (anti- sense strand) Transcription stops when it reaches the terminator sequence on DNA  Information Flow from DNA to RNA Transcription begins when RNA polymerase binds to a segment of DNA called a gene promoter. Once bound, RNA polymerase begins to unwind the DNA and transcribe the template strand (bottom strand in diagram); which strand serves as the template is dictated by the positioning of the promoter, which orients the polymerase. Transcription stops when RNA polymerase reaches  In Eukaryotes, mRNA Is Chemically Modified After Transcription Posttranscriptional processing modifies RNA and prepares it for export from the nucleus. The newly formed mRNA undergoes RNA splicing, which removes the introns, and is then allowed to leave the nucleus through the nuclear pore. Translation  Translation: Information Flow from mRNA to Protein Translation is the process of converting a sequence of bases in mRNA to a sequence of amino acids in a protein. Translation occurs at the ribosomes, which are made up of proteins and rRNA. The Base Sequence of mRNA Is Read as a Sequence Codons Each unique sequence of three bases is called a codon. When reading the code, the ribosomes begin at the start codon, AUG, and end at one of three stop codons: UAA, UAG, or UGA.  There Are 64 Codons That Make Up the Information in the Genetic Code The genetic code has several distinct characteristics: – It is unambiguous – It is redundant – It is virtually universal  After Ribosomes Bind the mRNA, Each Specific Amino Acid Is Delivered to the Ribosome-mRNA Complex by a tRNA Molecule Specialized to Deliver a Specific Type of Amino Acid An anticodon is a three-base sequence that determines which codons on the mRNA can be recognized by the tRNA. Each codon on the mRNA is recognized by a specific tRNA, and the ribosome adds the amino acid delivered by this tRNA to the growing amino acid chain.  Translation mRNA is translated into the “language” of proteins Codons are groups of three mRNA nucleotides that code for a particular amino acid 61 sense codons encode the 20 amino acids The genetic code involves degeneracy, meaning each amino acid is coded by several codons  Translation Translation of mRNA begins at the Shine-Delgarno sequence and the start codon: AUG (fMet) Translation ends at nonsense codons: UAA, UAG, UGA Codons of mRNA are “read” sequentially tRNA molecules transport the required amino acids to the ribosome tRNA molecules also have an anticodon that base- pairs with the codon Amino acids are joined by peptide bonds Translation Begins When a tRNA Molecule Recognizes and Pairs with the AUG RBS of the Start Codon Ribosomal The process continues Binding until a stop codon is Site reached and the mRNA and the completed amino acid chain both separate from the ribosome.  Protein Synthesis through Translation RBS Ribosomal Binding Site  The First Covalent Initiation Bond Between Amino Acids: The Polypeptid e Chain Begins Elongation  Chain Elongation Continues  Chain Terminatio n Extra slides on Transcription and Translation  Transcription in Prokaryotes Synthesis of a complementary mRNA strand from a DNA template Transcription begins when RNA polymerase binds to the promoter sequence on DNA Transcription proceeds in the 5' 3' direction; only one of the two DNA strands is transcribed (anti- sense strand) Transcription stops when it reaches the terminator sequence on DNA mRNA can be polycistronic.  Figure 8.7 The Process of Transcription TRANSCRIPTION DNA RNA polymerase RNA polymerase mRNA RNA nucleotides Protein DNA Promoter Sense RNA polymerase bound to DNA RNA Template strand of DNA Anti-sense Promoter RNA synthesis (gene begins) RNA polymerase RNA Terminator (gene ends) RNA polymerase binds to the RNA is synthesized promoter, and by complementary The site of synthesis DNA unwinds at base pairing of free moves along DNA; the beginning of nucleotides with the DNA that has been Transcription reaches a gene. nucleotide bases on transcribed rewinds. the terminator. the template strand of DNA. RNA and RNA Complete polymerase are RNA strand released, and the DNA helix re-forms.  Translation mRNA is translated into the “language” of proteins Codons are groups of three mRNA nucleotides that code for a particular amino acid 61 sense codons encode the 20 amino acids The genetic code involves degeneracy, meaning each amino acid is coded by several codons Figure 8.8 The Genetic Code  Translation Translation of mRNA begins at the Shine-Delgarno sequence and the start codon: AUG (fMet) Translation ends at nonsense codons: UAA, UAG, UGA Codons of mRNA are “read” sequentially tRNA molecules transport the required amino acids to the ribosome tRNA molecules also have an anticodon that base- pairs with the codon Amino acids are joined by peptide bonds  Figure 8.9 The Process of Translation Ribosome Ribosomal P Site subunit tRNA Anticodon Ribosomal Start Second mRNA subunit mRNA codon codon Components needed to begin On the assembled ribosome, a tRNA carrying the first translation come together. amino acid is paired with the start codon on the mRNA. The place where this first tRNA sits is called the P site. A tRNA carrying the second amino acid approaches.  Figure 8.9 The Process of Translation Peptide bond forms A site E site mRNA mRNA Ribosome moves along mRNA The second codon of the mRNA pairs with a tRNA The ribosome moves along the mRNA until the carrying the second amino acid at the A site. The first second tRNA is in the P site. The next codon to be amino acid joins to the second by a peptide bond. This translated is brought into the A site. The first tRNA attaches the polypeptide to the tRNA in the P site. now occupies the E site.  Figure 8.9 The Process of Translation (3 of 4) Growing tRNA released polypeptide chain mRNA mRNA The second amino acid joins to the third by another The ribosome continues to move along the mRNA, peptide bond, and the first tRNA is released from the E and new amino acids are added to the polypeptide. site.  Figure 8.9 The Process of Translation (4 of 4) Polypeptide released mRNA New protein mRNA Stop codon When the ribosome reaches a stop Finally, the last tRNA is released, and the ribosome codon, the polypeptide is released. comes apart. The released polypeptide forms a new protein.  APPLYING WHAT WE LEARNED: FROM GENE EXPRESSION TO CYCLOPS Environmental influences, such as ingestion of the corn lily by a pregnant animal, can cause gene expression to be altered, resulting in abnormalities such as cyclopia. List of Key Terms: Chapter 12 anticodon (p. 269) point mutation (p. 271) codon (p. 267) regulatory DNA (p. 272) deletion (p. 271) regulatory protein (p. 272) repressor (p. 273) elongation (transcription, p. 265; ribosomal RNA (rRNA) (p. 264) translation, p. 269) RNA polymerase (p. 265) exon (p. 266) RNA splicing (p. 267) frameshift (p. 271) start codon (p. 267) gene (p. 262) stop codon (p. 267) gene expression (p 272) substitution (p. 271) template strand (p. 265) gene promoter (p. 265) termination (transcription, p. 265; genetic code (p. 267) translation, p. 270) initiation (transcription, p. 265; terminator (p. 266) translation, p. 269) transcription (p. 264) insertion (p. 271) transfer RNA (tRNA) (p. 264) intron (p. 266) translation (p. 264) messenger RNA (mRNA) (p. 264) operator (p. 273) operon (p. 272) Class Quiz, Part 1 Which of the following is true? A. Transcription occurs in the cytoplasm and produces RNA. B. Transcription occurs in the nucleus and produces proteins. C. Translation occurs in the cytoplasm and produces proteins. D. Translation occurs in the nucleus and produces RNA. Class Quiz, Part 2 Dystrophin is the largest protein in the human body. In this concept map, which of the following fits best in the box labeled with an “X”? A. DNA B. polypeptide C. phenotype D. mRNA E. tRNA  Class Quiz, Part 3 Which of the following is not true about the genetic code? A. Every codon has a corresponding amino acid. B. Every codon consists of three bases. C. There are 64 possible codons. D. A single amino acid can have more than one codon. Class Quiz, Part 4 Frameshift mutations A. occur only if three bases are deleted. B. occur when one base is changed to another. C. don’t change the structure of the protein. D. can be caused by either the insertion or deletion of a single base. 12.1 Concept Check, Part 1 1. Do all genes code for mRNA and therefore for proteins? ANSWER: No. RNA-only genes are transcribed into RNA other than mRNA, and these RNAs have specialized functions other than coding for proteins. 12.1 Concept Check, Part 2 2. Compare the chemical structures of RNA and DNA. Which is more stable chemically, and how is that stability consistent with its function? ANSWER: RNA is single-stranded; it contains ribose and the bases A, G, C, and U. DNA is double-stranded; it contains deoxyribose and A, G, C, and T. DNA is more stable—a property it must have to serve as the storehouse of genetic information. 12.1 Concept Check, Part 3 3. What is the product of transcription? What is the product of translation? ANSWER: The product of transcription is an mRNA complementary to the DNA sequence of a gene. The product of translation is a polypeptide (protein chain) determined from the sequence of the mRNA. 12.2 Concept Check, Part 1 1. The template strand of a gene has the base sequence TGAGAAGACCAGGGTTGT. What is the sequence of RNA transcribed from this DNA, assuming RNA polymerase travels from left to right on this strand? ANSWER: ACUCUUCUGGUCCCAACA 12.2 Concept Check, Part 2 2. The dystrophin gene has 78 introns. Are these introns transcribed? Do they code for amino acids? ANSWER: All 78 introns are transcribed into a pre- mRNA, but they are subsequently spliced out. Because they are absent from the fully processed mature mRNA transcript, introns do not code for amino acids. 12.3 Concept Check, Part 1 1. Why is the start codon, AUG, so important? ANSWER: The start codon sets the reading frame, that is, it determines the grouping of the bases in the mRNA into triplets to be read as codons. 12.3 Concept Check, Part 2 2. What does it mean to say that the genetic code is redundant? ANSWER: There are 64 possible codons, but only 20 amino acids. In most cases, a single amino acid is specified by more than one codon, and this is what is meant by redundancy. For example, tyrosine is specified by either UAU or UAC. 12.4 Concept Check, Part 1 1. What is meant by “translation” of mRNA? ANSWER: Translation converts a sequence of bases in mRNA to a sequence of amino acids in a protein. 12.4 Concept Check, Part 2 2. Does each of the 64 codons specify a different amino acid? ANSWER: No. The three stop codons do not specify any amino acids, and a single amino acid may be specified by as many as six different codons. 12.5 Concept Check, Part 1 1. What is a mutation? Are all mutations harmful? ANSWER: A mutation is a change in the base sequence of an organism’s DNA. A mutation may have no detectable effects or may be harmful; in rare instances, it may even be advantageous. 12.5 Concept Check, Part 2 2. A single-base addition or deletion in a gene is likely to alter the protein product more than a single-base substitution, such as C for T, would. Why? ANSWER: Single-base addition or deletion shifts the reading frame, so all the amino acids downstream of such a mutation are altered. Single-base substitution alters, at most, a single amino acid. 12.6 Concept Check, Part 1 1. Why are most genes controlled at the level of transcription? ANSWER: Transcriptional regulation prevents gene expression when a gene’s product is not needed by a cell, enabling the cell to invest its resources elsewhere. 12.6 Concept Check, Part 2 2. If transcriptional control is the most favored method of gene regulation, why are not all genes controlled at the level of transcription? ANSWER: Transcriptional activation is relatively slow; controlling gene expression at a posttranscriptional step enables a cell to respond faster to environmental changes.