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Splash. Chapter Introduction The Genetic Code: Using Information 9.1 Genetic Material 9.2 Importance of Proteins Transcription 9.3 RNA Synthesis 9.4 RNA Processing Protein Synthesis 9.5 Translation 9.6 Transport and Modification of Proteins 9.7 Translation Errors Viruses

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  1. Splash

  2. Chapter Introduction The Genetic Code: Using Information 9.1 Genetic Material 9.2 Importance of Proteins Transcription 9.3 RNA Synthesis 9.4 RNA Processing Protein Synthesis 9.5 Translation 9.6 Transport and Modification of Proteins 9.7 Translation Errors Viruses 9.8 Genetic Information and Viruses 9.9 Impact of Viruses Chapter Highlights Chapter Animations Chapter Menu Contents

  3. Learning Outcomes By the end of this chapter you will be able to: A Explain the connection between DNA and RNA in protein synthesis; describe the genetic code and its role in protein synthesis. BExplain why proteins are important to biological systems. C Identify the stages of transcription and explain what occurs during each stage. D Summarize the events that occur in RNA processing. Learning Outcomes 1

  4. Learning Outcomes By the end of this chapter you will be able to: E Identify the stages of translation and explain what occurs during each stage. F Describe posttranslational modification and transport of proteins. G Infer the consequences of RNA translation errors. H Explain the relationship between viruses and host cells and describe the impact of viruses on living systems. Learning Outcomes 2

  5. A colored scanning electron micrograph of a group of human chromosomes (x6,875) Expressing Genetic Information • How does an organism use the information stored in its genetic material? • Does a cell express all of its genetic information all the time? Chapter Introduction 1

  6. A colored scanning electron micrograph of a group of human chromosomes (x6,875) Expressing Genetic Information • Living organisms store information in their genetic material. In a process called gene expression, organisms read and use the encoded information by directing the synthesis of proteins. When a virus infects a cell, the virus takes control of gene expression in the cell. Chapter Introduction 2

  7. End of the Introduction

  8. The Genetic Code: Using Information 9.1 Genetic Material • Genetic material consists of two nucleic acids—DNA and RNA—that are involved in gene expression. Gene expression depends on two features of their molecular structure: 1. nucleic acids consist of a long strand of repeating subunits that act as letters in a code 2. the subunit bases of one strand pair with the bases of another strand 9.1 Genetic Material 1

  9. The Genetic Code: Using Information 9.1 Genetic Material (cont.) • Living cells store genetic information in DNA which specifies the primary structures of proteins. By determining the primary structure of each protein, DNA indirectly dictates protein function. Proteins, in turn, carry out important cell activities. When a gene becomes active, an enzyme makes a temporary RNA copy of the information the DNA contains. 9.1 Genetic Material 2

  10. The Genetic Code: Using Information 9.1 Genetic Material (cont.) • Messenger RNA (mRNA) is the temporary copy of a gene that encodes a protein. The process of making an mRNA molecule is called transcription. In translation, the mRNA molecule provides the pattern that determines the order in which amino acids are added to the protein being made. Protein synthesis takes place on ribosomes which are made of proteins and ribosomal RNA (rRNA). Each amino acid that will be used in making the protein is attached to transfer RNA (tRNA). 9.1 Genetic Material 3

  11. Information stored in DNA is copied to mRNA, which in turn directs the synthesis of a particular protein. 9.1 Genetic Material 4

  12. The Genetic Code: Using Information 9.1 Genetic Material (cont.) • The genetic code describes how a sequence of bases in DNA or RNA translates into the sequence of amino acids in a protein. The nucleotides serve as the four “letters” of the DNA “alphabet.” A genetic code requires at least 20 different code words—one for each amino acid. 9.1 Genetic Material 5

  13. The Genetic Code: Using Information 9.1 Genetic Material (cont.) • Three nucleotides are grouped at a time allowing 64 triplet combinations, such as CTG, TAC, and ACA. Each nucleotide triplet in DNA directs a particular triplet to be formed in mRNA during transcription. In translation, a second base-pairing step is essential for reading the genetic code. 9.1 Genetic Material 6

  14. A molecule of transfer RNA (tRNA) with a specific amino acid attached reads each codon of a messenger RNA (mRNA) during protein synthesis (translation). The Genetic Code: Using Information 9.1 Genetic Material (cont.) • A triplet in mRNA, called a codon, pairs with a triplet on a tRNA molecule, called an anticodon,carrying the correct amino acid. 9.1 Genetic Material 7

  15. The genetic code is written in nucleotide triplets, or codons, in a strand of mRNA. Each triplet codon specifies an amino acid. For example, UGG codes for the amino acid tryptophan. Several amino acids have more than one codon. Some triplets are “punctuation” telling the system to start or stop translation. 9.1 Genetic Material 8

  16. The feathers responsible for the appearance of this Raggiana bird of paradise, Poradisaea raggiana, are composed mostly of the protein keratin. The Genetic Code: Using Information 9.2 Importance of Proteins • Many proteins, such as keratin, collagen, and myosin, serve as the material that makes up cell structures or tissues. 9.2 Importance of Proteins 1

  17. The Genetic Code: Using Information 9.2 Importance of Proteins (cont.) • Some proteins are enzymes, essential catalysts that make the chemical reactions of living systems happen fast enough to be useful. Proteins, such as hemoglobin, bind to specific molecules. 9.2 Importance of Proteins 2

  18. The Genetic Code: Using Information 9.2 Importance of Proteins (cont.) • Protein hormones, such as insulin, play a key role in communication within an organism. Hormones are chemical signals given off by cells in one part of an organism that regulate behavior of cells in another part of the organism. 9.2 Importance of Proteins 3

  19. A scanning electron micrograph of human pancreatic connective tissue (collagen), x39,000. The Genetic Code: Using Information 9.2 Importance of Proteins (cont.) • A protein’s structure determines its function, and information expressed from the code in DNA determines the structures of proteins. Collagen exists as long fibers that bind cells together in tissues. Many enzymes, such as lysozyme, have cavities or pockets that bind only specific substrate molecules. 9.2 Importance of Proteins 4

  20. End of Section 1

  21. Transcription 9.3 RNA Synthesis • Gene expression begins with RNA synthesis—when the transcription enzyme RNA polymerase joins RNA nucleotides according to the base sequence in DNA. Prokaryotes have one type of RNA polymerase. Eukaryotes have three RNA polymerases, each responsible for making different types of RNA. 9.3 RNA Synthesis 1

  22. Transcription 9.3 RNA Synthesis (cont.) • In eukaryotes, protein synthesis takes place outside the nucleus; however, mRNA, tRNA, and rRNA are built in the nucleus. During protein synthesis, two ribosomal subunits bind to each other and an mRNA to form an intact ribosome. 9.3 RNA Synthesis 2

  23. Each type of RNA carries out a different function in protein synthesis. This figure uses a linear symbol for mRNA to emphasize that its sequence corresponds to the linear sequence of amino acids in a protein. In reality, the mRNA is folded and twisted rather than being straight or rigid. 9.3 RNA Synthesis 3

  24. Each DNA nucleotide pairs with a particular RNA nucleotide. This pairing is the basis of the genetic code. Note that in RNA, uracil (U) replaces the thymine (T) of DNA. Transcription 9.3 RNA Synthesis (cont.) • Only one strand of the DNA, the coding or template strand, directs the synthesis of RNA. 9.3 RNA Synthesis 4

  25. Transcription 9.3 RNA Synthesis (cont.) • Transcription takes place in three stages: 1. Initiation—the enzyme RNA polymerase attaches to a specific region of the DNA 2. Elongation of the RNA—RNA polymerase partially unwinds the DNA, exposing the coding strand of the gene 3. Termination—RNA polymerase reaches the terminator region, or the end of the DNA to be transcribed and the enzyme and primary transcript are released from the DNA 9.3 RNA Synthesis 5

  26. The three stages in transcription of RNA from a DNA template Click the image to view an animated version. 9.3 RNA Synthesis 6

  27. A transmission electron micrograph of an unidentified operon of the bacterium Escherichia coli, x72,600. Ribosomes attach to mRNA, and protein synthesis begins even before transcription is complete. Transcription 9.4 RNA Processing • In prokaryotes, new mRNA is translated and broken down by enzymes within a few minutes. In eukaryotes, mRNA can last from minutes to days, depending partly on how the primary transcript is processed. 9.4 RNA Processing 1

  28. Transcription 9.4 RNA Processing (cont.) • All three types of RNA are processed in the nucleus of eukaryotes before they leave the nucleus. Enzymes add additional nucleotides and chemically modify or remove others. 9.4 RNA Processing 2

  29. Transcription 9.4 RNA Processing (cont.) • Enzymes attach a cap of chemically modified guanine nucleotides (methyl-guanine, or mG) to the starting end of the mRNA molecule. 9.4 RNA Processing 3

  30. Transcription 9.4 RNA Processing (cont.) • Other enzymes then replace part of the opposite end with a tail of 100–200 adenine nucleotides called a poly-A tail. 9.4 RNA Processing 4

  31. Transcription 9.4 RNA Processing (cont.) • The final step in mRNA processing involves removal of some internal segments of the RNA that do not code for protein called introns. The parts of the transcript that remain (and code for protein) are called exons. 9.4 RNA Processing 5

  32. Transcription 9.4 RNA Processing (cont.) • The process of removing introns and rejoining cut ends is called splicing. If introns are left in RNA, the consequences can be serious. 9.4 RNA Processing 6

  33. Mature tRNA resembles a cloverleaf (a), with the amino-acid binding site at the end of a stem and the anticodon at the loop on the opposite end. Base pairing between parallel parts of the tRNA molecule stabilizes the cloverleaf shape. The three-dimensional structure of the molecule is roughly L-shaped (b). Transcription 9.4 RNA Processing (cont.) • An important step in the processing of tRNA is the chemical modification of several nucleotides and folding into a cloverleaf shape. 9.4 RNA Processing 7

  34. Transcription 9.4 RNA Processing (cont.) • Ribosomal RNA is not involved in coding. The primary rRNA transcript is spliced and modified to produce mature rRNA molecules. 9.4 RNA Processing 8

  35. End of Section 2

  36. Protein Synthesis 9.5 Translation • On ribosomes, protein synthesis translates the codon sequence of mRNA into the amino-acid sequence of a protein. tRNA anticodons pair with the mRNA codons that encodes a particular amino acid. Attachment of the correct amino acid to its tRNA molecule is called tRNA charging. A molecule of ATP provides the energy to form this bond. 9.5 Translation 1

  37. Protein Synthesis 9.5 Translation (cont.) • Charged tRNA, mRNA, and the growing polypeptide chain come together at specific binding sites on a ribosome. At these sites, tRNA anticodons base-pair with mRNA codons, positioning the amino acids they carry so that they can bond to the growing polypeptide chain. 9.5 Translation 2

  38. Protein Synthesis 9.5 Translation (cont.) • One of the binding sites, the P site, holds the tRNA carrying the growing polypeptide chain. The A site holds the tRNA carrying the next amino acid to be added to the chain. Next to the P site is the exit site, or E site. An uncharged tRNA leaves the E site after its amino acid is added to the growing chain. 9.5 Translation 3

  39. A charged tRNA sits in the A site of the ribosome, bound to the correct mRNA codon by base pairing. A second tRNA, carrying a growing polypeptide, is in the P site, bound to the previous mRNA codon. The E site is not shown. A groove between the large and small subunits of the ribosome accommodates mRNA and the growing polypeptide chain. Protein Synthesis 9.5 Translation (cont.) 9.5 Translation 4

  40. Protein Synthesis 9.5 Translation (cont.) • Translation involves initiation, elongation, and termination, the same three stages as transcription. Initiation and elongation require energy supplied by GTP (guanosine triphosphate), a molecule closely related to ATP. 9.5 Translation 5

  41. Protein Synthesis 9.5 Translation (cont.) • During initiation of translation, the ribosome attaches at a specific site on the mRNA. 9.5 Translation 6

  42. Protein Synthesis 9.5 Translation (cont.) • During elongation, peptide bonds join each amino acid with the next in the sequence. A charged tRNA whose anticodon matches the next codon on the message enters the A site of the ribosome. 9.5 Translation 7

  43. Protein Synthesis 9.5 Translation (cont.) • This positions the amino acid it carries to form a peptide bond with the amino acid attached to the tRNA at the P site. 9.5 Translation 8

  44. Protein Synthesis 9.5 Translation (cont.) • When the bond forms, the polypeptide chain transfers to the tRNA at the A site 9.5 Translation 9

  45. Protein Synthesis 9.5 Translation (cont.) • The entire ribosome moves down the mRNA to position the next codon at the A site and the uncharged tRNA leaves the E site. The A site is now open and available for the next matching tRNA to bring in an amino acid. 9.5 Translation 10

  46. Protein Synthesis 9.5 Translation (cont.) • Translation terminates when a stop codon reaches the A site of the ribosome. A special protein known as a release factor binds to the stop codon in the A site. At this point, the ribosome lets go of the mRNA, the tRNA, and the release factor. 9.5 Translation 11

  47. Protein Synthesis 9.5 Translation (cont.) Transcription produces mRNA, tRNA, and rRNA. All three participate in translation. 9.5 Translation 12

  48. Protein Synthesis 9.6 Transport and Modification of Proteins • Many proteins must be chemically modified and folded into an active tertiary structure to be functional. Helper, or “chaperone,” proteins often help stabilize the polypeptide as it is folded. After translation, the protein must be transported to where it will function. 9.6 Transport and Modification of Proteins 1

  49. Protein Synthesis 9.6 Transport and Modification of Proteins (cont.) • Transport can start while the protein is still being translated. The process uses a signal that is part of the protein sequence, called the signal sequence. When translation is complete, the new protein is released from the ribosome into the inner ER. Proteins to be released from the cell pass from the ER to the vesicles of the Golgi apparatus. 9.6 Transport and Modification of Proteins 2

  50. Synthesis of proteins for secretion or insertion in a membrane Click the image to view an animated version. 9.6 Transport and Modification of Proteins 3

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