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Chapter 10 Introduction Molecular Biology of the Gene Viruses infect organisms by – binding to receptors on a host’s target cell, – injecting viral genetic material into the cell, and – hijacking the cell’s own molecules and organelles to produce new copies of the virus. The host cell is destroyed, and newly replicated viruses are released to continue the infection. PowerPoint Lectures for Campbell Biology: Concepts & Connections, Seventh Edition Reece, Taylor, Simon, and Dickey © 2012 Pearson Education, Inc. Lecture by Edward J. Zalisko © 2012 Pearson Education, Inc. Introduction Figure 10.0_1 Chapter 10: Big Ideas Viruses are not generally considered alive because they – are not cellular and The Structure of the Genetic Material DNA Replication The Flow of Genetic Information from DNA to RNA to Protein The Genetics of Viruses and Bacteria – cannot reproduce on their own. Because viruses have much less complex structures than cells, they are relatively easy to study at the molecular level. For this reason, viruses are used to study the functions of DNA. © 2012 Pearson Education, Inc. Figure 10.0_2 THE STRUCTURE OF THE GENETIC MATERIAL © 2012 Pearson Education, Inc. 1 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material Until the 1940s, the case for proteins serving as the genetic material was stronger than the case for DNA. In 1928, Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell. He found that – Proteins are made from 20 different amino acids. – DNA was known to be made from just four kinds of nucleotides. – when he exposed heat-killed pathogenic bacteria to harmless bacteria, some harmless bacteria were converted to disease-causing bacteria and – the disease-causing characteristic was inherited by descendants of the transformed cells. Studies of bacteria and viruses – ushered in the field of molecular biology, the study of heredity at the molecular level, and – revealed the role of DNA in heredity. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material In 1952, Alfred Hershey and Martha Chase used bacteriophages to show that DNA is the genetic material of T2, a virus that infects the bacterium Escherichia coli (E. coli). – Bacteriophages (or phages for short) are viruses that infect bacterial cells. – The sulfur-labeled protein stayed with the phages outside the bacterial cell, while the phosphorus-labeled DNA was detected inside cells. – Cells with phosphorus-labeled DNA produced new bacteriophages with radioactivity in DNA but not in protein. – Phages were labeled with radioactive sulfur to detect proteins or radioactive phosphorus to detect DNA. – Bacteria were infected with either type of labeled phage to determine which substance was injected into cells and which remained outside the infected cell. Animation: Hershey-Chase Experiment Animation: Phage T2 Reproductive Cycle © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.1A Figure 10.1A_1 Head DNA Head Tail Tail fiber Tail Tail fiber 2 Figure 10.1B Figure 10.1B_1 Phage Bacterium The radioactivity is in the liquid. Phage DNA DNA Batch 1: Radioactive protein labeled in yellow Bacterium Empty protein shell Radioactive protein Phage Empty protein shell Radioactive protein Centrifuge Phage DNA DNA Batch 1: Radioactive protein labeled in yellow Pellet 1 3 2 2 1 4 Radioactive DNA Batch 2: Radioactive DNA labeled in green Centrifuge Pellet The radioactivity is in the pellet. Figure 10.1B_2 Radioactive DNA Batch 2: Radioactive DNA labeled in green Figure 10.1C Empty protein shell The radioactivity is in the liquid. Phage DNA Centrifuge Pellet 3 1 A phage attaches 4 itself to a bacterial cell. 2 The phage injects 3 The phage DNA directs its DNA into the bacterium. the host cell to make more phage DNA and proteins; new phages assemble. 4 The cell lyses and releases the new phages. Centrifuge Pellet The radioactivity is in the pellet. Figure 10.1C_1 Figure 10.1C_2 3 1 A phage attaches itself to a bacterial cell. 2 The phage injects its DNA into the bacterium. The phage DNA directs the host cell to make more phage DNA and proteins; new phages assemble. 3 4 The cell lyses and releases the new phages. 3 10.2 DNA and RNA are polymers of nucleotides 10.2 DNA and RNA are polymers of nucleotides DNA and RNA are nucleic acids. Each type of DNA nucleotide has a different nitrogen-containing base: One of the two strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain). A nucleotide is composed of a – adenine (A), – cytosine (C), – nitrogenous base, – thymine (T), and – five-carbon sugar, and – guanine (G). – phosphate group. The nucleotides are joined to one another by a sugar-phosphate backbone. Animation: DNA and RNA Structure © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.2A Figure 10.2A_1 T A C T A C Sugar-phosphate backbone G T A C A C A C Covalent bond joining nucleotides T A G A A G T G C T Phosphate group A C G T T G A Nitrogenous base C Nitrogenous base (can be A, G, C, or T) G T A Sugar C G C G G T A A DNA double helix DNA nucleotide T Thymine (T) T A Phosphate group G G G G T T T C G A A Sugar (deoxyribose) C DNA nucleotide G T A A DNA double helix Two representations of a DNA polynucleotide Figure 10.2A_2 Figure 10.2A_3 Sugar-phosphate backbone A A Covalent bond joining nucleotides C DNA nucleotide T Phosphate group Nitrogenous base (can be A, G, C, or T) Nitrogenous base Sugar C Thymine (T) T Phosphate group G G G G Sugar (deoxyribose) DNA nucleotide Two representations of a DNA polynucleotide 4 Figure 10.2B Figure 10.2B_1 Thymine (T) Cytosine (C) Guanine (G) Adenine (A) Pyrimidines Purines Thymine (T) Cytosine (C) Pyrimidines Figure 10.2B_2 10.2 DNA and RNA are Polymers of Nucleotides RNA (ribonucleic acid) is unlike DNA in that it – uses the sugar ribose (instead of deoxyribose in DNA) and – RNA has the nitrogenous base uracil (U) instead of thymine. Guanine (G) Adenine (A) Purines © 2012 Pearson Education, Inc. Figure 10.2C Figure 10.2D Nitrogenous base (can be A, G, C, or U) Phosphate group Cytosine Uracil Adenine Guanine Uracil (U) Ribose Phosphate Sugar (ribose) 5 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix Figure 10.3A In 1952, after the Hershey-Chase experiment demonstrated that the genetic material was most likely DNA, a race was on to – describe the structure of DNA and – explain how the structure and properties of DNA can account for its role in heredity. © 2012 Pearson Education, Inc. Figure 10.3A_1 Figure 10.3A_2 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix In 1953, James D. Watson and Francis Crick deduced the secondary structure of DNA, using Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double helix. – X-ray crystallography data of DNA from the work of Rosalind Franklin and Maurice Wilkins and – Chargaff’s observation that in DNA, – the amount of adenine was equal to the amount of thymine and – the amount of guanine was equal to that of cytosine. – The sugar-phosphate backbone is on the outside. – The nitrogenous bases are perpendicular to the backbone in the interior. – Specific pairs of bases give the helix a uniform shape. – A pairs with T, forming two hydrogen bonds, and – G pairs with C, forming three hydrogen bonds. Animation: DNA Double Helix © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 6 Figure 10.3B Figure 10.3C Twist Figure 10.3D Figure 10.3D_1 C C G Hydrogen bond G C C T Base pair G G Base pair A A T C G A T T C G C Computer model C G A Partial chemical structure Ribbon model A G A T T T A Ribbon model Figure 10.3D_2 Figure 10.3D_3 Hydrogen bond G T C A A C T G Partial chemical structure Computer model 7 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix In 1962, the Nobel Prize was awarded to DNA REPLICATION – James D. Watson, Francis Crick, and Maurice Wilkins. – Rosalind Franklin probably would have received the prize as well but for her death from cancer in 1958. Nobel Prizes are never awarded posthumously. The Watson-Crick model gave new meaning to the words genes and chromosomes. The genetic information in a chromosome is encoded in the nucleotide sequence of DNA. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 10.4 DNA replication depends on specific base pairing In their description of the structure of DNA, Watson and Crick noted that the structure of DNA suggests a possible copying mechanism. DNA replication follows a semiconservative model. – The two DNA strands separate. – Each strand is used as a pattern to produce a complementary strand, using specific base pairing. Figure 10.4A_s1 A T C G G C A T T A A parental molecule of DNA – Each new DNA helix has one old strand with one new strand. Animation: DNA Replication Overview © 2012 Pearson Education, Inc. Figure 10.4A_s2 Figure 10.4A_s3 A T A C G C G C A T T A T A parental molecule of DNA T A T A T A T G C G C G C G C A T A T A T A T A T A G C G C G C G C G C A T A T A T T A T A T G C Free nucleotides The parental strands separate and serve as templates A A parental molecule of DNA G C Free nucleotides The parental strands separate and serve as templates A T G Two identical daughter molecules of DNA are formed 8 Figure 10.4B A G A A T 10.5 DNA replication proceeds in two directions at many sites simultaneously T C T T A Parental DNA molecule DNA replication begins at the origins of replication where – DNA unwinds at the origin to produce a “bubble,” Daughter strand Parental strand – replication proceeds in both directions from the origin, and – replication ends when products from the bubbles merge with each other. Daughter DNA molecules © 2012 Pearson Education, Inc. 10.5 DNA replication proceeds in two directions at many sites simultaneously 10.5 DNA replication proceeds in two directions at many sites simultaneously DNA replication occurs in the 5 to 3 direction. Two key proteins are involved in DNA replication. – Replication is continuous on the 3 to 5 template. 1. DNA ligase joins small fragments into a continuous chain. – Replication is discontinuous on the 5 to 3 template, forming short segments. 2. DNA polymerase – adds nucleotides to a growing chain and – proofreads and corrects improper base pairings. Animation: Origins of Replication Animation: Leading Strand Animation: Lagging Strand Animation: DNA Replication Review © 2012 Pearson Education, Inc. 10.5 DNA replication proceeds in two directions at many sites simultaneously © 2012 Pearson Education, Inc. Figure 10.5A Parental DNA molecule DNA polymerases and DNA ligase also repair DNA damaged by harmful radiation and toxic chemicals. DNA replication ensures that all the somatic cells in a multicellular organism carry the same genetic information. Origin of replication Parental strand Daughter strand “Bubble” Two daughter DNA molecules © 2012 Pearson Education, Inc. 9 Figure 10.5B Figure 10.5C P HO 5 4 3 2 1 T 3 4 1 5 C G G C P Parental DNA Replication fork 3 5 This daughter strand is synthesized continuously This daughter strand is 3 synthesized 5 in pieces P P P 5 3 P T 3 end 5 3 2 A P OH DNA polymerase molecule 3 end 5 end A DNA ligase P 5 end THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN Overall direction of replication 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits DNA specifies traits by dictating protein synthesis. The molecular chain of command is from – DNA in the nucleus to RNA and – RNA in the cytoplasm to protein. Transcription is the synthesis of RNA under the direction of DNA. Translation is the synthesis of proteins under the direction of RNA. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.6A_s1 Figure 10.6A_s2 DNA DNA Transcription RNA NUCLEUS NUCLEUS CYTOPLASM CYTOPLASM 10 Figure 10.6A_s3 10.6 The DNA genotype is expressed as proteins, which provide the molecular basis for phenotypic traits DNA The connections between genes and proteins Transcription – The initial one gene–one enzyme hypothesis was based on studies of inherited metabolic diseases. RNA NUCLEUS Translation – The one gene–one enzyme hypothesis was expanded to include all proteins. CYTOPLASM Protein – Most recently, the one gene–one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides. © 2012 Pearson Education, Inc. Figure 10.6B 10.7 Genetic information written in codons is translated into amino acid sequences The sequence of nucleotides in DNA provides a code for constructing a protein. – Protein construction requires a conversion of a nucleotide sequence to an amino acid sequence. – Transcription rewrites the DNA code into RNA, using the same nucleotide “language.” © 2012 Pearson Education, Inc. 10.7 Genetic information written in codons is translated into amino acid sequences Figure 10.7 DNA molecule Gene 1 – The flow of information from gene to protein is based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DNA and RNA as a series of nonoverlapping threebase “words” called codons. – Translation involves switching from the nucleotide “language” to the amino acid “language.” – Each amino acid is specified by a codon. – 64 codons are possible. – Some amino acids have more than one possible codon. Gene 2 Gene 3 DNA A A A C C G G C A A A A Transcription RNA Translation U U U G G C C G U U U U Codon Polypeptide Amino acid © 2012 Pearson Education, Inc. 11 Figure 10.7_1 10.8 The genetic code dictates how codons are translated into amino acids DNA A A A C U U U C G G C A A A A C G U U U Characteristics of the genetic code Transcription RNA Translation – Three nucleotides specify one amino acid. G G C – 61 codons correspond to amino acids. U – AUG codes for methionine and signals the start of transcription. Codon – 3 “stop” codons signal the end of translation. Polypeptide Amino acid © 2012 Pearson Education, Inc. Figure 10.8A 10.8 The genetic code dictates how codons are translated into amino acids Second base The genetic code is First base Third base – redundant, with more than one codon for some amino acids, – unambiguous in that any codon for one amino acid does not code for any other amino acid, – nearly universal—the genetic code is shared by organisms from the simplest bacteria to the most complex plants and animals, and – without punctuation in that codons are adjacent to each other with no gaps in between. © 2012 Pearson Education, Inc. Figure 10.8B_s1 Figure 10.8B_s2 Strand to be transcribed T A C T T Strand to be transcribed C A A A A T T A C T C DNA T C A A A A T C DNA A T G A A G T T T A T G A A G T T A G T T T A G Transcription RNA A U G A A G U U U U A G 12 Figure 10.8B_s3 Figure 10.8C Strand to be transcribed T A C T T C A A A A T C DNA A T G A A G T T T T A G Transcription RNA A U G A A G U U U U A G Translation Start codon Polypeptide Met Stop codon Lys Phe 10.9 Transcription produces genetic messages in the form of RNA Overview of transcription – An RNA molecule is transcribed from a DNA template by a process that resembles the synthesis of a DNA strand during DNA replication. – RNA nucleotides are linked by the transcription enzyme RNA polymerase. – Specific sequences of nucleotides along the DNA mark where transcription begins and ends. – The “start transcribing” signal is a nucleotide sequence called a promoter. 10.9 Transcription produces genetic messages in the form of RNA – Transcription begins with initiation, as the RNA polymerase attaches to the promoter. – During the second phase, elongation, the RNA grows longer. – As the RNA peels away, the DNA strands rejoin. – Finally, in the third phase, termination, the RNA polymerase reaches a sequence of bases in the DNA template called a terminator, which signals the end of the gene. – The polymerase molecule now detaches from the RNA molecule and the gene. Animation: Transcription © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.9A Figure 10.9B RNA polymerase Free RNA nucleotides RNA polymerase DNA of gene Terminator DNA Promoter DNA 1 Initiation 2 Elongation Area shown in Figure 10.9A 3 Termination Growing RNA C C A A A U C C A T A G G T Direction of transcription Newly made RNA T Template strand of DNA Completed RNA RNA polymerase 13 Figure 10.9B_1 Figure 10.9B_2 RNA polymerase Terminator DNA DNA of gene 2 Promoter DNA 1 Elongation Area shown in Figure 10.9A Initiation Growing RNA Figure 10.9B_3 10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA Messenger RNA (mRNA) – encodes amino acid sequences and 3 Termination Growing RNA – conveys genetic messages from DNA to the translation machinery of the cell, which in – prokaryotes, occurs in the same place that mRNA is made, but in – eukaryotes, mRNA must exit the nucleus via nuclear pores to enter the cytoplasm. – Eukaryotic mRNA has Completed RNA – introns, interrupting sequences that separate RNA polymerase – exons, the coding regions. © 2012 Pearson Education, Inc. 10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA Figure 10.10 Exon Intron Cap Eukaryotic mRNA undergoes processing before leaving the nucleus. – RNA splicing removes introns and joins exons to produce a continuous coding sequence. – A cap and tail of extra nucleotides are added to the ends of the mRNA to – facilitate the export of the mRNA from the nucleus, – protect the mRNA from attack by cellular enzymes, and Exon Intron Exon DNA RNA transcript with cap and tail Transcription Addition of cap and tail Introns removed Tail Exons spliced together mRNA Coding sequence NUCLEUS – help ribosomes bind to the mRNA. CYTOPLASM © 2012 Pearson Education, Inc. 14 10.11 Transfer RNA molecules serve as interpreters during translation Figure 10.11A Amino acid attachment site Transfer RNA (tRNA) molecules function as a language interpreter, – converting the genetic message of mRNA Hydrogen bond – into the language of proteins. Transfer RNA molecules perform this interpreter task by RNA polynucleotide chain – picking up the appropriate amino acid and – using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mRNA. Anticodon A tRNA molecule, showing its polynucleotide strand and hydrogen bonding A simplified schematic of a tRNA © 2012 Pearson Education, Inc. Figure 10.11B 10.12 Ribosomes build polypeptides Enzyme tRNA Translation occurs on the surface of the ribosome. ATP – Ribosomes coordinate the functioning of mRNA and tRNA and, ultimately, the synthesis of polypeptides. – Ribosomes have two subunits: small and large. – Each subunit is composed of ribosomal RNAs and proteins. – Ribosomal subunits come together during translation. – Ribosomes have binding sites for mRNA and tRNAs. © 2012 Pearson Education, Inc. Figure 10.12A Figure 10.12B Growing polypeptide tRNA molecules tRNA binding sites Large subunit Small subunit Large subunit P A site site Small subunit mRNA binding site mRNA 15 Figure 10.12C 10.13 An initiation codon marks the start of an mRNA message Translation can be divided into the same three phases as transcription: The next amino acid to be added to the polypeptide Growing polypeptide 1. initiation, mRNA tRNA 2. elongation, and 3. termination. Initiation brings together Codons – mRNA, – a tRNA bearing the first amino acid, and – the two subunits of a ribosome. © 2012 Pearson Education, Inc. 10.13 An initiation codon marks the start of an mRNA message Initiation establishes where translation will begin. Figure 10.13A Start of genetic message Cap Initiation occurs in two steps. 1. An mRNA molecule binds to a small ribosomal subunit and the first tRNA binds to mRNA at the start codon. – The start codon reads AUG and codes for methionine. – The first tRNA has the anticodon UAC. End 2. A large ribosomal subunit joins the small subunit, allowing the ribosome to function. Tail – The first tRNA occupies the P site, which will hold the growing peptide chain. – The A site is available to receive the next tRNA. © 2012 Pearson Education, Inc. Figure 10.13B 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Large ribosomal subunit Initiator tRNA P site mRNA U A C A U G A site U A C A U G Once initiation is complete, amino acids are added one by one to the first amino acid. Elongation is the addition of amino acids to the polypeptide chain. Start codon 1 Small ribosomal subunit 2 © 2012 Pearson Education, Inc. 16 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Each cycle of elongation has three steps. Elongation continues until the termination stage of translation, when 1. Codon recognition: The anticodon of an incoming tRNA molecule, carrying its amino acid, pairs with the mRNA codon in the A site of the ribosome. 2. Peptide bond formation: The new amino acid is joined to the chain. – the ribosome reaches a stop codon, – the completed polypeptide is freed from the last tRNA, and – the ribosome splits back into its separate subunits. 3. Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site into the P site. Animation: Translation © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.14_s1 Figure 10.14_s2 Polypeptide P site mRNA Amino acid A site Polypeptide P site Anticodon Codons 1 mRNA Amino acid A site Anticodon Codons 1 Codon recognition Codon recognition 2 Figure 10.14_s3 Peptide bond formation Figure 10.14_s4 Polypeptide P site mRNA Amino acid A site Polypeptide P site Anticodon Codons 1 mRNA Amino acid A site Anticodon Codons 1 Codon recognition Codon recognition mRNA movement Stop codon 2 New peptide bond 3 Translocation Peptide bond formation 2 New peptide bond 3 Peptide bond formation Translocation 17 10.15 Review: The flow of genetic information in the cell is DNA RNA protein 10.15 Review: The flow of genetic information in the cell is DNA RNA protein Transcription is the synthesis of RNA from a DNA template. In eukaryotic cells, Translation can be divided into four steps, all of which occur in the cytoplasm: – transcription occurs in the nucleus and 1. amino acid attachment, – the mRNA must travel from the nucleus to the cytoplasm. 2. initiation of polypeptide synthesis, 3. elongation, and 4. termination. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.15 Figure 10.15_1 Transcription DNA 1 mRNA Transcription RNA polymerase CYTOPLASM Translation Amino acid 2 Amino acid attachment Enzyme Transcription tRNA ATP DNA Anticodon Initiator tRNA Large ribosomal subunit Start Codon 3 Initiation of polypeptide synthesis Small ribosomal subunit mRNA 1 mRNA New peptide bond forming Growing polypeptide 4 Elongation 5 Termination Transcription RNA polymerase Codons mRNA Polypeptide Stop codon Figure 10.15_2 Figure 10.15_3 CYTOPLASM Translation Amino acid Amino acid attachment 2 New peptide bond forming Growing polypeptide Enzyme 4 Elongation tRNA ATP Codons mRNA Anticodon Initiator tRNA Large ribosomal subunit Start Codon mRNA Initiation of polypeptide synthesis Polypeptide 2 3 5 Small ribosomal subunit Termination Stop codon 18 10.16 Mutations can change the meaning of genes 10.16 Mutations can change the meaning of genes A mutation is any change in the nucleotide sequence of DNA. Mutations within a gene can be divided into two general categories. Mutations can involve 1. Base substitutions involve the replacement of one nucleotide with another. Base substitutions may – large chromosomal regions or – have no effect at all, producing a silent mutation, – just a single nucleotide pair. – change the amino acid coding, producing a missense mutation, which produces a different amino acid, – lead to a base substitution that produces an improved protein that enhances the success of the mutant organism and its descendant, or – change an amino acid into a stop codon, producing a nonsense mutation. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 10.16 Mutations can change the meaning of genes 10.16 Mutations can change the meaning of genes 2. Mutations can result in deletions or insertions that may – alter the reading frame (triplet grouping) of the mRNA, so that nucleotides are grouped into different codons, Mutagenesis is the production of mutations. Mutations can be caused by – lead to significant changes in amino acid sequence downstream of the mutation, and – spontaneous errors that occur during DNA replication or recombination or – produce a nonfunctional polypeptide. – mutagens, which include – high-energy radiation such as X-rays and ultraviolet light and – chemicals. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. Figure 10.16A Figure 10.16B Normal gene mRNA Protein Normal hemoglobin DNA C T A U G A Met Lys U U G G C G C Phe Gly Ala A Mutant hemoglobin DNA C A T T Nucleotide substitution A U G A Met mRNA A G U U A G C A G U U Lys Phe Ser G C A Ala mRNA U Deleted G A A G U A Normal hemoglobin Sickle-cell hemoglobin Val Glu Nucleotide deletion A U G A Met A G U Lys U G G C G Ala Leu C A U His Inserted Nucleotide insertion A U G A Met A G Lys U U G Leu U G G C G C Ala His 19 10.17 Viral DNA may become part of the host chromosome THE GENETICS OF VIRUSES AND BACTERIA A virus is essentially “genes in a box,” an infectious particle consisting of – a bit of nucleic acid, – wrapped in a protein coat called a capsid, and – in some cases, a membrane envelope. Viruses have two types of reproductive cycles. 1. In the lytic cycle, – viral particles are produced using host cell components, – the host cell lyses, and – viruses are released. © 2012 Pearson Education, Inc. © 2012 Pearson Education, Inc. 10.17 Viral DNA may become part of the host chromosome Figure 10.17_s1 Phage Attaches to cell Phage DNA 2. In the Lysogenic cycle – Viral DNA is inserted into the host chromosome by recombination. 4 The cell lyses, releasing phages 1 – Viral DNA is duplicated along with the host chromosome during each cell division. Bacterial chromosome The phage injects its DNA – The inserted phage DNA is called a prophage. Lytic cycle – Most prophage genes are inactive. Phages assemble 2 – Environmental signals can cause a switch to the lytic cycle, causing the viral DNA to be excised from the bacterial chromosome and leading to the death of the host cell. 3 Animation: Phage Lambda Lysogenic and Lytic Cycles The phage DNA circularizes New phage DNA and proteins are synthesized Animation: Phage T4 Lytic Cycle © 2012 Pearson Education, Inc. Figure 10.17_s2 Figure 10.17_1 Phage Attaches to cell Phage DNA Phage Attaches to cell Phage DNA 4 The cell lyses, releasing phages 1 Bacterial chromosome The cell lyses, releasing phages Lytic cycle Environmental stress The phage injects its DNA Many cell divisions Lysogenic cycle 2 The phage DNA circularizes Prophage 6 The lysogenic bacterium replicates normally Lytic cycle Phages assemble 2 OR 3 1 Bacterial chromosome The phage injects its DNA 7 Phages assemble 4 New phage DNA and proteins are synthesized 5 The phage DNA circularizes Phage DNA inserts into the bacterial chromosome by recombination 3 New phage DNA and proteins are synthesized 20 Figure 10.17_2 10.18 CONNECTION: Many viruses cause disease in animals and plants Phage Attaches to cell Bacterial chromosome Phage DNA 1 Viruses can cause disease in animals and plants. The phage injects its DNA Environmental stress 7 Many cell divisions Lysogenic cycle 2 The phage DNA circularizes 6 Prophage The lysogenic bacterium replicates normally, copying the prophage at each cell division Phage DNA inserts into the bacterial chromosome by recombination 5 DNA viruses and RNA viruses cause disease in animals. A typical animal virus has a membranous outer envelope and projecting spikes of glycoprotein. The envelope helps the virus enter and leave the host cell. Many animal viruses have RNA rather than DNA as their genetic material. These include viruses that cause the common cold, measles, mumps, polio, and AIDS. © 2012 Pearson Education, Inc. 10.18 CONNECTION: Many viruses cause disease in animals and plants 10.18 CONNECTION: Many viruses cause disease in animals and plants The reproductive cycle of the mumps virus, a typical enveloped RNA virus, has seven major steps: Some animal viruses, such as herpesviruses, reproduce in the cell nucleus. 1. 2. 3. 4. 5. entry of the protein-coated RNA into the cell, uncoating—the removal of the protein coat, RNA synthesis—mRNA synthesis using a viral enzyme, protein synthesis—mRNA is used to make viral proteins, new viral genome production—mRNA is used as a template to synthesize new viral genomes, 6. assembly—the new coat proteins assemble around the new viral RNA, and 7. exit—the viruses leave the cell by cloaking themselves in the host cell’s plasma membrane. © 2012 Pearson Education, Inc. Most plant viruses are RNA viruses. – To infect a plant, they must get past the outer protective layer of the plant. – Viruses spread from cell to cell through plasmodesmata. – Infection can spread to other plants by insects, herbivores, humans, or farming tools. There are no cures for most viral diseases of plants or animals. Animation: Simplified Viral Reproductive Cycle © 2012 Pearson Education, Inc. Figure 10.18 Figure 10.18_1 Glycoprotein spike Protein coat Plasma membrane of host cell 1 Entry 2 Uncoating 3 RNA synthesis by viral enzyme Protein coat CYTOPLASM Protein synthesis 5 mRNA RNA synthesis (other strand) Template Plasma membrane of host cell New viral genome New viral proteins Membranous envelope Viral RNA (genome) Viral RNA (genome) 4 Glycoprotein spike Membranous envelope Viral RNA (genome) 6 Assembly CYTOPLASM 1 Entry 2 Uncoating 3 RNA synthesis by viral enzyme Viral RNA (genome) Exit 7 21 Figure 10.18_2 4 Protein synthesis 5 mRNA RNA synthesis (other strand) Template New viral genome New viral proteins 6 Assembly 10.19 EVOLUTION CONNECTION: Emerging viruses threaten human health Viruses that appear suddenly or are new to medical scientists are called emerging viruses. These include the – AIDS virus, – Ebola virus, – West Nile virus, and – SARS virus. Exit 7 © 2012 Pearson Education, Inc. 10.19 EVOLUTION CONNECTION: Emerging viruses threaten human health Figure 10.19 Three processes contribute to the emergence of viral diseases: 1. mutation—RNA viruses mutate rapidly. 2. contact between species—viruses from other animals spread to humans. 3. spread from isolated human populations to larger human populations, often over great distances. © 2012 Pearson Education, Inc. Figure 10.19_1 Figure 10.19_2 22 10.20 The AIDS virus makes DNA on an RNA template Figure 10.20A AIDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus). Envelope Glycoprotein HIV Protein coat RNA (two identical strands) – is an RNA virus, – has two copies of its RNA genome, Reverse transcriptase (two copies) – carries molecules of reverse transcriptase, which causes reverse transcription, producing DNA from an RNA template. © 2012 Pearson Education, Inc. 10.20 The AIDS virus makes DNA on an RNA template After HIV RNA is uncoated in the cytoplasm of the host cell, 1. reverse transcriptase makes one DNA strand from RNA, 2. reverse transcriptase adds a complementary DNA strand, 3. double-stranded viral DNA enters the nucleus and integrates into the chromosome, becoming a provirus, 4. the provirus DNA is used to produce mRNA, Figure 10.20B Reverse transcriptase Viral RNA 1 DNA strand CYTOPLASM NUCLEUS Chromosomal DNA 2 Doublestranded DNA 3 Provirus DNA 4 5 Viral RNA and proteins RNA 6 5. the viral mRNA is translated to produce viral proteins, and 6. new viral particles are assembled, leave the host cell, and can then infect other cells. Animation: HIV Reproductive Cycle © 2012 Pearson Education, Inc. 10.21 Viroids and prions are formidable pathogens in plants and animals 10.22 Bacteria can transfer DNA in three ways Some infectious agents are made only of RNA or protein. Viral reproduction allows researchers to learn more about the mechanisms that regulate DNA replication and gene expression in living cells. – Viroids are small, circular RNA molecules that infect plants. Viroids – replicate within host cells without producing proteins and – interfere with plant growth. – Prions are infectious proteins that cause degenerative brain diseases in animals. Prions – appear to be misfolded forms of normal brain proteins, – which convert normal protein to misfolded form. © 2012 Pearson Education, Inc. Bacteria are also valuable but for different reasons. – Bacterial DNA is found in a single, closed loop, chromosome. – Bacterial cells divide by replication of the bacterial chromosome and then by binary fission. – Because binary fission is an asexual process, bacteria in a colony are genetically identical to the parent cell. © 2012 Pearson Education, Inc. 23 10.22 Bacteria can transfer DNA in three ways Figure 10.22A Bacteria use three mechanisms to move genes from cell to cell. DNA enters cell 1. Transformation is the uptake of DNA from the surrounding environment. A fragment of DNA from another bacterial cell 2. Transduction is gene transfer by phages. 3. Conjugation is the transfer of DNA from a donor to a recipient bacterial cell through a cytoplasmic (mating) bridge. Bacterial chromosome (DNA) Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromosome. © 2012 Pearson Education, Inc. Figure 10.22B Figure 10.22C Mating bridge Phage Sex pili A fragment of DNA from another bacterial cell (former phage host) Donor cell Figure 10.22D Recipient cell 10.23 Bacterial plasmids can serve as carriers for gene transfer Donated DNA Recipient cell’s chromosome Crossovers Degraded DNA Recombinant chromosome The ability of a donor E. coli cell to carry out conjugation is usually due to a specific piece of DNA called the F factor. During conjugation, the F factor is integrated into the bacterium’s chromosome. The donor chromosome starts replicating at the F factor’s origin of replication. The growing copy of the DNA peels off and heads into the recipient cell. The F factor serves as the leading end of the transferred DNA. © 2012 Pearson Education, Inc. 24 Figure 10.23A-B Figure 10.23A F factor (integrated) F factor (integrated) F factor (plasmid) Donor Donor Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Donor Origin of F replication Bacterial chromosome Bacterial chromosome F factor starts replication and transfer F factor starts replication and transfer of chromosome Recipient cell Only part of the chromosome transfers Recombination can occur Recipient cell The plasmid completes its transfer and circularizes Only part of the chromosome transfers The cell is now a donor 10.23 Bacterial plasmids can serve as carriers for gene transfer Recombination can occur Figure 10.23B F factor (plasmid) Donor Bacterial chromosome An F factor can also exist as a plasmid, a small circular DNA molecule separate from the bacterial chromosome. F factor starts replication and transfer – Some plasmids, including the F factor, can bring about conjugation and move to another cell in linear form. – The transferred plasmid re-forms a circle in the recipient cell. The plasmid completes its transfer and circularizes R plasmids – pose serious problems for human medicine by The cell is now a donor – carrying genes for enzymes that destroy antibiotics. © 2012 Pearson Education, Inc. Figure 10.23C You should now be able to Plasmids 1. Describe the experiments of Griffith, Hershey, and Chase, which supported the idea that DNA was life’s genetic material. 2. Compare the structures of DNA and RNA. 3. Explain how the structure of DNA facilitates its replication. 4. Describe the process of DNA replication. 5. Describe the locations, reactants, and products of transcription and translation. © 2012 Pearson Education, Inc. 25 You should now be able to You should now be able to 6. Explain how the “languages” of DNA and RNA are used to produce polypeptides. 11. Describe the step-by-step process by which amino acids are added to a growing polypeptide chain. 7. Explain how mRNA is produced using DNA. 12. Diagram the overall process of transcription and translation. 8. Explain how eukaryotic RNA is processed before leaving the nucleus. 9. Relate the structure of tRNA to its functions in the process of translation. 10. Describe the structure and function of ribosomes. © 2012 Pearson Education, Inc. 13. Describe the major types of mutations, causes of mutations, and potential consequences. 14. Compare the lytic and lysogenic reproductive cycles of a phage. 15. Compare the structures and reproductive cycles of the mumps virus and a herpesvirus. © 2012 Pearson Education, Inc. Figure 10.UN01 You should now be able to Sugarphosphate backbone 16. Describe three processes that contribute to the emergence of viral disease. Nitrogenous base G Phosphate group A 17. Explain how the AIDS virus enters a host cell and reproduces. C 18. Describe the structure of viroids and prions and explain how they cause disease. T 19. Define and compare the processes of transformation, transduction, and conjugation. 20. Define a plasmid and explain why R plasmids pose serious human health problems. Sugar Nucleotide Nitrogenous bases G RNA C G A T C G A U DeoxyRibose ribose Sugar Polynucleotide DNA DNA © 2012 Pearson Education, Inc. Figure 10.UN02 Figure 10.UN03 DNA Growing polypeptide Large ribosomal subunit is a polymer made from monomers called is performed by an enzyme called (b) (a) (c) Amino acid (d) tRNA RNA comes in three kinds called Anticodon mRNA Codons Small ribosomal subunit (e) (f) (g) is performed by structures called Protein molecules are components of use amino-acid-bearing molecules called (h) one or more polymers made from monomers called (i) 26 Figure 10.1_UN Figure 10.17_UN Figure 10.18_UN 27