1.  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. Chapter 10 Molecular Biology of the Gene © 2012 Pearson Education, Inc.

2.  Viruses are not generally considered alive because they –are not cellular and –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. Introduction © 2012 Pearson Education, Inc.

3. Figure 10.1 Influenza virus

4. THE STRUCTURE OF THE GENETIC MATERIAL © 2012 Pearson Education, Inc.

5. 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. –Proteins are made from 20 different amino acids. –DNA was known to be made from just four kinds of nucleotides.  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.

6. 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material  In 1928, Frederick Griffith discovered that a “transforming factor” could be transferred into a bacterial cell. He found that –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. © 2012 Pearson Education, Inc.

7. 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. –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. © 2012 Pearson Education, Inc.

8. 10.1 SCIENTIFIC DISCOVERY: Experiments showed that DNA is the genetic material –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. © 2012 Pearson Education, Inc.

9. Figure 10.1 Head DNA Tail Tail fiber

10. Figure 10.1B The Hershey-Chase experiment Phage Bacterium Batch 2: Radioactive DNA labeled in green DNA Radioactive protein Centrifuge Phage DNA Empty protein shell Pellet The radioactivity is in the liquid. Radioactive DNA Centrifuge Pellet The radioactivity is in the pellet. 4 3 21 Batch 1: Radioactive protein labeled in yellow

11. Figure 10.1C A phage replication cycle A phage attaches itself to a bacterial cell. 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. The cell lyses and releases the new phages. 1 3 4 2

12. 10.2 DNA and RNA are polymers of nucleotides  DNA and RNA are nucleic acids.  One of the two strands of DNA is a DNA polynucleotide, a nucleotide polymer (chain).  A nucleotide is composed of a –nitrogenous base, –five-carbon sugar, and –phosphate group.  The nucleotides are joined to one another by a sugar-phosphate backbone. © 2012 Pearson Education, Inc.

13.  Each type of DNA nucleotide has a different nitrogen-containing base: –adenine (A), –cytosine (C), –thymine (T), and –guanine (G). 10.2 DNA and RNA are polymers of nucleotides © 2012 Pearson Education, Inc.

14. DNA and RNA Structure

15. Figure 10.2A The structure of a DNA polynucleotide A A A A A A A C T T T T T T C C C C G G G G G C CG A T A DNA double helix T DNA nucleotide Covalent bond joining nucleotides A C T Two representations of a DNA polynucleotide G G G G C T Phosphate group Sugar (deoxyribose) DNA nucleotide Thymine (T) Nitrogenous base (can be A, G, C, or T) Sugar Nitrogenous base Phosphate group Sugar-phosphate backbone

16. Figure 10.2B The nitrogenous bases of DNA Thymine (T) Cytosine (C) PyrimidinesPurines Adenine (A) Guanine (G)

17. 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. © 2012 Pearson Education, Inc.

18. Figure 10.2C An RNA nucleotide Phosphate group Sugar (ribose) Uracil (U) Nitrogenous base (can be A, G, C, or U)

19. 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix  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.

20. Figure 10.3A Rosalind Franklin and her X-ray image of DNA

21. 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 –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. © 2012 Pearson Education, Inc.

22.  Watson and Crick reported that DNA consisted of two polynucleotide strands wrapped into a double helix. –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. 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix © 2012 Pearson Education, Inc.

23. Figure 10.3B Watson and Crick in 1953 with their model of the DNA double helix

24. Figure 10.3C A rope ladder model for the double helix Twist

25. Figure 10.3D Three representations of DNA Base pair Hydrogen bond Partial chemical structure Computer model Ribbon model

26. 10.3 SCIENTIFIC DISCOVERY: DNA is a double-stranded helix  In 1962, the Nobel Prize was awarded to –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.

27. DNA REPLICATION © 2012 Pearson Education, Inc.

28. 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. –Each new DNA helix has one old strand with one new strand. © 2012 Pearson Education, Inc.

29. Figure 10.4A A template model for DNA replication A parental molecule of DNA A C G C A T T A The parental strands separate and serve as templates Free nucleotides T A T T A A T A G G G C C A T C G C Two identical daughter molecules of DNA are formed A T T A C G G C

30. Figure 10.4B The untwisting and replication of DNA Parental DNA molecule Daughter strand Parental strand Daughter DNA molecules A T G C A T T A T C G A T G C T C G T A C G C A T G C A T G A A

31.  DNA replication begins at the origins of replication where –DNA unwinds at the origin to produce a “bubble,” –replication proceeds in both directions from the origin, and –replication ends when products from the bubbles merge with each other. 10.5 DNA replication proceeds in two directions at many sites simultaneously © 2012 Pearson Education, Inc.

32. Figure 10.5A Multiple bubbles in replicating DNA Parental DNA molecule Origin of replication “Bubble” Parental strand Daughter strand Two daughter DNA molecules

33.  DNA replication occurs in the 5 (carbon with a phosphate) to 3 (carbon with a hydroxyl) direction. –Replication is continuous on the 3 to 5 template. –Replication is discontinuous on the 5 to 3 template, forming short segments. 10.5 DNA replication proceeds in two directions at many sites simultaneously © 2012 Pearson Education, Inc.

34. Figure 10.5B The opposite orientations of DNA strands 5 end 3 end 5 4 3 2 1 1 2 3 4 5 P P P P P HO A T C G GC P P P A T OH 5 end 3 end

35. 10.5 DNA replication proceeds in two directions at many sites simultaneously  Two key proteins are involved in DNA replication. 1.DNA ligase joins small fragments into a continuous chain. 2.DNA polymerase –adds nucleotides to a growing chain and –proofreads and corrects improper base pairings. © 2012 Pearson Education, Inc.

36. Figure 10.5C How daughter DNA strands are synthesized Overall direction of replication DNA ligase Replication fork Parental DNA DNA polymerase molecule This daughter strand is synthesized continuously This daughter strand is synthesized in pieces 3 5 3 5 3 5 3 5

37. THE FLOW OF GENETIC INFORMATION FROM DNA TO RNA TO PROTEIN © 2012 Pearson Education, Inc.

38. 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.

39. Figure 10.6A The flow of genetic information in a eukaryotic cell DNA NUCLEUS CYTOPLASM RNA Transcription Translation Protein

40. 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.

41. 10.7 Genetic information written in codons is translated into amino acid sequences –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 three- base “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. © 2012 Pearson Education, Inc.

42. Figure 10.7 Transcription and translation of codons DNA molecule Gene 1 Gene 2 Gene 3 A Transcription RNA TranslationCodon Polypeptide Amino acid AAC C GG C AAAA UU GGCCG UUUU DNA U

43. 10.8 The genetic code dictates how codons are translated into amino acids  Characteristics of the genetic code –Three nucleotides specify one amino acid. –61 codons correspond to amino acids. –AUG codes for methionine and signals the start of transcription. –3 “stop” codons signal the end of translation. © 2012 Pearson Education, Inc.

44. 10.8 The genetic code dictates how codons are translated into amino acids  The genetic code is –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.

45. Figure 10.8A Dictionary of the genetic code (RNA codons) Second base Third base First base

46. Figure 10.8B T Strand to be transcribed A C T TC A A A A A T DNA AA T C T T T T GAG G RNA Transcription AAAA U U U U U G G G Translation Polypeptide MetLysPhe Stop codon Start codon

47. 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. © 2012 Pearson Education, Inc.

48. 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. © 2012 Pearson Education, Inc.

49. Figure 10.9-3 Initiation RNA synthesis begins after RNA polymerase attaches to the promoter. RNA polymerase DNA of gene Promoter Terminator DNA Newly formed RNA Template strand of DNA Unused strand of DNA Direction of transcription Elongation Using the DNA as a template, RNA polymerase adds free RNA nucleotides one at a time. Newly made RNA DNA strands reunite Direction of transcription Free RNA nucleotide DNA strands separate Termination RNA synthesis ends when RNA polymerase reaches the terminator DNA sequence. Terminator DNA RNA polymerase detaches Completed RNA

50. 10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA  Messenger RNA (mRNA) –encodes amino acid sequences and –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 –introns, interrupting sequences that separate –exons, the coding regions. © 2012 Pearson Education, Inc.

51. 10.10 Eukaryotic RNA is processed before leaving the nucleus as mRNA  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 –help ribosomes bind to the mRNA. © 2012 Pearson Education, Inc.

52. Figure 10.10 DNA Cap Exon IntronExon RNA transcript with cap and tail ExonIntron Transcription Addition of cap and tail Introns removed Tail Exons spliced together Coding sequence NUCLEUS CYTOPLASM mRNA

53. 10.11 Transfer RNA molecules serve as interpreters during translation  Transfer RNA (tRNA) molecules function as a language interpreter, –converting the genetic message of mRNA –into the language of proteins.  Transfer RNA molecules perform this interpreter task by –picking up the appropriate amino acid and –using a special triplet of bases, called an anticodon, to bond to the appropriate codons in the mRNA. © 2012 Pearson Education, Inc.

54. Figure 10.11A The structure of tRNA Amino acid attachment site Hydrogen bond RNA polynucleotide chain Anticodon A simplified schematic of a tRNA A tRNA molecule, showing its polynucleotide strand and hydrogen bonding

55. 10.12 Ribosomes build polypeptides  Translation occurs on the surface of the ribosome. –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.

56. Figure 10.12A The true shape of a functioning ribosome tRNA molecules Growing polypeptide Large subunit Small subunit mRNA

57. Figure 10.12B A ribosome with empty binding sites tRNA binding sites mRNA binding site Large subunit Small subunit P site A site

58. Figure 10.12C A ribosome with occupied binding sites mRNA Codons tRNA Growing polypeptide The next amino acid to be added to the polypeptide

59. 10.13 An initiation codon marks the start of an mRNA message  Translation can be divided into the same three phases as transcription: 1.initiation, 2.elongation, and 3.termination.  Initiation brings together –mRNA, –a tRNA bearing the first amino acid, and –the two subunits of a ribosome. © 2012 Pearson Education, Inc.

60. 10.13 An initiation codon marks the start of an mRNA message  Initiation establishes where translation will begin.  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. 2.A large ribosomal subunit joins the small subunit, allowing the ribosome to function. –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.

61. Figure 10.13A A molecule of eukaryotic mRNA Start of genetic message Cap End Tail

62. Figure 10.13B The initiation of translation Initiator tRNA mRNA Start codon Small ribosomal subunit Large ribosomal subunit P site A site Met AUG U A C 2 AUG U A C 1

63. 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation  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. © 2012 Pearson Education, Inc.

64.  Each cycle of elongation has three steps. 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. 3.Translocation: tRNA is released from the P site and the ribosome moves tRNA from the A site into the P site. 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation © 2012 Pearson Education, Inc.

65.  Elongation continues until the termination stage of translation, when –the ribosome reaches a stop codon, –the completed polypeptide is freed from the last tRNA, and –the ribosome splits back into its separate subunits. 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation © 2012 Pearson Education, Inc.

66. Figure 10.14 Polypeptide mRNA Codon recognition Anticodon Amino acid Codons P site A site 1 Peptide bond 2 formation Translocation 3 New peptide bond Stop codon mRNA movement

67. 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, –transcription occurs in the nucleus and –the mRNA must travel from the nucleus to the cytoplasm. © 2012 Pearson Education, Inc.

68. 10.15 Review: The flow of genetic information in the cell is DNA  RNA  protein  Translation can be divided into four steps, all of which occur in the cytoplasm: 1.amino acid attachment, 2.initiation of polypeptide synthesis, 3.elongation, and 4.termination. © 2012 Pearson Education, Inc.

69. Figure 10.15 DNA Transcription mRNA RNA polymerase Transcription Translation Amino acid Enzyme CYTOPLASM Amino acid attachment 2 1 3 4 tRNA ATP Anticodon Initiation of polypeptide synthesis Elongation Large ribosomal subunit Initiator tRNA Start Codon mRNA Growing polypeptide Small ribosomal subunit New peptide bond forming Codons mRNA Polypeptide Termination 5 Stop codon

70. 10.16 Mutations can change the meaning of genes  A mutation is any change in the nucleotide sequence of DNA.  Mutations can involve –large chromosomal regions or –just a single nucleotide pair. © 2012 Pearson Education, Inc.

71. 10.16 Mutations can change the meaning of genes  Mutations within a gene can be divided into two general categories. 1.Base substitutions involve the replacement of one nucleotide with another. Base substitutions may –have no effect at all, producing a silent mutation, –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.

72. 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, –lead to significant changes in amino acid sequence downstream of the mutation, and –produce a nonfunctional polypeptide. © 2012 Pearson Education, Inc.

73. 10.16 Mutations can change the meaning of genes  Mutagenesis is the production of mutations.  Mutations can be caused by –spontaneous errors that occur during DNA replication or recombination or –mutagens, which include –high-energy radiation such as X-rays and ultraviolet light and –chemicals. © 2012 Pearson Education, Inc.

74. Figure 10.16A The molecular basis of sickle-cell disease Normal hemoglobin DNAMutant hemoglobin DNA mRNA Sickle-cell hemoglobin Normal hemoglobin Glu Val C T T G A A CT GA A U

75. Figure 10.16B Normal gene Nucleotide substitution Nucleotide deletion Nucleotide insertion Inserted Deleted mRNA Protein Met Lys Phe Lys Phe Ala Gly Ser AUGAAGUUU GGC G CA GC G CA A G UUU AUGAA Met Lys Ala His Leu GUU AUGAA GGC G CA U U Met Lys Ala His Leu GUU AUGAA G G C UG G C

76. THE GENETICS OF VIRUSES AND BACTERIA © 2012 Pearson Education, Inc.

77. 10.17 Viral DNA may become part of the host chromosome  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.

78. 10.17 Viral DNA may become part of the host chromosome 2. In the Lysogenic cycle –Viral DNA is inserted into the host chromosome by recombination. –Viral DNA is duplicated along with the host chromosome during each cell division. –The inserted phage DNA is called a prophage. –Most prophage genes are inactive. –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. © 2012 Pearson Education, Inc.

79. Figure 10.17 Phage Attaches to cell Phage DNA Newly released phage may infect another cell The cell lyses, releasing phages The phage injects its DNA 1 2 4 3 5 6 Bacterial chromosome Many cell divisions Environmental stress Prophage Lysogenic cycle OR The phage DNA circularizes Lytic cycle Phage DNA inserts into the bacterial chromosome by recombination New phage DNA and proteins are synthesized Phages assemble The lysogenic bacterium replicates normally, copying the prophage at each cell division 4

80. 10.18 CONNECTION: Many viruses cause disease in animals and plants  Viruses can cause disease in animals and plants.  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.

81. 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: 1.entry of the protein-coated RNA into the cell, 2.uncoating—the removal of the protein coat, 3.RNA synthesis—mRNA synthesis using a viral enzyme, 4.protein synthesis—mRNA is used to make viral proteins, 5.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.

82. 10.18 CONNECTION: Many viruses cause disease in animals and plants  Some animal viruses, such as herpesviruses, reproduce in the cell nucleus.  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. © 2012 Pearson Education, Inc.

83. 2 Figure 10.18 The replication cycle of an enveloped RNA virus Viral RNA (genome) Glycoprotein spike Protein coat Membranous envelope Entry CYTOPLASM Uncoating Plasma membrane of host cell 1 3 5 4 6 Protein synthesis Viral RNA (genome) RNA synthesis by viral enzyme mRNA New viral proteins Assembly New viral genome Template RNA synthesis (other strand) Exit 7 6

84. 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, and –others. © 2012 Pearson Education, Inc.

85. 10.20 The AIDS virus makes DNA on an RNA template  AIDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus).  HIV –is an RNA virus, –has two copies of its RNA genome, –carries molecules of reverse transcriptase, which causes reverse transcription, producing DNA from an RNA template. © 2012 Pearson Education, Inc.

86. Figure 10.20A A model of HIV structure Envelope Glycoprotein Protein coat RNA (two identical strands) Reverse transcriptase (two copies)

87.  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, 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. 10.20 The AIDS virus makes DNA on an RNA template © 2012 Pearson Education, Inc.

88. Figure 10.20B The behavior of HIV nucleic acid in a host cell Viral RNA DNA strand Reverse transcriptase Double- stranded DNA Viral RNA and proteins 1 2 3 4 5 6 CYTOPLASM NUCLEUS Chromosomal DNA Provirus DNA RNA

89. 10.21 Viroids and prions are formidable pathogens in plants and animals  Some infectious agents are made only of RNA or protein. –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.

90. 10.22 Bacteria can transfer DNA in three ways  Viral reproduction allows researchers to learn more about the mechanisms that regulate DNA replication and gene expression in living cells.  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.

91. 10.22 Bacteria can transfer DNA in three ways  Bacteria use three mechanisms to move genes from cell to cell. 1.Transformation is the uptake of DNA from the surrounding environment. 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.  Once new DNA gets into a bacterial cell, part of it may then integrate into the recipient’s chromosome. © 2012 Pearson Education, Inc.

92. Figure 10.22A Transformation DNA enters cell A fragment of DNA from another bacterial cell Bacterial chromosome (DNA)

93. Figure 10.22B Transduction Phage A fragment of DNA from another bacterial cell (former phage host)

94. Figure 10.22C Conjugation Mating bridge Sex pili Donor cell Recipient cell