Introduction Chapter 10. Introduction. Viruses. Host cell is destroyed when new viruses are released.

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1 Introduction Chapter 10 Viruses bind to receptors on a host cell, inject viral genetic material hijacks the cell s own equipment to produce new copies of the virus. Host cell is destroyed when new viruses are released. Introduction Viruses are not generally considered alive because they are not cells cannot reproduce on their own. Easy to study because of simplicity So viruses are used to study the functions of DN. Figure 10.0_2 1

2 10.1 SCIENIFIC DISCOVERY: Experiments showed that DN is the genetic material Until the 1940s, it was believed that proteins were genetic material. B/c proteins are made from 20 different amino acids. DN only 4 bases. Studies of bacteria and viruses Lead to the field of molecular biology, the study of heredity at the molecular level, and revealed the role of DN in heredity SCIENIFIC DISCOVERY: Experiments showed that DN is the genetic material Read about Frederick riffith experiment Read about Hershey and Chase experiment Figure 10.1 Bacteriophages (or phages for short) are viruses that infect bacterial cells. Head DN ail ail fiber 2

3 Figure 10.1C phage replication cycle 1 phage attaches 2 he phage injects 3 itself to a bacterial its DN into the cell. bacterium. he phage DN directs the host cell to make more phage DN and proteins; new phages assemble. 4 he cell lyses and releases the new phages DN and RN are polymers of nucleotides Nucleic acids - DN and RN. polynucleotide, a nucleotide polymer (chain). Nucleotide is composed of a nitrogenous base, five-carbon sugar, and phosphate group. he nucleotides are joined to one another by a sugar-phosphate backbone DN and RN are polymers of nucleotides DN nucleotides have a different nitrogencontaining base: adenine (), cytosine (C), thymine (), and guanine (). nimation: DN and RN Structure 3

4 Figure 10.2 C C C C C Covalent bond joining nucleotides Sugar-phosphate backbone C Phosphate group Nitrogenous base Sugar Nitrogenous base (can be,, C, or ) C DN double helix DN nucleotide Phosphate group hymine () Sugar (deoxyribose) DN nucleotide wo representations of a DN polynucleotide Figure 10.2B hymine () Cytosine (C) Pyrimidines denine () uanine () Purines 10.2 DN and RN are Polymers of Nucleotides RN (ribonucleic acid) is unlike DN in that it Ribose (instead of deoxyribose in DN) and RN has uracil (U) instead of thymine. 4

5 Figure 10.2C Phosphate group Nitrogenous base (can be,, C, or U) Uracil (U) Sugar (ribose) 10.3 SCIENIFIC DISCOVERY: DN is a double-stranded helix Race was on to describe the structure of DN and explain how the structure and properties of DN can account for its role in heredity SCIENIFIC DISCOVERY: DN is a double-stranded helix In 1953, James D. Watson and Francis Crick deduced the secondary structure of DN, using X-ray crystallography data of DN from the work of Rosalind Franklin and Maurice Wilkins and Chargaff s rule the amount of adenine = the amount of thymine and the amount of guanine = that of cytosine. 5

6 Figure 10.3 Figure 10.3B 10.3 SCIENIFIC DISCOVERY: DN is a double-stranded helix Watson and Crick - DN consisted of two polynucleotide strands wrapped into a double helix. he sugar-phosphate backbone is on the outside. he nitrogenous bases are perpendicular to the backbone in the interior. Specific pairs of bases give the helix a uniform shape. pairs with, forming two hydrogen bonds, and pairs with C, forming three hydrogen bonds. nimation: DN Double Helix 6

7 Figure 10.3C wist Figure 10.3D Hydrogen bond Base pair Ribbon model Partial chemical structure Computer model 10.3 SCIENIFIC DISCOVERY: DN is a double-stranded helix Nobel Prize was awarded to Watson, Crick, and Wilkins in Rosalind Franklin probably would have received the prize, but she died from cancer in Nobel Prizes are never awarded posthumously. 7

8 10.4 DN replication depends on specific base pairing DN replication follows a semiconservative model. he two DN strands separate. Each strand is used as a pattern to produce a complementary strand, using specific base pairing. Each new DN helix has one old strand with one new strand. nimation: DN Replication Overview Figure 10.4_s3 C C C C C C C C C parental molecule of DN Free nucleotides he parental strands separate and serve as templates wo identical daughter molecules of DN are formed Figure 10.4B C Parental DN molecule Daughter strand Parental strand Daughter DN molecules 8

9 10.5 DN replication proceeds in two directions at many sites simultaneously DN replication begins at the origins of replication where DN 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 DN replication proceeds in two directions at many sites simultaneously DN replication occurs in the 5 to 3 direction. Replication is continuous on the 3 to 5 template. Replication is discontinuous on the 5 to 3 template, forming short segments DN replication proceeds in two directions at many sites simultaneously wo key proteins are involved in DN replication. 1. DN ligase joins small fragments into a continuous chain. 2. DN polymerase adds nucleotides to a growing chain and proofreads and corrects improper base pairings. nimation: Origins of Replication nimation: Leading Strand nimation: Lagging Strand nimation: DN Replication Review 9

10 10.5 DN replication proceeds in two directions at many sites simultaneously DN polymerases and DN ligase also repair DN damaged by harmful radiation and toxic chemicals. DN replication ensures that all the somatic cells in a multicellular organism carry the same genetic information. Figure 10.5 Parental DN molecule Origin of replication Parental strand Daughter strand Bubble wo daughter DN molecules Figure 10.5B 5 end 3 end P P P P C HO P C P P OH 3 end P 5 end 10

11 Figure 10.5C 5 3 Parental DN Replication fork DN polymerase molecule his daughter strand is synthesized continuously his daughter strand is synthesized in pieces 5 3 DN ligase Overall direction of replication 10.6 he DN genotype is expressed as proteins, which provide the molecular basis for phenotypic traits DN specifies traits by dictating protein synthesis. he molecular chain of command is from DN in the nucleus to RN and RN in the cytoplasm to protein. ranscription is the synthesis of RN under the direction of DN. ranslation is the synthesis of proteins under the direction of RN. Figure 10.6_s3 DN ranscription RN NUCLEUS ranslation CYOPLSM Protein 11

12 10.6 he DN genotype is expressed as proteins, which provide the molecular basis for phenotypic traits he connections between genes and proteins What an organism looks like, is based on it s genotype One gene one polypeptide hypothesis recognizes that some proteins are composed of multiple polypeptides. Figure 10.6B 10.7 enetic information written in codons is translated into amino acid sequences he sequence of nucleotides in DN provides a code for constructing a protein. Protein construction needs conversion of a nucleotide sequence to an amino acid sequence. ranscription rewrites the DN code into RN, using the same nucleotide language. 12

13 10.7 enetic information written in codons is translated into amino acid sequences ene to protein - based on a triplet code: the genetic instructions for the amino acid sequence of a polypeptide chain are written in DN and RN as a series of nonoverlapping three-base words called codons. ranslation 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. Figure 10.7 DN molecule ene 1 ene 2 ene 3 DN ranscription C C C RN ranslation U U U C C U U U U Codon Polypeptide mino acid Figure 10.7_1 DN ranscription C C C RN ranslation U U U C C U U U Codon U Polypeptide mino acid 13

14 10.8 he genetic code dictates how codons are translated into amino acids Characteristics of the genetic code hree nucleotides specify one amino acid. 61 codons correspond to amino acids. U codes for methionine and signals the start of transcription. 3 stop codons signal the end of translation he genetic code dictates how codons are translated into amino acids he genetic code is Redundant - more than one codon for some amino acids, unambiguous - any codon for one amino acid does not code for any other amino acid, nearly universal the genetic code is shared by organisms without punctuation -no gaps in between codons. Figure 10.8 Second base First base hird base 14

15 Figure 10.8B_s3 Strand to be transcribed DN C C C ranscription RN U U U U U ranslation Start codon Stop codon Polypeptide Met Lys Phe Figure 10.8C 15

16 10.9 ranscription produces genetic messages in the form of RN Overview of transcription n RN molecule is transcribed from a DN template by a process that resembles the synthesis of a DN strand during DN replication. RN nucleotides are linked by the transcription enzyme RN polymerase. Specific sequences of nucleotides along the DN mark where transcription begins and ends. he start transcribing signal is a nucleotide sequence called a promoter ranscription produces genetic messages in the form of RN ranscription begins with initiation, as the RN polymerase attaches to the promoter. During the second phase, elongation, the RN grows longer. s the RN peels away, the DN strands rejoin. Finally, in the third phase, termination, the RN polymerase reaches a sequence of bases in the DN template called a terminator, which signals the end of the gene. he polymerase molecule now detaches from the RN molecule and the gene. nimation: ranscription 16

17 Figure 10.9 RN polymerase Free RN nucleotides C C U C C Direction of transcription Newly made RN emplate strand of DN Figure 10.9B RN polymerase DN of gene erminator DN Promoter DN 1 Initiation 2 Elongation rea shown in Figure ermination rowing RN Completed RN RN polymerase Eukaryotic RN is processed before leaving the nucleus as mrn Messenger RN (mrn) encodes amino acid sequences and conveys genetic messages from DN to the translation machinery of the cell, which in prokaryotes, occurs in the same place that mrn is made, but in eukaryotes, mrn must exit the nucleus via nuclear pores to enter the cytoplasm. Eukaryotic mrn has introns, interrupting sequences that separate exons, the coding regions. 17

18 10.10 Eukaryotic RN is processed before leaving the nucleus as mrn Eukaryotic mrn undergoes processing before leaving the nucleus. RN splicing removes introns and joins exons to produce a continuous coding sequence. cap and tail of extra nucleotides are added to the ends of the mrn to Help the export of the mrn from the nucleus, protect the mrn from attack by cellular enzymes, and help ribosomes bind to the mrn. Figure DN RN transcript with cap and tail Cap Exon Intron Exon Intron Exon ranscription ddition of cap and tail Introns removed ail mrn Exons spliced together Coding sequence NUCLEUS CYOPLSM ransfer RN molecules serve as interpreters during translation ransfer RN (trn) molecules function as a language interpreter, converting the genetic message of mrn into the language of proteins. ransfer RN molecules perform this interpreter task by picking up the appropriate amino acid and using a special triplet of bases, called an anticodon, to recognize the appropriate codons in the mrn. 18

19 Figure mino acid attachment site Hydrogen bond RN polynucleotide chain nticodon trn molecule, showing its polynucleotide strand and hydrogen bonding simplified schematic of a trn Figure 10.11B Enzyme trn P Ribosomes build polypeptides ranslation occurs on the surface of the ribosome. Ribosomes coordinate the functioning of mrn and trn and, ultimately, the synthesis of polypeptides. Ribosomes have two subunits: small and large. Each subunit is composed of ribosomal RNs and proteins. Ribosomal subunits come together during translation. Ribosomes have binding sites for mrn and trns. 19

20 Figure rowing polypeptide trn molecules Large subunit Small subunit mrn Figure 10.12B trn binding sites Large subunit Small subunit P site site mrn binding site Figure 10.12C rowing polypeptide he next amino acid to be added to the polypeptide mrn trn Codons 20

21 10.13 n initiation codon marks the start of an mrn message ranslation can be divided into the same three phases as transcription: 1. initiation, 2. elongation, and 3. termination. Initiation brings together mrn, a trn bearing the first amino acid, and the two subunits of a ribosome n initiation codon marks the start of an mrn message Initiation establishes where translation will begin. Initiation occurs in two steps. 1. n mrn molecule binds to a small ribosomal subunit and the first trn binds to mrn at the start codon. he start codon reads U and codes for methionine. he first trn has the anticodon UC. 2. large ribosomal subunit joins the small subunit, allowing the ribosome to function. he first trn occupies the P site, which will hold the growing peptide chain. he site is available to receive the next trn. Figure Cap Start of genetic message End ail 21

22 Figure 10.13B Initiator trn mrn U C P site U C site Large ribosomal subunit U U 1 Start codon Small ribosomal subunit 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 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Each cycle of elongation has three steps. 1. Codon recognition: he anticodon of an incoming trn molecule, carrying its amino acid, pairs with the mrn codon in the site of the ribosome. 2. Peptide bond formation: he new amino acid is joined to the chain. 3. ranslocation: trn is released from the P site and the ribosome moves trn from the site into the P site. 22

23 10.14 Elongation adds amino acids to the polypeptide chain until a stop codon terminates translation Elongation continues until the termination stage of translation, when the ribosome reaches a stop codon, the completed polypeptide is freed from the last trn, and the ribosome splits back into its separate subunits. nimation: ranslation Figure 10.14_s4 Polypeptide mrn P site site Codons mino acid nticodon 1 Codon recognition mrn movement Stop codon 2 Peptide bond formation New peptide bond 3 ranslocation Review: he flow of genetic information in the cell is DN RN protein ranscription is the synthesis of RN from a DN template. In eukaryotic cells, transcription occurs in the nucleus and the mrn must travel from the nucleus to the cytoplasm. 23

24 10.15 Review: he flow of genetic information in the cell is DN RN protein ranslation 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. Figure DN ranscription mrn 1 ranscription RN polymerase ranslation CYOPLSM 2 mino acid attachment mino acid Enzyme trn P Initiator trn nticodon Large 3 Initiation of ribosomal subunit polypeptide synthesis Start Codon mrn Small ribosomal subunit rowing polypeptide New peptide bond forming 4 Elongation mrn Codons Polypeptide 5 ermination Stop codon Figure 10.15_1 DN ranscription mrn RN polymerase 1 ranscription 24

25 Figure 10.15_2 ranslation CYOPLSM 2 mino acid attachment mino acid Enzyme trn P Initiator trn Large ribosomal subunit nticodon 2 3 Initiation of polypeptide synthesis mrn Start Codon Small ribosomal subunit Figure 10.15_3 rowing polypeptide New peptide bond forming 4 Elongation mrn Codons Polypeptide 5 ermination Stop codon Mutations can change the meaning of genes mutation is any change in the nucleotide sequence of DN. Mutations can involve large chromosomal regions or just a single nucleotide pair. 25

26 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 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 mrn, 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 Mutations can change the meaning of genes Mutagenesis is the production of mutations. Mutations can be caused by spontaneous errors that occur during DN replication or recombination or mutagens, which include high-energy radiation such as X-rays and ultraviolet light and chemicals. 26

27 Figure Normal hemoglobin DN Mutant hemoglobin DN C C mrn mrn U Normal hemoglobin lu Sickle-cell hemoglobin Val Figure 10.16B Normal gene mrn Protein U U U U C C Met Lys Phe ly la Nucleotide substitution U U U U Met Lys Phe C C Ser la U Deleted Nucleotide deletion U U U C C U Met Lys Leu la His Inserted Nucleotide insertion U U U U C C Met Lys Leu la His Viral DN may become part of the host chromosome 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. 27

28 10.17 Viral DN may become part of the host chromosome 2. In the Lysogenic cycle Viral DN is inserted into the host chromosome by recombination. Viral DN is duplicated along with the host chromosome during each cell division. he inserted phage DN is called a prophage. Most prophage genes are inactive. Environmental signals can cause a switch to the lytic cycle, causing the viral DN to be excised from the bacterial chromosome and leading to the death of the host cell. nimation: Phage Lambda Lysogenic and Lytic Cycles nimation: Phage 4 Lytic Cycle Figure 10.17_s1 4 he cell lyses, releasing phages Phage ttaches to cell Phage DN 1 he phage injects its DN Bacterial chromosome Lytic cycle Phages assemble 2 he phage DN circularizes 3 New phage DN and proteins are synthesized Figure 10.17_s2 4 he cell lyses, releasing phages Phage ttaches to cell Phage DN 1 he phage injects its DN Bacterial chromosome 7 Environmental stress Many cell divisions Lytic cycle Lysogenic cycle Phages assemble 2 he phage DN circularizes Prophage 6 he lysogenic bacterium replicates normally OR 3 New phage DN and proteins are synthesized 5 Phage DN inserts into the bacterial chromosome by recombination 28

29 Figure 10.17_1 4 he cell lyses, releasing phages Phage ttaches to cell Phage DN 1 he phage injects its DN Bacterial chromosome Lytic cycle Phages assemble 2 he phage DN circularizes 3 New phage DN and proteins are synthesized Figure 10.17_2 Phage ttaches to cell Phage DN Bacterial chromosome 1 he phage injects its DN 7 Environmental stress Many cell divisions 2 he phage DN circularizes Lysogenic cycle Prophage 6 he lysogenic bacterium replicates normally, copying the prophage at each cell division 5 Phage DN inserts into the bacterial chromosome by recombination CONNECION: Many viruses cause disease in animals and plants Viruses can cause disease in animals and plants. DN viruses and RN viruses cause disease in animals. typical animal virus has a membranous outer envelope and projecting spikes of glycoprotein. he envelope helps the virus enter and leave the host cell. Many animal viruses have RN rather than DN as their genetic material. hese include viruses that cause the common cold, measles, mumps, polio, and IDS. 29

30 10.18 CONNECION: Many viruses cause disease in animals and plants he reproductive cycle of the mumps virus, a typical enveloped RN virus, has seven major steps: 1. entry of the protein-coated RN into the cell, 2. uncoating the removal of the protein coat, 3. RN synthesis mrn synthesis using a viral enzyme, 4. protein synthesis mrn is used to make viral proteins, 5. new viral genome production mrn is used as a template to synthesize new viral genomes, 6. assembly the new coat proteins assemble around the new viral RN, and 7. exit the viruses leave the cell by cloaking themselves in the host cell s plasma membrane CONNECION: Many viruses cause disease in animals and plants Some animal viruses, such as herpesviruses, reproduce in the cell nucleus. Most plant viruses are RN viruses. o 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. here are no cures for most viral diseases of plants or animals. nimation: Simplified Viral Reproductive Cycle Figure 10.18_1 lycoprotein spike Protein coat Viral RN (genome) Membranous envelope Plasma membrane of host cell 1 Entry CYOPLSM Viral RN (genome) 2 3 Uncoating RN synthesis by viral enzyme 30

31 Figure 10.18_2 4 mrn Protein synthesis New viral proteins 6 5 RN synthesis (other strand) emplate ssembly New viral genome Exit EVOLUION CONNECION: Emerging viruses threaten human health Viruses that appear suddenly or are new to medical scientists are called emerging viruses. hese include the IDS virus, Ebola virus, West Nile virus, and SRS virus EVOLUION CONNECION: Emerging viruses threaten human health hree processes contribute to the emergence of viral diseases: 1. mutation RN 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. 31

32 Figure he IDS virus makes DN on an RN template IDS (acquired immunodeficiency syndrome) is caused by HIV (human immunodeficiency virus). HIV is an RN virus, has two copies of its RN genome, carries molecules of reverse transcriptase, which causes reverse transcription, producing DN from an RN template. Figure Envelope lycoprotein Protein coat RN (two identical strands) Reverse transcriptase (two copies) 32

33 10.20 he IDS virus makes DN on an RN template fter HIV RN is uncoated in the cytoplasm of the host cell, 1. reverse transcriptase makes one DN strand from RN, 2. reverse transcriptase adds a complementary DN strand, 3. double-stranded viral DN enters the nucleus and integrates into the chromosome, becoming a provirus, 4. the provirus DN is used to produce mrn, 5. the viral mrn is translated to produce viral proteins, and 6. new viral particles are assembled, leave the host cell, and can then infect other cells. nimation: HIV Reproductive Cycle Figure 10.20B Viral RN 1 Reverse transcriptase CYOPLSM NUCLEUS DN strand Doublestranded DN Chromosomal DN Provirus DN Viral RN and proteins 5 RN Viroids and prions are formidable pathogens in plants and animals Some infectious agents are made only of RN or protein. Viroids are small, circular RN 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. 33

34 10.22 Bacteria can transfer DN in three ways Viral reproduction allows researchers to learn more about the mechanisms that regulate DN replication and gene expression in living cells. Bacteria are also valuable but for different reasons. Bacterial DN 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 Bacteria can transfer DN in three ways Bacteria use three mechanisms to move genes from cell to cell. 1. ransformation is the uptake of DN from the surrounding environment. 2. ransduction is gene transfer by phages. 3. Conjugation is the transfer of DN from a donor to a recipient bacterial cell through a cytoplasmic (mating) bridge. Once new DN gets into a bacterial cell, part of it may then integrate into the recipient s chromosome. Figure DN enters cell Phage fragment of DN from another bacterial cell Bacterial chromosome (DN) fragment of DN from another bacterial cell (former phage host) 34

35 Figure 10.22C Mating bridge Sex pili Donor cell Recipient cell Figure 10.22D Donated DN Crossovers Degraded DN Recipient cell s chromosome Recombinant chromosome Bacterial plasmids can serve as carriers for gene transfer he ability of a donor E. coli cell to carry out conjugation is usually due to a specific piece of DN called the F factor. During conjugation, the F factor is integrated into the bacterium s chromosome. he donor chromosome starts replicating at the F factor s origin of replication. he growing copy of the DN peels off and heads into the recipient cell. he F factor serves as the leading end of the transferred DN. 35

36 Figure B F factor (integrated) Donor Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Recipient cell F factor (plasmid) Donor Bacterial chromosome F factor starts replication and transfer Only part of the chromosome transfers he plasmid completes its transfer and circularizes Recombination can occur he cell is now a donor Figure F factor (integrated) Donor Origin of F replication Bacterial chromosome F factor starts replication and transfer of chromosome Recipient cell Only part of the chromosome transfers Recombination can occur Bacterial plasmids can serve as carriers for gene transfer n F factor can also exist as a plasmid, a small circular DN molecule separate from the bacterial chromosome. Some plasmids, including the F factor, can bring about conjugation and move to another cell in linear form. he transferred plasmid re-forms a circle in the recipient cell. R plasmids pose serious problems for human medicine by carrying genes for enzymes that destroy antibiotics. 36

37 Figure 10.23B F factor (plasmid) Donor Bacterial chromosome F factor starts replication and transfer he plasmid completes its transfer and circularizes he cell is now a donor Figure 10.23C Plasmids You should now be able to 1. Describe the experiments of riffith, Hershey, and Chase, which supported the idea that DN was life s genetic material. 2. Compare the structures of DN and RN. 3. Explain how the structure of DN facilitates its replication. 4. Describe the process of DN replication. 5. Describe the locations, reactants, and products of transcription and translation. 37

38 You should now be able to 6. Explain how the languages of DN and RN are used to produce polypeptides. 7. Explain how mrn is produced using DN. 8. Explain how eukaryotic RN is processed before leaving the nucleus. 9. Relate the structure of trn to its functions in the process of translation. 10. Describe the structure and function of ribosomes. You should now be able to 11. Describe the step-by-step process by which amino acids are added to a growing polypeptide chain. 12. Diagram the overall process of transcription and translation. 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. You should now be able to 16. Describe three processes that contribute to the emergence of viral disease. 17. Explain how the IDS virus enters a host cell and reproduces. 18. Describe the structure of viroids and prions and explain how they cause disease. 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. 38

39 Figure 10.UN01 DN Sugarphosphate backbone C Polynucleotide Nitrogenous base Phosphate group Sugar Nucleotide Nitrogenous bases Sugar DN C Deoxyribose RN C U Ribose Figure 10.UN02 rowing polypeptide Large ribosomal subunit mino acid trn mrn nticodon Codons Small ribosomal subunit Figure 10.UN03 DN is a polymer made from monomers called (a) (b) is performed by an enzyme called (c) (d) RN comes in three kinds called (e) (g) (f) use amino-acid-bearing molecules called is performed by structures called (h) molecules are components of Protein one or more polymers made from monomers called (i) 39

40 Figure 10.1_UN Figure 10.17_UN Figure 10.18_UN 40