Chapter 18: The Genetics of Viruses and Bacteria

1. Chapter 18: The Genetics of Viruses and Bacteria

2. 0.5 m Figure 18.1 T4 bacteriophage infecting an E. coli cell

3. Virus Bacterium Animalcell Animal cell nucleus 0.25 m Figure 18.2 Comparing the size of a virus, a bacterium, and an animal cell

4. Figure 18.3 Infection by tobacco mosaic virus (TMV)

5. Capsomereof capsid Membranousenvelope RNA Capsomere DNA Head Capsid Tail sheath DNA RNA Tail fiber Glycoprotein Glycoprotein 80  225 nm 18  250 mm 80–200 nm (diameter) 70–90 nm (diameter) 50 nm 20 nm 50 nm 50 nm (d) Bacteriophage T4 (a) Tobacco mosaic virus (b) Adenoviruses (c) Influenza viruses Figure 18.4 Viral structure

6. VIRUS Entry into cell and uncoating of DNA DNA Capsid Transcription Replication HOST CELL Viral DNA mRNA Viral DNA Capsid proteins Self-assembly of new virus particles and their exit from cell Figure 18.5 A simplified viral reproductive cycle

7. Attachment. The T4 phage usesits tail fibers to bind to specificreceptor sites on the outer surface of an E. coli cell. Entry of phage DNA and degradation of host DNA.The sheath of the tail contracts,injecting the phage DNA intothe cell and leaving an emptycapsid outside. The cell’sDNA is hydrolyzed. 1 2 4 3 5 Release. The phage directs productionof an enzyme that damages the bacterialcell wall, allowing fluid to enter. The cellswells and finally bursts, releasing 100 to 200 phage particles. Phage assembly Synthesis of viral genomes and proteins. The phage DNAdirects production of phageproteins and copies of the phagegenome by host enzymes, usingcomponents within the cell. Assembly. Three separate sets of proteinsself-assemble to form phage heads, tails,and tail fibers. The phage genome ispackaged inside the capsid as the head forms. Head Tail fibers Tails Figure 18.6 The lytic cycle of phage T4, a virulent phage

8. Phage DNA The phage attaches to a host cell and injects its DNA. Many cell divisions produce a large population of bacteria infected with the prophage. Phage DNA circularizes Phage Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Bacterial chromosome Lytic cycle Lysogenic cycle Certain factors determine whether The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. The cell lyses, releasing phages. Prophage Lytic cycle is induced Lysogenic cycle is entered or New phage DNA and proteins are synthesized and assembled into phages. Phage DNA integrates into the bacterial chromosome,becoming a prophage. Figure 18.7 The lytic and lysogenic cycles of phage , a temperate phage

9. Table 18.1 Classes of Animal Viruses

10. Glycoprotein Viral envelope Capsid Reversetranscriptase RNA(two identicalstrands) Figure 18.9 The structure of HIV, the retrovirus that causes AIDS

11. The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 1 HIV Membrane of white blood cell 2 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. HOST CELL 3 Reverse transcriptase catalyzes the synthesis ofa second DNA strand complementary to the first. Reverse transcriptase Viral RNA RNA-DNAhybrid 4 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 0.25 µm HIV entering a cell DNA NUCLEUS Provirus ChromosomalDNA RNA genomefor the nextviral generation 5 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. mRNA 6 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 7 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 9 New viruses bud off from the host cell. New HIV leaving a cell Figure 18.10 The reproductive cycle of HIV, a retrovirus

12. Figure 18.11 SARS (severe acute respiratory syndrome), a recently emerging viral disease (b) The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glycoprotein spikes protruding from the envelope. (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS.

13. Originalprion Prion Many prions Normalprotein Newprion Figure 18.13 Model for how prions propagate

14. Replicationfork Origin of replication Termination of replication Figure 18.14 Replication of a bacterial chromosome

15. EXPERIMENT Researchers had two mutant strains, one that could make arginine but not tryptophan (arg+ trp–) and one that could make tryptophan but not arginine (arg– trp+). Each mutant strain and a mixture of both strains were grown in a liquid medium containing all the required amino acids. Samples from each liquid culture were spread on plates containing a solution of glucose and inorganic salts (minimal medium), solidified with agar. Mixture RESULTS Mutantstrainarg+trp– Mutantstrainargtrp+ Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids. Figure 18.15 Can a bacterial cell acquire genes from another bacterial cell?

16. Mixture Mutantstrainarg+trp– Mutantstrainarg–trp+ No colonies(control) No colonies(control) Coloniesgrew CONCLUSION Because only cells that can make both arginine and tryptophan (arg+ trp+ cells) can grow into colonies on minimal medium, the lack of colonies on the two control plates showed that no further mutations had occurred restoring this ability to cells of the mutant strains. Thus, each cell from the mixture that formed a colony on the minimal medium must have acquired one or more genes from a cell of the other strain by genetic recombination.

17. Phage DNA B+ Phage infects bacterial cell that has alleles A+ and B+ A+ 1 Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. A+ 2 B+ Donorcell A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid. 3 A+ Figure 18.16 Generalized transduction

18. Phage DNA B+ Phage infects bacterial cell that has alleles A+ and B+ A+ 1 Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. A+ 2 B+ Donorcell A bacterial DNA fragment (in this case a fragment withthe A+ allele) may be packaged in a phage capsid. 3 A+ Crossingover Phage with the A+ allele from the donor cell infects a recipient A–B– cell, and crossing over (recombination) between donor DNA (brown) and recipient DNA (green) occurs at two places (dotted lines). 4 A+ A– B– Recipientcell The genotype of the resulting recombinant cell (A+B–) differs from the genotypes of both the donor (A+B+) and the recipient (A–B–). 5 A+ B– Recombinant cell Figure 18.16 Generalized transduction

19. 1 m Sex pilus Figure 18.17 Bacterial conjugation

20. F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F+ cell Bacterial chromosome F– cell A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow). DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid. 2 4 1 7 1 3 3 4 2 5 8 6 (a) Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient Hfr cell F+ cell F factor The circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line). The resulting cell is called an Hfr cell (for High frequency of recombination). B+ D+ C+ C+ A+ D+ A+ A+ B+ D+ D+ A+ C+ C+ B+ B+ A+ B+ C– C– C– C– F– cell B– B+ D– D– D– A+ B– B– D– B– A– A– A– A– A+ The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D. Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA The mating bridgeusually breaks well before the entire chromosome and the rest of the F factor are transferred. (b) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination Temporary partial diploid Recombinant F– bacterium C– C– B+ B– D– D– B+ B– A– A– A+ A+ The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell. Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). Figure 18.18 Conjugation and recombination in E. coli (layer 1)

21. F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F+ cell Bacterial chromosome F– cell A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow). DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid. 2 4 1 7 1 3 3 4 2 5 8 6 (a) Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient Hfr cell F+ cell F factor The circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line). The resulting cell is called an Hfr cell (for High frequency of recombination). B+ D+ C+ C+ A+ D+ A+ A+ B+ D+ D+ A+ C+ C+ B+ B+ A+ B+ C– C– C– C– F– cell B– B+ D– D– D– A+ B– B– D– B– A– A– A– A– A+ The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D. Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA The mating bridgeusually breaks well before the entire chromosome and the rest of the F factor are transferred. (b) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination Temporary partial diploid Recombinant F– bacterium C– C– B+ B– D– D– B+ B– A– A– A+ A+ The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell. Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). Figure 18.18 Conjugation and recombination in E. coli (layer 2)

22. F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F+ cell Bacterial chromosome F– cell A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow). DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid. 5 3 2 3 7 8 1 4 2 1 6 4 (a) Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient Hfr cell F+ cell F factor The circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line). The resulting cell is called an Hfr cell (for High frequency of recombination). B+ D+ C+ C+ Hfr cell A+ D+ A+ A+ B+ D+ D+ A+ C+ C+ B+ B+ A+ B+ C– C– C– C– F– cell B– B+ D– D– D– A+ B– B– D– B– A– A– A– A– A+ The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D. Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA The mating bridgeusually breaks well before the entire chromosome and the rest of the F factor are transferred. (b) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination Temporary partial diploid Recombinant F– bacterium C– C– B+ B– D– D– B+ B– A– A– A+ A+ The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell. Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). Figure 18.18 Conjugation and recombination in E. coli (layer 3)

23. F Plasmid Bacterial chromosome F+ cell F+ cell Mating bridge F+ cell Bacterial chromosome F– cell A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins tomove into the recipient cell. As transfer continues, the donor plasmid rotates(red arrow). DNA replication occurs inboth donor and recipientcells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. The plasmid in the recipient cell circularizes. Transfer and replication result in a compete F plasmid in each cell. Thus, both cells are now F+. A cell carrying an F plasmid(an F+ cell) can form amating bridge with an F– celland transfer its F plasmid. 5 3 2 3 7 8 1 4 2 1 6 4 (a) Conjugation and transfer of an F plasmid from an F+ donor to an F– recipient Hfr cell F+ cell F factor The circular F plasmid in an F+ cellcan be integrated into the circularchromosome by a single crossoverevent (dotted line). The resulting cell is called an Hfr cell (for High frequency of recombination). B+ D+ C+ C+ Hfr cell A+ D+ A+ A+ B+ D+ D+ A+ C+ C+ B+ B+ A+ B+ C– C– C– C– F– cell B– B+ D– D– D– A+ B– B– D– B– A– A– A– A– A+ The location and orientation of the F factor in the donor chromosome determine the sequence of gene transfer during conjugation. In this example, the transfer sequence for four genes is A-B-C-D. Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F– cell and transfer DNA. A single strand of the F factorbreaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA The mating bridgeusually breaks well before the entire chromosome and the rest of the F factor are transferred. (b) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F– recipient, resulting in recombination Temporary partial diploid Recombinant F– bacterium C– C– B+ B– D– D– B+ B– A– A– A+ A+ The piece of DNA ending up outside thebacterial chromosome will eventually be degraded by the cell’s enzymes. The recipient cell now contains a new combination of genes but no F factor; it is a recombinant F– cell. Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). Figure 18.18 Conjugation and recombination in E. coli (layer 4)