Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 18: The Genetics of Viruses and Bacteria.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.2 Comparing the size of a virus, a bacterium, and an animal cell 0.25  m Virus Animal cell Bacterium Animal cell nucleus

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.4 Viral structure 18  250 mm 70–90 nm (diameter) 80–200 nm (diameter) 80  225 nm 20 nm50 nm (a) Tobacco mosaic virus(b) Adenoviruses(c) Influenza viruses (d) Bacteriophage T4 RNA Capsomere of capsid DNA Capsomere Glycoprotein Membranous envelope Capsid DNA Head Tail fiber Tail sheath

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.5 A simplified viral reproductive cycle VIRUS Capsid proteins mRNA Viral DNA HOST CELL Viral DNA Entry into cell and uncoating of DNA Replication Transcription DNA Capsid Self-assembly of new virus particles and their exit from cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.6 The lytic cycle of phage T4, a virulent phage Attachment. The T4 phage uses its tail fibers to bind to specific receptor 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 into the cell and leaving an empty capsid outside. The cell’s DNA is hydrolyzed. Synthesis of viral genomes and proteins. The phage DNA directs production of phage proteins and copies of the phage genome by host enzymes, using components within the cell. Assembly. Three separate sets of proteins self-assemble to form phage heads, tails, and tail fibers. The phage genome is packaged inside the capsid as the head forms. Release. The phage directs production of an enzyme that damages the bacterial cell wall, allowing fluid to enter. The cell swells and finally bursts, releasing 100 to 200 phage particles. 1 2 4 3 5 Phage assembly Head Tails Tail fibers

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.7 The lytic and lysogenic cycles of phage, a temperate phage Many cell divisions produce a large population of bacteria infected with the prophage. The bacterium reproduces normally, copying the prophage and transmitting it to daughter cells. Phage DNA integrates into the bacterial chromosome, becoming a prophage. New phage DNA and proteins are synthesized and assembled into phages. Occasionally, a prophage exits the bacterial chromosome, initiating a lytic cycle. Certain factors determine whether The phage attaches to a host cell and injects its DNA. Phage DNA circularizes The cell lyses, releasing phages. Lytic cycle is induced Lysogenic cycle is entered Lysogenic cycleLytic cycle or Prophage Bacterial chromosome Phage DNA

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.9 The structure of HIV, the retrovirus that causes AIDS Reverse transcriptase Viral envelope Capsid Glycoprotein RNA (two identical strands)

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.10 The reproductive cycle of HIV, a retrovirus Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 7 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 6 The double-stranded DNA is incorporated as a provirus into the cell’s DNA. 4 Proviral genes are transcribed into RNA molecules, which serve as genomes for the next viral generation and as mRNAs for translation into viral proteins. 5 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. 3 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. 2 New viruses bud off from the host cell. 9 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 mRNA RNA genome for the next viral generation Viral RNA RNA-DNA hybrid DNA Chromosomal DNA NUCLEUS Provirus HOST CELL Reverse transcriptase New HIV leaving a cell HIV entering a cell 0.25 µm HIV Membrane of white blood cell The virus fuses with the cell’s plasma membrane. The capsid proteins are removed, releasing the viral proteins and RNA. 1

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.11 SARS (severe acute respiratory syndrome), a recently emerging viral disease (a) Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. (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.

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.15 Can a bacterial cell acquire genes from another bacterial cell? Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids. RESULTS 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. Mutant strain arg + trp – Mutant strain arg  trp + Mixture

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutant strain arg + trp – Mutant strain arg – trp + No colonies (control) Mixture Colonies grew 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. CONCLUSION

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.16 Generalized transduction Phage DNA Donor cell A+A+ B+B+ A+A+ B+B+ A+A+ Phage infects bacterial cell that has alleles A + and B + Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. A bacterial DNA fragment (in this case a fragment with the A + allele) may be packaged in a phage capsid. 1 2 3

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.16 Generalized transduction Phage DNA Donor cell Recipient cell A+A+ B+B+ A+A+ B+B+ A+A+ A+A+ B–B– A–A– B–B– A+A+ Recombinant cell Crossing over Phage infects bacterial cell that has alleles A + and B + Host DNA (brown) is fragmented, and phage DNA and proteins are made. This is the donor cell. A bacterial DNA fragment (in this case a fragment with the A + allele) may be packaged in a phage capsid. 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). The genotype of the resulting recombinant cell (A + B – ) differs from the genotypes of both the donor (A + B + ) and the recipient (A – B – ). 1 2 3 4 5

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.18 Conjugation and recombination in E. coli (layer 1) 1 A cell carrying an F plasmid (an F + cell) can form a mating bridge with an F – cell and transfer its F plasmid. A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). 2 DNA replication occurs in both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. 3 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 +. 4 F Plasmid Bacterial chromosome F – cell F + cell Hfr cell F factor The circular F plasmid in an F + cell can be integrated into the circular chromosome by a single crossover event (dotted line). 1 The resulting cell is called an Hfr cell (for High frequency of recombination). 2 Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F – cell and transfer DNA. 3 A single strand of the F factor breaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA 4 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. 5 The mating bridge usually breaks well before the entire chromosome and the rest of the F factor are transferred. 6 Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). 7 The piece of DNA ending up outside the bacterial 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. 8 Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of an F plasmid from an F + donor to an F – recipient (a) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombination (b) A+A+ B+B+ C+C+ D+D+ F – cell A–A– B–B– C–C– D–D– A–A– B–B– C–C– D–D– D–D– A–A– C–C– B–B– D–D– A–A– C–C– B–B– A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ D+D+ C+C+ A+A+ A+A+ B+B+ A–A– B–B– C–C– D–D– A–A– B+B+ C–C– D–D– A+A+ B+B+ B–B– A+A+ A+A+ B+B+ F + cell Mating bridge

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.18 Conjugation and recombination in E. coli (layer 2) 1 A cell carrying an F plasmid (an F + cell) can form a mating bridge with an F – cell and transfer its F plasmid. A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). 2 DNA replication occurs in both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. 3 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 +. 4 F Plasmid Bacterial chromosome F – cell F + cell Hfr cell F factor The circular F plasmid in an F + cell can be integrated into the circular chromosome by a single crossover event (dotted line). 1 The resulting cell is called an Hfr cell (for High frequency of recombination). 2 Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F – cell and transfer DNA. 3 A single strand of the F factor breaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA 4 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. 5 The mating bridge usually breaks well before the entire chromosome and the rest of the F factor are transferred. 6 Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). 7 The piece of DNA ending up outside the bacterial 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. 8 Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of an F plasmid from an F + donor to an F – recipient (a) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombination (b) A+A+ B+B+ C+C+ D+D+ F – cell A–A– B–B– C–C– D–D– A–A– B–B– C–C– D–D– D–D– A–A– C–C– B–B– D–D– A–A– C–C– B–B– A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ D+D+ C+C+ A+A+ A+A+ B+B+ A–A– B–B– C–C– D–D– A–A– B+B+ C–C– D–D– A+A+ B+B+ B–B– A+A+ A+A+ B+B+ F + cell Mating bridge

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.18 Conjugation and recombination in E. coli (layer 3) 1 A cell carrying an F plasmid (an F + cell) can form a mating bridge with an F – cell and transfer its F plasmid. A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). 2 DNA replication occurs in both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. 3 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 +. 4 F Plasmid Bacterial chromosome F – cell F + cell Hfr cell F factor The circular F plasmid in an F + cell can be integrated into the circular chromosome by a single crossover event (dotted line). 1 The resulting cell is called an Hfr cell (for High frequency of recombination). 2 Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F – cell and transfer DNA. 3 A single strand of the F factor breaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA 4 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. 5 The mating bridge usually breaks well before the entire chromosome and the rest of the F factor are transferred. 6 Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). 7 The piece of DNA ending up outside the bacterial 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. 8 Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of an F plasmid from an F + donor to an F – recipient (a) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombination (b) A+A+ B+B+ C+C+ D+D+ F – cell A–A– B–B– C–C– D–D– A–A– B–B– C–C– D–D– D–D– A–A– C–C– B–B– D–D– A–A– C–C– B–B– A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ D+D+ C+C+ A+A+ A+A+ B+B+ A–A– B–B– C–C– D–D– A–A– B+B+ C–C– D–D– A+A+ B+B+ B–B– A+A+ A+A+ B+B+ F + cell Mating bridge Hfr cell

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.18 Conjugation and recombination in E. coli (layer 4) 1 A cell carrying an F plasmid (an F + cell) can form a mating bridge with an F – cell and transfer its F plasmid. A single strand of the F plasmid breaks at a specific point (tip of blue arrowhead) and begins to move into the recipient cell. As transfer continues, the donor plasmid rotates (red arrow). 2 DNA replication occurs in both donor and recipient cells, using the single parental strands of the F plasmid as templates to synthesize complementary strands. 3 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 +. 4 F Plasmid Bacterial chromosome F – cell F + cell Hfr cell F factor The circular F plasmid in an F + cell can be integrated into the circular chromosome by a single crossover event (dotted line). 1 The resulting cell is called an Hfr cell (for High frequency of recombination). 2 Since an Hfr cell has all the F-factor genes, it can form a mating bridge with an F – cell and transfer DNA. 3 A single strand of the F factor breaks and begins to move through the bridge. DNA replication occurs in both donor and recipient cells, resulting in double-stranded DNA 4 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. 5 The mating bridge usually breaks well before the entire chromosome and the rest of the F factor are transferred. 6 Two crossovers can result in the exchange of similar (homologous) genes between the transferred chromosome fragment (brown) and the recipient cell’s chromosome (green). 7 The piece of DNA ending up outside the bacterial 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. 8 Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of an F plasmid from an F + donor to an F – recipient (a) Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombination (b) A+A+ B+B+ C+C+ D+D+ F – cell A–A– B–B– C–C– D–D– A–A– B–B– C–C– D–D– D–D– A–A– C–C– B–B– D–D– A–A– C–C– B–B– A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ C+C+ D+D+ A+A+ B+B+ D+D+ C+C+ A+A+ A+A+ B+B+ A–A– B–B– C–C– D–D– A–A– B+B+ C–C– D–D– A+A+ B+B+ B–B– A+A+ A+A+ B+B+ F + cell Mating bridge Hfr cell

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Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Chapter 18: The Genetics of Viruses and Bacteria.

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