1. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings GENETICS OF BACTERIA AND VIRUSES

2. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Overview: Microbial Model Systems Viruses called bacteriophages – Can infect and set in motion a genetic takeover of bacteria, such as Escherichia coli E. coli and its viruses – Are called model systems because of their frequent use by researchers in studies that reveal broad biological principles Beyond their value as model systems – Viruses and bacteria have unique genetic mechanisms that are interesting in their own right Figure 18.1 0.5  m

3. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Recall that bacteria are prokaryotes – With cells much smaller and more simply organized than those of eukaryotes Viruses – Are smaller and simpler still Figure 18.2 0.25  m Virus Animal cell Animal cell nucleus

4. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Structure of Viruses Viruses – Are very small infectious particles consisting of nucleic acid enclosed in a protein coat – Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host cell

5. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Genomes Viral genomes may consist of – Double- or single-stranded DNA – Double- or single-stranded RNA

6. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.4a, b 18  250 mm 70–90 nm (diameter) 20 nm 50 nm (a) Tobacco mosaic virus (b) Adenoviruses RNA DNA Capsomere Glycoprotein Capsomere of capsid Capsids and Envelopes A capsid – Is the protein shell that encloses the viral genome – Can have various structures

7. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings General Features of Viral Reproductive Cycles Viruses are obligate intracellular parasites – They can reproduce only within a host cell Each virus has a host range – A limited number of host cells that it can infect

8. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viruses use enzymes, ribosomes, and small molecules of host cells – To synthesize “baby” viruses VIRUS Capsid proteins mRNA Viral DNA HOST CELL Viral DNA DNA Capsid Figure 18.5 Entry into cell and uncoating of DNA Replication Transcription Self-assembly of new virus particles and their exit from cell

9. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Phages Phages – Are the best understood of all viruses – Go through two alternative reproductive mechanisms: the lytic cycle and the lysogenic cycle

10. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lytic Cycle The lytic cycle – Is a phage reproductive cycle that culminates in the death of the host – Produces new phages and digests the host’s cell wall, releasing the “baby” viruses

11. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The lytic cycle of phage T4, a virulent phage Phage assembly Head Tails Tail fibers Figure 18.6 Attachment. The T4 phage uses its tail fibers to bind to specific receptor sites on the outer surface of an E. coli cell. 1 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. 2 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. 3 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. 4 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. 5

12. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Lysogenic Cycle The lysogenic cycle – Incoporates the phage genome without destroying the host Temperate phages – Are capable of using both the lytic and lysogenic cycles of reproduction

13. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The lytic and lysogenic cycles of phage, a temperate phage

14. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Reproductive Cycles of Animal Viruses The nature of the genome – Is the basis for the common classification of animal viruses

15. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Classes of animal viruses Table 18.1

16. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings RNA as Viral Genetic Material The broadest variety of RNA genomes – Is found among the viruses that infect animals Retroviruses, such as HIV, use the enzyme reverse transcriptase – To copy their RNA genome into DNA, which can then be integrated into the host genome as a provirus Reverse transcriptase Viral envelope Capsid Glycoprotein RNA (two identical strands)

17. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The reproductive cycle of HIV, a retrovirus Figure 18.10 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 Reverse transcriptase catalyzes the synthesis of a DNA strand complementary to the viral RNA. 2 Reverse transcriptase catalyzes the synthesis of a second DNA strand complementary to the first. 3 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 The viral proteins include capsid proteins and reverse transcriptase (made in the cytosol) and envelope glycoproteins (made in the ER). 6 Vesicles transport the glycoproteins from the ER to the cell’s plasma membrane. 7 Capsids are assembled around viral genomes and reverse transcriptase molecules. 8 New viruses bud off from the host cell. 9

18. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Evolution of Viruses Viruses do not really fit our definition of living organisms Since viruses can reproduce only within cells – They probably evolved after the first cells appeared, perhaps packaged as fragments of cellular nucleic acid

19. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Diseases in Animals Viruses may damage or kill cells – By causing the release of hydrolytic enzymes from lysosomes Some viruses cause infected cells – To produce toxins that lead to disease symptoms

20. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Vaccines – Are harmless derivatives of pathogenic microbes that stimulate the immune system to mount defenses against the actual pathogen – Can prevent certain viral illnesses

21. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viral Diseases in Plants More than 2,000 types of viral diseases of plants are known Common symptoms of viral infection include – Spots on leaves and fruits, stunted growth, and damaged flowers or roots Plant viruses spread disease in two major modes – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent

22. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings DO NOW – What is the difference between the lytic and lysogenic cycle? – Describe the structure of a virus.

23. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 18.3: Rapid reproduction, mutation, and genetic recombination contribute to the genetic diversity of bacteria Bacteria allow researchers – To investigate molecular genetics in the simplest true organisms

24. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The Bacterial Genome and Its Replication The bacterial chromosome – Is usually a circular DNA molecule with few associated proteins In addition to the chromosome – Many bacteria have plasmids, smaller circular DNA molecules that can replicate independently of the bacterial chromosome

25. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Bacterial cells divide by binary fission – Which is preceded by replication of the bacterial chromosome Replication fork Origin of replication Termination of replication Figure 18.14

26. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mutation and Genetic Recombination as Sources of Genetic Variation Since bacteria can reproduce rapidly – New mutations can quickly increase a population’s genetic diversity

27. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Further genetic diversity – Can arise by recombination of the DNA from two different bacterial cells Mutant strain arg  trp + EXPERIMENT Figure 18.15 Only the samples from the mixed culture, contained cells that gave rise to colonies on minimal medium, which lacks amino acids. RESULTS 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 – Mixture

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

29. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Mechanisms of Gene Transfer and Genetic Recombination in Bacteria Three processes bring bacterial DNA from different individuals together – Transformation – Transduction – Conjugation

30. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transformation – Is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign DNA from the surrounding environment

31. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transduction In the process known as transduction – Phages carry bacterial genes from one host cell to another 1 Figure 18.16 Donor cell Recipient cell 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 – ). 2 3 4 5 Phage DNA A+A+ B+B+

32. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Conjugation and Plasmids Conjugation – Is the direct transfer of genetic material between bacterial cells that are temporarily joined Figure 18.17 Sex pilus 1  m

33. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings The F Plasmid and Conjugation Cells containing the F plasmid, designated F + cells – Function as DNA donors during conjugation – Transfer plasmid DNA to an F  recipient cell Figure 18.18a 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 Mating bridge 1 Conjugation and transfer of an F plasmid from an F + donor to an F – recipient (a) F – cell

34. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings R plasmids and Antibiotic Resistance R plasmids – Confer resistance to various antibiotics – This can be dangerous as we humans over use antibiotics!

35. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposition of Genetic Elements Transposable elements – Can move around within a cell’s genome – Are often called “jumping genes” – Contribute to genetic shuffling in bacteria

36. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Figure 18.19a (a) Insertion sequences, the simplest transposable elements in bacteria, contain a single gene that encodes transposase, which catalyzes movement within the genome. The inverted repeats are backward, upside-down versions of each other; only a portion is shown. The inverted repeat sequence varies from one type of insertion sequence to another. Insertion sequence Transposase gene Inverted repeat Inverted repeat 3535 3535 A T C C G G T… T A G G C C A … A C C G G A T… T G G C C T A … Insertion Sequences An insertion sequence contains a single gene for transposase – An enzyme that catalyzes movement of the insertion sequence from one site to another within the genome

37. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposons Bacterial transposons – Also move about within the bacterial genome – Have additional genes, such as those for antibiotic resistance Figure 18.19b (b) Transposons contain one or more genes in addition to the transposase gene. In the transposon shown here, a gene for resistance to an antibiotic is located between twin insertion sequences. The gene for antibiotic resistance is carried along as part of the transposon when the transposon is inserted at a new site in the genome. Inverted repeats Transposase gene Insertion sequence Antibiotic resistance gene Transposon 5353 5353

38. Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings What are antibiotics? Drugs used to kill bacteria How do they kill bacteria – The produce selective poisons. I – It has been chosen so that it will kill the desired bacteria, but not the cells in your body.cells – Each different type of antibiotic affects different bacteria in different ways. an antibiotic might inhibit a bacterium's ability to turn glucose into energy, or its ability to construct its cell wall. When this happens, the bacterium dies instead of reproducing..