1. Chapter 18.1 & 18.4 The Genetics of Viruses and Bacteria

2. VIRUSES 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 Bacterium Animal cell nucleus

3. Structure of Viruses Viruses –Are very small infectious particles consisting of nucleic acid enclosed in a protein coat and, in some cases, a membranous envelope Viral genomes may consist of –Double- or single-stranded DNA –Double- or single-stranded RNA

4. 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

5. Some viruses have envelopes –Which are membranous coverings derived from the membrane of the host cell Figure 18.4c 80–200 nm (diameter) 50 nm (c) Influenza viruses RNA Glycoprotein Membranous envelope Capsid

6. Bacteriophages, also called phages –Have the most complex capsids found among viruses Figure 18.4d 80  225 nm 50 nm (d) Bacteriophage T4 DNA Head Tail fiber Tail sheath

7. 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. Viruses use enzymes, ribosomes, and small molecules of host cells –To synthesize progeny 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. 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. 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 progeny viruses

11. 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. The Lysogenic Cycle The lysogenic cycle –Replicates the phage genome without destroying the host Temperate phages –Are capable of using both the lytic and lysogenic cycles of reproduction

13. 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 Figure 18.7

14. RNA as Viral Genetic Material The broadest variety of RNA genomes –Is found among the viruses that infect animals

15. 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 Figure 18.9 Reverse transcriptase Viral envelope Capsid Glycoprotein RNA (two identical strands)

16. 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

17. Individual bacteria respond to environmental change by regulating their gene expression E. coli, a type of bacteria that lives in the human colon –Can tune its metabolism to the changing environment and food sources

18. This metabolic control occurs on two levels –Adjusting the activity of metabolic enzymes already present –Regulating the genes encoding the metabolic enzymes Figure 18.20a, b (a) Regulation of enzyme activity Enzyme 1 Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Regulation of gene expression Feedback inhibition Tryptophan Precursor (b) Regulation of enzyme production Gene 2 Gene 1 Gene 3 Gene 4 Gene 5 – –

19. Operons: The Basic Concept In bacteria, genes are often clustered into operons, composed of –An operator, an “on-off” switch –A promoter –Genes for metabolic enzymes

20. An operon –Is usually turned “on” –Can be switched off by a protein called a repressor

21. The trp operon: regulated synthesis of repressible enzymes Figure 18.21a (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. Genes of operon Inactive repressor Protein Operator Polypeptides that make up enzymes for tryptophan synthesis Promoter Regulatory gene RNA polymerase Start codon Stop codon Promoter trp operon 5 3 mRNA 5 trpD trpE trpCtrpB trpA trpR DNA mRNA ED C BA

22. DNA mRNA Protein Tryptophan (corepressor) Active repressor No RNA made Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. (b) Figure 18.21b

23. Repressible and Inducible Operons: Two Types of Negative Gene Regulation In a repressible operon –Binding of a specific repressor protein to the operator shuts off transcription In an inducible operon –Binding of an inducer to an innately inactive repressor inactivates the repressor and turns on transcription LAC Operon

24. The lac operon: regulated synthesis of inducible enzymes Figure 18.22a DNA mRNA Protein Active repressor RNA polymerase No RNA made lacZ lacl Regulatory gene Operator Promoter Lactose absent, repressor active, operon off. The lac repressor is innately active, and in the absence of lactose it switches off the operon by binding to the operator. (a) 5 3

25. mRNA 5' DNA mRNA Protein Allolactose (inducer) Inactive repressor lacl lacz lacYlacA RNA polymerase PermeaseTransacetylase  -Galactosidase 5 3 (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. mRNA 5 lac operon Figure 18.22b

26. Inducible enzymes –Usually function in catabolic pathways Repressible enzymes –Usually function in anabolic pathways