Download Chapter 18 notes

Chapter 18: The Genetics of Viruses and Bacteria
Overview: Microbial Model Systems
 Bacteria are prokaryotes with cells much smaller and more simply organized than those of eukaryotes
 Viruses are smaller and simpler than bacteria
Structure of Viruses
 Viruses are not cells
 Viruses are very small infectious particles consisting of
 Nucleic acid enclosed in a protein coat
 A capsid is the protein shell that encloses the viral genome
 A capsid can have various structures
 In some cases, a membranous envelope
Viral Genomes
 Viral genomes may consist of
o Double- or single-stranded DNA
o Double- or single-stranded RNA
 Depending on its type of nucleic acid, a virus is called a DNA virus or an RNA virus
 Some viruses have membranous envelopes that help them infect hosts
o Viral envelopes, which are derived from the host cell’s membrane, contain a combination of viral and
host cell molecules
 Viral envelopes surround the capsids of many viruses found in animals
 Viral glycoproteins on the envelope bind to specific receptor molecules on the surface of a host
cell
 Bacteriophages, also called phages, are viruses that infect bacteria
o Have the most complex capsids
 Have an elongated capsid head that encloses their DNA
 A protein tailpiece attaches the phage to the host and injects the phage DNA inside
General Features of Viral Reproductive Cycles
 Viruses are obligate intracellular parasites
o Can reproduce only within a host cell
o Host range: limited number of host cells that it can infect, virus specific
 Viruses use enzymes, ribosomes, and small host molecules to synthesize progeny viruses
Reproductive Cycles of Phages
 Phages have two reproductive mechanisms
o Lytic cycle
o Lysogenic cycle
The Lytic Cycle
 Lytic cycle: phage reproductive cycle that culminates in death of the host cell
o Known as a virulent phage
o Produces new phages
o Digests the host’s cell wall, releasing the progeny viruses
 Bacteria have defenses against phages, including restriction enzymes that recognize and cut up certain phage
DNA
The Lysogenic Cycle
 Lysogenic cycle: phage reproductive cycle that replicates the phage genome without destroying the host
o The viral DNA molecule is incorporated by genetic recombination into the host cell’s chromosome
o This integrated viral DNA is known as a prophage
 Every time the host divides, it copies the phage DNA and passes the copies to daughter cells
 Phages that use both the lytic and lysogenic cycles are called temperate phages
RNA as Viral Genetic Material
 The broadest variety of RNA genomes is found in viruses that infect animals
 Retroviruses use reverse transcriptase to copy their RNA genome into DNA
o This violates the dogma of DNA > RNA > protein
o HIV is the retrovirus that causes AIDS
 The viral DNA that is integrated into the host genome is called a provirus
o Unlike a prophage, a provirus remains a permanent resident of the host cell
 The host’s RNA polymerase transcribes the proviral DNA into RNA molecules
 The RNA molecules function both as mRNA for synthesis of viral proteins and as genomes for new virus
particles released from the cell
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
 Vaccines are (usually) harmless derivatives of pathogenic microbes that stimulate the immune system to mount
defenses against the actual pathogen
 Vaccines can prevent certain viral illnesses
Viral Diseases in Plants
 More than 2,000 types of viral diseases of plants are known
 Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots
 Spread disease in two major modes:
o Horizontal transmission, entering through damaged cell walls
o Vertical transmission, inheriting the virus from a parent
Viruses, viroids, and prions are formidable pathogens in animals and plants
 Smaller, less complex entities called viroids and prions also cause disease in plants and animals
 Viroids: circular RNA molecules that infect plants and disrupt their growth
 Prions: slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals
o Prions propagate by converting normal proteins into the prion version
Mechanisms of Gene Transfer and Genetic Recombination in Bacteria
 Three processes bring bacterial DNA from different individuals together:
o Transformation
o Transduction
o Conjugation
Transformation
 Transformation is the alteration of a bacterial cell’s genotype and phenotype by the uptake of naked, foreign
DNA from the surrounding environment
 For example, harmless Streptococcus pneumoniae bacteria can be transformed to pneumonia-causing cells
Transduction
 In the process known as transduction, phages carry bacterial genes from one host cell to another
Conjugation and Plasmids
 Conjugation is the direct transfer of genetic material between bacterial cells that are temporarily joined
 The transfer is one-way:
o One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes
 “Maleness,” the ability to form a sex pilus and donate DNA, results from an F (for fertility)
factor as part of the chromosome or as a plasmid
 Plasmids, including the F plasmid, are small, circular, self-replicating DNA molecules
R plasmids and Antibiotic Resistance
 R plasmids confer resistance to various antibiotics

When a bacterial population is exposed to an antibiotic, individuals with the R plasmid will survive and increase
in the overall population
o Ampicillin
o Kyntamycin
Operons: The Basic Concept
 In bacteria, genes are often clustered into operons, composed of
o An operator, an “on-off” switch
o A promoter
o Genes for metabolic enzymes
 Repressor: switch operon on/off
 Corepressor: cooperates with a repressor to switch an operon off
Repressible and Inducible Operons: Two Types of Negative Gene Regulation
 A repressible operon is one that is usually on
o Binding of a repressor to the operator shuts off transcription
o The trp operon is a repressible operon
 An inducible operon is one that is usually off
o Molecule called an inducer inactivates the repressor and turns on transcription
o The Lac Z operon is an inducible operon