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 Vaccines can prevent certain viral illnesses

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Emerging Viruses Emerging viruses are those that appear suddenly or suddenly come to the attention of scientists Severe acute respiratory syndrome (SARS) recently appeared in China Outbreaks of “new” viral diseases in humans are usually caused by existing viruses that expand their host territory

LE Young ballet students in Hong Kong wear face masks to protect themselves from the virus causing SARS. The SARS-causing agent is a coronavirus like this one (colorized TEM), so named for the “corona” of glyco-protein spikes protruding form the envelope.

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 Some symptoms are spots on leaves and fruits, stunted growth, and damaged flowers or roots

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings

Plant viruses spread disease in two major modes: – Horizontal transmission, entering through damaged cell walls – Vertical transmission, inheriting the virus from a parent

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Viroids and Prions: The Simplest Infectious Agents Viroids are circular RNA molecules that infect plants and disrupt their growth Prions are slow-acting, virtually indestructible infectious proteins that cause brain diseases in mammals Prions propagate by converting normal proteins into the prion version

LE Normal protein New prion Prion Original prion Many prions

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 The well-studied intestinal bacterium Escherichia coli (E. coli) is “the laboratory rat of molecular biology”

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 Many bacteria also have plasmids, smaller circular DNA molecules that can replicate independently of the chromosome Bacterial cells divide by binary fission, which is preceded by replication of the chromosome

LE Origin of replication Replication fork Termination of replication

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 quickly increase genetic diversity More genetic diversity arises by recombination of DNA from two different bacterial cells

LE Mutant strain arg + trp – Mutant strain arg + trp – Mixture No colonies (control) No colonies (control) Colonies grew Mutant strain arg – trp + Mutant strain arg – trp +

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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

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

LE A+A+ Phage DNA A+A+ Donor cell B+B+ A+A+ B+B+ Crossing over A+A+ A–A– B–B– Recipient cell A+A+ B–B– Recombinant cell

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 The transfer is one-way: One cell (“male”) donates DNA, and its “mate” (“female”) receives the genes

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings “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

LE Sex pilus 5 µm

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 F + cells transfer DNA to an F  recipient cell Chromosomal genes can be transferred during conjugation when the donor cell’s F factor is integrated into the chromosome A cell with a built-in F factor is called an Hfr cell The F factor of an Hfr cell brings some chromosomal DNA along when transferred to an F – cell

LE 18-18_1 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient

LE 18-18_2 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor

LE 18-18_3 F plasmidBacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor Hfr cell F – cell

LE 18-18_4 F plasmid Bacterial chromosome F + cell Mating bridge F + cell Bacterial chromosome F – cell Conjunction and transfer of an F plasmid from and F + donor to an F – recipient F + cell Hfr cell F factor Hfr cell F – cell Temporary partial diploid Recombinant F – bacterium Conjugation and transfer of part of the bacterial chromosome from an Hfr donor to an F – recipient, resulting in recombiination

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposition of Genetic Elements The DNA of a cell can also undergo recombination due to movement of transposable elements within the cell’s genome Transposable elements, often called “jumping genes,” contribute to genetic shuffling in bacteria

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Insertion Sequences The simplest transposable elements, called insertion sequences, exist only in bacteria An insertion sequence has a single gene for transposase, an enzyme catalyzing movement of the insertion sequence from one site to another within the genome

LE 18-19a Insertion sequence Transposase gene 5 3 Inverted repeat 3 5 Inverted repeat

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Transposons Transposable elements called transposons are longer and more complex than insertion sequences In addition to DNA required for transposition, transposons have extra genes that “go along for the ride,” such as genes for antibiotic resistance

LE 18-19b Transposon Insertion sequence Insertion sequence Antibiotic resistance gene Transposase gene Inverted repeat

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Concept 18.4: Individual bacteria respond to environmental change by regulating their gene expression A bacterium can tune its metabolism to the changing environment and food sources This metabolic control occurs on two levels: – Adjusting activity of metabolic enzymes – Regulating genes that encode metabolic enzymes

LE Regulation of enzyme activity Regulation of enzyme production Enzyme 1 Regulation of gene expression Enzyme 2 Enzyme 3 Enzyme 4 Enzyme 5 Gene 2 Gene 1 Gene 3 Gene 4 Gene 5 Tryptophan Precursor Feedback inhibition

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings 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 An operon can be switched off by a protein called a repressor A corepressor is a small molecule that cooperates with a repressor to switch an operon off

LE 18-21a Promoter DNA trpR Regulatory gene RNA polymerase mRNA 3 5 Protein Inactive repressor Tryptophan absent, repressor inactive, operon on mRNA 5 trpE trpD trpC trpBtrpA Operator Start codon Stop codon trp operon Genes of operon E Polypeptides that make up enzymes for tryptophan synthesis D C B A

LE 18-21b_1 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor

LE 18-21b_2 DNA Protein Tryptophan (corepressor) Tryptophan present, repressor active, operon off mRNA Active repressor No RNA made

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Repressible and Inducible Operons: Two Types of Negative Gene Regulation A repressible operon is one that is usually on; binding of a repressor to the operator shuts off transcription The trp operon is a repressible operon An inducible operon is one that is usually off; a molecule called an inducer inactivates the repressor and turns on transcription The classic example of an inducible operon is the lac operon, which contains genes coding for enzymes in hydrolysis and metabolism of lactose

LE 18-22a DNA lacl Regulatory gene mRNA 5 3 RNA polymerase Protein Active repressor No RNA made lacZ Promoter Operator Lactose absent, repressor active, operon off

LE 18-22b DNAlacl mRNA 5 3 lac operon Lactose present, repressor inactive, operon on lacZ lacYlacA RNA polymerase mRNA 5 Protein Allolactose (inducer) Inactive repressor  -Galactosidase Permease Transacetylase

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Inducible enzymes usually function in catabolic pathways Repressible enzymes usually function in anabolic pathways Regulation of the trp and lac operons involves negative control of genes because operons are switched off by the active form of the repressor

Copyright © 2005 Pearson Education, Inc. publishing as Benjamin Cummings Positive Gene Regulation Some operons are also subject to positive control through a stimulatory activator protein, such as catabolite activator protein (CAP) When glucose (a preferred food source of E. coli ) is scarce, the lac operon is activated by the binding of CAP When glucose levels increase, CAP detaches from the lac operon, turning it off

LE 18-23a DNA cAMP lacl CAP-binding site Promoter Active CAP Inactive CAP RNA polymerase can bind and transcribe Operator lacZ Inactive lac repressor Lactose present, glucose scarce (cAMP level high): abundant lac mRNA synthesized

LE 18-23b DNA lacl CAP-binding site Promoter RNA polymerase can’t bind Operator lacZ Inactive lac repressor Inactive CAP Lactose present, glucose present (cAMP level low): little lac mRNA synthesized