5 - Bacterial Genetics

Question 1

How does genetic material in bacteria differ from our genetic material?

Answer 1

Usually single, circular chromosome.

DNA NOT compartmentalized, localized in bacterial cytoplasm.

Often contains plasmids in addition to chromosome.

Chromosome condensed by supercoiling.

Question 2

What is the mechanism of chromosome/plasmid replication and cell division on bacteria?

Answer 2

Single origin, 2nd round of replication can start before the cell divides.

Replicated DNA partitioned into daughter cell, binary fission occurs.

Question 3

How does transcription and translation differ in prokaryotes compared to eukaryotes?

Answer 3

Genes are encoded within operons (euk don’t have operons), there’s NO introns or exons (no splicing) and there’s only 1 RNA pol (euk have 3 RNA pol).

Transcription/transclation are coupled since there’s not compartments. Ribosome is 70S.

Question 4

What results from binary fission? How do bacteria generate genetic diversity?

Answer 4

Following binary fission, all daughter cells are clonal; however, clones can undergo mutations that confer resistance and then the cells can survive in an environment that they previously would not have survived in.

Question 5

How does drug resistance develop?

Answer 5

Replication errors induce mutations into gene.

DNA pol misincorporates nucleotide and does not correct this mistake though proofreading.

Most mutations do not confer a selective advantage.

Question 6

What is the freq of mutation in bacteria?

Answer 6

~1 mutation per 300 chromosome replications.

10^-6 or -7 mutations per genome per generation.

Question 7

Besides mutations, how else can bacteria develop resistance?

Answer 7

Vertical transmission to progeny.

Horizontal transmission: exchange of genetic transmission from one cell to another.

Question 8

Bacteria are promiscuous with their DNA, what does that mean?

Answer 8

The species barrier in bacteria is much less stringent than in eukaryotes.

Question 9

What benefit does gene exchange allow bacteria?

Answer 9

PRovides then with a mechanism to create genetic diversity. Only need a single organism in a population to survive.

Helps then out-compete or kill other organisms and provides selectrive advantage.

Question 10

Horizontal gene transfer largely responsible for rapid spread of ____ ____? what is an example?

Answer 10

Antibiotic resistnace.

Vancomycin resistance from VRE to VRSE.

Question 11

Exchange of genetic material between bacteria occurs with great frequency and efficiency where?

Answer 11

Outside and within a host.

Question 12

What are three exchangeable genetic elements?

Answer 12

1. plasmids
2. transposable genetic elements
3. Pathogenicity islands

Question 13

What is of most concern when it comes to genes being exchanged between bacteria?

Answer 13

Genes encoding virulence factors and antibiotic resistance determinants.

Question 14

What are plasmids? How can they be transferred?

Answer 14

ss or ds DNA molecules that replicate independently.
Single F (fertility) plasmids or multiple copies can be in cells.
Can be 1500 bp to 400,000 bp.

Can be transferred between bacteria via transformation, conjugation, and transduction.

Question 15

What are transposable genetic elements?

Answer 15

Linear DNA segments can be mobilized from one location to another; Often disrupt genes in recipient following transposition.

Cannot replicate on own. Once transferred element can transpose from one location to another in the DNA. Must be present on a plasmid or chromosome to be maintained and passed to daughters.

Question 16

Transposable elements possess ___ ____ ____ at their ends. What recognizes these?

Answer 16

Inverted terminal repeats (ITR)

Transposase (tnp) enzyme recognizes ITR and cuts the DNA, allowing transposition of element from one location to another. Tnp regulated by TF called tpnA.

Question 17

What are the different types of transposabel elements?

Answer 17

Composite transposons that insert between two insertion sequences.

Mu bacteriophage: insertion sequence carried in genome of bacteriophage. This phage can carry element from one cell to another.

Question 18

What is an example of an insertion sequence in the bacterial genome that regulates production of a virulence determinant?

Answer 18

Phase variation of fibriae production: transposase has undergone a mutation that has it locked in the front (promotor) part of the gene that encodes fimbriae.

Insertion sequence undergoes site-specific inversion resulting in a spontaneous switch between non-fimbriated and fimbriated forms.

Question 19

What does fimbriae promote the attachment to?

Answer 19

Urinary tract epithelial cell surface.

Question 20

What are genomic pathogenicity islands? What do they often encode?

Answer 20

Large segments of bacterial genome carried plasmid or by bacteriophage.

Often encode determinants that give bacteria survival advantage within a specific environment.

Question 21

How do genomic pathogenicity islands differ from the majority of chromosome? How are they acquired? What do they encode?

Answer 21

G + C content differs; Acquired by horizontal gene transfer from another organism.

10,000 bp to 200,000 bp

Can encode adherence factors, invasion genes, iron uptake systems, protein secretion systems, and toxins.

Question 22

Can genomic pathogenicity islands replicate by themself?

Answer 22

Nah, must transpose onto a replicon (plasmid or chrom) in the recipient to be maintained and passed to daughter cells following replication.

Question 23

What are the three mechanisms by which plasmids, transposable elements, and pathogenicity islands are transferred between bacteria?

Answer 23

1. Transformation: DNA uptake from environment
2. Conjugation: direct cell to cell contact (via sex pilus)
3. Transduction: mediated by bacteriophage.

Question 24

What type of bacteria can do transformation? What happens to the DNA before it can be internalized by the recipient? What needs to occur for stable inheritance?

Answer 24

Uptake of DNA from environment. Occurs in gram + and - bacteria.

DNA released from donor following lysis, dsDNA bound by recipient then processed to ss before internalization.

Stable inheritance requires recombination with host chrom.

Question 25

What is conjugation and what does it require? What type of bacteria can do this? What does donor retain?

Answer 25

DNA passed unidirectionally from donor to recipient via mating bridge; requires physical contact (pilus contracts to bring cells closer)

Gram + and - can do this.

Donor retains copy of original genetic material.

Question 26

What are bacteriophages? What is their life cycle?

Answer 26

Obligate intracellular parasites (bacterial viruses); they are single nucleic acid molecules protected by protein coat or capsid.

Variable life cycle: lytic, lysogenic, temperate (can do both).

Question 27

What is the lytic lifecycle? What is the lysogenic life cycle? What is a temperate lifecycle?

Answer 27

Lytic: make new progeny phage; phage genome replicated independently of host genome.

Lysogenic: vertical transmission of phage; phage genome integrated into and replicated with bacterial genome.

Temperate: can switch between these life cycles.

Question 28

What happens during lytic replication of phage? What are the progeny called and what do they do?

Answer 28

Segment of bacterial genome accidentally packaged into phage particle rather than phage genome.

Results in progeny phage termed transducing phage, which only carry bacterial DNA (no phage DNA present so they can’t make more phage)

Unpon infection of new bacterium, transduction phage releases fragment of bacterial genome into host cytoplasm (transduction)

Question 29

What is a transductant?

Answer 29

Stable inheritance of transferred material as a result of recombination between introduced bacterial DNA and endogenous host host DNA results.

Question 30

What are two virulence determinants that are carried on bacteriaphage? How do these differ from transducing phages?

Answer 30

Cholera toxin and Shiga toxin.

These are different from transducing phages because toxins are part of the phage genome. (and transducing phages only carry bacterial DNA without phage DNA)

Question 31

What are characteristics of the cholera toxin phage? What does it infect?

Answer 31

Filamentous bacteriophage that infects vibrio cholera.

Genome contains genes encoding cholera toxin ctxAB: comprised of core elements and repetitive sequence.

Question 32

Describe the gene that encodes cholera toxin phage?

Answer 32

CtxAB is made of a core element and repetitive sequence. Ctx is 5B to 1A (5 binding subunits and 1 enzymatic subunit).

Ace and Zot dual functions for phage morphogenesis and enterotoxic activity.

Question 33

What is the mechanism of cholera toxin activity?

Answer 33

A-5B toxin

1. B subunits bind ganglioside GM1 receptor on the surface of intestinal epithelial cells.
2. A subunit is internalized and interacts with G proteins regulating adenylate cyclase.
3. Ctx induces conversion of ATP to cAMP which results in enhanced secretion of water and electrolytes (rice water diarrhea).

Question 34

What is the structure of the Shiga toxin? What is it made by?

Answer 34

Temperate phage with icosahedral head and short tail. Genome carries genes encoding shiga toxin (stxAB).

Stx is 5B to 1A (5 binding subunits with 1 enzymatic subunit)

Stx made by both shigella dysenteriae and EHEC strains of E. coli (enterohemorrhagic e. coli).

Question 35

What does the shiga toxin induce (symptoms)?

Answer 35

Severe diarrhea, hemorrhagic colitis, and hemolytic-uremic syndrome (HUS).

Question 36

What is the mechanism of the shiga toxin (A-5B toxin) ?

Answer 36

Mechanism identical between chiga and shiga-like toxin (Sttx1 and Stx2)

1. B subunit binds Gb3 glycolipid
2. A subunit translocated into cytosol and modifies ribosome acceptor site.
3. Shiga toxin blocks protein synthesis