lec 4 Genetic change

Question 1

Bacterial cells are ________

Answer 1

Haploid, so any mutations are passed to daughter cells upon division

Question 2

What are some DNA replication errors that can occur?

Answer 2

DNA polymerase can mis incorporate nucleotides , DNA polymerase normally has proofreading activity and can repair this mismatch. Occasionally mistakes are not repaired and SNP single nucleotide polymorphisms arise. When stressed, the mechanism turns off*

Question 3

What are some types of DNA damaging agents that cause lesions in DNA?

Answer 3

BASE ANALOGS, 5- bromouracil, INTERCALATING AGENTS- acriflavine, ethidium bromide, arcydine orange, UV SUNLIGHT- pyrimidine dimers, CHEMICALS that react with DNA xrays- free radicals damage DNA

Question 4

What repairs DNA damage?

Answer 4

DNA repair mechanisms induced by the SOS repair the DNA damage

Question 5

When are error prone polymerases induced?

Answer 5

under conditions of starvation and stress( lots of DNA damage)

Question 6

What do Pol V and Pol IV do?

Answer 6

Error prone polymerases read past the lesions and do NOT proofread. This causes misincorporation of nucleotides. More errors occur and translation slows down. They do not have proof reading capabilities, so the mutations stay.

Question 7

How do bacteria exist in the human host?

Answer 7

Bacteria exist as populations of millions of cells as microbiota and as pathogens. The induction of error-prone polymerases allows cells in the population to vary in hopes that the mutations may be beneficial.

Question 8

What is the purpose of error prone polymerases?

Answer 8

They allow cells in the population to vary in hopes that the mutations may be beneficial.

Question 9

How does the use of multiple antibiotics to treat infections reduce chances of resistance?

Answer 9

The error prone polymerases give rise to antibiotic resistant mutants. Using multiple antibiotics to treat infections reduces the chance that these pathogens will become completely resistant to antibiotics. When antibiotics are combined, the chance that a bacterium will become resistant to both antibiotics increases greatly. 10 7 to 10 14

Question 10

Myobacterium tub. resistance to Rifampicin is 10 7 , with combined therapy, what happens?

Answer 10

Reduced resistance risk 107x 107= 10 14

Question 11

What do antibiotics target?

Answer 11

They target key functions in the cell, so mutations leading to antibiotic resistance make the bacteria less fit and they are lost from the population of bacteria. can create population of random mutants known as antibiotic resistant clones

Question 12

What is horizontal gene transfer?

Answer 12

It is an efficient way to generate antibiotic resistant bacteria. The major mechanisms of HGT are transformation, conjugation, and transduction.

Question 13

How do competent bacteria take up extracellular DNA from dead cells?

Answer 13

By transformation. Transformation is DNASE sensitive because the DNA is free outside the cell and can be degraded by the DNASE enzyme.

Question 14

How is DNA integrated?

Answer 14

By homologous recombination

Question 15

How are genes integrated into the chromosome?

Answer 15

Genes with mutations are integrated into the chromosome by homologous recombination. This requires a stretch of homology between the incoming DNA and the chromosomal DNA, REC A protein, heteroduplex formation, replication to resolve the heteroduplex.

Question 16

How can antibiotic resistance genes integrate with the DNA?

Answer 16

They can be integrated into the chromosome directly if there is enough homology or be introduced into the DNA if flanking on both sides by genes containing homology.

Question 17

What allows for the development of a pan genome?

Answer 17

Competence .

Question 18

What is a supergenome?

Answer 18

Bacteria with a supergenome have conserved core of genes present in all strains, genes from a pool of distributed genes that differ between strains.

Question 19

Transformation

Answer 19

transfer of free DNA

Question 20

Conjugation

Answer 20

plasmid transfer

Question 21

Transduction

Answer 21

transfer by viral delivery

Question 22

What are bacterial plasmids?

Answer 22

They are covalently closed, double stranded circular DNA. Usually not essential for bacterial growth. Replicate seperately from the chromosome. Narrow host range( present in limited number of bacterial species) broad host range plasmids replicate in many species, conjugative plasmids can transfer.

Question 23

How are plasmids transfered by conjugation prototype F-plasmid E Coli?

Answer 23

The F + cell is the donor and contains the F plasmid. The F- cell is the recipient cell and does not contain the plasmid. The F+ donor synthesizes a pilus, which is a Type IV secretion system*** One strand is transferred, then strands in both cells are replicated so both the donor and recipient now have the plasmid. The recipient converts to a donor, so the process is infectious.

Question 24

F- plasmid can integrate into what?

Answer 24

the chromosome= Hfr

Question 25

What are the three outcomes from an Hfr?

Answer 25

1. F+ the f plasmid excises precisely and nothing happens to the plasmid or chromosome. 2. F’ The F plasmid excises and carries off a small piece of chromosomal DNA with it. The fragment becomes part of the plasmid. 3. HFR mating- the pilus and Type IV secretion systems are induced and transfer event is initiated. The plasmid does not re-circularize and is lost. ( not infectious)

Question 26

How are plasmids detected?

Answer 26

They are detected on agarose gels as distinct bands. Plasmids can be larger or smaller than the chromosomal fragments present on the gel.

Question 27

Can gene transfer events occur between bacteria of different species and genera?

Answer 27

YES

Question 28

What are bacteriophages?

Answer 28

They are bacterial viruses that can switch between lytic and lysogenic cycles. In the lytic cycle, bacterial chromosomal DNA is often degraded into small pieces. The viruses replicate their nucleic acid, package the DNA and lyse the bacteria. In the lysogenic cycle, the phage integrate into the chromosome by non-homologous self directed recombin.

Question 29

What is HGT by generalized transduction?

Answer 29

When the bacteriophage packages a fragment of chrom. DNA instead of viral DNA. The fragment is injected into a new bacterial cell. The chromosomal fragment can be incorporated into the bacterial chrom by homologous recomb. The phage does not contain viral DNA ( contains chrom DNA) so bact cells are NOT INFECTED by the virus. **

Question 30

What is HGT / bacteriophage by specialized transduction?

Answer 30

When a lysogenic phages converts to the lytic cycle it can occasionally take a piece of chrom DNA . The piece of chrom DNA is carried with the phage DNA and is introduced into the bacterial chrom. when the phage reintegrates upon entering the lysogenic cycle. The integration mediated by the phage is non homologous recombination. VERY ERROR PRONE ONLY genes around the virus are transferred.

Question 31

What are transposons?

Answer 31

They carry multiple genes between the insertion sequences. Moves from one piece of DNA to another. Does NOT require RecA or homology= non-homologous recombination.

Question 32

What do conjugative transposons do?

Answer 32

Conjugative transposons encode genes encoding their conjugative transfer to other bacteria. They are also known as ICE integrative and conjugative elements. Move by conjugation.

Question 33

What is MRSA SCCmec?

Answer 33

a large chromosomal cassette containing the methicillin resistance genes and can contain many other resistance and virulence factors. NOVEL transposon like element whose mechanism of movement is unknown. Different cassettes are variable in genetic content.

Question 34

What is Mec A?

Answer 34

mec A makes bacterial resistance to methicillin

Question 35

What are pathogenicity islands?

Answer 35

Large mobile genetic elements encoding virulence factors. Present in pathogenic strains and absent in commensal strains of the same species. contain integrase. tend to be found in tRNA genes.

Question 36

What can antibiotic resistance cause?

Answer 36

Loss in fitness like slower growth rate or reduced survival in stationary phase. In the absence of antibiotic selection, antibiotic resistant strains are often outcompeted and lost from the bacterial population.

Question 37

Where are antibiotic resistant strains more common?

Answer 37

They are more common in hospitals where there is more antibiotic selection and less common in the community where there is less antibiotic selection.