Bacterial Genetic Variation (complete)

Question 1

What are the two mechanisms by which bacteria regulate gene expression in order to adapt to new environments?

Answer 1

1) Regulation of transcription

2) Control of transcription by DNA rearrangement

Question 2

Describe how bacteria regulate transcription when adapting to new environments

Answer 2

• Bacteria can increase/decrease transcription based on the new environments
• Done by DNA-binding proteins (interact with promoter regions)

Takes me back to the M2M days… but it makes sense, right?

Question 3

Describe how control transcription by DNA rearrangement when adapting to new environments

Answer 3

This is best illustrated with an example

• Salmonella contain 2 genes for making flagella (H1 and H2) — but they can only make 1
• The environment controls which one is expressed b/c the promoter can be inverted
• If the bacteria wants H2 expressed, it will produce an H1 repressor that minds to the operator portion of gene —» prevents transcription of H1

Question 4

What are the different mechanisms that generate genetic diversity within a bacterial species?

Answer 4

1) Spontaneous mutation
2) Recombination
3) Acquisition of new DNA segments

Question 5

Describe spontaneous mutation.

Answer 5

• Errors b/c of base pair changes, deletions, duplications — often don’t affect anything
• RARE: a mutation can do something positive
• Occur 1 in a billion but b/c bacteria grow in great density, it happens more often than you’d think

OVERALL: mostly not noticeable, but if it is, mostly bad, rarely good

Question 6

What are the two types of recombination seen in bacterial genetic variation?

Answer 6

1) Antigenic variation

2) Genetic exchange between related organisms

Question 7

Describe antigenic variation

Answer 7

Similar concept to the H1/H2 Salmonella gene inversion

• Some bacterial genes have multiple silent- non-expressed copies of variant genes for the same structure expression

Question 8

Describe genetic exchange between related organisms.

Answer 8

• Produce recombinants that have new phenotypic traits
• Relates to plasmid exchange or transposons
• More on this later

Question 9

What are the different ways to acquire new DNA segments?

Answer 9

1) Acquisition of transposable elements
2) Bacteriophage conversion
3) Acquisition of plasmids
4) Acquisition of pathogenicity islands

Question 10

What is a transposon? Discuss its replication. For what type of protein does it encode?

Answer 10

AKA: transposable element

• DNA segment that can move itself (or a copy of itself) from one chromosome to another
• Not capable of self-replication except as part of a plasmid, bacterial chromosome, or virus
• Typically encodes for one/more proteins that mediate transposition (TRANSPOSASE)

Question 11

Describe the process of transposition

Answer 11

• A transposon is introduced into a cell — as a component of plasmid/bacteriophage
• Distinct nucleotides will be recognized by enzyme and moved
• It may transpose and become stable and permanently integrated into the new chromosome

Question 12

What is an insertion sequence?

Answer 12

IS

• Transposons that encode transposase
• Play role in genome evolution
• Inactivates genes into which they transpose
• OR turn on gene expression adjacent to the genes they’ve transposed

Question 13

What are complex transposons?

Answer 13

• Carry add’l genes

- Encode antibiotic resistance, toxins, adhesions, and other virulence factors

Question 14

Describe bacteriophage conversion

Answer 14

• Certain virulence genes are on bacteriophages — not a normal part of bacterial genome
• THEREFORE a virulence factor is only carried/expression by bacterial strains that have been lysogenized and bacteriophage is maintained by bacterium

Question 15

What are bacterial plasmids?

Answer 15

• Self-replicating, extrachromosomal DNA elements
• Usually circular
• Range from a few genes to a few % of chromosome
• Non-essential

Question 16

What are the functions of bacterial plasmids?

Answer 16

Can be any of the following:

1) Resistance to antibiotics, heavy metals
2) Virulence factors
3) Metabolic functions
4) Self-transmission (can promote their own transfer from one cell to another!) WOWOW

Question 17

What are pathogenicity islands?

Answer 17

• large segments of DNA present in some bacterial species
• Encode genes that contribute to virulence — bacteria w/o PI not virulent or have different disease causing potential
• Molecular appearance suggests they were acquired by an unrelated organism

Question 18

Describe transformation as a mechanism of gene transfer. Identify the “salient” features.

Answer 18

• Occur in G(+) and G(-) bacteria
• Active component: naked DNA (chromosome fragments from lysing cells or plasmids!)
• Only able to be transformed into new cell at certain points of the growth cycle (competency)
• Occurs most frequently between the same species

Question 19

Describe transduction generally speaking

Answer 19

• Gene xfer mediated by a bacteriophage

- Closely related with lift cycles of bacteriophages

Question 20

What are the stages of bacteriophage reproduction?

Answer 20

1) Growth of virulent bacteriophage
2) Temperate phages and lysogenic bacteria
3) Mechanism of transduction
4) Bacteriophage conversion

Question 21

Describe the growth of virulent bacteriophage

Answer 21

• Virus adsorbs to bacterial cell surface
• Injects its nucleic acid into cell
• Viral genome is replicated, transcribed, and translated

Question 22

What is a lytic response?

Answer 22

Leads to:

1) Phage multiplication
2) Host cell lysis

Question 23

What is a lysogenic response?

Answer 23

• Host cell stays alive (unlike lytic response)

- The infecting phage DNA maintained by host cell in a non-infectious stage (PROPHAGE)

Question 24

Describe the life of a prophage

Answer 24

• Can be induced to enter lytic state —» viral replication/production, cell lysis
• Repressor protein maintains lysogenic stage, which blocks gene expression necessary for viral DNA replications/lytic development

Under stress —» lytic state

Question 25

What is a temperate phage?

Answer 25

• Do not invariably kill their host cells

- May elicit a lytic or lysogenic response

Question 26

Describe bacteriophage/lysogenic conversion

Answer 26

• Temp bacteriophages can encode genes that express during lysogenic phase —» appearance of a new phenotypic trait
• Genes controlling the new phenotype trait are found ONLY as a component of the phage genome

Question 27

Describe bacterial conjugation

Answer 27

• Form of genetic transfer dependent on physical contact between donor and recipient cells
• Mediated by bacterial plasmids

Question 28

Describe the plasmid F paradigm. What is the mechanism of conjugation?

Answer 28

• Plasmid F: prototype self-transmissible plasmid
• Contains genes that encode for:
  1) Autonomous replication of plasmid DNA
  2) Sex pili synthesis
  3) Conjugative xfer of F DNA to recipient cells
  4) Ability to integrate into bacterial chromosome

GO LOOK AT THE DIAGRAM!

Question 29

What are conjugate transposons?

Answer 29

• Mobile elements
• Mediate conjugation between pairs of cells
• The transferred DNA is the conjugative transposon!
• The thingy transposes to recipient chromosome

May encode for antibiotic resistance!!