L19: Microbial Genetics

Question 1

Describe two types of genetic material in bacteria

Answer 1

1. ) Bacterial chromosome: large circular DNA molecule containing all genes for survival
2. ) Plasmid: small circular DNA molecule separate from bacterial chromosome with copy number that varies from 1-100

Question 2

What is episome?

Answer 2

• When plasmid integrates into bacterial chromosome

Question 3

What is a polycistronic mRNA?

Answer 3

• One promoter drives expression of multiple genes

Question 4

Contrast repression and activation of bacterial genes

Answer 4

1. Repression = negative gene regulation. Repressor binds to the operator sequence, which is close to or overlapping the promoter. This association prevents polymerase from binding to promoter and prevents production of mRNA.
2. Activation = positive gene regulation. UAS (universal activator sequence) upstream of promoter binds activator, which then allows polymerase to bind to and produce mRNA

Question 5

Two types of gene transfer in bacteria

Answer 5

1. ) Vertical gene transfer – genes passed via replication to progeny
2. ) Horizontal gene transfer – genes passed via transformation, conjugation and transduction

Question 6

Why is horizontal gene transfer important to bacterial survival?

Answer 6

• Bacterial can become antibiotic resistant and produce new virulence factors more quickly via these processes than simply relying on vertical gene transfer

Question 7

What is homologous recombination? Proteins that facilitate homologous recombination in bacteria? Importance?

Answer 7

• Exchange of DNA between two DNA molecules based on homology
• Rec protein family including recA mediates this
• DNA incorporated into c/s passed to daughter cells and becomes permanent part of the genome

Question 8

Compare and contrast transformation, conjugation and transduction

Answer 8

1. ) Transformation: uptake of free DNA (from another bacterial cell that lysed for example) from the environment
2. ) Conjugation: DNA transferred directly from one bacterial cell to another through cell contact
3. ) Transduction: transfer of bacterial DNA via a bacteriophage

Question 9

What do bacterial cells need in order to undergo conjugation if they are the donor?

Answer 9

• Require F-factor (fertility factor), which carries the tra operon that encode components of the sex pilus that forms the physical bridge between it and another bacterium

Question 10

What is the oriT?

Answer 10

• Location on F-factor where single strand break occurs for transfer to another bacterium

Question 11

Can conjugation take place between any bacteria?

Answer 11

• Must be between F+ (bacterium carrying F-factor in plasmid or episome (Hfr-high frequency recombination) and one that doesn’t, F-.
• F+/Hfr is always the donor, F- is always the recipient

Question 12

In conjugation is transfer of genetic material bidirectional?

Answer 12

• No, always from Hfr/F+ to F-

Question 13

Describe conjugation between F+ and F- bacteria

Answer 13

• Sex pilus is formed on F+ bacterial cell and links into F- cell
• Break made at oriT in F+ donor and entire single strand (including tra operon at end) from plasmid is transferred to F- cell
• Previously F- cell is now F+

Question 14

Describe conjugation between Hfr and F- bacteria

Answer 14

• Sex pilus is formed on Hfr bacterial cell and links into F- cell
• Break made at oriT in Hfr donor and single strand starts moving into F- cell
• Conjugation doesn’t typically last long enough for entire genome to move into F- cell, so it doesn’t receive tra operon genes. Does however receive some other homologous genes and undergoes homologous recombination. Donor cell doesn’t undergo F- to Hfr sex change

Question 15

Two life cycles of bacteriophage

Answer 15

1. ) Lytic/virulent phage: bacteriophage inserts genetic material into host, undergoes replication, lysis bacterial chromosome and reproduces followed by lysing of host
2. ) Temperate phage: bacteriophage integrates its genetic material into bacterial chromosome in process known as lysogeny. Event causes excision of only phage DNA (generalized) from bacterial chromosome and entry into lytic life cycle. Specialized transduction refers to case when excision of phage DNA includes small portion of bacterial genome in its.

Question 16

Clinical relevance of lysogenic phages (prophages)

Answer 16

• Many diseases caused by bacteria are a result of toxins carried to the bacteria via lysogenic bacteriophages

Question 17

Out of the following processes, which require homologous recombination?

Answer 17

A.) transformation
B.) conjugation
C.) transduction
- Transformation and transduction (when lysogenic) require homologous recombination
- Conjugation between F+ and F- does not require homologous recombination, while between Hfr and F- it does

Question 18

3 categories of antibiotic resistance

Answer 18

1. ) Intrinsic: d/t inherent structural or functional characteristics not transferable between bacteria, not increasing among bacterial population – eg. mycoplasma lacks cell-wall
2. ) Chromosome-mediated: chromosomal genes encode antibiotic resistance via random mutations or horizontal gene transfer – eg. Ribosomal proteins, transpeptidases (PBPs), DNA gyrase
3. ) Plasmid-mediated: genes located on plasmid that encode antibiotic resistance – typically degrade or modify antibiotic or pump it out

Question 19

Describe the components of a resistance factor (R-factor) plasmid

Answer 19

1. ) Resistance transfer factor: equivalent to F-factor, contains oriT and tra operon necessary for conjugation to occur
2. ) Resistance determinant: gene for drug resistance

Question 20

Compare and contrast transposons, integrons and pathogenicity islands

Answer 20

1. ) Transposons: aka jumping genes, DNA elements that can transfer / copy themselves from one molecule of DNA to another. Require transposase gene (TnpA), which they carry with them that recognize palindromic sequences flanked on either end of the transposon.
2. ) Integrons: group of genetic elements that encode a site-specific recombination system that can capture antibiotic resistance gene cassettes and coordinately express them under control of the integrin promoter. Can be associated with transposons.
3. ) Pathogenicity islands: region of bacterial chromosome acquired through horizontal gene transfer that often carry coordinately regulated virulence genes (adhesins, toxins, iron uptake systems, invasins) surrounded by insertion sequences – certain environmental stimuli can trigger island to coordinate virulence gene expression

Question 21

Which of the following types of horizontal DNA transfer requires close contact between two bacterial cells?

1. ) transformation
2. ) specialized transduction
3. ) general transduction
4. ) conjugation
5. ) replication

Answer 21

• Conjugation via sex pilus

Question 22

Which term best describes a diverse group of genetic elements that can capture and coordinately express antibiotic resistance genes and play an important role in the development of multi-drug resistance in bacteria?

Answer 22

• Integrons

Question 23

In which of the following conjugal crosses is a recipient cell converted into a donor cell after the transfer of genetic information by conjugation?

1. ) Hfr x Hfr
2. ) F+ x F-
3. ) F+ x F+
4. ) Hfr x F-
5. ) Hfr x F+

Answer 23

• F+ x F-

Question 24

Which one of the following types of antibiotic resistance is not increasing and is not transferable via horizontal gene transfer?
A. plasmid-mediated resistance
B. chromosome-mediated resistance
C. intrinsic resistance

Answer 24

• C. Intrinsic resistance