Bacterial Genetics

Question 1

What is an operon?

Answer 1

Sequence of DNA that contains multiple genes under the control of a single promoter and termination sequence

Question 2

What is polycistronic mRNA?

Answer 2

mRNA that has information for more than one gene (or protein product)

Question 3

What is a biotype?

Answer 3

Group of organisms that have a specified genotype

Question 4

What is an insertional mutation?

Answer 4

Disruption of a gene by a transposon (jumping gene)

Question 5

What is an inversion mutation?

Answer 5

When the segment of DNA is removed and reinserted in the reverse direction

Question 6

How has M. tuberculosis become antibiotic resistant?

Answer 6

A mutation in the 30S ribosomal subunit (which normally binds aminoglycosides) results in the inability for the antibiotic to bind thus allowing protein synthesis to proceed normally

Question 7

What is antigenic variation and phase variation, how are they different?

Answer 7

Antigenic variation: is a change in the composition (AA sequence, w/o change in the structure) or a change in the structure of a molecule
Phase variation: is the turning “on” or “off” expression of a molecule (so part of the population will be expressing a gene while the other part will not, this keeps the immune system guessing)

Question 8

What are the 4 mechanisms by which phase variation can occur?

Answer 8

1. Phenotypic modulation due to a deletion
2. DNA inversion
3. Point mutations
4. Insertion of a gene

Question 9

What is the mechanism by which antigenic variation occurs?

Answer 9

Silent genes (without promoter) recombine with another part of the genome that has an expression locus which leads to the expression of a new/different antigenic molecule

Question 10

What are conjugative plasmids and how do they function?

Answer 10

They are plasmids which encode transfer enzymes and sex pili (Tra genes) for transfer of itself (the plasmid) to other cells. This can support the transfer of non-conjugative plasmids

Question 11

What are the two lifestyles that a bacteriophage can assume?

Answer 11

Lytic (virulent): Rapid replication after infection, which results in creation of progeny and lysis of the host
Lysogenic (Temperate): Integration of phage DNA inot the chromosome or the formation of a plasmid (prophage)

Question 12

How does the prophage function when in the lysogenic state?

Answer 12

Prophage is usually bound by a repressor which keeps the phage genes from replicating. However if the repressor is cleaved (perhaps by a signal) if will reenter the lytic state.

Question 13

What are integrons?

Answer 13

Genetic elements that contain a site at which gene cassettes can be integrated by site-specific recombination

Question 14

What is a transposable element?

Answer 14

Genes which have the ability to mediate their replication and move from one site to another within the genome

Question 15

What is the smallest and most simple of all of the transposable elements? How might they function in the genome?

Answer 15

Insertion sequence (IS)
They may contain promoters such that when the IS moves within the genome they enhance the expression of other genes

Question 16

What are transposons?

Answer 16

Complex transposable elements that contain genes for transposition but also include genes that encode for antibiotic resistance and other virulence properties

Question 17

What are pathogenicity islands (PAI)? Where in the genome are they sometimes located near?

Answer 17

They are essentially just larger transposons which carry multiple genes. They tend to be found near the tRNA genes

Question 18

What are the 2 enzymes that are encoded within a transposable element that allow it to move around?

Answer 18

Transposase and Resolvase

Question 19

What are the 3 methods of horizontal gene transfer?

Answer 19

1. Transformation
2. Conjugation
3. Transduction

Question 20

How does transformation occur?

Answer 20

Free floating (naked) DNA, usually from a lysed cell is taken up by competent bacteria

Question 21

What are competent bacteria?

Answer 21

Bacteria that are naturally capable of taking up exogenous DNA across their membrane without manipulation

Question 22

What is conjugation be specific

Answer 22

An intimate interaction between a donor male (F+) cell which contains the conjugative plasmid and a a recipient female (F-) cell which does not already contain the plasmid.
Transfer specifically involves a sex pili (encoded by Tra genes)

Question 23

What is transduction be specific

Answer 23

Also called lysogenic conversion, it is a horizontal gene transfer process by which a bacteriophage serves as the vector.
Bacteriophage when in the lytic phase will package more DNA than they require thus stealing certain genes from the “donor” cell, and when they infect the “recipient” cell and enter the lysogenic phase they transfer the genes

Question 24

What is an example of a human pathogen which has undergone transduction which results in virulence

Answer 24

Corynebactium diphtheriae (aka the bacteria which causes Diphtheria). Some of their population has been lysogenized by a bacteriophage containing the gene for the diphtheria toxin