Bacterial Genetics

Question 1

bacterial chromosomes, plasmids, and viruses are called ____ because they have sites for initiation of DNA synthesis, while insertion sequences, transposons, and pathogenicity islands do not

Answer 1

replicons (may also have sites for partition of replicated DNA into daughter cells)

insertion sequences, transposons, and pathogenicity islands only replicate when integrated into a replicon

Question 2

what is a provirus or prophage?

Answer 2

bacterial chromosome with viral genome integrated into it (via temperate bacteriophage)

later, provirus can excise from chromosome, replicate, and lyse cell

Question 3

virulence genes of bacteria, especially toxins, are often found in ____, such as the Shiga toxin of E. coli

Answer 3

proviruses

[Shiga toxin encoded by a phage]

Question 4

what do insertion sequences (IS) contain?

Answer 4

only the machinery for their own movement - can move from one location to another in DNA

gene for transposase, which recognizes inverted repeats at termini (sequence of nucleotides that is the reverse complement of another sequence farther down)

Question 5

how are transposons different than insertion sequences (IS)?

Answer 5

transposons (Tn) contain genes unrelated to transposition, such as antibiotic-resistance genes

basically, IS inserts near antibiotic-resistance gene, a second copy of IS inserts on the other side of the gene, and now the whole thing can move as a transposon (via transposase, enzyme contained in IS)

Question 6

what is a pathogenicity island (PI)?

Answer 6

very large transposon with 50-100 genes (a “complete kit” of virulence genes - can make a non-pathogenic bacteria pathogenic)

most pathogenic strains of bacteria contain multiple PI

Question 7

DNA transfer between bacteria: transformation vs conjugation vs transduction

Answer 7

transformation: DNA released via lysis

conjugation: DNA transfer via direct cell-cell contact (requires conjugative plasmid)

transduction: DNA packaged into virus, transferred via bacteriophage

Question 8

transfer of DNA between bacteria is one way, and often forms an intermediated called ____

Answer 8

merozygote

*note donor fragment is unstable and lost unless it recombines with recipient chromosome - requires DNA homology

Question 9

describe the process of conjugation via the F-plasmid (E. coli)

Answer 9

1. F (fertility) plasmid DNA transferred from donor (F+) to recipient (F-) via conjugation bridge
2. conjugation bridge breaks, recipient cell left with linear fragment
3. transferred DNA re-circularizes, this cell is now F+ also

Question 10

R-Factors in bacteria

Answer 10

F-like (fertility) plasmids with multiple antibiotic-resistance genes, transferred via conjugation

major issue in medicine, esp. tuberculosis

Question 11

transduction, bacterial DNA transfer via virus particle, occurs in 2 ways:

Answer 11

1. virus particle contains bacterial and not viral DNA (rare)
2. viral genome incorporates bacterial genes (and all progeny viruses contain it)

Question 12

do proviruses, insertion sequences (IS), and transposons require DNA homology?

Answer 12

no, because they have special enzyme-based mechanisms

enables virulence genes to spread to genetically unrelated bacteria

Question 13

antigenic phase variation

Answer 13

continual production of new antigenic variants to evade immune system specificity

created by programmed alterations in DNA (not random mutation) and more frequent than mutation and can be exactly reversed by same process that created it

major issue for vaccine development

Question 14

what are 3 mechanisms of antigenic phase variation of bacteria?

Answer 14

1. inversion of DNA segment
2. recombination between silent and expressed genes
3. stuttering by polymerase during copy of a repeat

Question 15

how does Salmonella alter its H antigen to evade the immune system?

Answer 15

antigenic phase variation of H antigen of flagella

only one gene of flagellin is expressed at a given time (H1 or H2), via catalyzed DNA inversion

(hin, aka h inversion, enzyme - separates H2 gene from its promoter to allow disinhibition of H1 expression)

Question 16

what 2 mechanisms of antigenic phase variation does Neisseria use to evade immune system?

Answer 16

1. phase variation in pili produced via recombination between expressed and silent pilin genes
2. phase variation in outer membrane PII/P2 protein causes variation in copy number of CTCTT repeats, which determines whether PII is produced (disrupts reading frame)