Term
| What are the 4 mechanisms of gene transfer in Bacteria? |
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Definition
1) Transformation (Streptococcus, Haemophilus and Neisseria)- Naked DNA released from lysed cells taken up and recombined into homologous region of recipient DNA.
2) Transduction- Bacteriophage-mediated gene transer from infected donor to infected recipient (homologous recombination).
3) Conjugation- cell:cell contact between male (conjugative plasmid) and female cells
4) Transposon Insertion- DNA segments (resistance, toxin production, ect) that repeatedly insert at different sites. They do NOT occur independently in a cell, but found as PART of a genome (contain ITRs). |
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Term
| What are the primary elements that make up Bacterial Plasmids? |
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Definition
1) Drug resistance genes
2) Replicon (origin, rep gene encoding initiator protein, copy control gene). Replication occurs independently of bacterial chromosome
** require host proteins and enzymes for replication
3) Sex pilus (Tra genes)
4) oriT site (nicking site for initiation of DNA transfer). |
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Term
| Explain the concept of "copy number" and how it relates to plasmid size. |
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Definition
Copy number is fixed for a given plasmid, and it is the number of plasmic molecules per chromosomes in a cell.
Large plasmids have low copy numbers (1-2), while small plasmids have large copy number (>10). |
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Term
| What are "compatible" plasmids? |
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Definition
| Different plasmids in same host cell that are maintained and replicated independently. |
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Term
| What are the different types of plasmids and how do they work? |
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Definition
1) Conjugative- initiate own transfer by cell:cell contact and can MOBILIZE non-conjugative plasmids
example- Resistance-plasmids (R-factor)
2) Virulence Plasmids- carry genes that encode toxins and virulence factors
example- pXO1 in Bacillus anthracis encodes anthrax toxin
pXO2 contains genes for capsule biosynthesis |
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Term
| What is the basic structure of an R-Factor and how can it confer resistance? |
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Definition
R-factors are stable and hardly ever "lost"
- tra genes encode the sex pilus for DNA transfer
- oriT gene for transfer initiation (nicking site)
- May also contain insertion sequences and transposons
1) Single DNA strand is transferred, 5' first, and is then replicated back to dsDNA by host and recipient.
- integron and transposon insertions can increase resistance. |
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Term
| What is an "integron" and how does it relate to drug resistance? |
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Definition
| Integrons are mobile DNA elements encoding an integrase and may carry antibiotic resistance genes under control of integron promor |
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Term
| What are the 2 different types of transposable elements in cells? |
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Definition
1) Insertion Sequences- small (750-1500 bp) elements that carry gnes that are required for their own movement
2) Transposons (>5 kb) carry additional genes and can be "Composite transposons" if they carry insertion sequences at their ends. |
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Term
| What is the basic structure of a typical transposon? |
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Definition
Tn3
1) tnpA encodes "transposase"
2) tnpR encodes repressor of transposition
3) AMPr encodes B-lactamase which confers ampicillin resistance.
4) IRs at ends required for transposition (cleavage site for mobilization) |
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Term
| What is the mechanism of transposition? |
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Definition
Can be intra or inter-molecular and R-factors contain different numbers of tranposons
1) Replicative- semiconservative replication of transposon involving duplication of Tn element
2) Conservative- Insertion of transposon at target site |
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Term
| How does the donor DNA strand in Transformation "replace" the recipient strand? |
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Definition
The ssDNA integrates into a homologous region in the recipient, and causes the recipient's dsDNA to complement it through homologous recombination.
DNA must be similar but not identical! |
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Term
| What factor limits inter-species gene transfer via Transduction? |
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Definition
Phage-docking issues!
A phage may replicate inside Neiserria, picking up DNA and lysig the cell, but it cannot dock on Pseudomonas and inject there without the correct proteins! |
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Term
| Which method of bacterial gene transfer enables inter-species exchange? |
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Definition
| Direct cell-cell contact in Conjugation (both gram + and gram -) |
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Term
| Describe two ways by which multi-resistance can arise extrachromasomally? |
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Definition
1) Multiple plasmids with different resistance genes
2) 1 R-factor with many resistance genes picked up through transposon insertion. |
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Term
| How can the number amount of tnpA expression be clinically significant? |
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Definition
| The amount of tnpA, or "transposase," is a measure of how much transposition of mobile elements is going on, and can indicate the rate of resistance development. |
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Term
True or False:
Both male and female cells of gram + and gram - bacteria contain conjugative plasmids. |
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Definition
False!
Only male cells, but they are present in gram + and gram - strains |
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