IA. General microbiology | 3. Bacterial genetics. Mode of gene transfers. Flashcards
I. Bacterial genetics
1. Where is bacterial DNA found in?
- Bacterial Chromosomes
- Plasmids
- Transposons
I. Bacterial genetics
2. What are the features of bacterial chromosome?
○ Main carrier
○ Present in all bacteria
○ Contains a single circle with ~4000 kilobase pairs
I. Bacterial genetics
3. What are the features of Plasmids?
○ Only found in some bacteria
○ Are for genese of special functions
○ Consists of circular double stranded DNA ~100 kbp
○ Drug resistance is often carried from one bacteria to another via plasmids
I. Bacterial genetics
4. What are the features of Transposons?
○ Smaller kbp sequence of DNA
○ Can migrate from one gene locus to another (plasmid → plasmid or plasmid → chromosome)
○ Are for special functions i.e. antibiotic resistance
○ No information for genetic replication in a transposon
I. Bacterial genetics
5. What is transposable element?
a DNA sequence that can change its position within a genome, sometimes creating or reversing mutations and altering the cell’s genetic identity and genome size
II. Mode of Gene Transfer
1. What are the 2 modes of gene transfer?
- Vertical gene transfer
- Horizontal gene transfer
II. Mode of Gene Transfer
2. What is Vertical gene transfer?
Transmission of genes from parent to offspring via sexual (or asexual) reproduction
II. Mode of Gene Transfer
3. What are the features of Horizontal gene transfer?
Transfers genes between organisms in a non-traditional reproductive method
- Used in genetic engineering
- Primary cause of antibiotic resistance
- Mechanisms: transformation, transduction, conjugation, and transposons
II. Mode of Gene Transfer
4A. What are the 4 mechanisms of Horizontal gene transfer?
Mechanisms: transformation, transduction, conjugation, and transposons
II. Mode of Gene Transfer - Horizontal gene transfer
4B. What are the features of Transformation?
- After cell death/lysis some bacteria release their DNA into the environment
- Other bacteria (especially those of same species) can come into contact and pick up these DNA fragments → incorporated into bacterial DNA by recombination
- Any non-integrated DNA is degraded
- End result is a recombinant cell (different from donor and recipient)
- Results in an increase in genetic diversity
- First shown in Griffith’s Experiment
II. Mode of Gene Transfer - Horizontal gene transfer
4C. What are the features of Transduction?
- Involves the transfer of DNA from one bacteria to another using a bacteriophage
- Phage is a virus that infects bacteria
+) E.g. T4 phage or lambda phage both infect E. coli - Phages are obligatory intracellular parasites
- Phages reproduce by either the lytic (kills host) or the lysogenic (does not kill host) cycle
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
5A. What are the 5 steps of lytic cycle?
- Attachment of T4 to receptors on E. coli cell wall.
- Penetration of the cell wall by tail core. Inject DNA into host.
- E. coli DNA is hydrolyzed. Phage DNA directs biosynthesis of viral parts using the host cell’s machinery.
- The phages mature as the parts are assembled.
- Lyses of E. coli and release of the new phages.
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
5B. What are the 5 steps of Lysogenic cycle?
- Phage attaches to E. coli and injects DNA
- Phage DNA circularizes and can enter either the lytic or the lysogenic cycle.
- The lytic cycle would occur as previously described.
- In the lysogenic cycle the circular phage DNA recombines with E. coli DNA and the phage DNA is now called prophage.
- E. coli undergoes cell division, copying prophage and passing to daughter.
+) With more divisions there are more cells with the prophage. - The prophage may exit the chromosome and start a lytic cycle at any time.
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
6. What are 6 steps of bacteria transduction?
- A phage attaches to a cell wall of bacterium and injects
- The bacterial chromosome is broken down and biosynthesis of phage DNA and protein occurs.
- Sometimes bacterial DNA can be packaged into the virus instead of phage DNA.
+) This phage is defective (can’t destroy another host cell) because it does not carry its own genetic material. - The cell lyses, releasing viruses.
- The phage carrying bacterial DNA infects another cell.
- Crossing over between donor and recipient DNA can occur producing a recombinant
cell.
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
7. What are the 2 types of transduction?
- Generalized transduction
- Specialized transduction
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
8. What is Generalized transduction?
Any bacterial genes are transferred , since the host chromosome is completely broken down, and whatever DNA gets packaged in the phage is what is transferred between cells.
II. Mode of Gene Transfer - Horizontal gene transfer - Transduction
9. What is Specialized transduction?
- Only certain genes are transferred
- Requires a phage that uses lysogenic cycle since the prophage needs to take some of the bacterial DNA with it
II. Mode of Gene Transfer - Horizontal gene transfer - conjugation
10A. What are the features of conjugation?
- is when the male cell extends his sex pili and one attaches to the female (F-)
- Requires an F plasmid, a special plasmid; F = fertile
- When the F plasmid transfers, it can take some of the host DNA with it which is also integrated into the attached bacteria
II. Mode of Gene Transfer - Horizontal gene transfer - conjugation
10B. What are the features of F plasmid used in conjugation?
- It can give a bacterium new genes that may help it survive in a changing environment
- F plasmid consists of 25 genes that mostly code for production of sex pili
- Bacteria with F plasmid = male / F+
- F plasmid can be an episome (Episome: when a plasmid becomes integrated into the host DNA)
- When the F plasmid transfers, it can take some of the host DNA with it which is also integrated into the attached bacteria
II. Mode of Gene Transfer - Horizontal gene transfer - conjugation
10C. F plasmid can be an episome
=> What is an epiosome?
Episome: when a plasmid becomes integrated into the host DNA
II. Mode of Gene Transfer - Horizontal gene transfer - conjugation
10D. Where does conjugation usually occur?
Usually only occurs in gram negative bacteria:
- E. coli
- bacteroids
- enterococci
- streptococci
- streptomycetes
- clostridia
