Mobile genetic elements Flashcards
Horizontal gene transfer mechanisms- What are they and give 3 exmaples + how do they contribute to changes in bacterial genome (mutation)
CLUE: TTC
Transferred in one direction (undirectional - donor –> recipient)
- Transformation - bacteria take up of DNA fragments from environment, need to be similar to integrate (homologous recombination)
- taking naked DNA from environment is known as competence (chemical treatment can also be used to induce competence)
- Least medically imp, DNA is vulnerable when naked outside of the cell
- Genetic recombination : Transfer of DNA from one organism to another
- Homologous recombination: Enzymatic mechanism to incorporate the new DNA fragment (requires close homology between the donor DNA and recipient chromosome DNA)
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Transduction - through bacteriophage infection; incorrect packaging of host DNA can transfer new DNA to recipient bacteria (generalised, specialised)
* DNA is resistant to environmental damage because DNA protected by phage protein coat
Two types:
Generalised:
Transfer and packaging of pure bacterial DNA
- Phage injects its DNA into a host bacteria
- Phage enzymes degrades DNA (chromosome)
- Cell synthesizes new phage that incoporates phage DNA + some host DNA mistakenly
- Transducing phage injects donor DNA into host cell
- Donor DNA is incorporated into recipients chromosome by recombination
- Can occur with a virulent (lytic) phage or temperate phage going through a lytic cycle
> Plasmids do not require homologous recombination
Specialised:
Transfer and packaging of small percentage of bacterial DNA with the viral genome
- Prophages are bacteriophage genomes integrated into the host genome (chromosome )
- Excised phage contains some bacterial DNA
- New viral particles are synthesized, some contain bacterial DNA in addition to phage DNA
- Cell lyses and releases new bacteriophages
- Defective phage attaches to another bacterial cell and DNA can be injected (phage and bacterial DNA are now integrated into the host chromosome)
- Occurs with temperate/lysogenic phase
> Phages carry genes for toxins and antibiotic resistance (cholera, botulism and diphteria toxin genes are carried by phages)
- Conjugation - direct transfer of plasmids between bacteria, highly efficient and major mechanism of spread of bacterial virulence and resistance
(A)
Donor has F (factor) F+
- Genes carried on plasmids
- Fertility factor
- Genes for conjugation
- found on a plasmid
- Recipient lacks F factor (F-)
(B)
Plasmid can be intergrated into the chromosome –> episome
- called HFR strains (high frequency transfer + recombination)
- Episome replication and conjugation may sometimes transfer
> Multiple antibiotic resistance plasmids
>Rapid spread of drug resistance
Explain the medical importance of bacterial mutation and mobile genetic elements
Mobile genetic elements(MGE) and mutations are sources of genetic variation.
> MEDICALLY SIGNIFICANT because bacteria can acquire new virulence and resistance genes
> MGE carry genes that can lead to environmental adaptation –> survival advantages ( metabolise new carbon source, deactivate antimicrobial substances)
What are the 3 types of mobile genetic elements and describe thier importance
(3rd type of MBE - What are Insertion Sequence Elements?)
1) Plasmids: (plasmids carry genes)
- circular DNA moelcules found in cytoplasm of many bacteria
- extra chromosomal (not part of the chromosome)
- Replicate independently of chromosome but use the same enzymes –> autonomous replication
- Plasmids integrate into the chromosome: episome
- Mobile pool of extra genes –> upto 100 extra genes
2) Bacteriophage
- Virus that infects bacteria –> leads to death of bacteria
- Two structures
- -* Virulent lytic:
- Reproduce and kill bacteria
-Temperate bacteriophages
- Integrate into chromosome and delay death
- Lysogenic life cycle –> can progress to lytic
- Lysogenic phase DNA inserts into the bacteriums DNA and becomes a non-infectious prophage –> temporarily not lethal to host
3) Transposable elements
- Jump from one DNA site to another
- Mobility mediated by enzymes
- Can result in duplication or removal
- Source of genetic variation and mutation
- Carry antibiotic resistant genes + disease causing genes for toxins
- Jump into and out of plasmids and chromosomes (within a bacteria)
What are Insertion Sequences Elements (IS)?
- Simple form of a transposon
- Only carry genes for transposition and flanked by inerted terminal repeats
- Significance: can cause mutation if inserted into a gene (can be inserted into plasmid)
Extra genes that transposons carry are flanked by IS elements, which have the capacity to translocate the transposon (from chromosome to plasmid) –> plasmid can then transfer these genes to another bacteria
What is the major sources of mutation in bacteria and what is the molecular basis of spontaneous mutations
1) Spontaenous - DNA replication process
2) Chemcial Mutagens - Induced e.g. radiation, chemicals
Mutations are readily expressed in bacteria due to:
- Bacteria being haploid i.e. only one copy of a gene
- Rapid growth rate
Molecular basis of spontaneous mutations:
>Point mutations: Single change in DNA coding sequence
>Deletion: can be large, even deletions of whole genes
>Insertions or duplications: effects vary from none –> reduced survival –> or death. Frame shift mutation –> can terminate gene
