-
Changes in genetic material
allow organism to prosper and adapt to conditions that favor survival
-
two major ways Bacteria bring about change
-
Mutation
alteration in bacterial chromosome by a change in the DNA
-
Recombination
chromosomal alteration by acquisition of new DNA from another organism
-
The Bacterial Chromosome
- Single molecule of DNA in a double stranded helix, existing in a closed loop
- Found in outer spacein cytoplasm called the “nuceloid”
- Locus – individual sites to which genetic activity can be attributed (one or several genes)
-
1.5mm long DNA fit into a cell 1.0um in diameter?
- DNA actually existsin “loop domain” structure
- Regions of chromosome attach to each other at Anchorage points resulting in a flower like structure
- Sturdy cell wall is able to withstand this pressure
-
Plasmids
- closed loops of DNA in cytoplasm
- not essential for the life of the cell, usually have no effect
- may confer selective advantages to microorganism that have them
-
plasmid advantages
- R factors – carry genesfor antibiotic resistance
- Bacteriocins – proteinstoxic to other bacteria
- Toxins: encode for toxin production
-
Bacterial Mutation
- usually involves a disruption of the base sequence in DNA molecule – OR a loss of a significant part of the gene
- ultimately leads to mistakes in protein synthesis – thus, mutation results in significant changes in cell’s biochemical functions
-
Two Basic Categories of Mutation
- Spontaneous mutation
- Induced mutations
-
Spontaneous mutation
due to natural events, NOT the result of human intervention or any other identifiable cause
-
Induced mutations
actively caused by agents called mutagens (ie: ultraviolet light, nitrous acid)
-
Mutation fighting mechanisms do exist in cell
- Diverse array of DNA repair enzymes (actlike checkpoints)
- Patrol DNA-Locate and repair alterations and distortions If these mechanisms fail, mutation ispermanent (once cell divides)
-
Transposable Genetic Elements
another source of mutation There are two types: Insertion sequences and Transposons
-
Insertion sequences
- small sequences of DNA found at 1 or more sites on a chromosome
- Insert copies somewhere else in chromosome
- Addt’l bases added atinsertion site (interruption of coding sequence and/or loss of fuction)
- Movement is reciprocal (meaningthe copy moves and the original stays in place)
-
Transposons (“Jumping Genes”)
a. Generally larger than
insertion sequence (meaning they contain more genes)
b. No copies involved (hence
the name jumping) original genes jump to new location
c. Disrupts protein
synthesis
d. Unlike insertion sequences : movement
of transposons is non-reciprocal. When
transposon moves, nothing replaces it
e. Can be a source of genes
coding for antibiotic resistance
- Generally larger than insertion sequence (meaning they contain more genes)
- No copies involved, original genes jump to new location
- Disrupts protein synthesis
- Unlike insertion sequences : movement of transposons is non-reciprocal
- When transposon moves, nothing replaces it e. Can be a source of genes coding for antibiotic resistance
-
Bacterial Recombination
The genetic process in which 2 organisms, a donor and a recipient, are involved (either sharing chromosome DNA or plasmid)
-
Bacterial Recombination- plasmid
- Exist independently in recipient’s cytoplasm
- Begin to multiply and encode proteins immediately
-
Bacterial Recombination- chromosomal DNA
- Transferred DNA pairs to complementary region of recipient
- chromosomal DNA and replaces it
- No change in quanity, only quality
-
Three forms of Bacterial Recombination
- Transformation
- Conjugation
- Transduction
-
Transformation
- Mixed population of bacteria
- A donor DNA fragment passes through cell wall and plasma membrane of recipient is called competency (when a recipient cell can take up new DNA)
- Significance:
- May increase pathogenicty of microorganism
- May confer drug resistance on microorganisms
-
Conjugation
- Two live bacteria come together, and donor transfers genetic material to recipient
- Two mating types:
- “F+” – denotes donor DNA
- “F-“ – denotes recipient DNA
- Transferred via sex pili
-
Transduction
- Requires participation ofbacteriophage (virus for bacteria)
- Phages that participatein transduction are also known as “transducing phages”
- All contain DNA
- Replication ofbacteriophage can follow 1 of 2 paths
-
(Transduction) Lytic cycle
- called lytic cycle b/c bacterium is lysed
- Phage invades bacterium
- Replicates itself
- Destroys host bacteriumas phages released
- lytic phage = virulent phage
-
(transduction) Lysogeny
- Phage DNA encodes repressor gene
- Repressor gene inhibits replication
- Then either goes to plasmid or integrates into host DNA
- lysogenic phage =temperate phage
|
|
