Test 3-3 Flashcards
-Describe the three types of genetic material that can be found in bacterial cells. -Explain the process of homologous recombination and describe why it is important for bacteria that have acquired new DNA molecules. -Compare and contrast the elements involved in the processes of transformation, conjugation and transduction. -Differentiate between intrinsic, chromosome-mediated and plasmid-mediated drug resistance mechanisms. -Describe the genetic elements involved in the construction of mu
episome
when a plasmid integrates itslef into the bacterial chromosome
temperate phage
bacteriophage that stabily integrate their DNA into host cell DNA
lytic phage
bacteriophage that replicates inside and lyses the bacterium
-immediately goes into viral mode
three forms of DNA transfer in bacteria
tnsformation - direct uptake of DNA material from environment
conjugation - sex pili
transduction - with viruses
Homologous recombination (HR) definition
An exchange of genes between different pieces of DNA.
-need to stabilize linear pieces of DNA that the bacteria wants to take up and use
-must be some similarity between the genes (like similar alleles)
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Homologous recombination (HR) purpose
Stabilizes genes from DNA the bacterium has acquired by transformation, conjugation or transduction.
Homologous recombination (HR) process
1) Short, linear pieces of DNA exchange genes with a homologous region of genes (i.e. alleles) on the bacterial chromosome.
2) A recombination protein is required for HR to occur. The most common is recombination protein A, or recA (SWAPS THE GENES)
3) Linear DNA (the OLD genes that the bacteria used to have) is broken down by cellular exonucleases, therefore it is not stable in the bacterium.
4) Genes that are incorporated into the bacterial chromosome become a permanent part of the cell’s genome.
transformation definition
Uptake of free DNA from the environment.
transformation purpose
Acquire new traits by utilizing the DNA from a dead bacterium.
transformation process
1) A dead bacterium releases its DNA into the environment.
2) Segments of the free DNA are taken up by a viable bacterium.
3) If the DNA segments share homology with the bacterial chromosome, some genes can be stabilized by HR.
conjugation definition
Genes are transferred from one bacterial cell to another through cell-to-cell contact.
conjugation purpose
Acquire new DNA from another viable bacterium.
conjugation process
1) “Fertility factors” supply all needed components, e.g. the sex pili which establishes a bridge between two bacterium.
2) Single-stranded DNA is transferred from a “donor” cell (which has the fertility factor) to a “recipient” cell.
3) Chromosomal DNA transfer: Genes have to be stabilized by HR, since the linear DNA would be degraded.
4) Plasmid DNA transfer: DNA circularizes and becomes stable without HR.
fertility factor
1) tra operon = encodes sex pili and other proteins needed for conjugation
2) oriT = a.k.a. “origin of transfer”; a single-strand break occurs here to initiate the transfer of DNA to another bacterium
F neg cells
Recipient cells only - cannot initiate conjugation bc no F-factor
in every cross there must be one ____ cell
F neg
F+ cells
have fertility factor as a plasmid
Hfr cells
have fertility factor integrated into bacterial chromosome – episome (high frequency)
F+ X F- conjugal cross
1) sex pilus first formed - bridge forms
2) (unwinding at oriT) oriT transfered (as single strand) first following the rest of the plasmid
3) last gene transfered is the tra operon
4) F-cell becomes F+ cells (sex change)
Hfr X F- conjugal cross
1) sex pilus fromed and bridge formed
2) (unwinding at oriT) oriT transfered (as single strand) first following the rest of the Hfr DNA
3) Hfr bridge is usually broken off before complete transfer is done so most likely only a partial transfer
4) F- cells needs to do HR to stabilize single strand now - F-factor does NOT COMPLETELY TRANSFER –> new DNA is incorporated
5) F- cell DOES NOT go through sex change
transduction definition
Transfer of bacterial DNA to another bacterium using a phage vector
phage types
1) Virulent phage – Undergoes lytic replication
2) Temperate phage – Can undergo lytic replication or lysogeny (integration of phage DNA into a specific site on the bacterial chromosome)
transduction process
1) A lytic phage mistakenly incorporates bacterial chromosomal DNA into its phage head.
2) When that phage infects another bacterium, that bacterial DNA can be utilized by the other bacterium if it is stabilized by HR.
Specialized transduction:
- NEEDS Temperate phages, i.e. phages that incorporate their DNA into their bacterial host DNA (a.k.a. “lysogeny”), can remove bacterial genes when the phage excises its DNA to undergo viral replication.
- Excised bacterial DNA can be transferred to a new bacterial host.
- Only bacteria genes that are close to the phage insertion site can be excised.
- Transferred bacterial genes must be stabilized by HR.
- limited to genes near the incorporated phage DNA
