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Microbiology Exam 1 > Microbial Genetics > Flashcards

Flashcards in Microbial Genetics Deck (25):

Bacterial Chromosome

-Single, large circular DNA (~1 to 6 megabases)
-Contains all genes for survival



-Small, circular DNA (~1.5 to 400 kilobases)
-Separate from bacterial chromosome
-Replicated and transferred to daughter cells
-Copy number varies from 1 to 100


What is an episome?

A plasmid that has been integrated into the bacterial chromosome


Importance of Horizontal Gene Transfer

Allows bacteria to develop abx resistance and produce new virulence factors more quickly compared to Vertical Gene Transfer and natural slow rate of mutations


Homologous Recombination

Exchange of DNA between two DNA molecules based on homology (sequence similarity) resulting in the incorporated DNA into the chromosome being passed on to daughter cells and becoming a permanent part of the genome

***Gene MUST be incorporated into the host plasmid/DNA in order to be passed on


Transformation (Definition and Process)


2. Process: DNA is released into environment (e.g. bacterial death), DNA is actively imported into the bacterial cell, homologous can occur if there is sequence similarity, donor DNA is passed to progeny


Conjugation (Definition and Process)

1. Direct transfer of DNA from one bacterial cell to another via CELL CONTACT

2. Process:

Donor: carries Fertility Factor (F-Factor) which contains the tra operon (encodes sex pilus); carries oriT, where a single strand break occurs for transfer of DNA

Either Chromosomal DNA can be transferred (requires homologous recombination in order to be passed on to progeny) OR Plasmid DNA can be transferred


Donors vs Recipients

F+ and Hfr (High frequency recombination; F factor integrated into bacterial chromosome) can both act as donors and mate with F- (No F factor)


Conjugation: F+ x F-

-Transfer is unidirectional
-F- undergoes sex change to F+
-Only one strand is transferred
-No bacterial genes are transferred
-tra region comes in last and then replication machinery forms second strand of DNA


Conjugation: Hfr x F-

-Hfr donates to F-
-Mating does not generally last long enough for tra gene to move in (so, F- DOES NOT undergo sex change)
-F- receives new bacteria genes


Transduction (Definition and Phage Life Cycles)

1. Transfer of bacterial DNA via a bacteriophage (virus)

2. Phage Life cycles:
-Lytic/Virulent phage --> lytic replication upon entry
-Temperate phage --> LYSOGENY (integrated into chromsome); can excise and convert to lytic replication when bacteria is dying


Generalized Transduction

-A lytic phage incorporates bacterial chromsomal DNA into its phage head
-When phage infects another bacteria, the DNA can be incorporated into its chromosome via homologous recombination or can be degraded and lost


Specialized Transduction

-A lysogenic phage incorporates into the chromosome as a PROPHAGE
-Event (e.g. UV damage) leads to conversion to lytic cycle
-Upon excision from the genome, small portion of chromosome is packaged into phage head

***Cause many diseases including cholera, botulism, diphtheria, shigellosis, and hemorrhagic colitis


What is the only type of DNA transfer in bacteria that uses FREE DNA?



Type of DNA in Generalized vs Specialized Transduction

Generalized: Any DNA

Specialized: Only genes adjacent to phage insertion


Is homologous recombination required for: Transformation? Conjugation? Transduction?

Transformation: Yes

Conjugation: Hfr x F - (Yes) / F+ x F- (NO)

Transduction: Yes


Why is homologous recombination not needed for F+ x F-?

Because the complete plasmid is transferred in this type, whereas, in Hfr type, only part of the plasmid is incorporated into the F plasmid and transferred (this is why there is no sex change with Hfr x F-)


Intrinsic Antibiotic Resistance

-NOT transferrable (horizontally) between bacteria
-NOT increasing among bacterial populations
-Example: lack of antibiotic target (e.g. Mycoplasma lack cell wall/peptidoglycan)


Chromosome-Mediated Antibiotic Resistance

Arises through 1) Random Mutation and 2) Acquisition of DNA through horizontal gene transfer and incorporation into the genome via homologous recombination

e.g. Ribosomal proteins, Penicillin-binding proteins, DNA gyrase


Plasmid-Mediated Antibiotic Resistance

Genes encoding antibiotic resistance located on a plasmid

Commonly, plasmids carry genes that encode proteins that degrade or modify an abx or an efflux pump to pump the abx out of the cell


Resistance Factor (R-Factor): Two components

1. Resistance Tranfer Factor (equivalent to F-factor, as it encodes all proteins needed for conjugation)

2. Resistance Determinant (gene(s) for drug resistance)



-Mobile DNA elements that transfer or copy themselves from one molecule of DNA to another
-Present in both Eukaryotic and Bacterial cells, as well as Viruses


What does the simplest Transposon contain? (3 things)

1. Indirect repeat sequences on each end (recognized by transpoase, TnpA)
2. A single gene for transposase, TnpA, an enzyme required for transposon movement (i.e. "Jumping")
3. Single or multiple antibiotic resistance genes


Integrons and Antibiotic Resistance Gene Cassettes

Integrons are a diverse group of genetic elements which encode SITE SPECIFIC RECOMBINATION SYSTEMS that can capture antibiotic resistance gene cassettes and express them under the control of an integron promoter

-Often found with large mobile genetic elements like PLASMIDS or TRANSPOSONS, as well as on the CHROMOSOME

***Play important role in MULTI DRUG RESISTANCE BACTERIA


Pathogenicity Island

Basically, a very large transposon

-Can transfer itself into a bacterial chromosome via horizontal gene transfer
-Often carry coordinately regulated VIRULENCE GENES surrounded by insertion sequences