Bacteria have a single circular chromosomeclasspages.warnerpacific.edu/bdupriest/BIO 250...• Some...
Transcript of Bacteria have a single circular chromosomeclasspages.warnerpacific.edu/bdupriest/BIO 250...• Some...
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• Bacteria have a single circular chromosome
• Double-stranded DNA
• Haploid for all genes
• Can have extra-chromosomal DNA
• Plasmids
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• Bacteria reproduce via binary fission
• Cannot undergo meiosis
• No synapsis, no crossing over
• When cell divides, the chromosome and any plasmid(s) will be duplicated and divided evenly
• Can still undergo genetic recombination
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• Horizontal Gene Transfer
• Transfer of genetic material from one bacterium to another of a different species
• Confers new genes/traits on bacteria that never had them before
• E.g., nutrient production, antibiotic resistance
• Conjugation
• Transformation
• Transduction
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• Vertical Gene Transfer
• Transfer of genetic material from one bacterium to another of the same species
• AKA – reproduction!
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• A) They have only one chromosome
• B) They have two circular chromosomes
• C) They must undergo meiosis prior to fertilization
• D) All of the above
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• A) Involves synapsis
• B) Occurs during Prophase I
• C) Involves replacing similar genes with new ones
• D) Occurs during reproduction
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Lederberg
and
Tatum,
1946
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Davis
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Physical contact occurs
via sex pilus
The gene product of the
F (fertility) factor confers
ability to donate part of the
chromosome during
conjugation
In most conjugations, this
transfer (recombination)
rarely occurs, even though
F factor is always transferred
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• A new species of bacteria gains the F factor
• The new species of bacteria, now F+, has not gained any other traits
• So…who cares?
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• Some bacterial strains were discovered to have a much higher rate of recombination than typically observed
• 1/104, rather than 1/107
• Hfr bacteria were found to have the F factor incorporated into chromosome, instead of as a plasmid
• Transfers in Hfr include chromosomal DNA, rather than plasmid DNA
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• Hfr transfers studied using the interrupted mating technique
• Mix two cultures, wait a specified period of time, then shear cells apart using a blender
• Repeat with different periods of time, but the same bacterial crosses
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• Transfer always begins just after the F factor insertion point Origin (O)
• Insertion of F factor is random, so for any newly-generated Hfr strain, the starting point of genes will be different
• The overall order of gene transfer will always be the same for newly-generated Hfr from the same parent strain
• Order of transfer is used to map bacterial chromosomes
• Use multiple Hfr strains to map one parent strain, since complete chromosome is never transferred
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• Who cares if F factors are transferred?
• It matters if that F factor subsequently inserts into the bacterial chromosome, converting an F+ into an Hfr
• That Hfr cell can now transfer genes to F- cells at a high rate
• Antibiotic resistance
• Nutrient production
• Theoretically any bacterial gene for which a similar gene is already present
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• Sometimes in Hfr, the F factor becomes excised from the chromosome
• At the same time, genes from chromosome can be carried with it
• This creates a new plasmid containing F factor AND (formerly) chromosomal genes
• This F’ cell can initiate conjugation, and transfer F factor AND genes easily
• So…
• F- F+ Hfr F’ easy transfer of genes
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• A) F-
• B) F+
• C) F’
• D) B and C
• E) All of the above
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• A) F-
• B) F+
• C) F’
• D) B and C
• E) All of the above
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• Many bacteria are able to pick up naked DNA out of the aqueous environment
• Must be competent
• Active transport process requires energy and specific transporters
• Integration of new DNA into host chromosomes forms heteroduplex DNA
• Recombination involves replacing ONE strand of DNA, not both
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• Bacteria can receive new genetic information via viral infection
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• A) conjugation
• B) transformation
• C) transduction
• D) all of the above