Chapter 7_Genetic Transfer And Mapping In Bacteria And Bacteriophages Flashcards
Bacteriophages (Phages)
Viruses that infect bacteria. They contain their own genetic material that governs the traits of the phage.
Genetic Transfer
A process by which one bacterium transfers genetic material to another bacterium.
Three Natural Mechanisms of Genetic Transfer in Bacteria
- Conjugation
- Transduction
- Transformation
Conjugation
Requires direct contact between a donor and a recipient cell. The donor cell transfers a strand of DNA to the recipient.
Transduction
When a virus infects a donor cell, it incorporates a fragment of bacterial chromosomal DNA into a newly made virus particle. The virus then transfers this fragment of DNA to a recipient cell, which incorporates the DNA into its chromosome.
Transformation
When a bacterial cell dies, it releases a fragment of its DNA into the environment. This DNA fragment is taken up by a recipient cell, which incorporates the DNA into its chromosome.
Minimal Medium
A growth medium that contains the essential nutrients for a wild-type bacterial species to grow.
What type of medium do most researchers study bacterial strains on?
The kind that cannot grow on minimal media.
Auxotroph
A strain that cannot synthesize a particular nutrient and needs that nutrient to be supplemented in its growth medium.
Prototroph
Does not need specific nutrient in its growth medium.
F Factor
(Fertility Factor) A small circular segment of genetic material; it is in addition to their circular chromosome.
F+ means it has the F factor
F- means it does not
Sex Pili
Made by F+ strains. Makes physical contact with F- strain.
Conjugation Bridge
Formed between two cells, provides a passageway for DNA transfer.
Relaxosome
- A protein complex. This complex first reorganizes a DNA sequence in the F factor known as the origin of transfer.
- Upon recognition, one DNA strand in the site is cut.
- Relaxosome catalyzs the separation of the DNA strands, and only the cut DNA strand is transferred to the recipient cell.
- As DNA strands separate, most of the proteins within the relaxosome are released, but one protein, called relaxase, remains bound to the end of the cut DNA strand.
- The complex between the single stranded DNA and relaxase is called a nucleoprotein because it contains both nucleic acid (DNA) and protein (relaxase).
Hfr Strain
High Frequency of Recombination
F’ Factor
(F Prime Factor) F factor that carries a portion of the bacterial chromosome and leaves behind some of the F factor DNA in the bacterial chromosome.
Researchers scale genetic maps from bacterial conjugation studies…
…in units of minutes. This unit refers to the relative time it takes for genes to first enter an F- recipient strain during a conjugation experiment.
Plasmid
One type of DNA that can exist independently of the chromosomal DNA (F factors). Most are circular, some are linear.
Episomes
Plasmids (such as F factors) that can integrate into the chromosome.
The DNA sequence of the origin of replicatino…
…influences how many copies of the plasmid are found within a cell.
Fertility Plasmids
(F factors), allow bacteria to mate with each other.
Resistance Plasmids
(R factors), contain genes that confer resistance against antibiotics and other types of toxins.
Degradative Plasmids
Carry genes that enable the bacterium to digest and utilize an unusual substance.
Col-plasmids
Contain genes that encode colicines, which are proteins that kill other bacteria.
Virulence Plasmids
Carry genes that turn a bacterium into a pathogenic strain.
Lytic Cycle
- Bacteriophage directs the synthesis of many copies of the phage genetic material and coat proteins.
- These components then assemble to make new phages.
- When synthesis and assembly are completed, the bacterial host cell is lysed (broken apart), releasing the newly made phages into the environment.
Virulent Phage
Follows only a lytic cycle, and thus infection results in the death of the host cell.
Lysogenic Cycle
- Phages integrate their genetic material into the chromosome of the bacterium (This integrated phage DNA is known as a prophage).
- Prophage can be dormant. If lysogenic prophage divides, genetic material is copied and both daughter cells inherit the prophage.
- Prophage may become activated to excise itself from the bacterial chromosome and enter the lytic cycle.
- When this happens, it promotes the synthesis of new phages and eventually lyses the host cell.
Temperate phage
A bacteriophage that usually exists in the lysogenic cycle.
Generalized Transduction
Any piece of the bacterial chromosomal DNA can be incorporated into the phage.
Cotransduction
If two genes are close together along the chromosome, a bacteriophage may package a single piece of the chromosome that carries both genes and transfer that piece to another bacterium.
If two genes are far apart along a bacterial chromosome…
…they will never be cotransduced because the bacteriophage cannot physically package a DNA fragment that is larger than 1% to 2.5% of the bacterial chromosome.
Competent Cells
Bacterial cells that are able to take up DNA
Competence Factors
Proteins encoded by Competent cells that facilitate the binding of DNA fragments to the cells surface, the uptake of DNA into the cytoplasm, and its subsequent incorporation into the bacterial chromosome.
What influences the competence of cells?
Temperature, ionic conditions, and nutrient availability.
Describe the steps of bacterial transformation
- DNA fragment binds to a cell surface receptor of a competent bacterium
- An extracellular endonuclease cuts the DNA into smaller fragments.
- One strand is degraded and a single strand is transported into the cell via an uptake system.
- The DNA strand aligns itself with a homologous region on the bacterial chromosome.
- The DNA strand is incoporated into the bacterial chromosome via homologous recombination.
- The heteroduplex DNA is repaired in a way that changes the lys- strand to create a lys+ gene (usually by eliminating the mutation that causes the lys- genotype)
Homologous recombination
If the DNA strand has a sequence that is similar to a region of DNA in the bacterial chromosome, the DNA may be incorporated into the chromosome by a process known as homologous recombination.
Heteroduplex
A region of DNA where the alignment of the lys- and lys+ alleles take place during homologous recombination. It contains one or more base sequence mismatches. It only exists temporarily. DNA repair enzymes in the recipient cell recognize the heteroduplex and repair it.
Nonhomologous (illegitimate) recombination
A DNA fragment that has entered a cell may not be homologous to any genes that are already found in the bacterial chromosome.
Competence stimulating peptide (CSP)
A short peptide. When cells are in the vicinity of one another, the concentration of CSP becomes high, which stimulates the cells, via a cell signaling pathway, to express the competence proteins needed for the uptake of DNA and its incorporation in the chromosome.
- NOTE: Because competence requires a high external concentration of CSP, cells are more likely to take up DNA from nearby cells that have died and released their DNA into the environment.
DNA uptake signal sequences
Allow bacterial species to promote the uptake of DNA among members of their own species. About 9 to 10 bp long.
Does transformation more often happen between the same species or different or similar species?
The same. This is because cells are much more likely to take up a DNA fragment with their own uptake sequence.
Cotransduction Frequency
= (1- d/L)^3
d = distance between two genes in minutes L = the size of the chromosomal pieces (in minutes) that the phage carries during transduction (For P1 transduction, this size is approximately 2% of the bacterial chromosome, which equals about 2 minutes)
Cotransformation Frequency
IF two genes are close together, this is expected to be high. Only used to map genes relatively close together.
Vertical Gene Transfer
The transmission of genes from mother cell to daughter cell or from parent to offspring
Horizontal Gene Transfer
Organism incorporates genetic material from another organism without being the offspring of that organism. Involves the exchange of genetic material between members of the same or different species.
What types of genes are acquired through horizontal gene transfer?
They are quite varied, though they commonly involve functions that are readily acted on by natural selection. (Antibiotic resistance, degrade toxic compound, pathogeneticity).
- NOTE: Most of the speciation in prokaryotes is due to horizontal transfer.
Acquired antibiotic resistance
Widespread and uncontrolled use of antibiotics has promoted the prevalence of antibiotic resistant strains of bacteria. May occur via genetic laterations in the bacteria’s own genome or by the horizontal transfer of resistance genes from a resistant strain to a sensitive strain.
How do resistance genes to antibiotics work?
They encode proteins that either break down the drug, pump the drug out of the cell, or prevent the drug from inhibiting cellular processes.
Viruses
Small particles that have genetic material and propagate only with the aid of a host cell.
- NOT living entities
Intragenic (Fine Structure) Mapping
Seeks to establish distances between two or more mutations within the same gene.
Plaque
An observable clear area where the bacteria have been lysed around the original site where a phage infected a bacterial cell.
Complementation Test
To determine if two different mutations that affect the same trait are in the same gene or in two different genes.
Noncomplementation
Two rII phage strains possess mutations in the same gene. When this happens, the cell cannot make a wild type gene A product when coinfected into an E. Coli K12(lambda) cell, and plaques do not form.
Complementation
If each rII mutation is in a different phage gene (gene A and gene B), a bacterial cell that is coinfected by both types of phages will have two mutant genes as well as two wild types. If the mutant phage genes behave recessively, the doubly infected cell will have a wild type phenotype.
- BASICALLY, the defective genes in each rII strain are complemented by the corresponding wild type genes.
When would intergenic complementation not work?
- If a mutation behaves dominantly
- Mutations that affect regulatory genetic regions rather than the protein coding region may not show complementation.
Cistron
The smallest genetic unit that gives a negative complementation test. In other words, if two mutations occur within the same cistron, they cannot complement each other.
Homoallelic Mutations
If two mutations happen to be located at exactly the same site within a gene, coinfection would not be able to produce any wild-type recombinants and so the map distance would be zero.
Deletion Mapping
A first step in localizing rII mutations to a fairly short region within gene A or gene B
Hot spots
Certain locations contain a relatively high number of mutations compared with other sites.
