Chapter 7_Genetic Transfer And Mapping In Bacteria And Bacteriophages

Question 1

Bacteriophages (Phages)

Answer 1

Viruses that infect bacteria. They contain their own genetic material that governs the traits of the phage.

Question 2

Genetic Transfer

Answer 2

A process by which one bacterium transfers genetic material to another bacterium.

Question 3

Three Natural Mechanisms of Genetic Transfer in Bacteria

Answer 3

• Conjugation
• Transduction
• Transformation

Question 4

Conjugation

Answer 4

Requires direct contact between a donor and a recipient cell. The donor cell transfers a strand of DNA to the recipient.

Question 5

Transduction

Answer 5

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.

Question 6

Transformation

Answer 6

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.

Question 7

Minimal Medium

Answer 7

A growth medium that contains the essential nutrients for a wild-type bacterial species to grow.

Question 8

What type of medium do most researchers study bacterial strains on?

Answer 8

The kind that cannot grow on minimal media.

Question 9

Auxotroph

Answer 9

A strain that cannot synthesize a particular nutrient and needs that nutrient to be supplemented in its growth medium.

Question 10

Prototroph

Answer 10

Does not need specific nutrient in its growth medium.

Question 11

F Factor

Answer 11

(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

Question 12

Sex Pili

Answer 12

Made by F+ strains. Makes physical contact with F- strain.

Question 13

Conjugation Bridge

Answer 13

Formed between two cells, provides a passageway for DNA transfer.

Question 14

Relaxosome

Answer 14

• 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).

Question 15

Hfr Strain

Answer 15

High Frequency of Recombination

Question 16

F’ Factor

Answer 16

(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.

Question 17

Researchers scale genetic maps from bacterial conjugation studies…

Answer 17

…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.

Question 18

Plasmid

Answer 18

One type of DNA that can exist independently of the chromosomal DNA (F factors). Most are circular, some are linear.

Question 19

Episomes

Answer 19

Plasmids (such as F factors) that can integrate into the chromosome.

Question 20

The DNA sequence of the origin of replicatino…

Answer 20

…influences how many copies of the plasmid are found within a cell.

Question 21

Fertility Plasmids

Answer 21

(F factors), allow bacteria to mate with each other.

Question 22

Resistance Plasmids

Answer 22

(R factors), contain genes that confer resistance against antibiotics and other types of toxins.

Question 23

Degradative Plasmids

Answer 23

Carry genes that enable the bacterium to digest and utilize an unusual substance.

Question 24

Col-plasmids

Answer 24

Contain genes that encode colicines, which are proteins that kill other bacteria.

Question 25

Virulence Plasmids

Answer 25

Carry genes that turn a bacterium into a pathogenic strain.

Question 26

Lytic Cycle

Answer 26

• 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.

Question 27

Virulent Phage

Answer 27

Follows only a lytic cycle, and thus infection results in the death of the host cell.

Question 28

Lysogenic Cycle

Answer 28

• 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.

Question 29

Temperate phage

Answer 29

A bacteriophage that usually exists in the lysogenic cycle.

Question 30

Generalized Transduction

Answer 30

Any piece of the bacterial chromosomal DNA can be incorporated into the phage.

Question 31

Cotransduction

Answer 31

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.

Question 32

If two genes are far apart along a bacterial chromosome…

Answer 32

…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.

Question 33

Competent Cells

Answer 33

Bacterial cells that are able to take up DNA

Question 34

Competence Factors

Answer 34

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.

Question 35

What influences the competence of cells?

Answer 35

Temperature, ionic conditions, and nutrient availability.

Question 36

Describe the steps of bacterial transformation

Answer 36

• 1. DNA fragment binds to a cell surface receptor of a competent bacterium
• 2. An extracellular endonuclease cuts the DNA into smaller fragments.
• 3. One strand is degraded and a single strand is transported into the cell via an uptake system.
• 4. The DNA strand aligns itself with a homologous region on the bacterial chromosome.
• 5. The DNA strand is incoporated into the bacterial chromosome via homologous recombination.
• 6. 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)

Question 37

Homologous recombination

Answer 37

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.

Question 38

Heteroduplex

Answer 38

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.

Question 39

Nonhomologous (illegitimate) recombination

Answer 39

A DNA fragment that has entered a cell may not be homologous to any genes that are already found in the bacterial chromosome.

Question 40

Competence stimulating peptide (CSP)

Answer 40

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.

Question 41

DNA uptake signal sequences

Answer 41

Allow bacterial species to promote the uptake of DNA among members of their own species. About 9 to 10 bp long.

Question 42

Does transformation more often happen between the same species or different or similar species?

Answer 42

The same. This is because cells are much more likely to take up a DNA fragment with their own uptake sequence.

Question 43

Cotransduction Frequency

Answer 43

= (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)

Question 44

Cotransformation Frequency

Answer 44

IF two genes are close together, this is expected to be high. Only used to map genes relatively close together.

Question 45

Vertical Gene Transfer

Answer 45

The transmission of genes from mother cell to daughter cell or from parent to offspring

Question 46

Horizontal Gene Transfer

Answer 46

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.

Question 47

What types of genes are acquired through horizontal gene transfer?

Answer 47

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.

Question 48

Acquired antibiotic resistance

Answer 48

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.

Question 49

How do resistance genes to antibiotics work?

Answer 49

They encode proteins that either break down the drug, pump the drug out of the cell, or prevent the drug from inhibiting cellular processes.

Question 50

Viruses

Answer 50

Small particles that have genetic material and propagate only with the aid of a host cell.
- NOT living entities

Question 51

Intragenic (Fine Structure) Mapping

Answer 51

Seeks to establish distances between two or more mutations within the same gene.

Question 52

Plaque

Answer 52

An observable clear area where the bacteria have been lysed around the original site where a phage infected a bacterial cell.

Question 53

Complementation Test

Answer 53

To determine if two different mutations that affect the same trait are in the same gene or in two different genes.

Question 54

Noncomplementation

Answer 54

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.

Question 55

Complementation

Answer 55

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.

Question 56

When would intergenic complementation not work?

Answer 56

• If a mutation behaves dominantly

- Mutations that affect regulatory genetic regions rather than the protein coding region may not show complementation.

Question 57

Cistron

Answer 57

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.

Question 58

Homoallelic Mutations

Answer 58

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.

Question 59

Deletion Mapping

Answer 59

A first step in localizing rII mutations to a fairly short region within gene A or gene B

Question 60

Hot spots

Answer 60

Certain locations contain a relatively high number of mutations compared with other sites.