Chapter 7: Bacterial and Viral Genetic Systems Flashcards
What are the advantages of bacteria for genetic studies?
- Small size
- Easy reproduction (division every 20 min)
- Easy to culture
- Small genome
- Many mutants available
How does the genome of bacteria differ from humans?
Most of the DNA contained within the genome of bacteria has a function (compact and efficient)
What is a minimal medium?
A medium that contains only the nutrients required by prototrophic bacteria
How can bacteria be grown?
- In liquid medium
- On solid medium
What are auxotrophs?
Stains that lack one or more enzymes necessary for synthesizing essential molecules and will grow only on medium supplemented with those essential molecules
How can mutant strains be isolated?
- Based on their nutritional requirements
- A colony that grows only on the supplemented medium has a mutation in a gene that encodes the synthesis of an essential nutrient
Describe the bacterial genome.
- Most possess a circular chromosome, containing a single double-stranded DNA molecule
- Most have a single CIRCULAR chromosome
What are plasmids? Do all bacteria contain plasmids?
- Small, usually circular DNA molecules
- Many, but not all
What is the function of plasmids? Are they essential?
- Plasmids carry genes that are NOT ESSENTIAL to bacterial function
- But may play an important role in the life cycle and growth of their bacterial hosts
What is the significance of the origin of replication within a plasmid?
- DNA replication is initiated at the origin
- Thus, a plasmid can replicate INDEPENDENTLY of its bacterial chromosome
What are episomes? Give an example.
- Plasmids that are capable of replicating freely and are able to integrate into the bacterial chromosomes
- Fertility factor of E. coli
What is the F factor of E. coli?
- Episome
- Regulates the transfer into the bacterial cell, replication, and insertion into the bacterial chromosome
- Genes REQUIRED for CONJUGATION
Without the __________, it is difficult to transfer genetic information from one cell to another?
F factor
Which of the following statements is true of plasmids?
A) They are composed of RNA
B) They normally exist outside of bacterial cells
C) They possess only a single strand of DNA
D) They contain an origin of replication
D) They contain an origin of replication
When does conjugation take place?
When genetic material passes DIRECTLY from one bacterium to another
When does transformation take place?
When a bacterium takes up DNA from the medium in which it is growing
When does transduction take place?
When bacterial viruses (bacteriophages, or phages) carry DNA from one bacterium to another
What happens in conjugation? What needs to occur?
- A plasmid or part of the bacterial chromosome passes from one cell (donor) to the other (recipient)
- A cytoplasmic bridge must form between two bacteria
What may occur following conjugation?
- Crossing-over may take place between the homologous sequences in the transferred DNA and the chromosome of the recipient cell
- Creation of recombinant chromosome
Which exchange process does not involve a donor cell? What does it involve instead?
- Transformation
- Free DNA (DNA fragments or plasmid)
What may occur following transformation?
Recombination may take place between the introduced genes and those of the bacterial chromosome
Explain the process of transduction.
1) A virus attaches to a bacterial cell, injects its DNA, and replicates, taking up the bacterial DNA
2) The bacterial cell lyses
3) The virus infects a new bacterium, carrying bacterial DNA with it
4) A crossover in the recipient leads to the creation of a recombinant chromosome
Which process of DNA transfer in bacteria requires a virus?
Transduction
What did Lederberg and Tatum’s experiment demonstrate?
That bacteria can transfer and recombine genetic information
Explain Lederberg and Tatum’s experiment.
1) Two auxotrophic bacteria, which differed in their functional genes required for nutrient synthesis
2) Each strain was unable to grow individually
3) When the strains were combined, colonies grew (proof of genetic exchange)
Explain Bernard Davis’ experiment.
1) Constructed a U-shaped tube, in which two auxotrophic strains were separated (medium could pass but not bacteria)
2) NO phototrophic bacteria were produced
What did Bernard Davis demonstrate?
That genetic exchange requires direct contact (conjugation) between bacterial cells
In most bacteria, what does conjugation depend on?
- The presence of the fertility factor in the donor cell (F+)
- The absence of the fertility factor in the recipient cell (F-)
Conjugation can ONLY take place between what?
A cell that possesses the F factor and a cell that lacks the F factor (bacterial chromosomes have nothing to do with this process)
What is the function of a sex pilus?
Connects F+ and F- cells during bacterial conjugation
What is transferred during conjugation between an F+ cell and an F- cell? What is the result?
- The F factor
- Produces 2 F+ cells
What does Hfr stand for?
High-frequency recombination
What are Hfr cells? What do they behave like?
- Cells in which the F factor is integrated into the bacterial chromosome
- Behave like F+ cells, forming sex pili and undergoing conjugation with F- cells
How do Hfr cells arise?
Crossing-over takes place between the F factor and the bacterial chromosome
How does an Hfr cell differ from an F+ cell?
In conjugation with an F- cell, the chromosome follows the F factor into the recipient cell
What happens during crossing-over following conjugation between an Hfr and F- cell?
The donated chromosome (linear) is degraded, and the recombinant recipient chromosome remains
How do F’ cells arise?
- When an F factor is excised from the bacterial chromosome of an Hfr cell
- A small amount of the bacterial chromosome may be removed with it
- These chromosomal genes will then be carried with the F plasmid
What are the outcomes of conjugating an F’ cell with an F- cell?
- The F plasmid is transferred to the F- cell
- The genes on the F plasmid, including those from the bacterial chromosome may be transferred to the F- recipient cells
What are the F factor characteristics and what is the role in conjugation of F+ cells?
- Present as separate circular plasmid
- Donor
What are the F factor characteristics and what is the role in conjugation of F- cells?
- Absent
- Recipient
What are the F factor characteristics and what is the role in conjugation of Hfr cells?
- Present, integrated into the bacterial chromosome
- High-frequency donor
What are the F factor characteristics and what is the role in conjugation of F’ cells?
- Present as separate circular plasmid, carrying some bacterial genes
- Donor
What cell types are present after conjugating an F+ and an F- cell?
- Two F+ cells
- F- cell becomes F+
What cell types are present after conjugating an Hfr and an F- cell?
- One Hfr cell
- One F- (no change)
- Rarely, the F- cell becomes F+ in this conjugation if the entire chromosome is transferred during conjugation
What cell types are present after conjugating an F’ and an F- cell?
- Two F’ cells
- F- cell becomes F’
How can interrupted conjugation be used to map bacterial genes?
The transfer times indicate the order and relative distances between genes and can be used to construct a genetic map
What does antibiotic resistance frequently arise from?
Genes located on R plasmids (small circular plasmids that can be transferred by conjugation)
What are competent cells?
Cells that take up DNA through their cell membrane
What does transformation require?
- Uptake of DNA from the surrounding medium
- Incorporation into a bacterial chromosome or plasmid
How many strands of DNA enter the cell during transformation?
- One strand of the DNA fragment enters
- The other is hydrolyzed
When the transformed cell replicates and divides, how many are transformed and non-transformed?
- 1 non-transformed
- 1 transformed
