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MBG 2040 > Chapter 9 - Bacterial and Viral Genetic Systems > Flashcards

Chapter 9 - Bacterial and Viral Genetic Systems Flashcards

1
Q

What are some examples of important contributions bacteria and viruses have made to genetics?

A
  • Evidence for nature of genetic material (DNA)
  • First characteristics/definitions of gene
  • First evidence for gene interactions at molecular level
  • Basic principles of inheritance
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2
Q

What makes bacteria valuable research objects?

A
  • Small size
  • Rapid reproduction
  • Selective media that can easily identify presence of an active allele
  • Simple structures/physiology
  • Genetic variability
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3
Q

What are bacterial viruses/bacteriophages?

A
  • Reproduce by infecting bacterial cells

- Helped discover some important genetic concepts

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4
Q

How do bacteria grow in liquid media?

A
  • Inoculate medium with bacteria

- Bacteria grow and divide

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5
Q

How do bacteria grow in solid media?

A
  • Growth medium is suspended in gelatin-like agar
  • Add a dilute solution of bacteria to petri plate
  • Spread evenly with glass rod
  • After incubation for 1-2 days, bacteria multiply, forming visible colonies
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6
Q

What do bacteriophages produce on plates with dense bacterial cultures?

A
  • Clearances/plaques within hours of infection
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7
Q

What does Bacteriophage T4 consist of?

A
  • Protein head
  • 168,800 base pairs and 150 characterized genes
  • Fairly large/complex genome for a virus
  • Phage goes through lytic phase (lyses cell to infect other cells); quick/simple experiments
  • No dormant phase
  • Tail fibres land and hook onto plasma membrane
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8
Q

What does Bacteriophage Lambda consist of?

A
  • 48,502 base pairs and 50 genes (about 1/3 of T4)

- May be lytic or lysogenic (inserts DNA into host and goes into latency/dormancy)

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9
Q

How does a retrovirus work?

A
  • Attaches at receptors in membrane
  • Viral core enters host
  • Viral RNA uses reverse transcriptase to make complementary DNA
  • Reverse transcriptase synthesizes 2nd DNA strand
  • Viral DNA enters nucleus and is integrated into host chromosome (forming provirus)
  • Proviral DNA is transcribed into viral RNA on activation
  • RNA is translated in cytoplasm
  • Viral RNA, proteins, new capsids and envelopes are assembled
  • Assembled virus buds from cell membrane
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10
Q

What is the lytic cycle?

A
  • Phage binds to bacterium
  • Phage DNA enters host
  • Host DNA is digested
  • Phage DNA replicates
  • Host cell transcribes/translates phage DNA -> proteins, phages are assembled
  • Phage encoded enzyme causes cell to lyse
  • New phages released to start cycle again
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11
Q

What is the lysogenic cycle?

A
  • Phage binds to bacterium
  • Phage DNA enters host
  • Phage DNA integrates into bacterial chromosome (becomes prophage)
  • Prophage is replicated as part of bacterial chromosome (can be many cell divisions)
  • Prophage may separate from chromosome and cell will enter lytic cycle after dormancy
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12
Q

Why would an infected cell go through dormancy?

A
  • Nutrients are scarce
  • Host is sick
  • Not happy with conditions
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13
Q

What do bacterial genomes look like?

A
  • Circular molecules
  • Several million base pairs
  • Double-stranded DNA molecule
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14
Q

Where is additional genetic material found in bacteria?

A
  • Plasmids (small circular DNA)

- Can replicate independently of bacterial chromosome

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15
Q

What are episomes?

A
  • Subclass of plasmid
  • Large circular DNA
  • Can integrate into bacterial chromosome for replication or remain separate
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16
Q

Is bacterial recombination possible?

A
  • Yes, but not through meiosis/mitosis
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17
Q

Is gene transfer unidirectional or bidirectional?

A
  • Unidirectional
18
Q

Gene mutations can be readily observed via… (4)

A
  • Colony colour and morphology
  • Nutritional mutants (cannot metabolise certain sugars)
  • Prototrophs (can make all AAs) and auxotrophs (cannot make certain AAs, need to be added to medium)
  • Antibiotic resistance
19
Q

What happens when a bacterial culture is plated onto petri dishes containing and lacking leucine? Why is this useful?

A

LACKING
- Only leu+ grow
CONTAINING
- leu+ and leu- grow

  • Leucine auxotrophs (leu-) are recovered and cultured for further study
  • A colony that grows only on supplemented medium has a mutation in gene that encodes synthesis of essential nutrient
20
Q

What are the 3 parasexual processes through which bacteria exchange genetic material?

A
  • Conjugation
  • Transformation
  • Transduction
21
Q

What happens in conjugation?

A
  • Direct transfer of DNA from one cell to another via cytoplasmic bridge
  • ‘sexual’ reproduction mediated by Fertility/F factor (an episome)
  • Cytoplasmic bridge forms b/n bacteria cells
  • DNA replicates and transfers from one cell to another
  • Transferred DNA replicates
  • A crossover in the recipient cell leads to the creation of a recombinant chromosome
  • Cell contact IS required
  • It is NOT sensitive to DNase
22
Q

What happens in transformation?

A
  • Transfer of free piece of DNA from one bacterium into another
  • Competent bacterial cells take up DNA from environment
  • Transferred DNA can be stably inherited
  • Naked DNA is taken up by recipient cell
  • Crossover in bacterium leads to recombinant chromosome
  • Cell contact is NOT required
  • It IS sensitive to DNase
23
Q

What happens in transduction?

A
  • Transfer of genes from one cell to another via bacteriophage
  • Bacteriophage can ‘hijack’ bacterial chromosome genes during phage assembly and donate hijacked genes to another cell
  • Virus attaches to bacterial cell, injects its DNA and replicates (taking up bacterial DNA)
  • Cell lyses
  • Virus infects new bacterium carrying bacterial DNA with it
  • Crossover in recipient leads to recombinant chromosome
  • Cell contact is NOT required
  • It is NOT sensitive to DNase
  • Transfer of genes from donor strain and recombination produce transductants in recipient bacteria
24
Q

How do we establish the need for contact?

A
  • U-tube experiment
  • 2 auxotrophic strains were separated by filter
  • No prototrophic bacteria were produced
  • Therefore genetic exchange requires direct contact b/n bacterial cells
25
Q

What is a competent bacterium?

A
  • Can bind exogenous DNA and transport it into the cell
26
Q

What can transformation be used for?

A
  • To determine distance b/n genes
  • Only 0.2-0.5% of chromosome undergoes transformation
  • If 2 genes are close, they will be present on same piece of DNA
  • Transformation is measured as a change in phenotype and it occurs with a certain frequency
27
Q

How is the rate of cotransformation related to distance between genes?

A

Rate of cotransformation is inversely proportional to distance between genes
- As distance increases, cotransformation decreases

28
Q

How can an F-factor exist?

A
  • As extra-chromosomal DNA
    OR
  • Insert into the bacterial chromosome and replicate as part of it
29
Q

What is a F- cell?

A

No F factor

30
Q

What is a F+ cell?

A

Autonomous F factor

31
Q

What is a Hfr cell?

A

Integrated F factor

32
Q

What is conjugation in E. coli mediated by?

A

Cytoplasmic ‘conjugation’ channel

33
Q

What happens in a F+ x F- cross?

A
  • F pili of F+ make contact with F- to pull cells together
  • Genes on F factor direct synthesis of conjugation bridge
  • One strand of DNA is cleaved at origin of F factor
  • Rolling-circle replication transfers one strand into F-
  • Replication occurs in both cells during transfer
  • Ends with 2 F+ bacteria
34
Q

What happens in a Hfr x F- cross?

A
  • F is nicked and 5’ end moves into F- cell
  • Transferred strand replicates and crossing over takes place b/n donated Hfr and original F- chromosome
  • Crossing over may lead to recombination
  • Linear chromosome is degraded
35
Q

What happens in Hfr1?

A
  • F is integrated b/n leu gene and azi gene

- So the genes are transferred beginning with leu

36
Q

What happens in Hfr5?

A
  • F is integrated b/n thi and his

- F has the opposite orientation in this chromosome, so the genes are transferred beginning with thi

37
Q

Conjugation b/n an F+ and F- cell usually results in what?

A

2 F+ cells

38
Q

What is a plasmid?

A
  • An extra-chromosomal circular piece of DNA
  • Can replicate independently of chromosome
  • In general, not required for survival of host cell and serve as auxiliary genetic elements
39
Q

What is an episome?

A
  • Genetic element that is not essential to the host and that can either replicate autonomously or be integrated into the bacterial chromosome
40
Q

What are examples of plasmids and episomes in E. coli?

A
  • F factor (episome; fertility factor)

- R plasmids (resistance plasmids)

41
Q

How are distance between genes and cotransduction related?

A
  • Genes close to one another are more likely to be cotransduced
  • Rate of cotransduction is inversely proportional to distance b/n genes

MBG 2040 (7 decks)

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