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H2 biology > 2.9: bacteria > Flashcards

2.9: bacteria Flashcards

1
Q

what is a bacterium?

A
  • small unicellular prokaryotes
  • lacks a true nucleus and membrane-bound organelles
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2
Q

what kind of DNA do bacteria have?

A

circular DNA

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3
Q

what type of ribosomes do bacteria have?

A

70S

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4
Q

how many oriR do bacteria have

A

single

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5
Q

what are plasmids?

A
  • extrachromosomal DNA
  • replicate independently of chromosomal DNA
  • involved in conjugation & antibiotic resistance
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6
Q

describe the process of DNA replication (before binary fission)

A
  1. DNA double helix unwinds and unzips @ oriR
  2. replicates via semi-conservative model
  3. bidirectional replication
  4. circular DNA -> interlocking structure -> topoisomerase cuts, reseals 2 DNA molecules
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7
Q

describe binary fission

A
  1. bacterial chromosome is attached to plasma membrane @ oriC
  2. replicates , 2nd oriC attaches to membrane adjacently
  3. cell elongates by growing membrane, depositing cell wall material btwn 2 chromos
  4. after growing, septum forms in btwn
  5. septum fuses -> 2 identical daughter cells
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8
Q

what is the significance of genetic variation?

A
  • rapidly changing environment
  • natural selection
  • occur due to point mutations/genetic transfer
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9
Q

what is conjugation?

A
  • unidirectional transfer of F plasmid
  • via direct contact
  • from F+ -> F-
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10
Q

what does the F plasmid contain

A

fertility factor
- enables sex pilus production

surface protein gene
- surface exclusion
- sex pilus will not attach

endonuclease gene
- cut strand of dsDNA @ oriT

regulatory gene

origin of replication
- rolling circle mechanism

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11
Q

describe the process of conjugation

A
  1. F+ produces sex pilus that binds to specific binding site @ F- cell
  2. sex pilus retracts, pulling cells tgt
  3. temporary cytoplasmic bridge built
  4. 1 strand of F plasmid dsDNA cleaved by endonuclease @ oriT, transferred to recipient cell
  5. F plasmid replicated via rolling circle mechanism
  6. free 3’ end of cleaved DNA extended by DNA pol -> new strand
  7. new strand displaces cleaved strand via 5’ end
  8. ss F plasmid transferred acts as template for 2nd DNA strand
  9. ds plasmid in recipient circularises

F- -> F+ !!

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12
Q

what is transformation

A

uptake of naked, foreign DNA in environment by competent bacterial cells

incorporated into genome

change in genotype and phenotype of cell

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13
Q

describe the process of transformation

A
  1. dsDNA of dead recognised
  2. bound by competence factors on CSM of recipient
  3. dsDNA fragment enters
  4. one strand degraded by endonuclease/exonuclease
  5. other dsDNA aligns w homologous regions of bacteria
  6. integrated via homologous recombination
  7. needs 2 crossover events

recombinant cell !!

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14
Q

how to induce competence

A

heat shock
- cold CaCl2
- brief heat
- transient pores

electroporation
- electric shock
- transient pores

induce DNA uptake !!

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15
Q

what is generalised transduction?

A
  • lytic cycle bacteriophages (virulent: T4 phage)
  • random transfer of donor bacterial genome -> recipient
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16
Q

describe generalised transduction

A
  1. bacteriophage binds to host cell receptor
  2. injects viral genome
  3. phage degrades bacterial chromosome into small fragments using phage nuclease
  4. random fragments of lysed bacterial genome may be wrongly packaged into phage capsid
    transducting (defective) phage !!
  5. post lysis of host cell: transducing phage attaches to new host
  6. injects viral DNA + DNA from old host
  7. homologous recombination
    recombinant bacterial cell !!
17
Q

what is specialised transduction?

A

only lysogenic cycle bacteriophages (temperate: lambda)

transfer of specific bacterial DNA

18
Q

describe the process of specialised transduction

A
  1. bacteriophage attaches to host
  2. injects viral genome -> forms prophage
  3. enters lysogenic cycle / spontaneously induced: lytic cycle
  4. prophage excised

incorrect excision: host genes excised accidentally

  1. both phage DNA and bacterial genes assembled into capsid.
  2. old host cell DNA + viral DNA into 2nd cell

recombinant genome !!

19
Q

what is an operon?

A

cluster of genes w related functions

all turned on/off tgt

promoter + operator + structural genes = single polycistronic mRNA

20
Q

what is a promoter?

A

RNA pol binding site

upstream of structural genes

21
Q

what is an operator?

A

repressor protein binding site

prevents RNA pol from binding to promoter and initiating transcription

22
Q

what is the purpose of regulation in bacteria?

A
  • economical use of energy and resources
  • prevents wastage
  • genes expressed only when necessary
  • bacteria can respond rapidly to changes in env
  • selective advantage to bacteria
23
Q

what is a repressible operon?

A
  • controls anabolic pathways
  • usually switched on
  • repressor protein synthesised in inactive form

eg trp operon

24
Q

what is the structure of a trp operon

A
  1. promoter
  2. operator
  3. structural genes

trpE, trpD, trpC, trpB, trpA

*outside operon: regulatory gene trpR (codes for inactive repressor protein)

25
Q

when tryptophan is ABSENT

A
  1. trpR synthesised in inactive form
  2. RNA pol binds to promoter
  3. synthesise structural genes -> polycistronic mRNA formed
  4. translation of polycistronic mRNA -> 5 enzymes responsible for tryptophan synthesis formed
  5. trp operon on : bacterium synthesises tryptophan as it contains 5 enzymes for trp anabolism
26
Q

when tryptophan is PRESENT IN EXCESS

A
  1. repressor protein in inactive form
  2. tryptophan accumulates -> acts as corepressor -> bind to allosteric site of inactive trp repressor
  3. repressor undergoes conformational change
  4. active repressor protein binds to operator at DNA binding site
  5. RNA pol cannot bind to promoter
  6. transcription cannot proceed
  7. trp operon off
  8. no polycistronic mRNA
  9. no 5 enzymes in tryptophan synthesis
  10. no synthesis of trp
27
Q

what is an inducible operon?

A
  • catabolic pathways
  • usually switched off
  • deactivated when inducer binds to it
28
Q

what is the structure of a lac operon?

A
  1. promoter
  2. operator
  3. structural genes: lacZ, lacY, lacA

*outside operon: lacI repressor

29
Q

what does lacZ code for?

A

beta-galactosidase

breaks down lactose into glucose and galactose
converts lactose -> allolactose

30
Q

what does lacY code for?

A

lactose permease

transport of lactose into bacterium from external environment

31
Q

what does the lacA gene code for?

A

transacetylase

may be involved in toxic by-product removal

32
Q

regulation of lac operon

A

dual regulation

negative: through lac repressor
positive: catabolite activator protein (CAP)

33
Q

(negative regulation) when LACTOSE and GLUCOSE ABSENT

A
  1. lacI gene constitutively transcribed
  2. continuous production of lac repressor protein
  3. active lac repressor protein binds to operator via DNA binding site
  4. absence of lactose: bound repressor prevents RNA pol from binding to promoter
  5. transcription of structural genes blocked

lac operon is switched OFF

34
Q

(negative regulation) when LACTOSE PRESENT

A
  1. lactose enters cell via permease
  2. converted to allolactose by beta-galactosidase
  3. allolactose acts as inducer + binds to allosteric site of active lac repressor
  4. inactivates repressor by altering conformation
  5. repressor inactive and detaches from operator
  6. RNA pol binds to promoter to initiate transcription of structural genes

lac operon SWITCHED ON

  1. single polycistronic mRNA
  2. produce 3 enzymes involved in lactose metabolism
35
Q

catabolite activator protein

A

DNA binding site: allows to bind to activator/CAP binding site in promoter

allosteric site specific for binding of cAMP

  • CAP naturally inactive
  • activated when bound to cAMP -> CAP-cAMPcomplex
36
Q

(positive regulation) GLUCOSE and LACTOSE both PRESENT

A
  1. inactive repressor bound to inducer
  2. lac operon switches on
  3. high glucose level INHIBITS adenylyl cyclase
  4. low levels of cAMP
  5. no binding of cAMP to CAP
  6. low levels of cAMP-CAP complex
  7. RNA pol binds to promoter less stably
  8. structural genes transcribed at lower rate
  9. enzymes synthesised at lower rate
  10. bacterium may use lactose BUT GLUCOSE PREFERRED RAHHH
37
Q

(positive regulation) GLUCOSE ABSENT, LACTOSE PRESENT

A
  1. absence of glucose: high adenlyl cyclase activity
  2. cAMP increase
  3. cAMP binds to allosteric site of CAP -> cAMP-CAP complex
  4. activating CAP which binds to CAP binding site in promoter

binding of cAMP-CAP complex to CAP binding site INCREASES AFFINITY OF PROMOTER FOR RNA POL
- increases rate of transcription initiation of lac operon
- structural genes transcribed at high rate
- enzymes synthesised at high rate
- bacterium take up and break down lactose at faster rate
- lactose is now main substrate for respiration

H2 biology (20 decks)

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