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Alevel Biology > Bacteria > Flashcards

Bacteria Flashcards

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1
Q

What is the function of bac chromosome?

A

1 circular, dsDNA in nucleoid region⇒ contains genes essential for survival (genes code for… )

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2
Q

What is the function of plasmids?

A

autonomously replicating, extrachromosomal circular DNA⇒ containing genes that confer advantages to bacteria living in stressful environments e.g. antibiotic resistance (/use new substrates for energy)

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3
Q

What is the function of 70S ribosomes?

A

synthesise proteins

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4
Q

What is the function of storage granules in cytoplasm?

A

containing nutrients and chemical reserves

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5
Q

What is the function of plasma membrane?

A

membranes chpt + where ETCs & ATP synthase embed to produce ATP in photosynthesis/ respiration

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6
Q

What is the function of peptidoglycan cell wall?

A

Protects cell from osmotic lysis

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7
Q

What is the function of capsule/slime layer?

A

Ensure phagocytes unable to recognise bac–> protect bac from being taken in by them
Enables bac to adhere to one another/surfaces→ biofilm
Prevent desiccation of bacteria as it contains water
Protection from antibiotics

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8
Q

What is the function of fimbriae?

A

attachment to sf or other bacteria/organisms

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9
Q

What is the function of pili?

A

for conjugation and motility

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10
Q

What is the function of the flagella?

A

for motility/propulsion by rotation

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11
Q

What are the advantages of asexual reproduction via binary fission?

A

+: selective advantage in stable, unchanging, favourable environment as it allows genotypes to rapidly reproduce and colonise a habitat (all adapted for the area)
+ fast reproduction
-: change in environment→ whole colony can’t survive

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12
Q

Describe the process of binary fission

A
  • Semi-conservative DNA replication begins at Ori:
    > DNA is unzipped by helicase by breaking hydrogen bonds between bases of the 2 strands. Ds separates→ replication bubble
    > Each parental strand serves as a template for synthesis of daughter strands
    > Free deoxyribonucleotides cbp with bases on template strand→ DNA polymerase forms phosphodiester bonds between adj dNTPs
    > Leading strand + lagging strand
  • As chromosome replicates, 2 newly formed Ori move to opposite poles of the cell & attach to plasma membrane
  • Cell concurrently elongates to prepare for division
  • Interlocking struc made up of 2 daughter DNA molecules formed with the completion of replication, as DNA is circular w no free ends→ Topoisomerase cuts, separate and reseal 2 DNA molecules
  • Cell budding off: Plasma membrane invaginates, new cell wall deposited→ parent cell divided into 2 daughter cells, each inheriting a complete genome
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13
Q

Compare mitosis and binary fission

A 
End pdt
Amt of DNA
DNA replication
Behaviour of chromosomes
- Attach to plasma membrane?
- Form tangled rings?
- Chromosome condensation?
- Specific positioning of chromosomes?
Spindle fibre
Type of division
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14
Q

Describe transformation

A

Fragments of foreign naked DNA (from lysed bac) in the surrounding medium are taken up by competent bac cell via surface proteins→ foreign DNA incorporated into bac chromosome via homologous recombination/ crossing over at homologous regions→ foreign DNA contains a different allele that is now expressed, bac cell transformed

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15
Q

What is meant by a competent bacterial cell?

A

natural ability to readily take up foreign DNA

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16
Q

How can bac become artificially competent?

A

immerse in a cold culture medium w high [CaCl2] + heat shock treatment

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17
Q

Describe generalised transduction

A
  • Phage infects bac, injecting its viral genome into the host cell
  • Bac DNA degraded into small fragments, one may be randomly packaged into a capsid head during assembly
  • cell lysis–> defective phage infects another bac by injecting bac DNA from prev host cell into new bac
  • Homologous recombination: foreign bac DNA replaces homologous region of recipient cell chromosome→ (possibly) expression of a different allele
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18
Q

Describe specialised transduction

A
  • Temperate phage infects bac, injecting its viral genome into the host cell
  • Viral DNA integrates into bac chromosome, forming prophage, which may be improperly excised to include adj segments of bac DNA during an induction event.
  • phage-host DNA hybrid packaged into capsid head during assembly
  • cell lysis–> defective phage infects another bac by injecting bac DNA from prev host cell into new bac
  • Homologous recombination: foreign bac DNA replaces homologous region of recipient cell chromosome→ (possibly) expression of a different allele
19
Q

In what ways are F plasmids different from bac chromosome?

A

Smaller

Has non-essential genes coding for advantageous traits e.g. antibiotic resistance

20
Q

Explain why conjugation can only be initiated by an F+ cell with an F plasmid / Role of plasmid

A

F+ cells possess F plasmid, which has an F factor that carries genes coding for proteins needed to form sex pili & subsequent cytoplasmic mating bridge
→ allow for conjugation & transfer of bac genes between bac

21
Q

Describe conjugation

A
  1. Sex pilus of F+ cell makes contact w F- cell & retracts→ bring F- cell closer, mating bridge forms
  2. Rolling circle DNA replication
    - One of the 2 strands of plasmid DNA is nicked & transferred from F+ to F– cell. Phosphodiester bond is broken.
    - Free 3’ OH end of the nick is used for strand elongation by DNA polymerase using intact strand as a template→ synthesis of new complementary strand
    - Elongation process is facilitated by the displacement of the 5’ end of nicked strand and is transferred to the recipient bacterium via the mating bridge
    - Completion of a unit length of plasmid DNA→ another nick releases original strand, which re-circularises
    - Recipient cell: ss F plasmid DNA re-circularises and serves as template for the synthesis of a new complementary daughter strand→ ds F plasmid→ F- cell becomes F+ cell
22
Q

Benefits of conjugation for recipient bac?

A
  • Gains new alleles that, when expressed, allow survival in a diff environment e.g antibiotic resistance
  • Use of a new metabolites/resources eg new C source by producing the relevant enzymes
23
Q

Compare transformation, generalised and specialised transduction, conjugation

A

Source of DNA/ Types of donor cell
Agent mediating transfer
Type of DNA transferred
Homologous recombination needed for permanent expression of foriegn gene?

24
Q

Define an operon

A

An operon is a cluster of genes with related functions, under the control of the same promoter, allowing for functionally related proteins to be synthesised as a unit

25
Q

Why is an operon necessary?/Advantages of gene regulation in prokaryotes?

A
  • Can be turned on or off according to certain conditions/changes→ respond rapidly and appropriately to environment
  • Bac only produces enzymes when required→ make economical use of energy and resources
  • Bac can use a variety of metabolites eg glucose metabolised preferentially over lactose, not economical to produce lac gene proteins in the presence of proteins
  • All above provides a selective advantage to bac→ maintain colonies despite env changes
26
Q

Distinguish between a structural and a regulatory gene

A

Structural: any gene that codes for a protein product that has an enzymatic function in a metabolic pathway
Regulatory: codes for protein (repressor, CAP) involved in the regulation of the expression of other/structural genes

27
Q

Explain what is a polycistronic mRNA

A

mRNA that has the base seq coding for aa seq of several proteins

28
Q

Compare polycistronic mRNA and pre-mRNA

A
  1. several ‘start’ and ‘stop’ codons VS 1 start and 1 stop codon
  2. No introns VS have introns
  3. Shine-Dalgarno seq upstream of each ‘start’ codon present VS absent
29
Q

Describe main struc features of the lac operon

A

Sruc genes

  • lac Z codes for β-galactosidase enzyme, which hydrolyses lactose to glucose and galactose + converts lactose to allolactose
  • lac Y codes for permease, which is a membrane transport protein that facilitates the movement of lactose from outside of cell to inside
  • lac A codes for transacetylase, whose function remains unknown

Common promoter upstream of the struc genes: RNA polymerase bind & initiate transcription
- Includes Catabolite Activator Protein (CAP) binding site where CAP-cAMP complex binds to upregulate transcription

Operator, where repressor binds to prevent access of RNA polymerase to genes, preventing transcription

Regulatory gene, lacI, codes for lac repressor protein (not part of operon but discussed together)

30
Q

Explain why the lac operon is an inducible operon and produces proteins that are involved in a catabolic (breakdown) pathway

A

Expression of the 3 genes is usually “off” but can be induced & hence is turned “on” in the presence of an inducer molecule
Genes only expressed when substrate is present, preventing wastage of resources

31
Q

Explain that the lac operon is under dual control i.e. negative regulation by the lac repressor and positive regulation by the catabolite activator protein (CAP)

A
  • For lactose-metabolising enzymes to be produced in appreciable quantity, lactose present + glucose in short supply
  • Ensures preferred carbon source, glucose, is used before other carbon sources, as considerable energy needed to synthesize additional lactose-metabolising enzymes eg β-galactosidase
  • State of lac repressor determines whether lac operon genes undergo transcription; state of CAP determines amt of transcription when operon is not repressed (on off vs vol control)
32
Q

Describe -ve gene regulation of the lac operon

A
  • Lactose is absent
  • Regulatory gene lacI is constitutively transcribed→ continued production of small amts of active lac repressor protein
  • Active lac repressor protein binds specifically to lac operator seq via protein’s DNA-binding site
  • RNA polymerase denied access & can’t bind to promoter
  • Transcription of structural genes of operon blocked→ lac operon “off” / repressed
33
Q

Explain why a basal level of β-galactosidase and permease is present even in the absence of lactose

A

repression of the lac operon by the repressor is ‘leaky’ due to weak interactions between operator and repressor→ repressor sometimes dissociates from operator
Permeases present to transport lactose from the surrounding medium into the cell→ Some lactose converted to allolactose, an inducer molecule, by β-galactosidase

34
Q

Describe how (inducer) allolactose, relieves the repression of the lac operon in the presence of lactose

A
  • Allolactose binds to allosteric site of lac repressor protein → 3D conf of DNA-binding site altered, no longer complementary in shape and charge operator, can’t bind to operator→ repressor inactivated
    RNA polymerase can access & bind to promoter, initiate transcription of struc genes of operon, to form (products)
    Struc genes transcribed as a single polycistronic mRNA
    All 3 enzymes translated from a single mRNA (all genes in operon always expressed/not expressed in unison)
    → lac operon “on”
35
Q

Describe how (inducer) allolactose, relieves the repression of the lac operon in the presence of lactose

A
  • Allolactose binds to allosteric site of lac repressor protein → 3D conf of DNA-binding site altered, no longer complementary in shape and charge operator, can’t bind to operator→ repressor inactivated
  • RNA polymerase can access & bind to promoter, initiate transcription of struc genes of operon, to form (products)
    > Struc genes transcribed as a single polycistronic mRNA
    > All 3 enzymes translated from a single mRNA
    → lac operon “on”
36
Q

Describe +ve gene regulation of the lac operon

A

↓ glucose→ ↓ ATP→ ↑ cyclic AMP/cAMP

cAMP binds to allosteric site of CAP→ CAP-cAMP complex→ activating CAP, which binds to CAP-binding site within promoter⇒ upregulate

Affinity of promoter region for RNA polymerase ↑ → ↑ rate of transcriptional initiation of lac operon struc genes → increased synthesis of (products)

37
Q

Describe how the presence of glucose regulates the expression of lac operon when lactose is present

A

↑ glucose→ ↓cAMP→ no binding of cAMP to allosteric site to form CAP-cAMP complex→ CAP remains inactive
→ CAP can’t bind to CAP-binding site in promoter→ affinity of lac operon promoter for RNA polymerase remains low→ low level of transcription and expression of lac operon struc genes

38
Q

Describe the organisation of the trp operon

A

Common promoter + operator
Struc genes: trp E, trp D, trp C, trp B and trp A code for 5 polypeptides that make up 3 enzymes involved in the synthesis of tryptophan

39
Q

Explain why trp operon is a repressible operon & produces proteins involved in an anabolic pathway (synthesis)

A

Normally “on” by default, and are turned “off” in the presence of an effector molecule, which is usually end product of anabolic pathway→ tryptophan is effector⇒ avoid devoting resources to unnecessary synthetic activities once end product accumulated to sufficient amts

Aka end-product repression

40
Q

Describe how the trp operon is regulated in low levels of tryptophan

A

Regulatory gene, trpR, is constitutively transcribed→ continued synthesis of inactive trp repressor protein, which has little affinity for trp operator & hence can’t bind to operator at DNA-binding site

RNA polymerase able to bind to promoter and transcription of struc genes occurs→ operon is default ‘on’

41
Q

Describe negative gene regulation of the trp operon in high levels of the co-repressor, tryptophan

A

Inactive trp repressor synthesised has little affinity for trp operator

Tryptophan accumulates & binds to allosteric site of trp repressor→ activate repressor, which binds to operator’s DNA-binding site
- Binding is complementary

RNA polymerase can’t bind to promoter, transcription of struc genes can’t occur→ operon is ‘off’ / can’t be expressed

42
Q

Compare lac operon/inducible operon and trp operon/repressible operon/

A 
Type of metabolic pathway
Structure
Type of operon→ purpose
Default operon expression
Type of regulation
Regulatory gene
Default state of repressor 
Effector molecule
Effect of effector mlc on operon

S:

  • several genes under the control of a single promoter
  • operator
  • enzymes involve in the same metabolic pathway
  • polycistronic mRNA
43
Q

Compare viruses and bacteria

A 
Cellular organisation
Genetic material
Macromolecular machinery
Metabolism
Reproduction
Size
Pathogenicity

Alevel Biology (16 decks)

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