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Biology Core Idea 2 > Bacteria > Flashcards

Bacteria Flashcards

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1
Q

Bacteria

A

Prokaryotes with no membrane-bound organelles

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2
Q

Parts of Bacteria

A
  1. Internal structure
  2. Surface structure
  3. Appendages
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3
Q
  1. Internal structure (5)
A
  1. Bacterial chromosome
    - Double-stranded, circular DNA
    - Within nucleoid region, no membrane
    - Associates with DNA binding proteins → loop domains → supercoiling
    - No introns
  2. Nucleoid
    - Region in bacteria cell where DNA confined to
  3. Ribosomes
    - 70S → granular appearance
  4. Storage granules
  5. Plasmid
    - Small, circular autonomously replicating DNA molecule
    - Confer advantages
    - Can be more than 1 copy
    - Can have different plasmids
    - Vectors in genetic engineering
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4
Q
  1. Surface structure (3)
A
  1. Cell membrane
    - Phospholipid bilayer
    - ETC and ATP synthase
  2. Cell wall
    - Peptidoglycan → long chains of sugar cross-linked by short peptide chains
    - Protects cell from osmotic lysis
    - Confers rigidity and shape
    - Gram stain
  3. Capsule
    - Glycocalyx (sugar coat) → exterior of cell wall
    - Capsule or slime layer
    - Protects bacteria from phagocytosis as it can’t be recognised
    - Enables bacteria to adhere to one another or particular surfaces → biofilm
    - Prevent desiccation
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5
Q
  1. Appendages
A
  1. Fimbriae (Fimbria)
    - Short, bristle-like fibres
    - Attachment to surfaces/other bacteria/organisms
  2. Pili (Pilus)
    - Longer and fewer in number than 1
    - Conjugation → sex pilus
    - Motility → makes contact with surface and retracts to pull
  3. Flagella
    - Long appendages for motility
    - Hollow cylindrical protein thread → propels by rotation
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6
Q

Binary Fission

A

Asexual reproduction produces genetically identical bacteria

  1. DNA replication begins at the origin of replication (ori) → DNA unzipped by breaking H bonds between bases of the 2 strands to form replication bubble
  2. DNA replicates by semi-conservative replication → each original strand serves as template for synthesis of daughter strands by cbp
  3. 2 newly formed ori move to opposite poles of the cell and attach to the plasma membrane
  4. Cell elongates to prepare for division.
  5. DNA is circular with no free ends → 2 daughter DNA molecules will be interlocked with completion of replication.
  6. Enzyme topoisomerase cuts, separates and reseals the 2 DNA molecules
  7. Invagination of plasma membrane and the deposition of new cell wall (division septum) eventually divides parent cell into two daughter cells → each inherits a complete genome (genetically identical)
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7
Q

Genetic variation

A

Generate genetic variation through forming new combination of new alleles → enhancing reproductive success

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8
Q

Genetic variation methods (3)

A
  1. Transformation
  2. Transduction (General/Specialised)
  3. Conjugation
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9
Q

Transformation definition

A

Uptake of naked, foreign DNA from the surrounding environment, resulting in a change of the bacterial cell’s genotype and phenotype

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10
Q

Transformation (7)

A
  1. Fragments of foreign naked DNA from dead lysed bacterial cells
  2. Naturally competent bacteria with cell-surface proteins bind and transport DNA into the cell.
  3. Artificially, bacteria can be made competent through immersion in a medium with CaCl2 followed by a heat shock treatment
  4. Foreign DNA incorporated into chromosome through crossing over at 2 homologous regions found on the bacterial chromosome (homologous recombination)
  5. Result: recombinant cell
  6. If different alleles for a gene were exchanged, the new allele will be expressed → permanent change in genotype & phenotype
  7. Recombinant genome will be passed on to all subsequent offspring through binary fission
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11
Q

Transduction definition

A

Bacterial DNA from one host cell is introduced into another bacterial host cell by a bacteriophage due to aberrations in the phage reproductive cycle

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12
Q

Generalised Transduction (5)

A
  1. A phage infects a bacterium, injecting its viral genome (DNA) into the host cell
  2. Bacterial DNA degraded into small fragments, one of which may be randomly packaged into a capsid head during the spontaneous assembly of new viruses
  3. Upon cell lysis, the defective phage will infect another bacterium and inject bacterial DNA from the previous host cell into the new bacterium
  4. Foreign bacterial DNA can replace homologous region in the recipient cell’s chromosome if crossing over/homologous recombination* takes place, possibly allowing expression of a different allele from previous host.
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13
Q

Specialised Transduction (6)

A
  1. Temperate phage infects bacterium, injecting viral genome into host cell
  2. The viral DNA is integrated into bacterial chromosome forming a prophage which may be improperly excised to include adjacent segment of bacterial DNA during an induction event
  3. Bacterial DNA may be packaged into a capsid head during the spontaneous assembly of new viruses
  4. Upon cell lysis, defective phage will infect another bacterium and inject bacterial DNA from previous host cell into new bacterium
  5. Foreign bacterial DNA can replace homologous region in the recipient cell’s chromosome if crossing over/homologous recombination take place, possibly allowing expression of a different allele from previous host
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14
Q

Conjugation definition

A

Direct transfer of genetic material from one bacterial cell to another through a mating bridge between the two cells via the transfer of F plasmid from an F+ donor to F– recipient cell

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15
Q

Conjugation (5)

A
  1. Sex pilus (coded for by F factor) of F+ bacterial cell makes contact with a F- cell and retracts to bring the 2 cells closer
  2. The hollow pilus then acts as a cytoplasmic mating bridge between the 2 cells
  3. One of the 2 strands of the plasmid DNA is nicked and transferred from the F+ cell to the F- cell through the bridge
  4. The single stranded F plasmid DNA circularises in F - cell and is used as a template to synthesise a complementary strand for a double-stranded
    plasmid DNA. The F- recipient cell is now a F+ cell
  5. Replication of the plasmid occurs via rolling circle DNA replication occurs
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16
Q

Rolling circle DNA replication (5)

A
  1. 1 strand of ds F plasmid nicked by nuclease
  2. Free 3’OH end then used as a primer for strand elongation by DNA polymerase using intact strand as a template
  3. Elongation process is facilitated by the displacement of the 5’ end of the nicked strand and is transferred across the mating bridge to the recipient bacterium
  4. Upon completion of a unit length of the plasmid DNA (after 1 round), another nick occurs to release the
    original strand
  5. In the recipient cell, the single strand of F plasmid DNA re-circularises and serves as a template for the synthesis of a complementary daughter stand to form a double stranded circular DNA.
17
Q

Gene Regulation

A
  • Rate at which certain protein molecules are synthesised varies according to circumstances and demand
  • Predominates at transcriptional level → efficient with minimal wastage
  • Economical use of energy and resources
  • Responsiveness to environment
  • Selective advantage
18
Q

Operon

A

A cluster of genes with related functions, regulated in such a way that all the genes in the cluster are turned on and off together

  1. Common promoter
  2. Operator
  3. ≥ 1 structural genes → controlled as a unit to produce a single polycistronic mRNA
19
Q

Promoter

A

RNA polymerase binding site, upstream of structural genes

20
Q

Operator

A

Repressor protein binding site to prevent RNA polymerase from binding to the promoter and intiating transcription

21
Q

Polycistronic mRNA

A
  • mRNA that contains base sequence coding for the amino acids sequence of several proteins.
  • Contains multiple start and stop codons (one per polypeptide)
  • Gives rise to a total of ___ different polypeptides which can be translated from a single mRNA
22
Q

Structural gene

A

Any gene that codes for a protein product that forms part of a structure or has an enzymatic function

23
Q

Regulator gene

A
  • Any of several kinds of nucleotide sequences involved in the control of the expression of structural genes
  • Codes for a protein involved in regulating the expression of other genes e.g. repressor, CAP
  • Has its own promoter and terminator sequences
  • Not within operon, usually far away, but gene products that control the expression are diffusible
24
Q

Effector

A
  • Small molecule that binds to a specific protein, causing a conformational change and hence regulating its biological activity.
  • In this context, includes inducer (allolactose in lac operon) and corepressor (tryptophan in trp operon)
25
Q

Lac operon

A
  • E, coli → intestines of humans and mammals
  • Inducible operon
  • Catabolic pathway
  • Breakdown of lactose
  • Dual control → negative/positive regulation
26
Q

Lac operon structure

A
  • LacZ, lacY, lacA
  • Promoter → RNA pol bonding site
  • Operator → lac repressor binding site
  • Operator overlaps with promoter
  • Catabolite Activator Protein (CAP) binding site within promoter
27
Q

LacZ codes for:

A

β-galactosidase

28
Q

β-galactosidase function (2)

A
  1. Hydrolyse lactose into glucose and galactose

2. Convert lactose into allolactose, an isomer

29
Q

LacY codes for:

A

Permease, a membrane transport protein

30
Q

Permease function

A

Facilitates movement of lactose from outside the cell to inside

31
Q

LacA codes for:

A

Transacetylase → function not very well known, metabolises certain disaccharides

32
Q

Lac operon regulatory gene

A

LacI that codes for lac repressor protein

33
Q

Lac operon effector

A

Inducer lactose, substrate

34
Q

Lac operon default (4)

A
  1. Repressed by default
  2. Regulatory gene lacI constitutively transcribed → lac repressor protein
  3. Lac repressor protein produced in active form → binds to lac operator via DNA-binding site → denying RNA polymerase access to promoter
  4. Transcription of structural genes blocked
35
Q

Lac operon negative regulation (6)

A
  1. In the absence of lactose, a basal level of beta galactosidase and permease is present in the cell: repression of lac operon by lac repressor is leaky
  2. Lactose enters the cell by permease
  3. And is converted to allolactose by β-galactosidase
  4. Allolactose acts as an inducer and binds to allosteric site of lac repressor → change in confirmation → inactive
  5. The inactive repressor can no longer bind to operator of lac operon (no longer complementary in shape and charge)
  6. Promoter site available for RNA polymerase to
    bind
36
Q

Lac operon positive regulation (3)

A
  1. When glucose is absent → high levels of cAMP is
    present → binds to CAP → activated CAP binds to promoter of lac operon → ↑ affinity of RNA pol to promoter
  2. Transcription frequency of structural genes lac Z,
    lac Y and lac A to produce beta-galactosidase,
    permease and transacetylase respectively to
    breakdown lactose thus ↑
  3. (Give one example of a product and what it does)
37
Q

Trp operon

A
  • Repressible operon (end-product repression)
  • Anabolic pathway
  • Synthesis of amino acid tryptophan (corepressor)
  • 5 structural genes (trpA-E)
  • Regulatory gene → trpR → trp repressor
38
Q

Trp operon default (3)

A
  1. Regulatory gene, trpR, constitutively transcribed → trp repressor protein
  2. Trp repressor protein synthesised in inactive form → unable to bind to operator
  3. RNA pol able to bind to promoter → operon is default “on”
39
Q

Trp operon negative regulation

A
  1. When tryptophan is present in high concentrations in the
    cell, it acts as a corepressor and binds to the of trp repressor
  2. This causes a change in conformation of trp repressor
    and trp repressor become active → can bind to the operator (complementary in shape and charge)
  3. Prevents binding of RNA polymerase to promoter and transcription of structural genes & expression of operon
  4. Synthesis of tryptophan is reduced/stopped

Biology Core Idea 2 (9 decks)

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