Bacteria Flashcards
Describe the structure of a typical bacteria [3]
o70S ribosome (as with prokaryotes)
o NO membrane bound organelles/regions
o Plasmid- Small, Circular Extrachromosomal DNA
- May contain genes that confer advantages
e.g Antibiotics resistance
Define Binary Fission
Asexual Reproduction that produces genetically identical cells
Why is binary fission beneficial?
Selective advantage in stable, favourable environment where it can rapidly replicate & colonise
Describe the process of binary fission [6] (consider the reason for each step)
- DNA rep. begins at Ori → DNA unzipped by breaking H bonds btwn
complementary bases to form replication bubble - Semi-conservative replication of DNA as each original strand serves as template for
synthesis of daughter strands by complementary base pairing - The 2 newly formed Ori moves to opposite poles of cells & attach to plasma membrane
- As DNA is circular with no free ends, an interlocking structure formed by 2
daughter DNA molecule formed when rep. is completed - Topoisomerase cuts, separate & reseal the 2 DNA molecules
- Plasma membrane invaginates & new cell wall divides parent cell into 2
Point of comparisons between Binary Fission & Mitosis [5]
- End products
- Amt of DNA formed
- When does rep. occur?
- Process description (Chr Behaviour)
- Spindle Fibre involved?
Define Transformation
- Uptake of naked,
foreign DNA from
surrounding
environment → - genotype ∆, hence
- phenotype ∆
Define Transduction
- Process of which bacterial DNA from 1 host cell is
- introduced into another by a bacteriophage due to
aberrations in phage reproductive cycle
When does generalised/specialised transduction happen?
Specialised: Temperate phage infection…
Generalised: ANY phage
What is a temperate phage in the syllabus?
Lambda phage
Describe how does Transformation occur? [4]
- Foreign DNA
fragments from
dead lysed
bacterial cells
enter bacterium - Naturally
competent
bacteria1 have
cell-surface
proteins that
binds &
transport DNA
into cell - Foreign DNA
incorporated into
bacterial chr via
Homologous
Recombination,
forming a (con’t) - recombinant bacteria able to express new alleles → permanent ∆ in gen/phenotype
Describe how does generalised tranduction happen?
- Phage infects
bacterium, injecting
its viral genome
DNA into host cell - Phage enzymes
degrade bacterial
DNA into small
fragments. A small
fragment of
degraded host cell
DNA is randomly
packaged into
capsid during
assembly of phage
genome - Upon host cell lysis,
defective phage
released can infect
another bacterium,
injecting previous
host’s bacterial
DNA into new host - Foreign bacterial
DNA can replace
homologous region
of new host’s chr as
homologous
recombination
occurs - Recombinant bacteria able to express new alleles → permanent ∆ in gen/phenotype
Describe how does SPECIALISED transduction happen? [6]
- Temperate phage (lambda)
infects bacterium,
injecting its viral
genome into host
cell & viral DNA is
integrated into
bacterial chr,
forming prophage - Upon induction,
Viral DNA may be
improperly
excised to include
adjacent bacterial
DNA* - Phage-Bacterium
hybrid DNA may be
packaged in capsid
during assembly of
phage - Upon host cell lysis,
defective phage
infect another
bacterium - New alleles from
previous host can
replace
homologous region
of new host cell via
homologous
recombination (or
integrase
incorporate) - Recombinant bacteria able to express new alleles → permanent ∆ in gen/phenotype
Define conjugation [2]
- Direct transfer of genetic material from
- 1 F+
donor bacteria cell to another through
mating bridge to another F-recipient
bacterium
Describe how does conjugation happen? [5]
- Sex pilus of F+ bacterium makes
direct contact with F- cell & retracts
to bring the 2 cells closer - Hollow pilus act as cytoplasmic
mating bridge btwn 2 cells - 1 of the 2 strands of plasmid DNA is
nicked & transferred from F+ to F- cell via mating bridge via Rolling
Circle Replication - SS F plasmid DNA circularises in Fcell & is used as a template to
synthesize complementary strand - Recombinant bacteria able to express new alleles → permanent ∆ in gen/phenotype
Points of Comparison for Conjugation vs Transformation vs Transduction (General/specialised) vs Conjugation [4]
- Source of DNA introduced (phage involved?)
- Conditions
- Type of DNA transferred (random?)
- Homo Recombination?
How can a bacteria gain variation? [4]
- Random mutation
- Transformation
- Generalised/Specialised Transduction
- Conjugation
Why does bacteria need to regulate its genes?
- Unicellular bacteria have limited resources
- easily influenced by environment
Advantages of regulation? [3]
(i) ensure economical use of resources & energy, no wastage,
(ii) allow bacteria to respond & adapt to ∆s,
(iii) Operons can turn on/off as all functionally related protein are synthesized in 1 unit →
selective advantage who can respond & survive better
Define operon
Cluster of genes with related functions, regulated such that genes in cluster are turned on/off together
Define polycistronic mRNA
mRNA with multiple stop & start codons , codes for diff p.p that are related to e/o
When is Lac operon UPREGULATED?
Is this Positive/Negative Regulation?
In ABSENCE of glucose
→ When [glucose] decreases, [cAMP]
increases
Positive regulation
When is LAC operon DOWNREGULATED
Is this Positive/Negative Regulation?
In ABSENCE of Lactose
No allolactose inducer → lac repressor remains
active → binds to operator → prevent RNA pol.
accessing promoter → prevent transcription
Negative regulation
Why is there some activity of Beta-Galactosidase even if the Lac operon is DOWNREGULATED? [4]
- Basal level of β-galactosidase & permease
exists due to leaky repression* - Lactose enters cell via permease → converted
to allolactose by β-galactosidase - Allolactose act as Inducer & bind to Allosteric
Site of lac repressor → causing conformation
∆ & inactivating repressor - Inactive lac repressor unable to bind to
operator → promoter now available to RNA
Polymerase to bind* → transcription of
structural genes occurs
Why is repression of Lac operon described as leaky? [2]
- Interactions btwn repressor & operator are weak
- Repressor can dissociate from operator from time to time → basal lvl of permease, β-galactosidase
→ small amt of lactose can enter & converted to allolactose → inactive repressor → transcription
Define end-product inhibition [2]
- A form of feedback inhibition where the
- *end product of a metabolic pathway inhibits the enzyme involved in early reactions
Describe whether the following operon is repressible/inducible
1. Lac
2. Trp
- Inducible (cuz its usually INACTIVE due to active repressor produced)
- Repressible (cuz its constitutively ACTIVE due to inactive repressor produced)
Describe the type of regulation of the Trp Operon
Negative Regulation in the form of end-product inhibition
When is the Trp operon DOWNREGULATED [2]
In presence of HIGH [Trp]
1. Tryptophan acts as co-repressor, binds to
allosteric site of Trp repressor →
activating it
2. Activated Trp repressor binds to Operator in
Promoter → prevents RNA polymerase
from binding to promoter → prevent
transcription of structural genes
When is the Trp operon UPREGULATED [2]
In presence of LOW [Trp]
1. Low Tryptophan → no co-repressor to
activate repressor → Trp repressor remains
inactive
2. Unable to bind to operator in promoter →
RNA polymerase able to access promoter →
transcription continues → Trp synthesized
Given an Lac Operon activity to [glucose] graph, why will there be a delay in increase in activity of Lac operon to utilise Lactose once glucose is fully used up?
time needed for transcription & translation of Lac operon given glucose is used up
