e) Describe the structure of a bacterial chromosome including the arrangement of DNA
within bacterial cells.
f) Describe the process of binary fission, transformation, transduction and conjugation
in bacteria and explain the role of F plasmids in bacterial conjugation.
g) Distinguish between structural and regulatory genes, and between repressible and
inducible enzymes
h) Describe the concept of a simple operon (using lac operon as an example).
Describe the structure of a bacterial chromosome including the arrangement of DNA
within bacterial cells.
Describe features of a prokaryotic cell
Describe Structure bacteria cells
1. small  1-10µm
2. cellular organisation
3. cell wall – peptidoglycan = sugars cross linked by short polypeptide chains
4. bi-lipid cell membrane
5. Capsule – covers cell = A layer of polysaccharide. Often present in pathogenic
bacteria that protects the bacteria cell from environmental - antibiotics or
phagocytosis by white blood cells, helps in adherence to particular surfaces
6. double stranded circular DNA, associated with proteins, supercolied to form
bacteria chromosome
7. in Nucleoid region No nuclear membrane
8. plasmids – circular DNA, non-essential genes, contains advantageous genes,
divide autonomously
9. 70s ribosomes
10. pilli – for attachment, form sex pilli for conjugation
11. flagella (one or more) – for motility
Describe the process of binary fission,
1. DNA double helix separates to 2 strands at origin of replication
2. forms leading strand and lagging strand for replication
3. 2 copies of chromosomes separate
4. cell elongates
5. plasma membrane grows inwards at the middle dividing cell into two as new cell
wall also forms between the 2 daughter cell
6. binary fission produces clones
TRANSFORMATION,
1. = uptake of naked foreign DNA form surrounding environment resulting in
change in bacteria genotype
2. source of DNA = dead lysed bacteria cells
3. bacteria have sell-surface proteins that recognize ant take up DNA of closely
related species
4. (artificially induced by immersion in high Ca++
5. foreign genes incorporated through crossing over at homologous region
TRANSDUCTION
1. process where phage carry bacteria DNA fragments form one cell to another host
cell
2. in lytic cycle phage enzymes hydrolyse bacteria DNA into fragments
3. during assembly a small piece gets packaged in capsid
4. lysis release phage to infect another bacterium
5. Bacteria DNA is injected into new host by phage
6. (the phage is non infectious because it lacks phage genes as bacteria genes
have been packaged instead)
7. foreign bacteria DNA aligns on homologous region of host chromosome
8. cross over may results in recombination
(SPECIALISED TRANSDUCTION can skip)
1. occurs with phage in lysogenic cycle
2. phage incorporate DNA in bacteria genome = prophage
3. phage switches to lytic cycle
4. phage DNA excised out together with some bacteria genes
5. progeny infects new bacterium
6. bacteria DNA delivered into new cell
7. crossing over
AND CONJUGATION IN BACTERIA and explain the role of F plasmids in bacterial
conjugation.
1. = one way direct transfer of genetic material form on bacteria cell to another
via a cytoplasmic bridge
2. donor cell has F-plasmid / with F-factor genes for pilli production
3. donor cell = F+ cell form cytoplasmic bridge to recipient cell
4. F-plasmid DNA separates into 2 single strands
5. one strand is transferred across the bridge
6. both act as templates for synthesis of daughter strands
7. plasmid DNA in recipient cell circularizes to form F plasmid
8. result in 2 daughter cells with F+
(Hfr SKIP)
1. high frequency recombination
2. donor cell with F-plasmid incorporated into chromosome
3. replication of DNA initiated at site within F-factor
4. single strand moves into recipient cell
5. bridge usually breaks before whole bacteria DNA can move across
6. single strands act as template in both cells
7. DNA aligns with homologous segment of F- and recombination occurs
h) Describe the concept of a simple operon (using lac operon as an example).
Operon
A functionally integrated genetic unit for the control of gene expression in bacteria.
It consists of one or more genes that encode one or more polypeptide(s) and the
adjacent site (promoter and operator) that controls their expression by regulating the
transcription of the structural genes.
lac operon
RJC
Regulator gene
Operator
Promoter
lacI
Structural genes
lacZ
lacY
lacA
Figure 9 showing the organization of the lac operon
Lac OPERON SYSTEM
1. E.coli uses 3 enzymes to take up and metabolise lactose.
2. The structural genes for these 3 enzymes are clustered in the lac operon
3. lacZ  codes of β-galactosidase  which hydrolyses lactose to glucose and
glalactose
4. lacY  codes for permease  a membrane protein that transports lactose into
the cell
5. LacA  function unknown
6. Upstream of the structural genes lie a single promoter sequence which provides
a site for RNA polymerase to bind and initiate transcription.
7. the operator allows binding of a repressor protein to switch the operon off
8. a regulatory gene lacI outside the operon codes for a repressor protein that
switches off the operon by binding to the operator.
9. the operon is usually “off” until stimulated by lactose.
IN PRESENCE OF LACTOSE
lac operon
Regulatory gene
P
Operator
Structural genes
Promoter
lacI
lacZ lacY
lacA
DNA binding site
Transcription
Repressor
protein (active
form)
3
Allosteric site
Polycistronic
4
mRNA
Inducer
(Allolactose)
Translation
2
Transacetylase
Permease
5
-galactosidase
Repressor protein
(inactive form)
RJC
1
Allolactose
lactose
lactose
Plasma
membrane
Figure 13. The action of inducer relieving the repression of lac operon in the presence
of lactose
1. A few molecules of β-galactosidase are present in the cell
2. lactose is converted by β-galactosidase to allolactose.
3. lactose is also converted by β-galactosidase to glucose (used mainly for
glycolysis) + galactose
4. allolactose binds to repressor protein inactivating it.
5. promoter is unblocked and RNA polymerase can bind and transcribe structural
genes
6. Operon is “induced” by presence of lactose
GLUCOSE {extra}
Promoter
RNA
CAPpolymerase Operator
binding site binding site
Regulatory gene
P
Structural genes
lacZ
lacI
CAP (active)
Repressor protein
3
RNA polymerase
binds efficiently
2
cAMP-binding
sites
Repressor protein
(inactive)
Glucose 
cAMP
1
lacY
lacA
Transcription
Translation
Transacetylase
Permease
-galactosidase
CAP (inactive)
RJC
Enzymes involved in
lactose catabolism
Scenario: Lactose high, glucose low, cAMP high. When glucose levels are low, each
CAP polypeptide has cAMP bound to its allosteric site. The active form of CAP binds to
the DNA at the CAP-binding site, activating transcription.
Figure 18. Transcription of lac operon requires both lactose as well as the absence
of glucose
1. for lactose utilizing enzymes to be produced in high quantities glucose must be in
short supply
2. E. coli preferentially uses glucose
3. when glucose level in cells is low, cyclic AMP levels accumulate.
4. cAMP binds to catabolite activator protein (CAP) allosteric site
5. this activates CAP allowing it to bind to promoter site
6. binding of active CAP stimulates gene expression (increase affinity of RNA
polymerase to gene)
7. when glucose levels increase cAMP levels drop; CAP detaches and transcription
levels proceed at low level.
tryp OPERON {EXTRA}
= repressible operon.  represses system
1. tryp Operon has 5 structural genes which produce enzymes for the production of
aa tryptophan.
2. Regulator gene produces inactive tryp repressor protein
3. if tryptophan levels increase, tryptophan binds to tryp repressor and activates it
4. active repressor binds to operator and switches operon off.
5. this is an example of end product repression
g) Distinguish between structural and regulatory genes, and between repressible and
inducible enzymes
A structural gene is a gene that codes for any RNA or protein product (e.g. structure,
enzyme) other than a regulatory element (i.e. regulatory protein).
Regulatory gene = A gene encoding a protein that plays a role in controlling the activity
of other genes.
Repressible systems - A repressible system is on except in the presence of some
molecule (called a co repressor) that suppresses gene expression. The molecule is
said to "repress expression".
A repressible enzyme is one whose synthesis is down regulated or "turned off"
by the presence of (for example) the end product of a pathway that the
enzyme .
Inducible systems - An inducible system is off unless there is the presence of some
molecule (called an inducer) that allows for gene expression
Inducible enzyme - An enzyme that is normally present in minute quantities/
upregulated or "turned on" within a cell, but whose concentration increases
dramatically when a substrate compound is added.

The lac operon is thus an inducible operon. A constitutively expressed repressor
protein keeps the structural genes turned “off”, until an inducer (in this case,
allolactose) inactivates the repressor permitting the transcription of the genes.