A2 Biology Unit 5 complete

1. Biology A2
Unit 5
Revision

2. Teacher 1 –
Control in
Organisms
3.5.1  3.5.5

3. Survival and Response
Organisms increase their chance of survival by
responding to their environment. Their behaviour
is most likely going to be in search of food or
shelter from predators.
responses to unidirectional stimuli.
Movement toward or away from the stimulus such as
gravitropism/phototropism.
e.g. When a seed germinates the root go down as they are positively
gravitropic, whereas the shoot goes upwards and is negatively
gravitropic

4. Taxes and kinesis are simple responses that can maintain a mobile
organism in a favourable environment such as one with plentiful
food supplies or no predators.
movement along a gradient of intensity such as
temperature:
Towards the high intensity (positive taxis)
Away from the high intensity (negative taxis)
e.g. Daphnia move towards greater light intensity as the phytoplankton
they feed on are found closer to the waters surface where they
photosynthesise.
change in the rate of movement (not the direction) in
response to a stimulus change.
e.g. Woodlice move faster when they are exposed to high light intensity
to increase their chance of finding shade again and not losing water
as quickly.

5. Auxin (IAA/Indoleacetic acid)
Auxins act by binding to a specific site on the plasma membrane of a cell
activating a H+ pump. The cell wall therefore becomes acidified which
activates enzymes. These enzymes then weaken the cell wall which results
in an increase in internal pressure; the cell becomes elongated/ GROWS.
The movement of a plant stem towards light is a result of auxin being
redistributed (diffused) to the shaded side of the shoot. The cells on this
side then grow more than the other side causing a curvature towards the
light (positive phototropism).
Movement of
auxin

6. Auxins effect on roots
Auxin has an opposite effect on roots. It is a low
concentration of auxin which causes increased
growth rate in roots. In both root and shoot the
auxin is redistributed (diffused) to the lowers side.
The auxin causes
decreased growth in
the lower side of the
root causing
downwards curvature
(positive gravitropism).
The auxin causes
increased growth in the
lower side of the shoot
causing upwards
curvature (negative
gravitropism).

7. THE NERVOUS SYSTEM

8. Nerve Types
Check Point - Humans have three types of neurones:
•Sensory neurones have long axons and carry nerve impulses from sensory receptors to the central
nervous system.
•Motor neurones also have long axons and carry nerve impulses from the central nervous system to
effectors such as muscles.
•Inter-neurones/Relay neurones are usually much smaller cells, with many interconnections. Within
the central nervous system they carry impulses between the motor and sensory neurones.
A reflex arc

9. Structure of a myelinated motor
neurone

10. The myelin sheath
The myelin sheath consists of other nerve cells
and allows the nerve impulse to travel quicker.
In the CNS,
In the PNS,
Oligodendrocytes.
Schwann cells.

11. Resting potential
An electrical potential difference
is maintained between the
outside and inside of the
neurone. The inside of the
axon is 70mV more negative
than the outside. (The
membrane is polarised)
This is maintained by:
o Large anions such as
negatively charged proteins
inside the axon
o Passive diffusion of Na+ and
K+ ions across the membrane
o Active transport of the Na+
and K+ ions by a sodiumpotassium pump found in the
membrane of the neurone.

12. A Nerve Impulse
1. Nerve impulse reaches the neurone.
2. ‘Voltage-activated’ sodium channels open.
3. There is an influx of Na+ ions – when the threshold value
is met - which causes the inside of the neurone to become
positively charged (compared with the outside).
DEPOLARISATION
4. The action potential continues down the axon.
5. The gated sodium channels close and the potassium
channels open.
6. The efflux of K+ causes REPOLARISATION of the
membrane.
An action potential is an ALL OR NOTHING RESPONSE, unless
the threshold value is reached there will be no action
potential.

14. Why does the action potential only
travel in one direction?
• The period following the initiation of an action potential when
another action potential cannot be generated is called the
ABSOLUTE REFRACTORY PERIOD.
This absolute refractory period includes:
- depolarisation
- repolarisation because the sodium gated channels are
inactivated when they are closed.
• It is also difficult to initiate another action potential during
hyperpolarisation (this is where an excess of K+ ions leave the neurone
during repolarisation and the membrane potential is lower than resting
potential) but as its not impossible this period is called the
RELATIVE REFRACTORY PERIOD.

15. Speed of Transmission
There are several factors which affect speed of a nerve impulse:
o Temperature Higher temperature = faster transmission, until
enzymes and transport proteins begin to denature.
o Diameter of neurone Larger diameter = faster transmission.
o Myelination myelinated neurones conduct nerve impulses faster
than non-myelinated neurones because the action potential ‘jumps’
between over the myelin sheath and only needs to be conducted at
the nodes of Ranvier.
The transmission of an
action potential in a
myelinated neurone is
called SALTATORY
PROPAGATION.

16. Synapse
A nerve impulse can only travel in one
direction at the synapse because:
o Only the pre-synaptic neurone has
neurotransmitter-containing
vesicles.
o Only the post-synaptic membrane
has receptors for the
neurotransmitter.
o There is a diffusion gradient of
neurotransmitter between the two
neurones (across the synaptic
cleft).
The secretion of neurotransmitters
directly onto the target cell results
in a rapid, short-lived and localised
response.

17. At an excitatory synapse...
1.
2.
3.
4.
5.
6.
7.
The action potential arrives at the pre-synaptic membrane.
Calcium channels open and Ca2+ ions enter the neurone.
This causes the secretary vesicles to move towards and fuse with
the membrane and the neurotransmitter, acetylcholine is released
into the synaptic cleft.
Acetylcholine diffuses across the Cholinergic Synapse and binds to
receptors on the post-synaptic membrane.
This causes sodium-ion channels in the post-synaptic neurone to
open leading to the action potential being continued down the
next neurone.
The neurotransmitter is then hydrolysed by the enzyme
acetylcholinesterase found in the synaptic cleft and the products
are reabsorbed into the pre-synaptic neurone.
Acetylcholine is the resynthesised.

18. At an inhibitory synapse...
When the inhibitory neurotransmitter binds to
the post-synaptic membrane, K+ and Clchannels open and the inside of the neurone
becomes more negative. Therefore, the
membrane is less likely to reach threshold and
an action potential is unlikely.

19. Summation
Summation is the method of signal transduction between
neurons, which determines whether or not an action
potential will be triggered by the summation (adding
together) of postsynaptic potentials.
Temporal Summation Temporal summation
occurs when two or more action potentials (nerve
impulses) arrive in rapid succession along a single
pre-synaptic neurone.
Spatial Summation Spatial summation occurs
when two or more separate inputs arrive almost
simultaneously from different pre-synaptic
neurones. The individual pre-synaptic potentials add
together.

20. Neuromuscular Junction and Muscle
Fibres

21. Skeletal Muscle – What Does It Look Like?
Microscopic structure
of skeletal muscle.
Gross structure of
skeletal muscle.

22. The Sliding Muscle Theory of Muscle
Contraction
A band
No I or H zone when
the muscle is
contracted.
H zone
I band
Thin filaments are made up of actin and
tropomyosin.
Thick filaments are myosin.
Both together form myofibrils which
bundle up to form muscle tissue.

23. Contraction of Muscles
When an impulse
reaches the
neuromuscular
junction and the
neurotransmitter
binds to the
post-synaptic
receptors, Ca2+
ions are released
around the actin
molecules.
These ions bind to
calcium ion
binding sites
which causes
tropomyosin to
move and
expose the

24. ATP molecules are hydrolysed to ADP and inorganic
phosphate. The energy released is transferred to the myosin
heads which – with the ADP attached - move and bind to the
exposed binding sites on the actin molecule forming a cross
bridge. The ADP is released
ATP
ADP
m
Pi
Cross bridge cycle
The complete cycle
of a myosin crossbridge bending,
binding, sliding and
returning to its
original position.
Contracted
Relaxed

25. ATP and Photocreatine
ATP and Phosphocreatine provide energy for muscle
fibres.
ATP
Hydrolysis of ATP provides energy for movement of the myosin heads to form
cross bridges.
ATP also provides energy for the reabsorption of Ca2+ ions by active transport
(after they have caused the movement of tropomyosin.)
Phosphocreatine
Is stored in muscle and acts as a reserve supply of phosphate (needed to
make ATP). Phosphocreatine enables phosphate to immediately combine
with ADP to regenerate the ATP used in muscle contraction.
This phosphocreatine store is replenished using phosphate from ATP when
muscle is relaxed.

26. Slow and Fast Twitch Muscle Fibres
SLOW TWITCH
MUSCLE FIBRES
FAST TWITCH MUSCLE
FIBRES
Structure
Lots of mitochondria, stores of
myoglobin (oxygen store)
Thicker and more myosin, lots of
enzymes for anaerobic respiration,
phosphocreatine
Location
Muscles such as calf muscles
Muscles such as the bicep
General
Properties
Endurance
Less power
Aerobic respiration
High fatigue resistance
Powerful, short-term contractions
Intense exercise
Anaerobic respiration
Low fatigue resistance

27. Drugs at the Synapse
Don’t need to know specific drug action, however...
- If the drug has a similar shape to
the neurotransmitter it will
probably bind to the post-synaptic
receptor and block impulse
transmission.
- Some drugs will bind to
acetylcholinesterase and stop it
from breaking down the
neurotransmitter, therefore
the impulse will carry on being
initiated (excitatory).

28. Why Have Nerve Impulses?
Simple reflexes avoid damage to the body, for
example, when your hand flinches away from a
hot object.
Nerve impulses can control heart rate which allows
the heart to speed up when oxygen demand
increases.
All receptors respond to specific stimuli e.g. Rod
and cone cells result in a nerve impulse to the
brain which produces images which allow us to
see.

29. Control of Heart Rate
Controlled by a region of the brain called the
medulla oblongata.
The medulla oblongata has two centres:
•Centre which increases heart rate = linked to
sinoatrial node by sympathetic nervous system
•Centre which decreases heart rate = linked to
sinoatrial node by the parasympathetic nervous
system
CHEMICAL and PRESSURE changes in the
blood stimulate parts of the brain via
receptors.

30. Control of Heart Rate cont.
• Found in the wall of the carotid arteries.
• They are sensitive to pH changes that results from CO2
concentration changes (exercise is a cause of increased CO2)
• When the chemoreceptors detect this change they increase the
frequency of nerve impulses to the medulla oblongata
• This centre increases impulse frequency via the sympathetic
nervous system to the sinoatrial node of the heart  increased heart
rate.
• The increases blood flow increases amount of CO2 removed at the
lungs and therefore pH returns to normal.
• The chemoreceptors detect the change and reduce the impulse
frequency back to normal levels.

31. Control of Heart Rate cont.
• Found in the wall of the carotid arteries and the aorta.
• When blood pressure is higher than normal  they send
nervous impulse to the medulla oblongata which decreases
heart rate by sending impulses via the parasympathetic
nervous system to the sinoatrial node of the heart; lowering
the pressure back to normal.
• When blood pressure is lower than normal  they send
nervous impulse to the medulla oblongata which increases
heart rate by sending impulses via the sympathetic nervous
system to the sinoatrial node of the heart; raising the pressure
back to normal.

32. Pacinian Corpuscle
Another example of a receptor
is the Pacinian Corpuscle.
Which is found all over the
body and sends a nerve
impulse to the brain when
stimulated allowing us to
have the sense of touch.
When the Pacinian corpuscle is
exposed to pressure, stretchmediated sodium channels
become deformed (and
open) leading to the
establishment of a generator
potential.

33. The Eye
1= Bipolar Cell
2= Cone
3= Rod

34. RODS
CONES
Monochromatic Vision
Colour Vision
Good Sensitivity
Poor Sensitivity
Many rods connected to one
Each cone is connected to one
bipolar cell  poor acuity = poor bipolar cell  good acuity =
resolution
good resolution
Is used for peripheral vision as
found all over the retina.
Found mainly on the fovea,
which means that can only
detect images in centre of
retina.

35. PRINCIPALS OF
HOMEOSTASIS
Homeostasis in mammals involves physiological control systems that maintain the
internal environment within restricted limits. (e.g. Body temperature at around 37 .)
Negative Feedback:
Negative feedback systems maintain systems at a preset level by detecting deviations from that level
and initiating corrective mechanisms to restore it.
e.g. Body temperature and blood glucose concentration are controlled by negative feedback.
The body has separate mechanisms for controlling these deviations in each directions providing greater
control.
e.g. Different areas of the brain for temperature loss and temperature gain
Positive Feedback:
Positive feedback systems exaggerate a deviation from the preset and is often associated with the break
down of control systems.
e.g. During hyperthermia, the negative feedback system breaks down and a positive feedback loop is
established meaning that the body’s core temperature continues to decrease.
e.g. During childbirth, contractions of the uterus wall releases oxytocin which then increases the
contractions further until the child is born.

36. ...
...are substances that stimulate their target cells via the blood
system. This results in a slow, long-lasting and widespread
response.

37. ...
...are released from cells and only affect cells in the immediate
vicinity. They are usually released by injured or infected cells. Their
secretion causes blood vessels around the area to dilate
(inflammation)
Histamine
-Produced by mast cells in response to injury or allergen.
-Causes capillaries to dilate and their walls to become more permeable which
allows some of the plasma to leave the blood.
-This increased permeability causes swelling and makes it easier for phagocytes to
exit the blood and ingest the dead tissue and bacteria found in the wound.
Prostaglandins
-Produced by mast cells in response to injury or allergen.
-Cause warmth, pain and redness around the injured area.
-As well as causing vasodilation of arterioles, prostaglandins promote blood
clotting which minimises blood loss from the wound and stop entry of
microorganisms.

38. Homeostasis – Body Temperature Control
Increase in skin/blood temperature
Nerve impulse sent to the heat loss centre
of the hypothalamus via the autonomic
nervous system.
Vasodilation of skin arterioles which
increases radiation of heat.
Increased sweating which increases heat
loss by evaporation.
Decrease in skin/blood temperature
The maintenance of body temperature
and blood pH is important as too high or
low a temperature/pH could denature the
enzymes of the body or prevent them
from working efficiently.
Nerve impulse sent to the heat gain centre
of the hypothalamus via the autonomic
nervous system.
Vasoconstriction of skin arterioles which
decreases radiation of heat.
Stop sweating to avoid the heat loss through
evaporation.
Piloerector muscles contract, hairs stand up
on body to insulate heat

39. Ectotherms V Endotherms
• Homeotherms/Endotherms - These are organisms that
that regulate their own body temperature internally using
biological mechanisms. Their internal body temperature is
independent of the external temperature. (Don't use the
term 'warm-blooded').
• Poikilotherms/Ectotherms - These are organisms that
cannot regulate their own body temperature internally.
Their internal temperature fluctuates with the external
temperature. (Don't use the term 'cold-blooded'). They
use behavioural mechanisms to control their core body
temperature.

40. Homeostasis – Blood Glucose Concentration
Blood glucose concentration is affected by many factors such as the
body’s ability to produce insulin, the amount of exercise taken, alcohol
consumed, diet etc.
It is important to maintain a constant blood glucose concentration
because:
-Too high plasma glucose conc. results in kidney malfunction and tissue
dehydration.
-Too low plasma glucose conc. results in fatigue, paleness and possibly losing
consciousness (coma).
- A deviation from healthy blood glucose conc. could result in excess water loss or
gain by red blood cells due to the change in water potential.
-Cells around the body could shrink or burst if tissue fluid were to contain too much
or too little glucose.

41. Maintained by two hormones; insulin and
glucagon which are found in the Islets of
Langerhans in the pancreas. Both hormones bind
to liver cell receptor proteins which releases
enzymes which catalyse the interconversion of
glucose and glycogen.
Insulin
(Glycogenesis)
Glucose
Glycogen
(Glycogenolysis)
Glucagon

42. Increase in glucose conc  Decrease in glucose conc 
Detected by islet cells
α-cells decrease glucagon
secretion
Detected by islet cells
α-cells increase glucagon
secretion
-cells
ᵝ increase insulin
-cells
ᵝ decrease insulin secretion.
secretion.
...both insulin and glucagon bind to receptor proteins on the surface of liver cells and
through a cascade system activate enzymes which control the interconvertion of
glucose and glycogen: Once they bind to the receptor, a G-protein is activated which
turns ATP into cyclic AMP (the second messenger). This cyclic AMP then activates
enzymes which...
...break down the glucose into
glycogen. (Glycogenesis)
...which convert glycogen to
glucose and this is released into
the blood.
(Glycogenolysis)

43. The effect of other hormones on
plasma glucose concentration
Adrenaline
- Increases the breakdown of glycogen to glucose.
- Increases the release of glucose into the
bloodstream to allow increased energy release in
the muscles.
Not on
spec.
Thyroxin
- Increases metabolic rate leading to an increase in
energy requirements.

44. Diabetes
What is it?
- Type 1 diabetes is where the ß cells in the islets of Langerhans are damaged
and cannot produce sufficient insulin so blood glucose conc. becomes
erratic.
- Type 2 diabetes is where not enough insulin is produced by the ß cells AND
the receptors on the liver cells become insensitive to it.
- Symptoms for both Type 1 + 2 diabetes include urinating more, fatigue, loss
of weight and excessive thirst.
How can it be controlled?
- Type 1 diabetes requires insulin being administered (by injection)
- Type 2 can be treated with oral medication
- Both types can be controlled by a carefully managed diet and regular
exercise.

45. Menstrual Cycle
FSH
• The hormone FSH is secreted by the pituitary gland. FSH makes two things happen:
• it causes an egg follicle to mature in an ovary
• it stimulates the ovaries to release the hormone oestrogen
Oestrogen
• The hormone oestrogen is secreted by the ovaries. Oestrogen makes three things happen:
• It causes the repair of the lining of the uterus wall in preparation for the implantation of a
blastocyte.
• it stops FSH being produced - so that only one egg matures in a cycle
• it stimulates the pituitary gland to release the hormone LH
LH (luteinising hormone)
• The hormone LH is secreted by the pituitary gland.
• The hormone LH causes the mature egg to be released from the ovary.
Progesterone
• Progesterone is a hormone secreted by the corpus luteum (left over after the egg is released
from the follicle) in the ovaries.
• Progesterone maintains the lining of the uterus during the middle part of the menstrual cycle
and during pregnancy.

47. Teacher 2 –
Genetic
Control in
Cells
3.5.6  3.5.8

48. The genetic code
Universal, non-overlapping, degenerate, base triplet code.
DNA triplet codes for
same amino acid in all
living organisms.
Each triplet is distinct
from other triplets and
only helps code for a
single amino acid.
Three nucleotides which
make up the code for a
specific amino acid.
Each amino acid has
several triplets which will
code for it.

49. DNA
STRUCTURE
Hydrogen bonds
between the
complementary
bases hold the
helix together.
Covalent bonds
hold the sugar
phosphate
backbone
together.
Anti-parallel
strands

50. mRNA and tRNA
STRUCTURE
• Both have Uracil (U) base
instead of Thymine (T).
• Both single stranded instead of
double stranded.
• Both have ribose sugar
backbone rather than
deoxyribose sugar backbone.

51. Transcription – In the nucleus
DNA helix is unwound by DNA helicase.
RNA polymerase produces a pre-mRNA strand using
complementary base pairing and RNA nucleotides
found in the nucleus. (The pre-mRNA is complementary to the
anti-sense strand of DNA so therefore produces the protein that the sense
strand codes for.)
(DNA helix re-winds.)
Introns and promoter regions are removed from the premRNA which is then spliced back together to form
mRNA.(and capped with a nucleotide which ribosomes can recognise.)
mRNA leaves the nucleus through the nuclear pores and
enters the cytoplasm...

52. Translation – In the cytoplasm
mRNA reaches a ribosome.
tRNA molecules that have complementary
anti-codons to the codons on the mRNA strand
bind to those codons bringing their specific
amino acid.
Peptide bonds form between the amino acids
carried by the tRNA’s.
The ribosome moves along the mRNA strand one
codon at a time and tRNA’s continuously deliver
amino acids to the ribosome until a stop codon is
reached and the polypeptide chain is complete.
(The amino acid chain then leaves the ribosome ready for
modification.)

53. Overview of Transcription/Translation
DNA -
AGT.CTT.GAC.ACT
mRNA – UCA.GAA.CUG.UGA
(mRNA is spliced after removal of introns)
tRNA – AGU, CUU, GAC, ACU
Amino Acid sequence – Ser, Leu, Asp, Thr
Protein

54. Gene Mutation
Occur spontaneously during DNA replication and
involve the deletion or substitution of bases.
Chance increased by mutagens such as
carcinogenic chemicals and radiation.
Some mutations results in a changes amino acid
sequence (different or inactive protein).
Others do not change the amino acid sequence
because of the degenerate nature of the code.

55. Stem Cells
Totipotent cells  all genes in this unspecialised
cell have the potential to be expressed. A zygote
has totipotency. Able to grow / divide rapidly.
During development a totipotent cell translates
only 3-5% of its DNA and becomes specialised.
PLANTS
Many mature plant cells maintain totipotency and
have the ability to develop in vivo into whole plants
or into plant organs under correct conditions. This is
why taking ‘cuttings’ can result in new plants.
During micropropagation tiny samples of plant tissue
can be grown on an agar plate with the correct
nutrients and hormones to form explants.

56. Stem Cells - continued
Only a few totipotent cells remain in the adult
human body - stem cells.
These are found in the bone marrow.
These can be used to treat genetic disordersthe stem cells, when under the correct
conditions can form new tissue if humans
have damaged their own. Bone marrow
transplants are used for the treatment of
leukaemia etc.

57. Genes which control cell division
Proto-oncogenes  control the rate of cell
division.
..... but mutates to become....
Oncogenes  stimulate the cells to divide too rapidly causing a
tumour to develop.
Tumour suppressor genes  slows cell division
..... but mutates to become....
Inactive, so cell division continues and a tumour develops.

58. Regulation of transcription and
translation
Genes are only expressed once a specific
transcriptional factor moves from the cytoplasm
into the nucleus. (These are usually proteins.)
The transcriptional factor binds to the promoter
region near the required gene.
RNA polymerase binds to the DNA transc. factor
complex and is activated causing it to move away
from the complex and along the gene.
The RNA polymerase now transcribes the strand of
DNA. (The gene is now being expressed.)

59. Oestrogen –Its effect
on transcription
Oestrogen diffuses through the plasma membrane of a cell and binds
with an oestrogen receptor forming a complex (alters shape of the
receptor).
The complex (transcriptional factor) moves from the cytoplasm to the
nucleus where it binds to the promoter region and activates the
transcription of a target gene.
There are drugs with a similar shape to oestrogen which block the
receptors and stop transcription. This is a treatment for breast
cancer sufferers.
oestrogen
oestrogen
oestrogen
oestrogen
cytoplasm
nucleus

60. siRNA
A short doublestrand of RNA which
interferes with
(represses) the
expression of a
specific gene.
Post-transcriptional
interference.

61. Creating DNA / DNA fragments
Reverse transcriptase
enzyme
mRNA
extracted
from cells
where
desired
DNA is
found.
The DNA is cut at specific palindromic
recognition sequences using restriction
enzymes.
Free DNA
nucleotides
From mRNA
Using Restriction
Endonucleases

62. PCR
Used to make multiple copies of DNA fragments. In
a PCR machine which constantly heats up and
cools down to break and reform hydrogen bonds.
Template DNA,
Primers,
DNA polymerase enzyme,
DNA nucleotides

63. 95ºC
37ºC
72ºC

64. In VIVO and In VITRO cloning of genes
In VIVO
The use of restriction enzymes and
ligases to insert DNA fragments into
a vector (virus or liposome) which
are then transferred into host cells.
These transformed host cells can be
grown and therefore clone the
desired DNA fragments.
‘Sticky ends’ are left by the restriction
endonuclease which allow the
exposed bases pairs to anneal to
other strands with complementary
‘sticky ends’.
Delivers the gene, already in the
organism- good for making products
e.g. Insulin.
In VITRO
The use of PCR in cloning DNA
fragments. Can be used for
paternity tests/crime
investigations etc.
PCR is fully automated
which makes it a quick and
relatively cheap method of
DNA cloning.
Billions of copies of a small
DNA sample can be made.

65. Genetic Fingerprinting
An organisms genome contains many non-coding repetitive base
sequences which are very unlikely to be repeated in another
individual.
DNA fragments are cloned through PCR and can be analysed to find
genetic relationships such as a paternity test, to diagnose a genetic
disease or to find genetic variation in a population.
Genetic fingerprinting is also used in forensic science to match
hair/blood samples to suspects and in agriculture to produce
genetically superior crops. Animal breeding can also be
supplemented with genetic manipulation to produce animals with
desirable characteristics.

Unethical to change the genotype of an animal: some changes can result in a
decrease in their quality of life e.g. cows with huge udders for increased milk
production.
 The medical benefits of using recombinant plasmid technology and virus vectors
outweigh the ethical issues, as the quality of life of the sufferer could be greatly
improved.

66. Gene Therapy
Germ-Line Gene Therapy
Replacing or supplementing defective gene in a
fertilised egg.
Somatic-Cell Gene Therapy
Targeting affected tissue (so not passed on to next
generation) by introducing additional gene.
Effect is short-lived as cells treated will eventually die off.
Can induce immune response.
Not effective if a characteristic is controlled by more than one
gene.

67. The Use of Vectors somatic-cell gene therapy
Genes that code for useful substances, such as
hormones, enzymes and antibiotics, are often
transferred into micro-organisms, which then produce
large quantities of these substances. The gene is cut
out from the DNA of the donor organism using a
restriction endonuclease enzyme. This cuts out the
relevant section of the organism's DNA, leaving sticky
ends, that will enable the gene to be inserted into a
small circular piece of bacterial DNA called a plasmid.
Plasmids are often used as vectors to take the selected
gene into bacterial cells. The same restriction
endonuclease is used to cut the plasmid. This

68. Virus Vectors
A virus is made harmless and the desired gene is
introduced to it. The virus is then breathed in
by the sufferer of a genetic disease and the
virus implants the gene into their infected cells.
This DNA is incorporated into their cells
and the gene is activated.
Liposome Vectors
Liposomes are tiny spheres or phospholipids and other chemicals.
Because of their small size they can pass easily through plasma
membranes and carry the desired gene with them.

69. How do we know which bacteria now
contain the desired gene?
GENETIC MARKERS in the plasmids, such
as genes that confer antibiotic resistance,
enable genetic engineers to identify
bacteria that have successfully taken up
the selected gene.
The insertion of the new gene can split the
gene for antibiotic resistance so any
bacteria which were once resistant but are
now killed by the antibiotic have

70. How do we know which bacteria now
contain the desired gene? Cont.
USING DNA PROBES
Incubate the recombinant plasmids with bacteria,
then dilute the bacterial suspension and culture on
an agar plate, The different bacteria will colonise
different areas of the Petri dish.
Blot the Petri dish with filter paper and break open
the cells on the filter paper and split the DNA
strands.
Incubate with specific DNA probes which have a
complementary base sequence to part of the
desired DNA – these will bind to the desired base
sequence.
Using an X-ray (which highlight where the DNA
probes are on the sample) the position on the
filter paper corresponds with where the colonies of

72. Determining the Base Sequence of the Desired Gene.
Once located, the base sequence of the gene can be determined by...
DNA Sequencing
The ‘Sanger Method’ (the chain terminator technique) is used to find out the exact order of the nucleotide in
the DNA section.
DNA nucleotides
and a small quantity
of ‘T terminator’
nucleotides
The DNA
fragments which
want sequencing
DNA nucleotides
and a small quantity
of ‘C terminator’
nucleotides
Primer to start
DNA synthesis
DNA
polymerase
DNA nucleotides
and a small quantity
of ‘A terminator’
nucleotides
The DNA
fragments which
want sequencing
DNA nucleotides
and a small quantity
of ‘G terminator’
nucleotides
Primer to start
DNA synthesis
DNA
polymerase
The DNA
fragments which
want sequencing
The DNA
fragments which
want sequencing
Primer to start
DNA synthesis
DNA
polymerase
Primer to start
DNA synthesis
DNA
polymerase

73. The ‘Sanger Method results in many DNA fragments. In the ‘A tube’ the
DNA polymerase will form DNA strands with the DNA nucleotides
but at some point will pick up an ‘A terminator’ nucleotide which
will block further nucleotides from forming the strand.
In the ‘A tube’ at
the end there will
be...
In the ‘T tube’ at
the end there will
be...
In the ‘G tube’ at
the end there will
be...
In the ‘C tube’ at
the end there will
be...
The sequence can then be worked out from the position of the fragments on
the agar plate.

74. Restriction Mapping
Using a restriction enzyme on large DNA sections provides
fragments. The more fragments, the more cuts were made by
the restriction enzymes. The number of cuts equal the number
of recognition sites on that DNA strand.
If these fragments are put through gel electrophoresis then the
relative distance between cut sites is determined.
A restriction map is made of the original DNA.

75. More Recent Developments...
The specification says:
Candidates should understand the principle of these methods
[DNA sequencing and restriction mapping]. They should be aware that
methods are continuously updated and automated.
One of these developments
involves tagging the nucleotides
with fluorescent dyes. These are
all run on the same
electrophoresis gel lane. As the
nucleotides migrate they pass a
laser that detects the dye. The
whole system is automated and
can be linked to a PCR machine
for small samples.

76. Genetic Counselling
Most human diseases results from a mutated genes or from genes
which are useful in one context but not in another.
e.g. Sickle cell anaemia causes a decrease in oxygen concentration capacity but people with the
disease cannot contract malaria.
Genetic counselling enables people to make decision about
themselves and offspring:
The person is genetically screened - using
specific DNA probes - for the presence of
genetic conditions. Through genetic
counselling the person can decide whether to
have pre-emptive treatment if for example
they have oncogenes which will lead to
tumours. Partners who are both carriers for a
genetic disease can decide if the risk of passing
on the gene is too great to reproduce.

77. Enzymes in Genetics
• RNA polymerase – joins RNA nucleotides
(transcription).
• DNA polymerase – joins DNA nucleotides
(copying DNA).
• Ligase – joins 2 strands of DNA (S-P backbone).
• Reverse transcriptase – joins DNA nuc’s to make
copy of mRNA.
• Restriction endonucleases – cut DNA in specific
places – create sticky ends.