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AQA Biology Unit 5

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Annie Davey

on 16 June 2015

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Transcript of AQA Biology Unit 5

Feedback mechanisms
Genetic control of protein structure/function
Homeostasis
The maintenance of a constant internal environment in organisms.
Control of gene expression
DNA Technology
AQA Biology Unit 5
Muscle Contraction
Response to stimuli
Coordination
Actin
Thinner and consists of 2 strands twisted round one another.
Myosin
Thicker and consists of long rod-shaped fibres with bulbous heads that project to the side.
Tropomyosin
Fibrous strands that wrap around the actin filament.
Slow twitch fibres:
Fast twitch fibres:
Less powerful contraction
Over a long period of time
Adapted to aerobic respiration
Powerful contraction
Over a short period of time
Adapted to anaerobic respiration
1. Stimulation
Action potential
reaches many neuromuscular junctions
Calcium ion channels open

Ions move into
synaptic knob
Synaptic vesicles
fuse with presynaptic membrane
Release
Acetylcholine
into synaptic cleft
Diffuses
across
Binds
with receptors (postsynaptic membrane)
Depolarisation
(postsynaptic)
2. Contraction
Action potential
travels through tubules
Opens calcium ion channels

Ions flood into muscle cytoplasm
(down diffusion gradient)
Tropomyosin
pulls away from binding site on actin
Myosin head (+ADP)
binds
to actin filament (forming cross-bridge)
'Power Stroke'
changes angle pulls actin filament
Releases ADP
ATP attaches
to myosin head(detaches from actin)
Ions
activate ATPase
(enzyme)
Hydrolysis of ATP
to ADP (energy for myosin head to return to original position)
Cycle is repeated as long as stimulation continues
.
3. Relaxation
Stimulation ceases
Active transport of calcium ions(into endoplasmic reticulum)
Tropomyosin blocks actin filament
Contraction ceases
Sliding filament theory:

Muscle contracts:
I band narrows
H Zone narrows
Z lines get closer together
Energy Supply
Via hydrolysis of ATP
Needed for:
Movement of myosin head
Active transport of calcium ions
Homeostatic control:
Set Point
Receptor
Controller
Effector
Feedback Loop
Negative Feedback:
Causes corrective measures to be turned off.
Returns system to original level.
Positive Feedback:
Causes corrective measures to be turned off.
System deviates further from original level.
Oestrous cycle
FSH
Development of follicles containing eggs.
Causes production of...
Oestrogen
Rebuilds uterus lining
Causes pituitary gland to produce...
LH
Causes ovulation
Causes ovary to produce... (Via Corpus Luteum)
Progesterone
Maintains uterus lining
Inhibits production of FSH
Progesterone
FSH
Oestrogen
LH
Low Levels
High Levels
= Negative Feedback
= Positive feedback
Check list:
Response to stimuli = Done

Coordination = To do

Muscle Contraction = Done
Homeostasis = Done
Feedback Mechanisms = Done
Genetic control of protein structure and function = To do
Control of gene expression = To do
DNA Technology = Done


Recombinant DNA
Combined DNA of 2 different organisms. Resulting organism known as GMO-Genetically Modified organism.
DNA technology of gene transfer:
Stages:
Isolation
DNA fragments that have the gene for the desired protein
Insertion
DNA fragment into vector
Transformation
transferal of DNA into suitable host cells
Identification
host cells that have successfully taken up the gene
gene markers
Growth/Cloning
population of host cells.
Reverse Transcriptase
Catalyses the production of cDNA from RNA
Complementary DNA
Restriction Endonuclease
Cuts a DNA double strand at a specific sequence of bases (recognition sequence).
Leaving either:
Blunt ends
cut occurs between two opposite base pairs
Sticky Ends
cut at a palindrome, leaving exposed pairs on each strand.
Importance of Sticky Ends
When using the same restriction endonuclease it means that DNA from one organism can be combined with DNA from another. (DNA Ligase is also used to join the phosphate-sugar framework.)
In vivo
gene cloning
Once appropriate fragment of DNA has been cut from the rest of the DNA it is joined to a carrying unit (Vector).
used to transport DNA into host cell.
Most commonly used = Plasmid
Incorporated using a restriction edonuclease to cut complementary section from both.
Permanently joined using DNA ligase.
Once DNA has been incorporated the plasmids then have to be reintroduced into bacteria cells.
This is called transformation:
plasmids and bacteria cells mixed together in a medium containing calcium ions.
Calcium ions and change in temperature make the bacteria permeable allowing plasmids to pass through the cell membrane into cytoplasm.
Not all bacteria cells will contain the fragmented DNA.

To identify the ones that have taken up the plasmid. This entails using the gen for antibiotic resistance(unaffected by introduction of new gene). The bacteria cells that have not taken up the plasmids will not be resistant and will die.
The next task is to identify the cells that have incorporated the new genes
Gene Markers
Identify whether a gene has been taken up by a bacteria cell.
Antibiotic resistance markers
Technique is called replica plating
Uses the antibiotic resistance gene that was cut to incorporate new gene
The antibiotics used will destroy the gene that contains the new gene
To remedy the fact that the bacteria with the desired gene will be destroyed, the colonies of cells are placed in exactly the same position on two plates
There are a number of different ways to do this and all involve using a second separate gene on the plasmid.
Fluorescent markers
Involves transferal of GFP (green fluorescent protein-makes jellyfish glow)
Desired gen transplanted into center of GFP gene.
The bacteria which have taken up the gene will not fluoresce.
Enzyme markers
Desired gene transplanted into gene that makes lactase
The bacteria that don't change the colourless substrate blue will contain the desired gene.
In vitro gene cloning
Requires:
DNA fragment to be copied
DNA polymerase
joins nucleotides
Primers
short sequences of nucleotides
Nucleotides
contain each of the 4 bases
Thermocycler
varies temperatures precisely over a period of time
Polymerase chain
3 Stages:
Separation of DNA Strand
Temp increased to 95°C causing the 2 strands of DNA fragments separate
Addition (annealing) of the primers
Mixture cooled to 55°C causing primers to join to complementary bases at the end on the DNA fragment.
Primers also prevent the 2 strands rejoining
Synthesis of DNA
Temp increased to 72°C
Optimum temp for DNA polymerase to add complementary nucleotides along each of the separated DNA strands (begins at primer until it reaches the end of the chain)
Advantages
Extremely rapid
Doesn't require living cells
Advantages
Useful when wanting to introduce a gene into another organism
No risk of contamination
Very accurate
Cuts out specific genes
Produces transformed bacteria that can be used to produce large quantities of gene products
Use of Recombinant DNA technology
Genetic modification is used for:
increasing the yield from animals or plant crops
improving nutrient content in food
introducing resistance to disease and pests
making crop plants tolerant to herbicides
developing tolerance to environmental conditions
making vaccines
producing medicines for treating diseases
Cystic Fibrosis
Caused by a mutant recessive allele. It is a deletion mutation where the DNA bases adenine-adenine-adenine are missing.
A normal CFTR protein transports chloride ions across the epithelial membranes, and water follows via osmosis keeping the membranes moist. The deletion of bases means CFTR can't perform this role.
This can cause symptoms such as...
breathing difficulties
higher risk of infection accumulation of thick mucus in pancreatic ducts leads to fibrous cysts
possible infertility in males
Treatment using gene therapy:
2 ways gene therapy can be used to treat cystic fibrosis:
gene replacement
defective gene replaced with healthy gene
gene supplementation
1+ of the healthy gene added alongside defective gene.
2 different techniques of gene therapy:
germ-line therapy
replacing/supplementing defective gene in fertilised egg. Currently prohibited.
Somatic-cell gene therapy
targets affected tissue only
Delivering cloned CFTR genes:
Using a harmless virus:
Adenoviruses: cause respiratory diseases by injecting their DNA into epithelial cells of lungs and therefore make useful vectors for the transfer of the normal CFTR gene into host cells.
they are made harmless by interfering with a gene in their replication.
CFTR gene incorporated into adenoviruses DNA
Inhaled via nostrils
Adenoviruses inject their DNA into epithelial cells of the lungs and therefore the CFTR gene.
Wrapping the gene in lipid molecules:
Lipid molecules can pass through the phospholipid portion of cell surface membrane.
CFTR gene inserted into plasmid vectors
introduced back into bacterial hosts
bacteria cloned
plasmids are extracted and wrapped in lipid molecules to form a liposome
Sprayed into nostrils as an aerosol drawn into lungs during inhalation
Severe Combined Immunodeficiency
Treatment using gene therapy:
Normal ADA gene inserted into retrovirus
Retrovirus grown in host cells
Mixed with patients T-cells
Inject a copy of normal ADA gene into T-cells
T-cells reintroduced into patient's blood
Caused by a defect in the gene that codes for the enzyme ADA, which destroys toxins that would otherwise kill white blood cells.
Locating and sequencing genes
DNA Probes
Short single stranded section of DNA that has a label attached to make it identifiable.
Radioactively labeled probes
made up of nucleotides with isotope 32P
identified using photograph plate exposed by radioactivity
Fluorescently labeled probes

emit light/fluoresce under certain conditions
Used to identify genes in the following way:
Probe has bases complementary with portion of DNA that makes up the gene being identified
DNA being tested is seperated into 2 strands
DNA strands mixed with probe (DNA Hybridisation)
Site is identified via probe
DNA Sequencing
The Sanger method
uses modified nucleotides that cannot attach to the next base in the sequence when they are being joined together
Act as terminators ending synthesis of a DNA strand
4 different ones are used each with one of the 4 bases
Sequencing process:
4 test tubes each containing
1. many single stranded fragments of DNA to be sequenced (acts as template)
2. mixture of nucleotides with all 4 bases
3. small quantity of one of the 4 terminator nucleotides
primer to start process of DNA synthesis (radioactively or or fluorescently labeled)
DNA polymerase to catalyse DNA synthesis
As the binding of nucleotides is a random process
DNA synthesis may be terminated after only a few nucleotides or after a long fragment of DNA has been synthesised
hence the DNA fragments in each test tube will be of varying lengths
Control mechanisms
The set point
the desired level at which the system operates.
Monitored by a...
Receptor
detects any deviation from set point
informs...
The controller
coordinates information from various receptors
sends instruction to appropriate...
Effector
brings about changes needed to return system to the set point
return to normality creates a...
Feedback loop
informs receptor of the changes to the system brought about by the effector
Regulation of body temperature
Conduction
The transfer of energy through matter from particle to particle.
heat causes particle to vibrate gaining kinetic energy which then cause adjacent particles to vibrate so energy is transferred through the material.
Covection
The transfer of heat as the result of the warmed matter itself.
heat causes fluid to expand and move, carrying the heat it has absorbed with it.
Radiation
Energy tranferred by electromagnetic waves.
when the waves hit an object they usually heat it up.
Regulation of body temperature in ectotherms
Control body temperature by:
Exposing themselves to the sun (Lizards)
Taking shelter
Gaining warmth from the ground
Generating metabolic heat
Colour variations (dark = absorb heat / light = reflects heat)
Regulation of body temperature in endotherms
Gaining heat
(cold environment)
Vasoconstriction
Shivering
Raising of hair (erector muscles)
Increased metabolic rate
Decrease in sweating
Behavioural mechanisms
Losing heat
(warm environment)
Vasodilation
Increased sweating
Lowering of body hair (erector muscles)
Behavioural mechanisms
Control of body temperature
Stimulus is detected by thermoreceptors
Infomation passed onto hypothalamus (coordinator)
within hypothalamus there is a thermoregulatory cetre consisiting of two parts
heat gain centre
activated by a fall in blood temp
controls mechanism that increase body temp
heat loss centre
activated by a rise in blood temp
controls mechanisms that decrease body temp
Causes effector to produce appropriate response
Normal blood temp
Warm receptors (skin)
Cold receptors (skin)
Hypothalamus
Heat gain centre
Heat loss centre
Mechanisms for heat loss
Mechanisms for heat gain
Decrease
Increase
Feedback
Feedback
Regulation of blood glucose
Second messenger model
Role of pancreas
Beta cells
smaller
produce insulin
detects a rise in blood glucose level
secrete insulin directly into the blood plasma
Body cells have glycoprotein receptors on their cell-surface membranes that bind with insulin
insulin brings about:
change in tertiary structure of the glucose transport protein channels
they open
more glucose allowed into cells
increased number of carrier molecules in the cell-surface membrane
activation of enzymes that covert glucose to glycogen and fat
Blood glucose level is lowered by:
increased rate of absorption into cells
increased respiratory rate of cells
increased rate of conversion of glucose to glycogen and fat
Example of negative feedback
The pancreas contains groups of hormone-producing cells known as islets of Langerhans.
There are 2 types of these cells:
Alpha cells
larger
produce glucagon
detects a fall in blood glucose level
secrete glucagon directly into the blood plasma
only liver cells respond
Glucagon increases blood glucose by
activates enzyme that converts glycogen to glucose
increases conversion of amino acids and glycerol into glucose
Example of negative feedback
Role of Adrenalin in the regulation of blood glucose.
Adrenalin raises the blood glucose by:
activating enzyme that causes the breakdown of glycogen to glucose
inactivating an enzyme that synthesises glycogen from glucose
Normal blood glucose level
Detected by alpha cells, which produce glucagon
Detected by beta cells, which produce insulin
Uncontrolled quantity of glucose enters from intestines
Conversion of amino acids to glucose
Conversion of glycogen to glucose
Increased cellular respiration
Absorption of glucose into cells
Conversion of glucose to glycogen and fat
Blood glucose level rises
Blood glucose level falls
Negative feedback
Blood glucose level falls
Blood glucose level rises
Glucagon
Insulin
Hormone is the first messenger
binds to a specific receptors on the cell-surface membrane
forming a hormone-receptor complex
Hormone-receptor complex
activates an enzyme inside the cell
results in the production of a chemical that acts as a second messenger
Second messenger
causes a series of chemical changes that produce the required response
Diabetes and it's control
Type 1
insulin dependent
body is unable to produce insulin
usually begins in childhood
develops quickly
symptoms are obvious
Control:
injections of insulin
dose of insulin matched to glucose intake
blood glucose levels monitored by using biosensors
management of carbohydrate intake
Type 2
insulin independent
glycoprotein receptors on the body cells lose responsiveness to insulin
may also be due to an inadequate supply of insulin from the pancreas
develops slowly
symptoms are usually less severe
Control:
regulating intake of carbohydrates matching it to amount of exercise
may be supplemented by injections of insulin or drugs that stimulate insulin production
Gel electrophoresis
DNA fragments placed onto agar gel
Voltage is applied
The larger the fragments the more slowly they move
The smaller fragments will move further
Sheet of photographic film is placed over agar gell for several hours
Radioactivity from each of the DNA fragment exposes film and shows where it is situated on the gel
Only DNA fragments up to around 500 bases are done this way
Restriction Mapping
Involves cutting DNA with a series of different restriction endonuclease
fragments produced are separated by gel electrophoresis
Distance between recognition sites can be determined by the patterns of fragments that are produced
Automation of DNA sequencing and restriction mapping
Both are now routinely carried out by automatic machines; computers analyse the data they produce
the four types of terminators are labeled with fluorescent dye
DNA synthesis takes place in a single test tube and is speeded up using PCR cycles
Electrophoresis is carried out in a single narrow capillary gel
Results are scanned by lasers and interpreted by computer software
Genetic screening and counseling
Screening
Process that can determine whether an unborn child might be affected by a genetic disorder
It is possible to test simultaneously for many different genetic disorders
Hundred of different DNA probes are fixed in an array on a glass slide
Sample of DNA is then added to the array
Any complementary sequences will bind to one or more probes
Can also detect oncogenes (responsible for cancer)
some people inherit mutated tumour suppressor genes and are at greater risk of developing cancer
C
T
A
GG
C
T
A
G
A
T
CC
G
A
T
Mutated gene
T
CC
G
T
CC
G
T
CC
G
T
CC
G
T
CC
G
G
T
AA
GG
C
T
C
G
T
AA
GG
C
T
C
T
CC
G
G
T
AA
GG
C
T
C
T
CC
G
1. Order of nucleotides determined by DNA sequencing. Genetic libraries now store the genes responsible for common genetic diseases.
2. Fragment of DNA with complementary bases to the mutated portion of the gene is produced.
3. DNA probe is formed by radioactively labeling the DNA fragment.
4. PCR techniques used to produce multiple copies of DNA probe.
5. Probe added to single-stranded DNA fragments of person being screened.
6. If mutated gene is present, the probe will bind to the complementary sequence.
7. These DNA frag,ents will now be labeled with the probe and can be distinguished from the rest of the DNA fragments by use of X-ray film.
8.If complementary fragments are present the DNA probe will be taken up and the X-ray film will be exposed.
Counselling
It is where advice and information are given that enable people to make personal decisions about themselves or their offspring.

Is closley linked with genetic screening:
the screening results provide the genetic counsellor with a basis for informed decision.
Genetic Fingerprinting
Extraction
DNA is extracted by separating it from the rest of the cell (as the amount of DNA is usually small, it's quantity can be increased by polymerase chain reaction.
Digestion
DNA is cut into fragments using restriction endonucleases.
Seperation
Fragments of DNA are separated according to size by gel electrophoresis. The gel is then immersed in alkali to separate double strands into single strands. Single strands are transferred on to a nylon membrane by a technique called southern blotting which involves:
a thin nylon membrane is laid over the gel
the membrane is covered with several sheets of absorbent paper
drawing up the liquid containing the DNA by capillary action
This transfers the DNA fragments to the nylon membrane in precisely the same relative positions they occupied on the gel
DNA fragments then fixed to the membrane using ultraviolet light
Hybridisation
Radioactive (or fluorescent) DNA probes are used to bi with the core sequences. The process is carried out with different probes, each of which binds with a different core sequence.
Development
An X-ray film is put over the nylon membrane. The film is exposed by the radiation from the radioactive probes. A series of bars is revealed. The pattern will be unique to every individual (except identical twins)
Taxes
Direction is determined by the direction to stimulus. Positive-towards, negative-away.
Kineses
Results in the increase of random movement in an unfavourable environment (unpleasant stimulus)
Tropism
Growth movement of a plant in response to a directional stimulus. Positive-towards, negative-away.
Nervous System (NS)
Central NS
Peripheral NS
Spinal cord
Brain
Sensory NS
Motor NS
Voluntary NS
Autonomic NS
Sympathetic NS
Parasympathetic NS
Carry nerve impulses towards the CNS
Carry nerve impulses away from CNS
Carries nerve impulses to muscles (voluntary control)
Carries nerve impulses to glands, smooth muscles and cardiac muscle (involuntary)
Speeds up activity
Slows down activity
Reflex ARC
Stimulus
Receptor
Creates nerve impulse
Sensory neurone
Passes nerve impulse to spinal cord
Intermediate neurone
Links motor and sensory neurones
Motor neurone
Carries nerve impulses away from spinal cord to a muscle (effector)
Effector
Response
Medulla Oblongata
Control of heart rate
Centre that increases heart rate (sympathetic NS)
Centre that decreases heart rate (parasympathetic)
Receptors respond to pressure/chemical changes in the blood.
Chemoreceptors
found in the wall of the carotid artery and the aorta
detects fall in pH
increases impulses to medulla oblongata
increases impulses to sinoatrial node
increases heart rate
increases blood flow
more CO2 removed
pH of blood rises
reduced impulses to medulla onblongata
reduced impulses to sinoatrial node
decreases heart rate to normal
High blood pressure
transmission of a nervous impulse to the medulla oblongata which sends impulses via the parasympathetic nevous system to sinoatrial node, decreasing heart rate.
Low blood pressure
transmission of a nervous to the medulla oblongata which sends impulses via the sympathetic nervous system to the sinoatrial node, increasing heart rate.
Excercise
More CO2
Lower pH
increased impulses to medulla oblongata
inceased impulses to SAN
increased heart rate
CO2 removed
CO2 level returns to normal
Pacinian Corpuscle
It responds only to mechanical pressure and produces a generator potential by acting as a transducer. It contains a stretch-mediated sodium channel in its plasma membrane, their permeability to sodium changes when they change shape.
Resting potential: pacinian corpuscle is in a resting state the channels are too narrow for sodium to pass through
Pressure is applied and the membrane is stretched widening the sodium channels and sodium ions diffuse through
The membrane becomes depolarised, producing a generator potential
This creates an action potential that passes along the neurone.
There are 2 main types of light receptors found in the retina. They both act as transducers by converting light energy into the electrical energy of a nerve impulse.
Chemical mediators:
Histamine:
stored in certain white blood cells and is released during injury or in response to an allergen and causes dilation of small arteries and arterioles and increased permeability of capillaries, leading to localised swelling, redness and itching.
Prostagladins:
found in cell membranes and also cause dilation of small arteries and arterioles. They are released during injury increasing permeability of capillaries. Also they effect blood pressure and neurotransmitters affecting pain sensation.
Control of tropisms by IAA
Cells in the tip of the shoots produce IAA, which is transported down the shoot
Initially it is transported to all sides
Light causes IAA to move to the shaded side of the shoot
A greater concentration of IAA builds up on the shaded side

IAA causes the cells on the shaded side to elongate more than on the light side
Shaded side grows faster causing the root to bend towards the light.

IAA also causes roots to grow in the direction of gravity, but a high concentration of IAA decreases growth in root cells.

Cell body:
production of proteins and neuro-transmitters.
Dendrons:

carry nerve impulses toward the cell body
Axon:
carries nerve impulses away from the cell body
Shwann cells:
protect and provide electrical insulation for the axon
Myelin sheath:
forms a covering to the axon
Nodes of Ranvier:

gaps where there is no myelin sheath
Sensory Neurones
Transmits nerve impulses from receptor to an intermediate neurone
one dendron
one axon
Intermediate Neurone
Transmits impulses between neurones
numerous short dendrons
Motor Neurone
Transmit nerve impulses from an intermediate or sensory neurone to an effector
long axon
many short dendrites
Resting potential
Movement of ions across a memebrane are controlled in the following way:
Phospholipid bilayer prevents ions diffusing across it
intrinsic proteins contain ion channels that pass through them some...
have 'gates' which can be open or closed
remain open
sodium-potassium pump intrinsic proteins that actively transport potassium ions into the axon and sodium ions out
At
resting potential
the inside of an axon is negatively charged relative to the outside of the axon, in this condition the axon is known as being
polarised
.
This potential difference is established in the following ways:
sodium ions actively transported out
potassium ions actively transported in
3 sodium ions move out for every 2 potassium ions that move in
Chemical gradient more sodium ions in tissue fluid surrounding the axon than in the cytoplasm (opposite case for potassium)
potassium diffuses out faster than sodium diffuses back in further increasing potential difference
Electrical gradient causes potassium ions to move into the axon as the inside is more negative than the outside
Equilibrium is reached where both electrical and chemical gradients are balanced so there is no net movement of ions
Action potential
Action potential, a stimulus is recieved by a receptor/nerve ending, causing a temporary reversal of charges on the axon membrane negative charge inside becomes positive. The axon is depolarised.
At resting potential some potassium voltage-gated channels are open but the sodium voltage-gated channels are closed
Energy from the stimulus causes some of the sodium voltage-gated channels to open
Sodium diffuses into the axon through these channels along their electrochemical gradient
Triggering a reversal in the potential difference across the membrane
More sodium channels open causing an even greater influx of sodium ions by diffusion
Once the action potential is established the sodium channels close preventing further influx of sodium ions
Voltage gates on the potassium ion channels begin to open
Electrical gradient that was preventing further outward movement of potassium ions is reversed so more potassium channels open and more potassium ions diffuse out causing repolarisation of the axon
This causes a temporary over-shoot of the electrical gradient (inside of axon is more negative than usual = hyperpolarisation)
Gates on potassium ion channels close
Sodium-potassium pumps cause sodium to be pumped out and potassium to be pumped in
Resting potential is re-established and the axon is said to be repolarised
Passage of an action potential in an unmyelinated axon:
At resting potential the concentration of...
potassium ions is higher inside the membrane
sodium ions is higher outside the membrane
positive ions overall is greater on the outside making it positive relative to the inside (axon membrane is polarised)
Stimulus causes sudden influx of sodium ions reversing the charge on the axon membrane = action potential axon is depolarised
Opening of sodium voltage-gated channels further along the axon
influx of sodium ions in this region causes depolarisation
Behind the new region the sodium channels close and the potassium channels open causing potassium to leave the axon along their electrochemical gradient
The action potential (depolarisation) is propogated in the same way further along the axon
The outward movement of potassium has continued to the extent that the axon membrane behind the action potential has become repolarised.
Repolarisation of the axon allows sodium ions to be actively transported out returning the axon to its resting potential.

Passage of an action potential in a myelinated axon:
myelin acts as an electrical insulator, preventing action potential from forming
Gaps in the myelin sheath are known as the Nodes of Ranvier and action potentials occur at these points
The action potential in effect jumps from node to node in a process called salatory conduction
An action potential passes along faster than a long an unmyelinated axon
Factors affecting the speed of an action potential:
Messenger RNA
It is manufactured when DNA forms a mirror copy of part of one of its two strands.
Once formed it leaves the nucleus via pores in the nuclear envelope and enters the cytoplasm where it associates with ribosomes, acting as a template on which proteins are built.
It is easily broken down so exists only while it is needed to manufacture a specific protein.
Transfer RNA
Made up of around 80 nucleotides. Single stranded chain folded into a clover-shape with one end of the chain extending above the other, which is where the amino-acid attaches. At the opposite side of the t-RNA molecule is a sequence of amino-acids aka anticodon. The anticodon differs for each amino-acid.
Transcription
Splicing
Translation
DNA helicase acts on a specific reason of the DNA molecule to break the hydrogen bonds between bases to seperate and expose the nucleotide bases
RNA polymerase moves along the template strand, causing the nucleotides to to join with individual complementary nucleotides from the pool which is present in the nucleus
As the RNA polymerase adds the nucleotides one at a time to build up a strand of pre-mRNA, the DNA strands rejoin behind it (only about 12 bases are exposed at any one time)
When the RNA polymerase recognises the stop codon, it detaches and the pre-mRNA is then complete.
Removes the introns, which do not code for bases and would otherwise interfere with polpeptidesynthesis, leaving behind only the exons, which do code for proteins. The process of splicing forms mRNA from pre-mRNA.
Full transcript