Transcription as the production of mRNA from DNA. The role of RNA polymerase. The splicing of pre-mRNA to form mRNA in eukaryotic cells. Translation as the production of polypeptides from the sequence of codons carried by mRNA. The role of ribosomes and tRNA.The genetic code as base triplets in mRNA which code for specific amino acids. The genetic code is universal, non-overlapping and degenerate. The structure of molecules of messenger RNA (mRNA) and transfer RNA (tRNA). Candidates should be able to compare the structure and composition of DNA, mRNA and tRNA.

DNA is has the information needed to make proteins in its code, however proteins are made using ribosomes which are in the cytoplasm and DNA is in the nucleus and is too big to get out.

To solve this problem, sections of DNA that are needed to create a protein are translated into mRNA which is single stranded and so can fit through the nuclear pores.

The two strands of DNA are separated by DNA helicase breaking the hydrogen bonds between bases. RNA polymerase then moves along one of the strands, the template strand, attaching complimentary bases (so a Guanine where there is a Thymine and visa versa, a Adenine where there is a Thymine and a Uracil where there is a Adenine). The resulting molecule is called pre-mRNA. This process is called transcription.

This pre-mRNA contains exons (parts of the genetic code) and introns (a bit of the base sequence which doesn't code for anything). The introns are removed (and exons may be rearranged) in a process called splicing to create mRNA (happens in eukaryotic cells). The sequence of bases on mRNA is the genetic code.

mRNA then leaves the nucleus and is attracted to a ribosome.

tRNA is a molecule with one end that is complimentary to a codon on mRNA (the anticodon) and one end that is attached to the amino acid that that part of mRNA codes for.

The ribosome brings together the right piece of tRNA to the mRNA in a process called translation. The first tRNA is attached, then the second: the amino acids they are carrying then become attached to each other using an enzyme and ATP. The third tRNA then does the same thing, as it does the first bit of tRNA detaches from the mRNA but leaves its amino acid attached to the second amino acid. The process continues like this until the ribosome reads the stop codon which makes the ribosome, mRNA and tRNA all detach leaving behind a polypeptide chain.

The mammalian oestrous cycle is controlled by FSH, LH, progesterone and oestrogen. The secretion of FSH, LH, progesterone and oestrogen is controlled by interacting negative and positive feedback loops. Candidates should be able to interpret graphs showing the blood concentrations of FSH, LH, progesterone and oestrogen during a given oestrous cycle. Changes in the ovary and uterus lining are not required.

A follicle is an egg, cells that create oestrogen and fluid.

On day 1 you get your period (for 5 days). FSH is maturing a follicle.

The follicle is producing low levels of oestrogen which develop the womb lining and keep FSH and LH production down.

As the follicle develops, it begins to produce more oestrogen. When oestrogen production reaches threshold it increases the production of FSH and LH.

The peak in LH causes the follicle to rupture, releasing the egg (ovulation) (day 14). The egg travels towards the uterus.

The follicle becomes a corpus luteum, which secretes progesterone and oestrogen.

Progesterone maintains the womb lining. It also inhibits LH and FSH (preventing another egg from being released). (This is in preparation for fertilisation, but if that doesn't happen then:)

After a few days the corpus luteum withers and stops producing progesterone so the womb lining breaks down (this causes the period days 1-5).

Negative feedback restores systems to their original level. The possession of separate mechanisms involving negative feedback controls departures in different directions from the original state, giving a greater degree of control. Positive feedback results in greater departures from the original levels. Positive feedback is often associated with a breakdown of control systems, e.g. in temperature control. Candidates should be able to interpret diagrammatic representations of negative and positive feedback.

A feedback is when a receptor senses a change in stimuli due to a response that it coordinated. This means that it can make an informed decision to change the response it is coordinating.

Negative feedback is when feedback makes the response stop.

There is a norm for conditions, and if this is deviated from in either direction a different response will be coordinated. A response in either direction (if there is too much or too little of something) will have its own negative feedback loop. For example, the alpha cells stop producing glucagon when the blood glucose concentration is back up to normal, and the beta cells stop producing insulin when the blood glucose concentration is back down to normal.

Positive feedback is when feedback makes the response carry on, making the conditions get further and further from the norm. One example of this is in neurones when sodium is detected sodium ion channels are opened so more can flood in.

Mostly positive feedback is a bad thing caused by a disease or due to a break down, for example hypothermia.

The factors that influence blood glucose concentration. The role of the liver in glycogenesis and gluconeogenesis. The role of insulin and glucagon in controlling the uptake of glucose by cells and in activating enzymes involved in the interconversion of glucose and glycogen. The effect of adrenaline on glycogen breakdown and synthesis. The second messenger model of adrenaline and glucagon action. Types I and II diabetes and control by insulin and manipulation of the diet.

Types I and II diabetes and control by insulin and manipulation of the diet.

Factors that influence concentration:

• The amount you take in as carbohydrates in food
• The amount broken down from glycogen which is a molecule stored in the liver (glycogenolysis)
• The amount produced by the body from glycerol and amino acids (gluconeogenesis)

The pancreas has pieces of tissue called islets of Langerhans which contain alpha (α) and beta (β) cells, both of which play a role in the control of blood glucose levels.

β cells

Detect when blood glucose is too high and secrete insulin which:

• Bind to glycoprotein receptors of cells which makes them change the shape of their protein channels to let more glucose in (taking it out of the blood)
• Activate enzymes that convert glucose into glycogen (glycogenesis) and fat
• Increase the rate of respiration so more glucose is broken down by cells

α cells

Detect when blood glucose is too low and secrete glucagon which

• Binds to receptors on liver cells causing it to
• activate an enzyme to convert glycogen to glucose (glycogenlysis)
• converts amino acids and glycerol into glucose (gluconeogenesis)

Adrenaline

This also has a role to play in the control of blood glucose. It is produced by the adrenal glands during stress.

It increases blood glucose by:

• Binding to receptors on the liver which
• activates an enzyme that converts glycogen to glucose (glycogenlysis)
• deactivates an enzyme which makes glycogen from glucose.

Second messenger model

• Adrenaline and glucose are first messengers which bind to receptors on the outside of the liver
• This activates an enzyme to produce another messenger on the inside of the liver
• This messenger then activates or deactivates the desired enzymes to control glucose levels

Type 1 diabetes

• Insulin dependent
• Fast and noticible
• The body is unable to produce insulin
• Possibly because the β cells are being attacked by the immune system
• There is no uptake of glucose into cells
• Insulin is injected to control it

Type 2 diabetes

• Insulin independent
• Slow and subtle
• Glycoprotein receptors stop responding to insulin
• Cause by a bad diet
• Controlled by dieting to restrict carbohydrates
• or Drugs to stimulate more insulin to be produced
• or Drugs to slow the rate of absorbtion of glucose from the intestine

The contrasting mechanisms of temperature control in an ectothermic reptile and an endothermic mammal. Mechanisms involved in heat production, conservation and loss. The role of the hypothalamus and the autonomic nervous system in maintaining a constant body temperature in a mammal.

Ectothermic
These are animals that mostly gain heat from their surroundings.

• Sunlight/shade
• Absorbent/reflective colours

Endothermic
These are animals that mostly gain heat from metabolic processes.

• Vasoconstriction/dialation (changing the amount of blood that goes near the surface and loses heat)
• Surface area to volume ratio
• Shivering
• Hair raising/lowering (by erector muscles)
• Sweating
• Speeding/slowing metabolic rate

These responses have to be coordinated by the body.

• A change in heat in the environment is detected by thermoreceptors in the skin which send a message to the hypothalamus through the autonomic nervous system (the heat gain centre if it is too cold or the heat loss centre if it is too hot).
• A change in core temperature is detected in the hypothalamus (again the heat gain centre if it is too cold or the heat loss centre if it is too hot).
• Which ever area is activated will coordinate a series of responses to correct the temperature 

Homeostasis in mammals involves physiological control systems that maintain the internal environment within restricted limits. The importance of maintaining a constant core temperature and constant blood pH in relation to enzyme activity. The importance of maintaining a constant blood glucose concentration in terms of energy transfer and water potential of blood.

Homeostasis is the control of internal conditions.

It makes sure that the cells in the body are functioning efficiently.

It does this by bringing conditions, for example temperature, PH and concentration of ions, back to a suitable level.

If the conditions were not controlled some processes would be disturbed for example proteins could denature and stop functioning or osmosis could cause cells to burst.

Having these things internally controlled means that an organism can live different environments and still function properly.

If blood glucose levels are too low there is not enough to respire sufficiently and supply cells with energy, if it is too high it decreases the water potential in the blood meaning water moves out of cells.

The detailed structure of a synapse and of a neuromuscular junction. Candidates should be able to explain • unidirectionality • temporal and spatial summation • inhibition. The sequence of events involved in transmission across a cholinergic synapse and across a neuromuscular junction. When provided with information, candidates should be able to predict and explain the effects of specific drugs on a synapse. Recall of the names and mode of action of individual drugs will not be required.

At a synapse a signal is passed from one neurone to another, or between a neurone and an effector (neuromuscular junction), by chemical signals.

The chemicals used are called neurotransmitters and they are created by smooth endoplasmic reticulum using energy made by mitochondria. Golgi apparatus packages these neurotransmitters up into vesicles.

When an action potential reaches the end (/knob) of the first neurone (/presynaptic neurone) it sets off a sequence of events:

• The increase in positivity causes voltage gated calcium ion channels to open
• Calcium ions bind vesicles to the membrane of the knob, which causes the neurotransmitters within them to be released into the cleft
• The neurotransmitters bind to receptors on the postsynaptic neurone

There are different neurotransmitters which complement different receptors, in the diagram above there are sodium ion channel receptors, which would bind to the neurotransmitter acetylcholine (known as a cholinergic synapse) and cause the following effects:

• The sodium ion channels open causing (positive) sodium to move in (down a concentration gradient)
• The positivity in the cell is increased, making an action potential more likely to start (excitation)

However, if there were chloride ion channels which bonded to the neurotransmitter GABA then:

• The chloride ion channels open causing (negative) chlorine to move in
• The positivity in the cell is decreased making an action potential less likely to start (inhibition)

Excitatory synapse

Inhibitory synapse

After the neurotransmitter has effected the receptor it can be broken down into different components, at which travel back into the presynaptic knob. Here they are joined back together using ATP produced by mitochondria.

Because receptors are only on the postsynaptic membrane, and neurotransmitters are only released from the presynaptic neurone, signals can only move one way (unidirectionality) entering a neurone at the dendrites and leaving a neurone at the axon terminals.

Synapses allow signals to go through multiple neurones in order to create multiple responses to a stimuli by letting a presynaptic neurone release neurotransmitters to multiple postsynaptic neurones.

There are also synapses where multiple presynaptic neurones meet a single postsynaptic neurone. The advantage of this is that if they are both sending neurotransmitters, the effect is stronger. So for example if one presynaptic neurone sends acetlyecholine accross it will make the postsynaptic neurone let in some Na+ and become more positive, but it may not reach the threshold to create a new action potential; however if another presynaptic neurone were to send neurotransmitters accross too this would make the postsynaptic neurone let in even more Na+ and become even more positive, enough to cause a new action potential. This is called spatial summation.

Temporal summation would be if there was only one presynaptic neurone sending neurotransmitters, but the first time there were not enough to let in enough Na+ to start an action potential, however there are multiple action potentials coming down it so it conitnues to send neurotransmitters and in the end there are enough to let in enough Na+ to start an action potential.

Drugs can effect signals at synapses in several ways:

• Mimic neurotransmitters
• Inhibit enzymes that break neurotransmitters
• Inhibit the release of neurotransmitters
• Block receptors