Gene mutations might arise during DNA replication. The deletion and substitution of bases. Gene mutations occur spontaneously. The mutation rate is increased by mutagenic agents. Some mutations result in a different amino acid sequence in the encoded polypeptide. Due to the degenerate nature of the genetic code, not all mutations result in a change to the amino acid sequence of the encoded polypeptide. The rate of cell division is controlled by proto-oncogenes that stimulate cell division and tumour suppressor genes that slow cell division. A mutated proto-oncogene, called an oncogene, stimulates cells to divide too quickly. A mutated tumour suppressor gene is inactivated, allowing the rate of cell division to increase.

When a cell of the body is creating a new cell (mitosis) it's DNA is copied, a mistake could happen resulting in a new cell with a mutation in it's DNA.

A base might be left out. This means that every codon read after the missing base would be wrong because the reading of the code has been shifted along and triplets will be read with different bases (known as a frame shift). This affect more codons the earlier it is in the code.

A base might be substituted with another base:

• A mis-sense mutation is if the changed base makes a codon code for a different amino acid
• A nonsense mutation is if it makes a stop codon then during translation amino acids will only be added to the chain up to that point making a shorter protein
• A silent mutation is if it makes a new codon that codes for the same amino acid (so the proteins made are not affected)

The primary structure of a protein (the amino acids) determines the tertiary/quaternary structure of a protein, so a change in the primary structure could result in a protein that is the wrong shape to carry out its function.

Mutations are spontaneous, this means they happen randomly. However, the rate that they happen at can be increased by certain things, like some radiation and some chemicals, these are called mutagenic agents.

Mutations can occur on the section of DNA that codes for proteins which regulate cell division. These genes are proto-oncogenes which make proteins that stimulate cell division and tumour suppressor genes which make proteins that slow cell division.

Mutations on these genes result in oncogenes which create proteins that permanently activate protein receptors on the cell surface membrane that stimulate the cell to divide too fast and inactive tumour suppressor cells. This means cells will divide too often and so not just replace dead cells, but create extra cells leading to a tumour.

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.

Candidates should be able to compare the structure and composition of DNA, mRNA and tRNA.

The sequence of bases on mRNA is the genetic code. Each set of three bases corresponds to the bases on one tRNA molecule (the anticodon) the top of the tRNA molecule is attached to an amino acid, in this way every triplet of bases- called a codon- codes for an amino acid.

The genetic code is:

• Universal- it is the same in all organisms
• Non-overlapping- each base is read only once
• Degenerate- amino acids have several different codons that code for them

tRNA is a folded strand that creates a shape with the anti-codon on one side and the attachment site for an amino acid on the other.

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