Year 2 Model Answers

Question 1

Describe predator-prey relationships

Answer 1

Predator populations peaks after prey population
Prey population increases due to low numbers of predators
more food for predators so numbers increase
increased predation reduces number of prey
number of predators decreases due to lack of food / starvation;

Question 2

Describe Succession

Answer 2

Pioneer species colonises an area with hostile conditions
This leads to changes in the abiotic factors
The conditions become less hostile
Other species are able to colonise the area
Conditions continue to change and become less hostile and new organisms outcompete the pioneer species increasing biodiversity
Eventually conditions become favourable to a climax community

The climax community has stable abiotic factors, stable populations, stable communities

Question 3

Why are there are energy losses between trophic levels in consumers?

Answer 3

Energy is not transferred between consumers because:
Some parts aren’t eaten (bones) and some parts are eaten and are not absorbed (faeces)
Some parts are eaten, absorbed but excreted (urine)
Some biomass us broken down and lost as heat in respiration (temp regulation/movement)

Question 4

Why don’t plants convert energy from light to GPP?

Answer 4

Sunlight is not converted to biomass because:
Some light is the wrong wavelength (e.g. green)
Some doesn’t hit a chlorophyll molecule/transmitted
Most light is reflected by other molecules in the atmosphere
Other limiting factors may be involved

Question 5

What is Biomass?

Answer 5

Dry mass of carbon in a organisms in a particular area

Question 6

How do farmers increase productivity?

Answer 6

Food chains/webs are simplified (pests are removed)
Respiration of livestock is reduced (movement limited, temperature regulated)

Question 7

How is Biomass determined?

Answer 7

A sample of organism is dried.
The sample is then weighed at regular intervals (e.g. every day)
Until the mass remains constant

Question 8

Describe the Nitrogen Cycle

Answer 8

Nitrogen gas in the air is converted into ammonia in the soil by nitrogen fixing bacteria
Some nitrogen fixing bacteria in leguminous plant root nodules have a mutualistic relationship with plants and convert nitrogen gas to ammonia then nitrates directly

Nitrates in the soil are absorbed by plant roots and converted to nitrogen containing compounds e.g. amino acids and DNA
Nitrogen containing compounds in plants may be absorbed when eaten by consumers

Proteins from waste and dead material are broken down/hydrolysed to ammonia in soil by enzymes released by saprobionts during ammonification

Ammonia in the soil is oxidised to nitrites, then nitrates, by nitrifying bacteria in the soil in aerobic conditions. These nitrates can be absorbed by the plants

If the soil is waterlogged, the lack of oxygen leads to denitrification where nitrates are converted back to gaseous nitrogen by denitrifying bacteria

Question 9

Describe ways the Nitrogen Cycle is Optimised

Answer 9

Using crop rotation to plant leguminous plants – these will replenish nitrates in the soil
Using crop rotation to replenish soil nutrients
Ploughing aerates soil to ensure more oxidised ammonia  nitrates in nitrification
Preventing waterlogging reduces anaerobic conditions so less denitrification occurs
Selective breeding can be used to optimise growing conditions
Fertilisers can be added to increase concentration of minerals e.g. nitrates

Question 10

Describe the Phosphorous Cycle

Answer 10

Plants absorb phosphorous form the soil
Consumers eat the plants and absorb phosphorous
Dead and waste (faeces etc.) material is decomposed releasing phosphorous into the soil
Runoff from farm fertiliser means excess phosphorous enters bodies of water (lakes/rivers etc.)
Phosphorous sediment in water is uplifted forming rocks on the surface
Weathering releases phosphorous from the rock into the water and soil
Leeching of phosphorous from soil/weathered rock causes phosphorous to enter the water

Question 11

Describe Eutrophication

Answer 11

Excess nitrates runoff into bodies of water
Excess growth of algae/Algal bloom forms on the surface of water
Reduced light so aquatic plants die less photosynthesis
Saprobionts respire aerobically while decomposing dead matter
Less oxygen for fish and other organisms to respire so they die

Question 12

Describe the Light Dependent Reaction

Answer 12

In photoionisation, light excites the electrons in chlorophyll II and they move to carrier proteins in the thylakoid membrane.
The electrons are replaced by the e- produced by splitting water (photolysis), which also produces oxygen and H+.
Electrons move along carrier proteins in a series of redox reactions losing energy as they go. This is used to pump H+ into the thylakoid space creating a chemiosmotic gradient.
H+ move down the gradient through ATP synthase during photophosphorylation producing ATP from ADP+Pi
The electrons are donated to chlorophyll I and more are excited by light, travelling along another ETC until they reduce NADP to NADPH with H+ from photolysis.

Question 13

Describe the Light Independent Reaction

Answer 13

CO2 is fixed, combining with RuBP using the enzyme Rubisco
This produces two glycerate-3-phosphate (GP)
GP is reduced to Triose phosphate (TP)
Using energy from ATP and reduced NADP
TP can be regenerated to RuBP using energy from ATP,
1C from TP is converted into organic molecules e.g. glucose, amino acids, glycerol

Question 14

Describe Glycolysis

Answer 14

In the cytoplasm
Phosphorylation of glucose using ATP to make it more reactive;
Lysis of the phosphorylated glucose intermediate to form Triose Phosphate
Oxidation from TP to pyruvate by losing H+ and e-
Net gain of 2 ATP;
NAD reduced/NADH formed

Question 15

Describe the Link Reaction

Answer 15

In the mitochondrial matrix
Pyruvate is oxidised using coenzyme A
CO2 released
NAD is reduced
Acetyl CoA is formed

Question 16

Describe the Krebs cycle

Answer 16

Acetyl CoA reacts with a 4C acceptor molecule
The 6C intermediate is decarboxylated and oxidised, removing CO2 and reducing NAD
The resulting 5C intermediate is also decarboxylated and oxidised removing CO2 and reducing 2xNAD, reducing FAD and generating 1x ATP in a series of REDOX reactions.
Until the original 4C acceptor is formed again.

Question 17

Describe Oxidative Phosphorylation

Answer 17

FADH and NADH are oxidised and lose e- and H+
The e- are passed from carrier protein to carrier protein in the mitochondria inner membrane in a series of redox reactions
This releases energy
The energy is used to pump H+ through the membrane into the inner membrane space building a chemiosmotic gradient
H+ moves back through the membrane through ATP synthase
ADP + Pi  ATP
Oxygen is the terminal electron acceptor forming water

Question 18

Describe Anaerobic Respiration in Mammals

Answer 18

Pyruvate is reduced to lactate
NADH is oxidised during this process

This prevents NAD running out and allows ATP to continue being made in glycolysis

Question 19

Describe Anaerobic Respiration in yeast

Answer 19

Pyruvate is reduced to ethanal then ethanol
NADH is oxidised during this process
CO2 is produced

This prevents NAD running out and allows ATP to continue being made in glycolysis

Question 20

Describe Taxis

Answer 20

In invertebrates
Movement in a direction
Movement toward (positive) or away (negative) from stimulus
So organisms can survive and reproduce

E.g. chemotaxis, phototaxis,

Question 21

Describe Kinesis

Answer 21

In invertebrates
Directionless movement
Movement isn’t in a direction
Usually, to do with rate of turning
Increased rate of turning leads to an organism remaining in favourable conditions
So organisms can survive and reproduce

Question 22

How does IAA affect the shoots?

Answer 22

IAA produced in tip
IAA diffuses down the shoot
IAA accumulates/moves to the shaded side
Leading to cell elongation
Shoot elongates toward the light

Question 23

How does IAA affect the roots?

Answer 23

IAA produced in the tip
IAA diffuses down the root
IAA accumulates/moves to the base of the root (due to gravity)
IAA inhibits elongation in root
Root elongates downwards

Question 24

Describe how the Pacinian corpuscle works

Answer 24

Pressure is applied and the lamella is deformed
Stretch mediated sodium ion channels open
Na+ diffuse into axon
Leading to depolarisation and action potential if threshold is exceeded

Question 25

Describe convergence in rod cells

Answer 25

Rods are found around the outside of the retina, away from the fovea Light stimulus triggers depolarisation in the rod cells Many rods converge and are connected to a single sensory neurone Depolarisation undergoes (spatial) summation to trigger an action potential In low light there is enough light to lead to action potential However, this reduces visual acuity

Question 26

Describe acuity in cone cells

Answer 26

Cone cells are located in the fovea (behind the retina to maximise light stimulus) Every cone cell is connected to a single sensory neurone (no convergence) Each cone sends a single impulse to the brain With high visual acuity Three types of cones detect three different wavelengths of light

Question 27

Describe the electrical control of heart rate

Answer 27

Sino Atrial Node initiates a wave of electrical impulses across both atria causing them to contract Non-conductive tissue prevents impulse going straight to ventricle Atrio Ventricular Node delay impulse so ventricles can fill AV Node sends wave of electrical impulses down Bundle of His Ventricles contract from bottom up.

Question 28

Describe how heart rate responds when CO2 increases

Answer 28

pH in blood lowers Chemoreceptors detect it CO2 needs to be removed Sensory neurone takes more impulses to medulla Medulla sends more impulses along the sympathetic nerve Sympathetic nerve causes SAN to increase Heart rate increases

Question 29

Describe how heart rate responds when CO2 decreases

Answer 29

pH in blood increases Chemoreceptors detect it Sensory neurone takes more impulses to medulla Medulla sends more impulses along the parasympathetic nerve Parasympathetic nerve causes SAN to decrease Heart rate decreases

Question 30

Describe how heart rate responds when blood pressure decreases

Answer 30

Baroreceptors detect pressure increase Too low pressure means not enough oxygen etc. will get to cells Sensory neurone takes more impulses to medulla Medulla sends more impulses along the sympathetic nerve sympathetic nerve causes SAN to increase Heart rate increases

Question 31

Describe how heart rate responds when blood pressure increases

Answer 31

Baroreceptors detect pressure increase Too high pressure can damage artery walls, so needs to be restored Sensory neurone takes more impulses to medulla Medulla sends more impulses along the parasympathetic nerve parasympathetic nerve causes SAN to decrease Heart rate decreases

Question 32

What is difference between nervous and hormonal communication?

Answer 32

Nervous is fast Response is short lived Requires neurone, neurotransmitters and nervous impulses Hormonal is slow Response is long lasting Hormones are produced in glands, travel in the blood to an organ/tissue with a target receptor

Question 33

Describe how resting potential is restored/maintained

Answer 33

Na+/K+ pumps 3 Na+ out of the axon and 2 K+ into the axon using ATP Membrane is more permeable to K+ so K+ moves out through channels and less permeable to Na+ Axon has a more negative potential difference inside

Question 34

Describe action potential

Answer 34

Na+ ions enter the axon (e.g. Na+ receptors at synapses, stetch mediated-sodium channels) The axon membrane is depolarised Potential difference across axon becomes more positive Potential difference will reach threshold triggering action potential Voltage gated Na+ channels will open Na+ moves into the axon by facilitated diffusion Voltage-gated Na+ channels close, Voltage-gated K+ channels open K+ leaves the axon by facilitated diffusion – axon is repolarised Axon membrane becomes hyperpolarised in the refractory period Na+/K+ pump restores resting potential

Question 35

What is the refractory period?

Answer 35

The period between firing of one action potential and the next The axon is hyperpolarised (very negative) on the inside Refractory period ensures: Action potentials occur in one direction Limits number of action potentials Keeps action potentials separated

Question 36

Describe the threshold/all or nothing principle

Answer 36

There must be a certain level of stimulus for a action potential to be triggered. There must be enough depolarisation to pass the threshold Regardless of how strong a stimulus is, the same action potential is produced. A larger stumulus can be detected by having more frequent action potentials.

Question 37

Describe saltatory conduction

Answer 37

The myelin sheath insulates the axon (ions cannot go in / out in these areas). The impulse must ‘jump’ from ‘node to node’ This speeds up the transmission of AP’s This is called Saltatory conduction only occurs in axons with a myelin sheath

Question 38

Describe the factors that affect the speed of neurotransmission

Answer 38

Temp – speeds it up if increased, more diffusion, ATP required for Na+/K+ pump and enzymes are sped up to a point… Diameter of axon – speeds it up, less leakage from axon maintains p.d. Presence of myelin – speeds it up, saltatory conduction, impulse jumps cutting out some of the transmission time.

Question 39

Describe the events at a cholinergic synapse

Answer 39

Action potential reaches pre-synaptic membrane allowing Ca2+ channels to open and Ca2+ enters by facilitated diffusion. Vesicles containing Acetylcholine (Ach) fuse with the pre-synaptic membrane. Ach diffuses across the synaptic cleft Ach binds to the receptors on the post-synaptic membrane. Na+ channels open and Na+ moves in by facilitated diffusion Leading to depolarisation Acetylcholine esterase hydrolyses Ach to acetic acid and choline to be returned to the presynaptic neurone

Question 40

Describe the effect of more chloride ions entering the post synaptic neuron

Answer 40

Cl-are negatively charged They make the post synaptic neurone hyperpolarised More Na+ needs to enter the post synaptic neuron for depolarisation of the neurone to pass threshold Action potential is inhibited

Question 41

Describe the effect of more potassium ions leaving the post synaptic neuron

Answer 41

K+ are positively charged When they move out they make the post synaptic neurone hyperpolarised More Na+ needs to enter the post synaptic neuron for depolarisation of the neurone to pass threshold Action potential is inhibited

Question 42

Describe the effects of some drugs that work at synapses

Answer 42

Drugs bind receptors instead of Ach because they are complementary – they prevent Na+ from entering, threshold isn’t reached reducing action potentials. Drugs prevent acetylcholinesterase from breaking down Ach by aching as an inhibitor (inhibitor model answer applies here e.g. blocks active site or changes shape so the active site is no longer complementary) this prevents Ach from being removed from receptors, more Na+ diffuses in increases action potentials. Drugs may be complementary to the NT (substrate) and bind it, this changes shape of the substrate and means it can’t bind the receptor. Less Na+ moves in, doesn’t pass threshold reducing action potentials.

Question 43

Describe the events at a NMJ

Answer 43

Vesicles of neurotransmitter fuse with the pre-synaptic membrane Neurotransmitter e.g. Acetylcholine diffuse across the neuromuscular junction Neurotransmitters bind to the Na+ receptors causing Na+ to enter the sarcolemma (muscle tissue) depolarising it Depolarisation is transmitted through the T-tubules causing Ca2+ ions to be released from the sarcoplasmic reticulum Ca2+ binds to tropomyosin in the leading to muscle contraction

Question 44

Describe Sliding filament theory of muscle contraction

Answer 44

Calcium ions diffuse into myofibrils from the sarcoplasmic reticulum Ca2+ binds to tropomyosin changing its shape and move This change exposes the myosin binding sites on the actin Myosin ‘heads’ bind to the Actin forming a cross bridge. Ca2+ activates ATPase hydrolysing ATP. Myosin ‘heads’ tilt in a rowing motion pulling actin molecules. ATP can now bind to the Myosin head, changing the shape and breaking the cross bridge ATP is hydrolysed by the head providing energy for the head to flip back/re-cock ADP and Pi remains on the head

Question 45

Describe how a muscle contracts

Answer 45

When a muscle contracts, the sarcomere shortens, and the Z lines are pulled closer together This happens because actin filaments slide between myosin filaments During contraction, the I band, sarcomere and the H zone decrease in length The A band remains the same

Question 46

What is the role of phosphocreatine

Answer 46

Phosphocreatine is stored in muscle When contraction demands are too high, respiration cannot meet the demands for ATP Phosphocreatine is stored in the muscle and is a source of phosphate The phosphate is used to generate ATP immediately (but it will run out!) Phosphocreatine needs to be regenerated when muscles relax

Question 47

Describe fast twitch muscles

Answer 47

More powerful contractions Only for a short time Thicker filaments More filaments Stores of glycogen Anaerobic respiration Store of phosphocreatine

Question 48

Describe slow twitch muscle fibres

Answer 48

Less powerful contractions over a longer period of time Aerobic respiration Stores of myoglobin (stores oxygen) Rich blood supply Many mitochondria

Question 49

What are hormones

Answer 49

Hormones are released by glands They travel in the blood They bind to complementary receptors on specific target cells

Question 50

Describe the effect of insulin

Answer 50

Binds to specific receptors in the liver and muscles Increases permeability of membranes to glucose – carrier proteins are activated Activates enzymes to turn glucose into glycogen in glycogenesis (in liver for storage) Increases the rate of respiration in muscles Increases the conversion of glucose into fat

Question 51

Describe the effect of glucagon

Answer 51

Binds to specific receptors in the liver and muscles Activates enzymes to turn glycogen into glucose (especially in the liver) in glycogenolysis Promotes making glucose form fatty acids and amino acids in gluconeogenesis Decreases the rate of respiration in muscles

Question 52

Describe second messenger model

Answer 52

Adrenaline binds to the specific receptors on target cells (the liver) The enzyme adenyl cyclase changes shape and is activated Adenyl cyclase catalyses conversion of ATP to cyclic AMP Cyclic AMP activates a protein kinase Cyclic AMP and protein kinase stimulates glycogenolysis (conversion of glycogen to glucose) This provides more glucose for respiration

Question 53

Describe Ultrafiltration

Answer 53

Blood passes through the afferent arteriole to the glomerulus The efferent arteriole has a narrower lumen so this generates a high hydrostatic pressure Small molecules (ions/water/glucose) are forced through gaps in the capillary endothelium They pass through the basement membrane into the renal capsule Large proteins and cells remain in the blood

Question 54

Describe selective reabsorption

Answer 54

Na+ is actively pumped out of the cell into the blood by the sodium potassium pump This lowers the concentration of sodium in the Proximal convoluted tubule Na+ moves into the cell from the lumen by facilitated diffusion and glucose/amino acids/salts are co-transported with it Glucose/amino acids/salts are then transported into the blood by facilitated diffusion Absorption of the solutes increased the water potential in the lumen Water is absorbed from the proximal convoluted tubule to the blood by osmosis

Question 55

Describe the loop of Henle

Answer 55

Water cannot move out of the ascending limb. Na+ and Cl- are ACTIVELY TRANSPORTED out of the ascending limb of the loop of Henle. Water moves out of the descending limb via Osmosis due to the high conc of Na+ / Cl- (lowering the water potential) in the tissue fluid. Na+ moves into the descending limb. This loss of water means that the conc of Na+ / Cl- is increased (less dilute solution). As the fluid moves down the descending limb and start to ascend, Na+ / Cl- diffuse out of the loop. This causes more water to be lost from the lowest descending part of the loop via osmosis. The longer the loop, the greater the water potential gradient so more can be absorbed.

Question 56

Describe how ADH is secreted

Answer 56

Water potential in the blood decreases Osmoreceptors in the hypothalamus shrink Stimulates the hypothalamus Increases ADH release by posterior pituitary gland ADH carried in the blood to the distal convoluted tubule and the collecting duct

Question 57

Describe the effect of ADH

Answer 57

ADH binds to complementary receptors on specific target cells in the collecting duct and distal convoluted tubule (DCT) The enzyme phosphorylase is activated by this binding Leading to aquaporins fusing with the membrane of the epithelial cells in the collecting duct (DCT) Membrane permeability to water increases Water moves from a high water potential in the collecting duct to a low water potential in the blood via osmosis This leads to increased concentration of urine (so less urine is produced)

Question 58

Describe Succession

Answer 58

Pioneer species colonises an area with hostile conditions This leads to changes in the abiotic factors The conditions become less hostile Other species are able to colonise the area Conditions continue to change and become less hostile and new organisms outcompete the pioneer species increasing biodiversity Eventually conditions become favourable to a climax community The climax community has stable abiotic factors, stable populations, stable communities

Question 59

What issues does lack of conservation cause?

Answer 59

CO2 levels and impact on climate change Reduces Biodiversity Reduces habitats Useful organisms may become extinct (e.g. for medicines)

Question 60

Why do should we conserve the environment?

Answer 60

Economic – able to keep using the area/tourism Biological – reduces biodiversity, food sources and habitats Ethical – we should prevent damage by humans and help those species effected by us Aesthetic – looks pretty, is a nice place to be

Question 61

Describe MRR

Answer 61

Take a sample of a population Mark them in a way that is not toxic so will not harm them or hinder their chances of survival Release them back into the population Allow time for them to redistribute Collect a second sample Count how many are marked in the second sample (N1 x N2) / N marked in 2nd sample

Question 62

Describe Allopatric Speciation

Answer 62

Geographical isolation; Separate gene pools Variation due to mutation; Different selection pressures; Selection for advantageous allele; Differential reproductive success / (selected) organisms survive and reproduce; Leads to change in allele frequency; Cannot produce fertile offspring

Question 63

Describe Sympatric Speciation

Answer 63

Same location - Reproductive isolation; Separate gene pools Variation due to mutation; Different selection pressures; Selection for advantageous allele; Differential reproductive success / (selected) organisms survive and reproduce; Leads to change in allele frequency; Cannot produce fertile offspring

Question 64

Describe the action of reverse transcriptase

Answer 64

Mature mRNA is extracted and converted to cDNA by Reverse Transcriptase cDNA is converted to double stranded DNA by DNA polymerase mRNA is found in large amounts in cells and contains no introns, so the gene products are easily expressed even by bacteria

Question 65

Describe the action of restriction endonucleases

Answer 65

DNA is cut at specific recognition sites Cuts can form ‘sticky ends’ these are complementary and can be used for joining DNA together

Question 66

Describe how a gene machine works

Answer 66

Desired gene sequence is designed using a computer – it is checked for safety and standards. Small pieces of DNA (oligonucleotides) are synthesised and joined together to make a sequence of DNA Using sticky ends these pieces of DNA can be inserted into a vector that can be used in cloning. Gene machines are quicker as they don’t need to isolate DNA/mRNA first

Question 67

Describe genetic engineering

Answer 67

Isolate DNA/mRNA from an organism using a restriction endonuclease or reverse transcriptase to get DNA Cut plasmid and isolated DNA with the same restriction endonuclease to get complementary sticky ends A promoter and terminator region may need to be added to the gene Use DNA ligase to join the desired gene to a plasmid – forming phosphodiester bonds Include marker gene e.g. antibiotic resistance Transform host using Ca2+ ions and a heat shock so plasmid passes through the membrane Allow bacteria to grow (colonies) then (replica) plate onto medium where the marker gene is expressed Bacteria / colonies not killed have antibiotic resistance gene/don’t fluoresce/have enzyme action and (probably) the wanted gene;

Question 68

Describe PCR

Answer 68

DNA heated to 90 to 95°C to separate the strands DNA is cooled to 55°C so that primers can bind Free nucleotides attach by complementary base pairing Temperature is increased to 72°C and DNA polymerase joins nucleotides together forming a phosphodiester bond Cycle is repeated and DNA is copied at an exponential rate

Question 69

Describe Gel Electrophoresis

Answer 69

DNA is cut at areas of tandem repeats using restriction endonucleases DNA fragments are placed in wells at the top of an agar gel. An electric current is applied over it. DNA is negatively charged due to the phosphate group The DNA moves towards the positive electrode, but at different rates. Small fragments move further through the gel A ladder/marker can be used to determine the size of the DNA fragments

Question 70

Describe DNA Fingerprinting

Answer 70

Extracted DNA is cut with a restriction endonuclease at sites of variable number tandem repeats/minisatellites DNA is separated by gel electrophoresis, shorter fragments run further on the gel Use Southern Blotting to transfer DNA to a nylon membrane Use an alkaline solution to make DNA single stranded Add a single stranded probe tagged with radioactive/fluorescent molecule Visualise the DNA using and X-ray film or UV light

Question 71

Describe the different types of stem cells

Answer 71

Totipotency – can specialise into any type of cell including placenta, found in early stages of an embryo Pluripotency – can specialise into most types of cells, but not form placenta, found in early embryos but not the earliest Multipotency – can form some types of cells e.g. red and white blood cells, found in adult mammals Unipotency – can only form one type of cell, found in adults

Question 72

That is a transcription factor?

Answer 72

A transcription factor is a protein that binds the promoter region of DNA to turn on Gene Expression. They recruit RNA polymerase to bind the promoter.

Question 73

What are transcription factors controlled by?

Answer 73

Transcription can be controlled by: Inhibitors binding the TFs changing their shape so they can’t bind to the promoter Sometimes other molecules (e.g. proteins/phosphates/steroid hormones) need to bind to change the shape of the TF to help it to bind

Question 74

Describe how oestrogen works as a transcription factor

Answer 74

Oestrogen is a steroid hormone so is lipid soluble so diffuses across the phospholipid bilayer Oestrogen binds to the complementary receptor on the transcription factor Binding causes a change in the tertiary structure The transcription factor moves into the nucleus through the nuclear pore The transcription factor can now bind to a specific promoter base sequence of DNA Binding of the transcription factor switches the gene on by encouraging RNA polymerase to bind and starting transcription

Question 75

Describe how siRNA works

Answer 75

dsRNA is cut by an enzyme into small dsRNA sections called siRNA (small interfering) siRNA becomes ssRNA and combines with an enzyme The complementary ssRNA binds to mRNA with the enzyme The mRNA is cut NO TRANSLATION!

Question 76

Describe a proto-oncogene

Answer 76

Mutation changes the proto-oncogene into an oncogene: Receptor protein permanently activated Oncogene may code for a growth factor that is produced in large amounts Excessive uncontrolled cell division

Question 77

Describe a tumour suppressor gene

Answer 77

Mutation changes the sequence of the bases in the tumour suppressor gene. Tumour suppressor gene is no longer transcribed or tertiary structure of protein is altered Cell division is no longer controlled (This is often caused by increased methylation, but we’ll get to that when we do epigenetics)

Question 78

Describe the effect of decreased acetylation on DNA

Answer 78

Decreased Acetylation Histone Deacetylase removes acetyl groups from histones Increases the positive charges on histones Increases their attraction to the negative phosphate groups. Association between histone and DNA is stronger so Tighter coiling Transcription factors and RNA Polymerase can’t bind as easily Transcription of gene less likely and gene is switched off

Question 79

Describe the effect of increased methylation on DNA

Answer 79

Increased Methylation Methylation is addition of methyl group to cytosine bases of DNA DNA coils more tightly Transcription factors and RNA Polymerase can’t bind as easily Transcription of gene less likely and gene is switched off