A2 Biology Term 3

Question 1

Define recombinant DNA

Answer 1

DNA containing genetic material from different sources.

Question 2

Describe genetic engineering

Answer 2

The technique of changing the genetic constitution of an organism through the transfer or synthesis of genes so that the host is able to express the gene product.

Question 3

Describe PCR

Answer 3

Polymerase Chain reaction:
1. Double stranded DNA chains are separated by heating to 95 degrees.

2. Primer is made that matches end sequences of DNA fragment to be copied. Cooled to 40 degrees to allow primer to stick.
3. Add heat tolerant Taq polymerase, found in extremophiles, and nucleotides. Heat to 72 degrees.
4. Nucleotides are added to the primers using the single strand as a template.

Question 4

Describe electrophoresis

Answer 4

• Process used to separate molecules such as proteins and fragments of nucleic acids.
• Separated based on charge and mass
• DNA fragments have negative charge and so migrate towards anode, distance moved depends on mass.
• Larger mass means moves more slowly
• fragments in gel are made visible using specific dye.

Question 5

Describe why plasmids are used in gene cloning

Answer 5

• many bacteria have plasmids
• small, circular and double-stranded DNA molecule
• Passed on to daughter cells when bacterium divides

Question 6

Describe the steps in genetic engineering

Answer 6

1. Obtaining a copy of gene by isolating mRNA
   - mRNA for gene is isolated from sample of tissue
   - reverse transcriptase used to form strand of DNA
   - DNA polymerase converts this into double stranded DNA known as cDNA
   - PCR is then used
2. Inserting DNA using plasmid vector
   - DNA of plasmid is cut using restriction enzyme which cuts at restriction site leaving exposed sequences known as sticky ends.
   - Sticky ends also formed on gene and ligase causes splicing together into continuous ring.
3. Inserting plasmid vector into host bacterium
   - done by zapping with electricity or treatment with ice cold CaCl2
4. Identifying transformed bacteria prior to cloning
   - bacteria that produce significant product are marked
   - plasmids may be marked with antibiotic resistance gene
   - fluorescent or easily stained substances are used
5. Switching on gene action
   - promoter is required and so is spliced into plasmid alongside gene.

Question 7

Describe how microarrays are used

Answer 7

1. mRNA extracted from cells
2. cDNA created from mRNA and tagged with dye
3. cDNA binds to probes on the microarray.
4. Can analyse different cells and see how each type of cDNA is expressed on different genes.

Question 8

Define bioinformatics

Answer 8

Consists of the mapping of genomes, analysis of the proteins expressed by genomes and application of biological science. (Genomics, Proteomics and Biotechnology).

Question 9

Describe how bioinformatics is used in prevention of parasites and disease

Answer 9

• entire genome sequences have been analysed of pathogens such as P. falciparum.
• This development is a major step in attempts to create vaccines.

Question 10

Describe the role of bioinformatics in creating human proteins

Answer 10

• Human proteins have been produced using recombinant DNA techniques
• genes for insulin are made this way for those with diabetes
• Factor VIII is formed in hamster kidney tissue for haemophiliacs who are deficient in this.
• SCID may be due to lack of AD but now stem cells taken from umbilical cord at birth can be engineered to produce the protein.

Question 11

Describe the role of bioinformatics in genetic screening

Answer 11

• Used to identify genetic diseases within patients.
• Prenatal screening used to identify genetic conditions in the unborn child.
• Prenatal screening has two methods:
  1. Chorionic villus sampling: Sample of placenta is taken.

2. Amniocentesis: Sample of fetal cells in amniotic fluid is examined.

Question 12

Describe how gene therapy is used for cystic fibrosis

Answer 12

• healthy CFTR gene is needed in lung epithelium
• can be delivered in an aerosol spray which contains liposomes to which the genes are attached.
• liposomes fuse with cell surface membrane
• can use viral vector instead which has been genetically engineered to inactivate or remove the alleles that cause disease.

Question 13

Describe the different types of vectors that can be used in gene therapy

Answer 13

Liposomes: tiny lipid bilayer droplets to which gene is attached.

Inactivated virus: triggered allergic or other immune responses and so discontinued

Naked DNA: Host cells have to be capable of taking up the DNA. Has to be competent.

Question 14

Describe the social implications of gene technology

Answer 14

Advantages:

• improved, cheaper medicines
• improved food supplies
• improved nutritional quality of foods
• a cleaner environment
• improved treatment of genetic diseases

Disadvantages:

• unexpected reductions in crop yields due to ecological disturbance
• farmers made dependent on specific varieties
• reduced natural biodiversity
• reduced effectiveness of antibiotics as more bacteria become resistant

Question 15

State the ethical issues raised by genetic screening

Answer 15

• who should decide who may be screened or tested?
• who should meet the substantial costs of screening?
• should results be confidential?

Question 16

Describe how DNA profiling occurs

Answer 16

1. Sample of cells taken and DNA is extracted. PCR is used to amplify sample.
2. DNA is cut into fragments that does not cut within satellite DNA containing VNTRs
3. Fragments are separated using gel electrophoresis
4. Gel is treated to split DNA into single strands and then copy is transferred to a membrane.
5. Radioactively labelled DNA probes are added to the membrane to bind to particular bands of DNA
6. Membrane is overlaid with X-ray film which becomes fogged due to radiation
7. Fragment position is seen and profile is formed.

Question 17

Describe the ways in which genetic engineering improves quality and yield of crop

Answer 17

Bt Maize:

• Soil bacterium is a source of insecticide known as Cry toxin.
• Genes for the production of the toxin are transferred to selected crops creating Bt crops.
• Less insecticide is required but resistance to the toxin has been developed.

Vitamin A-enhanced rice (Golden Rice):

• rice is staple for many people and lacks beta-carotene leading to decreased vision and poorly functioning immune system in these people.
• beta-carotene is converted to retinol (vitamin A) in body.
• Gives these people sufficient vitamin A intake.

GM Salmon:

• Includes growth hormone gene
• results in faster growth rate and is maintained all year around.

Question 18

Outline the use of herbicide and insecticide resistant crops.

Answer 18

Benefits:

• eradicates weeds which increases yield
• requires fewer sprays
• provides good weed control as ploughing not required

Dangers:

• Fewer weed species may break wildlife food chains
• Ties grower to one herbicide
• may adversely affect humans
• superweeds may develop

Question 19

Discuss implications of GM organisms in food production

Answer 19

• social and legal controls required
• may adversely affect environment
• may pose health risks
• who has access to this technology?

Question 20

Define homeostasis and describe its importance

Answer 20

• It is the ability to maintain a constant internal environment
• It is important as things like temperature, pH and blood content must be maintained within narrow limits.

Question 21

Describe negative feedback

Answer 21

• A type of control in which the conditions being regulated are brought back to a set value as soon as deviation has occured.

Change occurs -> Receptor measures level -> Information from receptor is compared to set value at co-ordinator -> Effector brings change in opposite direction to input -> Condition is restored to set value.

Question 22

Describe how thermoregulation occurs

Answer 22

If temperature is low:

• heat gain centre of hypothalamus inhibits heat loss centre
• Sweat is not produced
• Vasoconstriction
• Shivering
• Hair erector muscles contracted to trap still air which does not conduct heat.
• Hormone thyxoxin stimulates oxygen consumption and basal metabolic rate of body organs.

If temperature is high:

• heat loss centre of hypothalamus inhibits heat gain centre
• Sweat produced which evaporates causing cooling
• Vasodilation
• Panting
• Hair erector muscle relaxed and still air is not trapped
• Metabolic rate is slowed

Question 23

Describe how blood glucose concentration is controlled

Answer 23

Level is low: alpha cells of islets of Langerhans are stimulated and hormone glucagon is secreted. This reduces rate of respiration and also activates enzymes that convert glycogen and amino acids to glucose (gluconeogenesis).

Level is high: beta cells of islets of Langerhans is stimulated and hormone insulin is secreted. Glycogenesis occurs mainly in liver and skeletal muscle fibres. Glucose converted to fatty acids and fats in liver cells. Cell respiration is also increased.

Question 24

Describe the role of cyclic AMP in the effects of adrenalin and glucagon

Answer 24

• The hormones bind to specific receptors in the cell surface membrane activating a membrane-embedded G-protein.
• Activated G protein activates an enzyme known as adenylyl cyclase.
• Adenylyl cyclase catalyses formation of cyclic AMP from ATP in cell cytosol. About 10^4 molecules are formed.
• The cAMP activates kinase enzyme which activates about 10^6 phosphorylase enzyme molecules.
• This enzyme converts about 10^8 glycogen into glucose-1-phosphate.

Question 25

Describe how dipsticks and biosensors work

Answer 25

Dip Sticks:

• glucose is oxidised to gluconic acid and H2O2 by glucose oxidase.
• Reduced chromagen found in the stick is oxidised with the hydrogen peroxide using the peroxidase enzyme to form a dye.
• The more glucose the more dye formed.

Biosensor:

• glucose is also oxidised to form H2O2
• electrode measures drop in oxygen and electrical signal is generated.
• Size of signal is proportional to concentration of glucose.

Question 26

Describe the formation of urea

Answer 26

1. proteins are broken down to constituent amino acids and deamination occurs where amino group is removed. Deamination occurs in liver and forms ammonia.
2. Ammonia + Carbon Dioxide -> urea, excreted from the body

Question 27

Describe the structure of a nephron

Answer 27

• has a Malpighian body which consists of cup-shaped renal capsule and capillary network known as glomerulus.
• the body connects to the proximal convoluted tubule
• this connects to a loop of Henle which has a descending limb followed by an ascending limb
• Loop connects to distal convoluted tubule.
• Tubule is followed by a collecting duct.

Question 28

Describe the process of ultrafiltration

Answer 28

• Input capillary is wider than output capillary which raises the blood pressure
• water and soluble components of plasma then pass out through extremely fine sieve-like wall structure between podocytes.
• Sieve is made up of endothelium of capillary and epithelium of the capsule
• Fluid that is filtered out is similar to blood plasma but without the protein molecules.

Question 29

Describe how reabsorption occurs in the proximal convoluted tubule

Answer 29

• sugars and amino acids are reabsorbed through active pumping of protons out of cell and moving back through cotransporter protein along with the molecule.
• mineral ions move through active transport, facilitated diffusion and some exchange of ions.
• diffusion of urea
• few proteins in filtrate move by pinocytosis
• some movement of water by osmosis

Question 30

Describe how water conservation occurs in the loop of Henle

Answer 30

• ions are actively transported out of ascending limb
• this decreases the water potential and causes water to move into the medulla from the descending limb
• this causes ions to move into the descending limb
• this means that the water can be absorbed by nearby collecting ducts reducing water loss.

Question 31

Describe osmoregulation

Answer 31

too much water in blood plasma:

• osmoreceptors of hypothalamus not stimulated and thirst receptors not stimulated
• pituitary secretes less ADH and there is no feeling of thirst
• collecting duct walls become less permeable so medulla absorbs less water from collecting ducts
• dilute urine formed

too little water in blood plasma:

• thirst receptors stimulated and osmoreceptors stimulated.
• leads to feeling of thirst and pituitary secretes more ADH
• collecting duct walls more permeable , so medulla absorbs more water from collecting ducts leading to concentrated urine being formed.

Question 32

Describe different ways that urine is analysed

Answer 32

Glucose: using Clinistix or biosensor -> indicator of diabetes

Protein: dipstick test for albumin -> indicator of proteinura -> indicator of kidney disease

Ketones: dipstick test Ketostix -> diabetes, eating disorders

Question 33

Describe the structure and function of motor and sensory neurones

Answer 33

Motor neurones:

• cell body within the brain or spinal cord
• has branched cell processes that extend from the body known as dendrites.
• dendrites receive impulses from other neurones and conduct them towards the cell body.
• The neurone has a single axon that transmits impulses away from the cell body and to transmit the impulse to effector organs.

Sensory neurones:

• single dendron which brings impulse towards the cell body.
• Axon takes impulse away from cell body.
• Neurone transmits impulses from receptor to spinal cord or brain.

Question 34

Describe how impulses are generated through stimulus

Answer 34

• when strong pressure is applied it causes layers of collagen of a capsule to be deformed.
• This changes the permeability of the membrane at the nerve ending and sodium ions flow in.
• This causes the membrane to be depolarised and if a threshold value is reached an impulse is triggered in the sensory neurone.

Question 35

Describe how the reflex arc works

Answer 35

• sense organ detects stimulus and this generates an impulse that is transmitted by a sensory neurone.
• the impulse travels along axons and dendrons of a sequence of neurones and moves from a motor neurone to an effector organ
• the impulse causes a response in the effector

Question 36

Describe the resting potential of a neurone

Answer 36

• Active transport moves potassium ions in across the neurone membrane and sodium ions out across the membrane through a sodium-potassium pump.
• This has no effect on the potential difference across the membrane.
• Facilitated diffusion also occurs of these ions, however, the membrane is far more permeable to potassium ions flowing out than sodium ions flowing in.
• This means the inside is more negatively charged than the outside and so the neurone is polarised.
• A resting potential of about -70 mV is set up.

Question 37

Describe how an action potential allows an impulse to be transmitted

Answer 37

• the membrane contains ion channels that have a central pore with a gate. During a resting potential these channels are closed.
• the energy of the stimulus first opens the gates of the sodium channels causing sodium ions to diffuse in down their electrochemical gradient. This influx is very rapid.
• The inside of the cell becomes more positive going from a potential of -70 mV to +40 mV creating an action potential that travels along the whole length of the neurone fibre.
• The potassium ion channels then open and the ions rapidly diffuse out and resting potential is re-established.
• For a brief period after the action potential is generated the neurone fibre is not excitable. This is known as the refractory period
• A stimulus must also be above a minimum intensity known as the threshold of stimulation. Otherwise no action potential is generated.

Question 38

Explain the importance of the myelin sheath

Answer 38

• The myelin sheath has electrical resistance and so prevents depolarisations.
• This means depolarisation can only occur at the nodes of Ranvier where the axon membrane is exposed.
• The action potentials then have to jump from node to node.
• This is known as saltatory conduction and speeds up the rate of transmission greatly.

Question 39

Define a synapse and describe how transmission occurs across it.

Answer 39

A synapse is a gap that is found where two neurones meet and consists of the swollen tip of an axon and the dendrite or cell body of another neurone.

Transmission:
1. Action potential arriving at synaptic knob opens calcium ion channels in pre-synaptic membrane.

2. Calcium ions cause vesicles of transmitter substance to fuse with pre-synaptic membrane and release substance into synaptic cleft.
3. Transmitter substance diffuses across synaptic cleft and binds with receptor protein in post-synaptic membrane. This opens opens the ion channel and sodium ions move into cytoplasm of post-synaptic neurone. This causes depolarisation. This build up of action potential is known as facilitation.
4. The substance on the receptor is inactivated by an enzyme allowing the ion channel to close causing resting potential to be re-instated.
5. The inactivated products of the transmitter re-enters the pre-synaptic knob which is re-synthesised into transmitter substance and packaged for reuse.

Question 40

Why are synapses required?

Answer 40

• filter out low-level stimulus of limited importance
• protect effectors from over-stimulation as continuous transmission of action potentials exhausts supply of transmitter substances.
• allows transmission in one direction
• summates the action potentials of all the different types of neurones from which action potential comes.

Question 41

Describe the ultrastructure of skeletal muscle

Answer 41

• muscle fibres are multinucleate and has many parallel myofibrils.
• The myofibrils are found within a plasma membrane known as the sarcolemma and within cytoplasm known as sarcoplasm.
• sarcolemma infolds to form sarcoplasmic reticulum.
• mitochondria is packed between the myofibrils

Question 42

Describe how a motor nerve ending makes connection with a muscle fibre

Answer 42

• on arrival of an action potential at the neuromuscular junction vesicles of acetylcholine are released.
• transmitter molecules bind to receptors on sarcoplasm
• this triggers release of calcium ions from sarcoplasmic reticulum into the cytoplasm around the myofibrils via the T-tube system.
• the calcium ions then remove the blocking molecules on the binding sites of the actin filaments allowing contraction to occur.

Question 43

Describe how skeletal muscle contracts

Answer 43

1. As impulse reaches, calcium ions are released which react with additional protein troponin which triggers removal of blocking molecule, tropomyosin.
2. A charged bulbous head which is a myosin (thick filament) head with ADP and Pi reacts with binding site of the actin molecule (thin filament).
3. ADP is released from the bulbous head triggering rowing movement of head which tilts at an angle of 45 degrees. This pushes the actin filament along causing the myofibril to be shortened. This is the power stroke step.
4. A molecule of ATP binds to the bulbous head. ATPase enzyme in the head hydrolyses the ATP forming ADP and Pi meaning the head is charged again. The charged head detaches and straightens.

Question 44

Describe the menstrual cycle

Answer 44

1. FSH is secreted by the pituitary gland and stimulates development of several immature egg cells. Only one will develop into mature secondary oocyte.
2. Developing follicle secretes oestrogen. Oestrogen stimulates the build up of the endometrium which prepares the uterus for implantation of an embryo. It also inhibits the further secretion of FSH so no other follicles develop.
3. Rising level of oestrogen stimulates secretion of LH and some FSH. LH stimulates ovulation and the secondary oocyte is released from the ovary.
4. LH then stimulates the conversion of the vacant follicle into another gland called the corpus luteum. The corpus luteum secretes progesterone.
   - progesterone continues to build up the endometrium and also inbits further secretion of LH and FSH.
5. Levels of FSH and LH decrease causing the corpus luteum to degenerate. The levels of progesterone and oestrogen also fall meaning the endometrium breaks down and is lost through the vagina.

Question 45

Describe how the contraceptive pill works

Answer 45

• the pill decreases the release of FSH and LH from the pituitary gland.
• This restricts the growth of the follicles in the ovaries and so a secondary oocyte is not released.
• The endometrium still builds up and breaks down but menstruation is shorter and lighter as corpus luteum has not grown.
• the effect of the pill is then to prevent ovulation, thicken the mucus in the cervix making it difficult for the sperm to reach the egg and to make the uterine lining thinner so it is less likely to accept the egg.