A2 Biology Unit 5 page 1 AQA A2 Biology Unit 5 Contents Specification 2 Human Nervous system Nerve Cells 4 The Nerve Impulse 6 Synapses 0 Receptors 14 Muscle 17 Animal Responses 24 Control of Heart Rate 28 The Hormone System 30 Homeostasis 33 Temperature Homeostasis 34 Blood Glucose Homeostasis 38 Control of Mammalian Oestrus 42 Plant Responses 44 Molecular Genetics The Genetic Code 48 Protein Synthesis 50 Gene Mutations 54 Stem Cells 57 Control of Gene Expression 63 Biotechnology 66 DNA sequencing 71 Southern Blot 76 In vivo cloning 80 Genetically Modified Organisms 85 Gene Therapy 89 Genetic Screening and Counselling 92 These notes may be used freely by A level biology students and teachers, and they may be copied and edited. Please do not use these materials for commercial purposes. I would be interested to hear of any comments and corrections. Neil C Millar ([email protected]) Head of Biology, Heckmondwike Grammar School High Street, Heckmondwike, WF16 0AH Jan 2010 HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 2 Biology Unit 5 Specification Control Systems are substances that stimulate their target cells via the blood Organisms increase their chance of survival by responding system. This results in slow, long-lasting and widespread to changes in their environment. responses. The second messenger model of adrenaline and glucagon action. Histamine and prostaglandins are local The Nerve Impulse chemical mediators released by some mammalian cells that The structure of a myelinated motor neurone. The affect only cells in their immediate vicinity. establishment of a resting potential in terms of differential membrane permeability, electrochemical gradients and the Homeostasis movement of sodium and potassium ions. Changes in Homeostasis in mammals involves physiological control membrane permeability lead to depolarisation and the systems that maintain the internal environment within generation of an action potential. The all-or-nothing restricted limits. principle. The passage of an action potential along non- myelinated and myelinated axons, resulting in nerve Negative and Positive feedback impulses. The nature and importance of the refractory • Negative feedback restores systems to their original period in producing discrete impulses. Factors affecting the level. The possession of separate mechanisms involving speed of conductance: myelination and saltatory conduction; negative feedback controls departures in different axon diameter; temperature. directions from the original state, giving a greater degree of control. Synapses • Positive feedback results in greater departures from the The detailed structure of a synapse and of a neuromuscular original levels. Positive feedback is often associated with junction. The sequence of events involved in transmission a breakdown of control systems, e.g. in temperature across a cholinergic synapse and across a neuromuscular control. junction. Explain unidirectionality, temporal and spatial Interpret diagrammatic representations of negative and summation and inhibition. Predict and explain the effects of positive feedback. specific drugs on a synapse (recall of the names and mode of action of individual drugs will not be required). Temperature Homeostasis The importance of maintaining a constant core temperature Receptors and constant blood pH in relation to enzyme activity. The Receptors only respond to specific stimuli. The creation of a contrasting mechanisms of temperature control in an generator potential on stimulation. ectothermic reptile and an endothermic mammal. • The basic structure of a Pacinian corpuscle as an Mechanisms involved in heat production, conservation and example of a receptor. Stimulation of the Pacinian loss. The role of the hypothalamus and the autonomic corpuscle membrane produces deformation of stretch- nervous system in maintaining a constant body temperature mediated sodium channels leading to the establishment in a mammal. of a generator potential. • Differences in sensitivity and visual acuity as explained by Blood Glucose Homeostasis differences in the distribution of rods and cones and the The factors that influence blood glucose concentration. The connections they make in the optic nerve. importance of maintaining a constant blood glucose concentration in terms of energy transfer and water Muscle potential of blood. The role of the liver in glycogenesis and The sliding filament theory of muscle contraction. Gross and gluconeogenesis. The role of insulin and glucagon in microscopic structure of skeletal muscle. The ultrastructure controlling the uptake of glucose by cells and in activating of a myofibril. The roles of actin, myosin, calcium ions and enzymes involved in the interconversion of glucose and ATP in myofibril contraction. The role of ATP and glycogen. Types I and II diabetes and control by insulin and phosphocreatine in providing the energy supply during manipulation of the diet. The effect of adrenaline on muscle contraction. The structure, location and general glycogen breakdown and synthesis. properties of slow and fast skeletal muscle fibres Control of Mammalian Oestrus Animal Responses The mammalian oestrous cycle is controlled by FSH, LH, A simple reflex arc involving three neurones. The progesterone and oestrogen. The secretion of FSH, LH, importance of simple reflexes in avoiding damage to the progesterone and oestrogen is controlled by interacting body. Taxes and kineses as simple responses that can negative and positive feedback loops. Candidates should be maintain a mobile organism in a favourable environment. able to interpret graphs showing the blood concentrations Investigate the effect of external stimuli on taxes and kineses of FSH, LH, progesterone and oestrogen during a given in suitable organisms. oestrous cycle. Control of Heart Rate Plant Responses The role of receptors, the autonomic nervous system and Tropisms as responses to directional stimuli that can effectors in controlling heart rate. maintain the roots and shoots of flowering plants in a favourable environment. In flowering plants, specific growth Hormones factors diffuse from growing regions to other tissues. They Nerve cells pass electrical impulses along their length. They regulate growth in response to directional stimuli. The role stimulate their target cells by secreting chemical of indoleacetic acid (IAA) in controlling tropisms in neurotransmitters directly on to them. This results in rapid, flowering plants. short-lived and localised responses. Mammalian hormones HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 3 Genetics • Small interfering RNA (siRNA) as a short, double-strand The Genetic Code of RNA that interferes with the expression of a specific The genetic code as base triplets in mRNA which code for gene. Interpret data provided from investigations into specific amino acids. The genetic code is universal, non- gene expression. overlapping and degenerate. The structure of molecules of messenger RNA (mRNA) and transfer RNA (tRNA). Genetic Engineering Techniques Candidates should be able to compare the structure and • The use of restriction endonucleases to cut DNA at composition of DNA, mRNA and tRNA specific, palindromic recognition sequences. The importance of “sticky ends”. Protein Synthesis • conversion of mRNA to cDNA, using reverse • Transcription as the production of mRNA from DNA. transcriptase The role of RNA polymerase. The splicing of pre-mRNA • The base sequence of a gene can be determined by to form mRNA in eukaryotic cells. restriction mapping and DNA sequencing. • Translation as the production of polypeptides from the • Interpret data showing the results of gel electrophoresis sequence of codons carried by mRNA. The role of to separate DNA fragments. ribosomes and tRNA. • The use of the polymerase chain reaction (PCR) in Show understanding of how the base sequences of nucleic cloning DNA fragments. acids relate to the amino acid sequence of polypeptides, • The use of labelled DNA probes and DNA hybridisation when provided with suitable data. Interpret data from to locate specific genes. Candidates should understand experimental work investigating the role of nucleic acids. the principles of these methods. They should be aware Recall of specific codons and the amino acids for which they that methods are continuously updated and automated. code, and of specific experiments, will not be tested. • The technique of genetic fingerprinting in analysing DNA fragments that have been cloned by PCR, and its use in Gene Mutations determining genetic relationships and in determining the Gene mutations might arise during DNA replication. The genetic variability within a population. Explain the deletion and substitution of bases. Gene mutations occur biological principles that underpin genetic fingerprinting spontaneously. The mutation rate is increased by mutagenic techniques. An organism’s genome contains many agents. Some mutations result in a different amino acid repetitive, non-coding base sequences. The probability sequence in the encoded polypeptide. Due to the of two individuals having the same repetitive sequences degenerate nature of the genetic code, not all mutations is very low. Explain why scientists might use genetic result in a change to the amino acid sequence of the fingerprints, in the fields of forensic science, medical encoded polypeptide. Evaluate the effect on diagnosis and diagnosis, animal and plant breeding. treatment of disorders caused by hereditary mutations and • The use of ligases to insert DNA fragments into vectors, those caused by acquired mutations. which are then transferred into host cells. • The identification and growth of transformed host cells Oncogenes and Cancer to clone the desired DNA fragments. The rate of cell division is controlled by proto-oncogenes The relative advantages of in vivo and in vitro cloning. that stimulate cell division and tumour suppressor genes that slow cell division. A mutated proto-oncogene, called an Genetically Modified Organisms oncogene, stimulates cells to divide too quickly. A mutated The use of recombinant DNA technology to produce tumour suppressor gene is inactivated, allowing the rate of transformed organisms that benefit humans. Interpret cell division to increase. Interpret information relating to the information relating to the use of recombinant DNA use of oncogenes and tumour suppressor genes in the technology. Evaluate the ethical, moral and social issues prevention, treatment and cure of cancer. associated with the use of recombinant technology in agriculture, in industry and in medicine. Balance the Stem Cells humanitarian aspects of recombinant DNA technology with Totipotent cells are cells that can mature into any body cell. the opposition from environmentalists and antiglobalisation During development, totipotent cells translate only part of activists. their DNA, resulting in cell specialisation. • In mature animals only a few totipotent cells, called stem Gene Therapy cells, remain. These can be used in treating some genetic The use of gene therapy to supplement defective genes. disorders. Evaluate the use of stem cells in treating Candidates should be able to evaluate the effectiveness of human disorders. gene therapy. • In mature plants, many cells remain totipotent. They have the ability to develop in vitro into whole plants or Genetic Screening into plant organs when given the correct conditions. Many human diseases result from mutated genes or from Interpret data relating to tissue culture of plants from genes that are useful in one context but not in another, e.g. samples of totipotent cells sickle cell anaemia. DNA sequencing and PCR are used to produce DNA probes that can be used to screen patients Regulation of Gene Expression for clinically important genes. The use of this information in • Transcription of target genes is stimulated only when genetic counselling, e.g., for parents who are both carriers specific transcriptional factors move from the cytoplasm of defective genes and, in the case of oncogenes, in deciding into the nucleus. The effect of oestrogen on gene the best course of treatment for cancers. transcription. HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 4 The Human Nervous System Humans, like all living organisms, can respond to changes in the environment and so increase survival. Humans have two control systems to do this: the nervous system and the endocrine (hormonal) system. We’ll look at the endocrine system later, but first we’ll look at the nervous system. The human nervous system controls everything from breathing and standing upright, to memory and intelligence. It has three parts: detecting stimuli; coordinating; and effecting a response: Stimuli are changes in the external or internal environment, such as light waves, pressure or blood sugar. Humans can detect at least nine external stimuli: and dozens Stimulus of internal stimuli, so the commonly-held believe that humans have just five senses is obviously very wide of the mark! Receptor cells detect stimuli. Receptor cells are often part of sense organs, such as the ear, eye or skin. Receptor cells all have special receptor proteins on their cell Receptor membranes that actually do the sensing, so “receptor” can confusingly mean a protein, a cell or a group of cells. The coordinator is the name given to the network of interneurones connecting the sensory and motor systems. It can be as simple as a single interneurone in a reflex arc, Coordinator or as complicated as the human brain. Its job is to receive impulses from sensory neurones and transmit impulses to motor neurones. Effectors are the cells that effect a response. In humans there are just two kinds: muscles and glands. Muscles include skeletal muscles, smooth muscles and cardiac muscle, and they cause all movements in humans, such as walking, talking, breathing, Effector swallowing, peristalsis, vasodilation and giving birth. Glands can be exocrine – secreting liquids to the outside (such as tears, sweat, mucus, enzymes or milk); or endocrine – secreting hormones into the bloodstream. Responses aid survival. They include movement of all kinds, secretions from glands and Response all behaviours such as stalking prey, communicating and reproducing. We’re going to be looking at each of these stages in turn, but first we’ll look at the cells that comprise the nervous system. HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 5 Nerve Cells dendrites The nervous system composed of nerve cells, or neurones. A neurone has a cell body with extensions leading off it. Several dendrons carry nerve impulses e towards the cell body, while a single long axon carries the nerve impulse away n o dendron r from the cell body. Axons and dendrons are only 10µm in diameter but can be u e N up to 4m in length in a large animal (a piece of spaghetti the same shape would y r be 400m long)! A nerve is a discrete bundle of several thousand neurone axons. o ns cell body e S Nerve impulses are passed from the axon of one neurone to the dendron of another at a synapse. Numerous dendrites provide a large surface area for axon connecting with other neurones. synapse Most neurones also have many companion cells called Schwann cells, which are dendrites cell body wrapped around the axon many times in a spiral to form a thick lipid layer e nucleus n called the myelin sheath. The myelin sheath provides physical protection and o r u electrical insulation for the axon. There are gaps in the sheath, called nodes of e n r Ranvier, which we’ll examine later. e axon t n I synapse Humans have three types of neurone: dendrites • Sensory neurones have long dendrons and transmit nerve impulses from cell body sensory receptors all over the body to the central nervous system. • Effector neurones (also called motor neurones) have long axons and e myelin n transmit nerve impulses from the central nervous system to effectors o sheath r u (muscles and glands) all over the body. ne Schwann r cell o • Interneurones (also called connector neurones or relay neurones) are much t o M node of smaller cells, with many interconnections. They comprise the central Ranvier nervous system. 99.9% of all neurones are interneurones. synaptic terminals HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 6 The Nerve Impulse Neurones transmit simple on/off signals called impulses (never talk about nerve signals or messages). These impulses are due to events in the cell membrane, so to understand the nerve impulse we need to revise some properties of cell membranes. The Membrane Potential All animal cell membranes contain a protein pump called the Na+K+ATPase. This uses the energy from ATP splitting to simultaneously pump 3 sodium ions out of the cell and 2 potassium ions in. If this was to continue unchecked there would be no sodium or potassium ions left to pump, but there are also sodium and potassium ion channels in the membrane. These channels are normally closed, but even when closed, they “leak”, allowing sodium ions to leak in and potassium ions to leak out, down their respective concentration gradients. 3Na+ + outside cell Na Na+K+ATPase K membrane - inside closed closed (leak) ATP ADP+P (leak) i 2K+ The combination of the Na+K+ATPase pump and the leak channels cause a stable imbalance of Na+ and K+ ions across the membrane. This imbalance causes a potential difference across all animal cell membranes, called the membrane potential. The membrane potential is always negative inside the cell, and varies in size from –20 to –200mV in different cells and species. The Na+K+ATPase is thought to have evolved as an osmoregulator to keep the internal water potential high and so stop water entering animal cells and bursting them. Plant cells don’t need this pump as they have strong cells walls to prevent bursting (which is why plants never evolved a nervous system). The Action Potential In nerve and muscle cells the membranes are electrically excitable, which means that they can change their membrane potential, and this is the basis of the nerve impulse. The sodium and potassium channels in these cells are voltage gated, which means that they can open and close depending on the size of the voltage across the membrane. The nature of the nerve impulse was discovered by Hodgkin, Huxley and Katz in Plymouth in the 1940s, for which work they received a Nobel Prize in 1963. They used squid giant neurones, whose axons are almost 1 mm in diameter (compared to 10 µm normally), big enough to insert wire electrodes so that they could measure the potential difference across the cell membrane. In a typical experiment they would apply HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 7 an electrical pulse at one end of an axon and measure the voltage changes at the other end, using an oscilloscope: stimulating recording stimulator electrodes electrodes oscilloscope squid giant axon isotonic bath The normal membrane potential of these nerve cells is –70mV (inside the axon), and since this potential can change in nerve cells it is called the resting potential. When a stimulating pulse was applied a brief reversal of the membrane potential, lasting about a millisecond, was recorded. This brief reversal of the membrane potential is actually the nerve impulse, and is also called the action potential: +80 Action Potential ) V m ( +40 al 1 2 ti depolarisation repolarisation n e 1 ms ot 0 p time e n a br -40 m e m Resting -80 Potential The action potential has 2 phases called depolarisation and repolarisation. 1. Depolarisation. The sodium channels open for 0.5ms, causing - open out sodium ions to diffuse in down their gradient, and making the inside of the cell more positive. This is a depolarisation because the Na K normal voltage polarity (negative inside) is reversed (becomes in positive inside). closed + (leak) Na+ 2. Repolarisation. The potassium channels open for 0.5ms, K+ + causing potassium ions to diffuse out down their concentration out gradient, making the inside more negative again. This is a Na K repolarisation because it restores the original polarity. in closed open - (leak) Since both channels are voltage-gated, they are triggered to open by changes in the membrane potential itself. The sodium channel opens at–30mV and the potassium channel opens at 0V. The Na+K+ATPase pump runs continuously, restoring the resting concentrations of sodium and potassium ions. HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 8 How do Nerve Impulses Start? In the squid experiments the action potential was initiated by the stimulating electrodes. In living cells they are started by receptor cells. These all contain special receptor proteins that sense the stimulus. The receptor proteins are sodium channels that are not voltage-gated, but instead are gated by the appropriate stimulus (directly or indirectly). For example chemical-gated sodium channels in tongue taste receptor cells open when a certain chemical in food binds to them; mechanically-gated ion channels in the hair cells of the inner ear open when they are distorted by sound vibrations; and so on. In each case the correct stimulus causes the sodium channel to open; which causes sodium ions to diffuse into the cell; which causes a depolarisation of the membrane potential, which affects the voltage-gated sodium channels nearby and starts an action potential. How are Nerve Impulses Propagated? Once an action potential has started it is moved (propagated) along an axon automatically. The local reversal of the membrane potential is detected by the surrounding voltage-gated ion channels, which open when the potential changes enough. direction of just opened next to nerve impulse Na+ channels - refactory open + + - - + + membrane - - - - axon + + membrane - - + + - - - - + + + + resting action resting potential potential potential The ion channels have two other features that help the nerve impulse work effectively: • After an ion channel has opened, it needs a “rest period” before it can open again. This is called the refractory period, and lasts about 2ms. This means that, although the action potential affects all other ion channels nearby, the upstream ion channels cannot open again since they are in their refractory period, so only the downstream channels open, causing the action potential to move one way along the axon. • The ion channels are either open or closed; there is no half-way position. This means that the action potential always reaches +40mV as it moves along an axon, and it is never attenuated (reduced) by long axons. In other word the action potential is all-or-nothing. HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 9 How can Nerve Impulses convey strength? How do impulses convey the strength of the stimulus? Since nerve impulses are all-or-nothing, they cannot vary in size. Instead, the strength of stimulus is indicated by the frequency of nerve impulses. A weak stimulus (such as dim light, a quiet sound or gentle pressure) will cause a low frequency of nerve impulses along a sensory neurone (around 10Hz). A strong stimulus (such as a bright light, a loud sound or strong pressure) will cause a high frequency of nerve impulses along a sensory neurone (up to 100Hz). low frequency impulses high frequency impulses for weak stimulus for strong stimulus How Fast are Nerve Impulses? Action potentials can travel along axons at speeds of 0.1-100 ms-1. This means that nerve impulses can get from one part of a body to another in a few milliseconds, which allows for fast responses to stimuli. (Impulses are much slower than electrical currents in wires, which travel at close to the speed of light, 3x108 ms-1.) The speed is affected by 3 factors: • Temperature. The higher the temperature, the faster the speed. So homeothermic (warm-blooded) animals have faster responses than poikilothermic (cold-blooded) ones. • Axon diameter. The larger the diameter, the faster the speed. So marine invertebrates, which live at temperatures close to 0°C, have developed thick axons to speed up their responses. This explains why squid have their giant axons. • Myelin sheath. Only vertebrates have a myelin sheath surrounding their neurones. The voltage-gated ion channels are found only at the nodes of Ranvier, and between the nodes the myelin sheath acts as a good electrical insulator. The action potential can therefore jump large distances from node to node (1 mm), a process that is called saltatory propagation. This increases the speed of propagation dramatically, so while nerve impulses in unmyelinated neurones have a maximum speed of around 1 ms-1, in myelinated neurones they travel at 100 ms-1. direction of nerve impulse + - - + - + + - + - - + myelin node of Ranvier sheath -ions channels here only HGS Biology A-level notes NCM 8/09 A2 Biology Unit 5 page 10 Synapses The junction between two neurones is called a synapse. An action potential cannot cross the gap between the neurones (called the synaptic cleft), and instead the nerve impulse is carried by chemicals called neurotransmitters. These chemicals are made by the cell that is sending the impulse (the pre-synaptic neurone) and stored in synaptic vesicles at the end of the axon. The cell that is receiving the nerve impulse (the post-synaptic neurone) has chemical-gated ion channels in its membrane, called neuroreceptors. These have specific binding sites for the neurotransmitters. voltage-gated calcium channel Ca2+ 1 neuroreceptors mitochondria (chemical-gated ion channels) 2 axon of dendrite of presynaptic 3 postsynaptic neurone neurone Na+ 4 synaptic vesicles 6 containing neurotransmitter synaptic cleft 5 (20nm) 1. At the end of the pre-synaptic neurone there are voltage-gated calcium channels. When an action potential reaches the synapse these channels open, causing calcium ions to diffuse into the cell down their concentration gradient. 2. These calcium ions cause the synaptic vesicles to fuse with the cell membrane, releasing their contents (the neurotransmitter chemicals) by exocytosis. 3. The neurotransmitters diffuse across the synaptic cleft. 4. The neurotransmitter binds to the neuroreceptors in the post-synaptic membrane, causing the ion channels to open. In the example shown these are sodium channels, so sodium ions diffuse in down their gradient. 5. This causes a depolarisation of the post-synaptic cell membrane, called the post-synaptic potential (PSP), which may initiate an action potential. 6. The neurotransmitter must be removed from the synaptic cleft to stop the synapse being permanently on. This can be achieved by breaking down the neurotransmitter by a specific enzyme in the synaptic cleft (e.g. the enzyme cholinesterase breaks down the neurotransmitter acetylcholine). The breakdown products are absorbed by the pre-synaptic neurone by endocytosis and used to re-synthesise more neurotransmitter, using energy from the mitochondria. Alternatively the neurotransmitter may be absorbed intact by the pre-synaptic neurone using active transport. HGS Biology A-level notes NCM 8/09