Unit 5 Revision

1. Unit 5 Revision

2. Chapter 9 – Responses to Stimuli Fill in the blanks to show your understanding of stimulus and response: Stimulus  __________  __________  __________  response • Write a definition for tropism: • List the different types of tropism: • . • . • . • Explain the difference between: • positive tropism: • negative tropism: Growth movement of a plant in response to a directional stimulus effector receptor co-ordinator • Write a definition for taxes: • Explain the difference between: • positive taxes: • negative taxes: • Write a definition for kineses: • Give an example of kinesis: Random movement in response to a stimulus. The more unpleasant the stimulus the faster the organism moves. A directional response to a stimulus Phototropism Geotropism Hydrotropism Woodlice move more rapidly and change direction more often when they are in unfavourable, dry conditions. This is to increase their chances of finding a more favourable area. In moist conditions they slow down and change direction less often to remain in that area. Growth of the plant towards the stimulus. Movement is towards the stimulus e.g. algae move towards light Growth of the plant away from the stimulus. Movement is away from the stimulus e.g. earthworms move away from light

3. Chapter 9 – Nervous Control Complete the diagram to show the organisation of the nervous system: Label the diagram to show how the nerves are arranged in the spinal cord: Relay (Intermediate) neurone Sensory neurone Contains sensory neurones Peripheral Nervous System Central Nervous System Nervous system Spinal Cord Spinal nerve Motor neurone Brain Contains Motor neurones Sensory Nervous System Motor nervous system Autonomic Nervous System Voluntary nervous system

4. Chapter 9 – Reflex Arcs • Describe a reflex arc • Stimulus (heat from a hot object) •  Receptor (temperature receptors in the skin) •  Sensory Neurone (passes nerve impulse to the spinal cord) •  Relay Neurone (links the sensory to the motor neurone) •  Motor Neurone (carries nerve impulse from the spinal cord to the muscle) •  Effector (muscle contracts) •  Response (hand moves away) • Why are reflex arcs important? • Involuntary – they do not require input from the brain, leaving it free to carry out more complex responses • Protection – the fast response helps to protect the body from harm • Rapid – short neurone pathway, only 2 synapses

5. Chapter 9 – Control of Heart Rate • State and describe the two parts to the autonomic nervous system • Sympathetic Nervous System – stimulates effectors and therefore speeds up activity. Helps us cope in stressful situations (e.g. strenuous exercise) by heightening awareness and preparing us for activity (fight or flight) • Parasympathetic Nervous System – inhibits effectors and therefore slows down activity. It is our ‘rest and digest’ response. • The two nervous systems are antagonistic. What does this mean? • They normally oppose each other. If one system contracts a muscle, the other relaxes it.

6. The Brain Controls Changes in the Heart Rate Medulla oblongata (in the brain) Cardiac Centre Cardiac Inhibitory Centre Cardiac Accelerator Centre Parasympathetic nerve linked to SAN Sympathetic nerve linked to SAN Decreases heart rate Increases heart rate

7. Chapter 9 – Control of Heart Rate • Explain the role of chemoreceptors in control of heart rate • Found in the wall of the carotid arteries (arteries to the brain) and aorta • If there is increased CO2 in the blood then pH of the blood is lowered • Chemoreceptors detect this and increase the frequency of nerve impulses to the medulla oblongata (centre that increases heart rate) • This centre sends more nerve impulses via the sympathetic nervous system to the SAN. This increases heart rate. • The increased blood flow means that more CO2 is removed by the lungs, so the CO2 levels in the blood return to normal and pH rises. • Once the pH is back to normal then the chemoreceptors detect this and reduce the frequency of nerve impulses to the medulla oblongata. • The centre that increases heart rate sends less nerve impulses to the SAN and so heart rate is lowered.

8. Chapter 9 – Control of Heart Rate • Heart rate is also controlled by pressure receptors (in the carotid arteries and aorta). Explain what these do when the blood pressure is higher than normal: • The pressure receptors send nerve impulses to the centre that decreases heart rate (in the medulla oblongata). This centre sends impulses via the parasympathetic nervous system to the SAN, which decreases heart rate. • Explain what these do when the blood pressure is lower than normal • The pressure receptors send nerve impulses to the centre that increases heart rate (in the medulla oblongata). This centre sends impulses via the sympathetic nervous system to the SAN, which increases heart rate.

9. Chapter 9 - Receptors Describe the structure and function of the Pacinian corpuscle: Structure: Pacinian corpuscles respond to pressure and are found in the skin. They contain a single sensory neurone in the centre of layers of tissue, each separated by a gel. The sensory neurone has a stretch-mediated sodium channel in its membrane. Their permeability to sodium will change when they change shape. Function: Normally the stretch-mediated sodium channels do not allow sodium to pass. The neurone is at resting potential. When pressure is applied then the membrane around the sensory neurone stretches. This stretching widens the sodium channels and allows sodium ions to diffuse into the neurone. Influx of sodium ions causes depolarisation (this is the generator potential, which in turn causes an action potential to pass along the neurone.

10. Chapter 9 - Receptors Describe the structure and function of rod and cone cells in the retina: Rod Cells: Rod cells only produce black and white images. Many rod cells share a single bipolar cell and sensory neurone. This allows them to respond to light at very low intensity (they can add together to overcome the threshold needed to send an impulse). A pigment (rhodopsin) must be broken down to cause sodium channels to open in the neurone) Cone Cells: Cone cells produce colour images. Each cone cell has its own bipolar cell and sensory neurone. This means they will only respond to light at high intensity (they cannot add together to overcome the threshold needed to send an impulse). A pigment (iodopsin) must be broken down to create a generator potential. Rod-shaped Cone-shaped Greater numbers than cone cells Fewer numbers than rod cells Fewer at the edge, most at the fovea More at the edge, none at the fovea Poor Good Sensitive Not sensitive

11. Chapter 10 – Co-ordination Nerve impulses Chemicals called hormones Blood system Neurones Relatively slow Very rapid All parts of the body, only target organs respond Specific parts of the body Widespread Localised Slow Rapid Often long-lasting Short-lived Effect may be permanent and irreversible Effect is temporary and reversible

12. Chapter 10 – Co-ordination • What are chemical mediators? • Chemicals released from certain mammalian cells that affect cells in the immediate vicinity • Normally released by infected or injured cells and cause arterioles to dilate, leading to swelling. • Two types: histamine (response to an allergen or injury) and prostaglandins (response to injury) • What are plant growth factors? • Plant hormones that affect growth • Made by cells throughout the plant, rather than organs • Produced in small quantities and affect the tissues close to them. • Example = IAA (indoleacetic acid) which causes cell elongation.

13. Chapter 10 - IAA • Describe the sequence of events that causes a plant to grow towards the light • Cells in the tip produce IAA, which is transported down the shoot • Light causes the IAA to move to the shaded side of the shoot • Concentration of IAA becomes high on the shaded side • IAA causes elongation of cells on the shaded side • The shaded side grows faster, causing the shoot to bend towards the light

14. Chapter 10 - Neurones Describe the structure and function of the parts of a neurone: Axon – Single, long fibre that carries nerve impulses away from the cell body. Dendrites Cell Body – contains the nucleus and RER. Produces proteins and neurotransmitters. Nucleolus Dendrites Dendrites – branched fibres that carry nerve impulses towards the cell body. Axon Nucleus Cell Body Myelin Sheath – covers the axon and is made up of the lipid-rich membranes of the Schwann cells. Speeds up transmission of nerve impulses. Myelin Sheath Schwann Cells Schwann Cells – surround the axon, protect it and provide electrical insulation. Node of Ranvier Node of Ranvier – gaps between adjacent Schwann cells where there is no myelin sheath. Myelin Sheath

15. Chapter 10 – The Nerve Impulse Explain what resting potential is, including the role of potassium and sodium ions in its generation: The inside of an axon is negatively charged relative to the outside. The resting potential is the usual state of the neurone. It is -65mV. The resting potential is established by the sodium-potassium pump, which actively transports 3 Na+ ions out of the axon for every 2 K+ ions that are actively transported in. The membrane is much more permeable to K+ ions than Na+ ions. So, potassium ions diffuse back out of the axon faster than sodium ions diffuse back in.

16. Chapter 10 – The Nerve Impulse Using the graph, explain what happens when an action potential passes down a neurone. • Resting potential (-65mV), sodium voltage-gated channels are closed. • Sodium voltage-gated channels start to open due to energy from a stimulus and Na+ ions diffuse into the membrane. • More sodium voltage-gated channels open and more Na+ ions diffuse in, causing depolarisation of the membrane. • The membrane potential reaches +40mV and the sodium voltage-gated channels close. Potassium voltage-gated channels start to open. • More potassium voltage-gated channels open and K+ ions diffuse out and the membrane repolarises. • Temporarily, the membrane becomes more negative than the resting potential (due to extra K+ ions diffusing out) – hyperpolarisation. The potassium voltage-gated channels close and the sodium-potassium pump takes back over and returns the membrane to the resting potential.

17. Chapter 10 – Passage of an Action Potential Explain, using diagrams, how an action potential moves along an unmyelinated axon: As Na+ ions flood into the membrane the charge inside the membrane is reversed (-  +). This establishes a localised circuit which causes sodium voltage-gated channels a little further down the axon to open (causing a -+ change). The sodium voltage-gated channels behind now close and the resting potential is re-established (+-) Explain, using diagrams, how an action potential moves along a myelinatedaxon: In myelinated neurones the sheath acts as an electrical insulator, preventing action potentials from forming here. The action potential passes in a similar way to the unmyelinated neurone, but it has to ‘jump’ from node of Ranvier to node of Ranvier (saltatory conduction)

18. Chapter 10 – Speed of a Nerve Impulse • List factors that will affect the speed of an action potential • The myelin sheath, the diameter of the axon, temperature • What are the reasons for the refractory period (hyperpolarisation)? • 1) Action potential will only pass in one direction • 2)Discrete impulses – Action potentials can’t be formed immediately after the first one, so they are spread out • 3) Limits the number of action potentials • Explain the ‘all-or-nothing’ principle • A stimulus has to be above a certain threshold value if it is to cause an action potential. Below the threshold value no action potential occurs and therefore no impulse is generated. It doesn’t matter how much above the threshold value the stimulus is it will still only generate one action potential.

19. Chapter 10 - Synapses

20. Chapter 10 - Synapses • State the functions of synapses • Transmit impulses from one neurone  one neurone, one neurone  many neurones, many neurones  one neurone • How do synapses ensure the nerve impulse only travels in one direction? • Neurotransmitter is only made in the presynaptic neurone, only the post-synaptic neurone has receptors for the neurotransmitter • Explain Spatial and Temporal Summation • Spatial – a number of different presynaptic neurones together release enough neurotransmitter to exceed the threshold and trigger an action potential • Temporal – a single presynaptic neurone releases neurotransmitter many times over a short time period, the total of this can then exceed the threshold and trigger an action potential

21. Chapter 10 - Synapses • Explain how inhibition may occur at a synapse • Cl- ion channels may be made to open in the post-synaptic membrane. Cl- ions diffuse into the membrane, making it more negative (hyperpolarisation) and decreasing the likelihood of an action potential being created. • How can drugs affect synapses? • Stimulate the nervous system by creating more action potentials in postsynaptic neurones (may be by mimicking a neurotransmitter) • Inhibit the nervous system by creating fewer action potentials in postsynaptic neurones (may be by inhibiting neurotransmitter release)

22. Chapter 10 – Transmission Across Synapses 1. An action potential arrives at presynaptic membrane. Voltage gated calcium channels open, calcium ions enter along a concentration gradient. Afterwards these are pumped out using ATP. 2. Calcium ions cause synaptic vesicles to fuse with the presynaptic membrane, releasing acetylcholine into the synaptic cleft. 3. Acetylcholine diffuses cross the synaptic cleft and binds to specific receptor sites in the post synaptic membrane.

23. Chapter 10 - Synapses 4. Sodium channels open. Sodium ions diffuse rapidly along a concentration gradient into the postsynaptic membrane causing depolarisation. 5. The enzyme acetylcholinesterase hydrolyses acetylcholine into choline and ethanoic acid (acetyl). These diffuse back into the presynaptic neurone . 6. Acetylcholine is resynthesised using ATP from the mitochondria. The break down of acetylcholine prevents it from continuously generating a new action potential.

24. Chapter 11 - Muscles • Describe the 3 types of muscle • Skeletal Muscle (voluntary, attached to skeleton via tendons), Cardiac Muscle (myogenic, only in the heart), Smooth Muscle (involuntary, in the gut, uterus and arteries) • Explain the differences between fast and slow twitch fibres • Slow twitch – contract more slowly, adapted for endurance work and aerobic respiration (large store of myoglobin, supply of glycogen, rich supply of blood vessels, numerous mitochondria) • Fast twitch – contract more rapidly, adapted for intense exercise and anaerobic respiration (thicker muscle filaments, high concentration of enzymes needed for anaerobic respiration, store of phosphocreatine which can rapidly generate ATP from ADP in anaerobic conditions)

25. Chapter 11 – Structure of Muscles • Complete the diagram

26. Chapter 11 – Structure of Muscles Describe the zones found in a sarcomere • Light bands are I-bands only actin is found in these bands • Dark bands are A-bands actin and myosin overlap in these bands • In the middle of each A-band is a lighter part called the H-zone • In the centre of each I-band is the Z-line, where the actin filaments join • The section of muscle between Z-lines is called a sarcomere • Pattern = Z I A H A I Z • Explain the difference between actin and myosin • They are both protein filaments • Actin is thinner and has 2 strands twisted around each other • Myosin is thicker and has long rod-shaped fibres with bulbous heads Z line --I band-- ----A band----- H zone

27. Label the diagram of the neuromuscular junction

28. Chapter 11 – Neuromuscular Junctions • What is a neuromuscular junction? • The point where a motor neurone meets a skeletal muscle fibre • How does a neuromuscular junction work? • Impulse reaches the neuromuscular junction • Synaptic vesicles fuse with the pre-synaptic membrane, releasing acetylcholine. • Acetylcholine diffuses to the post-synaptic membrane, changing the permeability to sodium ions • Influx of sodium ions results in depolarisation • An action potential occurs in the muscle fibre • Muscle fibre contracts • Acetylcholinesterase breaks down the acetylcholine to prevent over-stimulation of the muscle

29. Chapter 11 – Contraction of Skeletal Muscle • What is the sliding filament mechanism? • Actin and myosin slide past one another when the muscle contracts • What is the evidence for this? • Sarcomere gets shorter • More overlap • Z-lines get closer together • I-band gets narrower • H-zone gets narrower

30. Chapter 11 – Contraction of Skeletal Muscle • What are the 3 main proteins involved? • Myosin – globular, bulbous head and a long tail • Actin – a globular protein where the molecules are twisted into a helix • Tropomyosin – long, thin threads wrapped around actin • Explain more detail on the sliding filament mechanism • Heads of myosin form cross-bridges with the actin filaments (attach to binding sites) • Myosin heads flex together and pull the actin along the myosin • They detach • Return to original angle and re-attach (uses ATP) • Repeats 100 times a second

31. Chapter 11 - Muscle Contraction – Sliding Filament Mechanism

32. Chapter 11 - 3 Stages of Muscle Contraction Explain the stimulation, contraction and relaxation of muscles 1. Stimulation • Neuromuscular junctions – acetylcholine diffuses across the cleft and binds to receptors causing depolarisation 2. Contraction • Action potential carried through t-tubules • Ca2+ ions are released into the muscle cytoplasm from the ER and tropomyosin molecules move away from binding sites • Myosin (with ADP attached) binds to actin and move it along (releases ADP) • ATP attaches to the myosin, causing it to detach from the actin • Ca2+ ions activate ATPase, which hydrolyses ATP  ADP giving the energy for the myosin head to return to its original position • The Myosin (with ADP attached) can now bind further along the actin and the cycle continues. 3. Relaxation • Ca2+ ions actively transported back to the ER (energy from hydrolysis of ATP) and tropomyosin blocks the actin again

33. Chapter 11 – Energy Supply • How do muscles ensure they have enough energy for contraction? • Muscles need a lot of energy when they contract • Supplied by the hydrolysis of ATP • Because of the great demand for energy in certain cases (e.g. Fight or flight responses) then it is required that ATP be generated anaerobically as well • This is achieved by using phosphocreatine • Phosphocreatine is stored in the muscle and helps to regenerate ATP

34. Chapter 12 - Homeostasis Explain what homeostasis is: Maintenance of a constant internal environment. There are continuous variations around a set point and homeostasis is the ability to return to that set point. Explain why homeostasis is important: Essential to keep enzymes functioning, prevent damage to cells from water potential changes. It also allows organisms to be more independent of the external environment. State some of the factors that are controlled by homeostasis: Temperature, pH, water potential, tissue fluid

35. Chapter 12 – Regulation of Body Temperature Complete the feedback loop for controlling body temperature in mammals: State the main ways in which heat is gained by organisms: State the main ways in which heat is lost by organisms: Production of heat (higher metabolic rate), gain of heat from environment (by CCR) Normal body temp Cold receptors in skin Evaporation of water, loss of heat to the environment (by CCR) Warm receptors in skin Explain how body temperature if regulated in ectotherms: Explain how body temperature is regulated in endotherms: Adapting their behaviour to changes in the external temperature e.g. exposing themselves to the sun, sheltering, gaining warmth from the ground, generating metabolic heat, colour variations Hypothalamus Heat gain centre Heat loss centre Vasoconstriction, shivering, hair raised, higher metabolic rate Vasodilation, sweating, hair lowered, lower metabolic rate Gain heat = vasoconstriction, shivering, raising hair, higher metabolic rate, behaviour Lose heat = vasodilation, sweating, lowering hair, behaviour

36. Chapter 12 – Regulation of Blood Glucose • What are common characteristics of all hormones? • Produced by glands and secreted directly into the blood stream • Carried in the blood to target cells which have receptors on their cell-surface membranes which have a complementary shape to the hormone • Effective in very small quantities, but often have widespread and long-lasting effects • Explain the second-messenger model of hormone action • The hormone is the first messenger and binds to receptors on target cells (forms a hormone-receptor complex) • The hormone-receptor complex activates an enzyme in the cell that causes the production of a chemical (the second messenger) • The second messenger causes a series of chemical changes to get to the response needed. • Give 2 examples of hormones that work via this second messenger model • Adrenaline and Glucagon

37. Chapter 12 – Regulation of Blood Glucose Explain the role of the pancreas in regulating blood glucose: State the sources of blood glucose: Explain the role of insulin in regulating blood glucose: Explain the role of glucagon in regulating blood glucose: Explain the role of adrenaline in regulating blood glucose: The pancreas has islets of Langerhans which contain α cells and β cells. α cells produce glucagon and βcells produce insulin. • If the α cells detect a fall in blood glucose then they secrete glucagon into the blood. Glucagon combines with receptors on liver cells and causes: • Activation of an enzyme that converts glycogen to glucose (glycogenolysis) • Increase in the conversion of amino acids and glycerol into glucose (gluconeogenesis) The diet - from breakdown of other carbohydrates. From the breakdown of glycogen (glycogenolysis) which has been stored in the liver. From gluconeogenesis – production of new glucose from sources other than carbohydrates. • At times of excitement or stress adrenaline is produced by the adrenal glands and raises the blood glucose level by: • Activating an enzyme that causes breakdown of glycogen to glucose in the liver (glycogenolysis) • Inactivating an enzyme that synthesises glycogen from glucose. If the βcells detect a rise in blood glucose then they secrete insulin into the blood. Insulin combines with receptors on cells and causes: - Opening of glucose transport protein channels (glucose enters cells) - Activation of enzymes that convert glucose to glycogen and fat

38. Chapter 12 – Regulation of Blood Glucose Complete the feedback loop for controlling body temperature in mammals: To increase To decrease β cells, which release insulin α cells, which release glucagon Detected by… Detected by… falls rises Blood glucose Blood glucose Conversion of glycogen to glucose, conversion of amino acids to glucose Conversion of glucose to glycogen, conversion of glucose to fat, absorption of glucose into cells, more respiration Response… Response… Normal blood glucose level 90mg100cm-3 blood Blood glucose Blood glucose rises, negative feedback falls, negative feedback

39. Chapter 12 - Diabetes

40. What is negative feedback? Negative feedback is when the feedback causes the corrective measures to be turned off. This returns the system to a normal level. Chapter 13 - Negative Feedback Receptors Detect the change Control Centre Coordination Effector Have an effect on the system Input Fall in some parameter Output Rise in some parameter Negative Feedback = Corrective measures turned off

41. Chapter 13 – Positive Feedback • What is positive feedback? • Positive feedback is when the feedback causes the corrective measures to stay turned on • Examples? • Neurones: influx of sodium ions increases the permeability of the neurone, causing more sodium ions to move in, which further increases the permeability etc. This allows a very fast build-up of action potential to respond very quickly to a stimulus

42. Chapter 13 – Control of the oestrous cycle State the role of each hormone: FSH: LH: Oestrogen: Progesterone: Fill in the diagram to show how the hormones interact in the menstrual cycle: Stimulates the development of follicles in the ovary, which contain eggs, and stimulates the follicles to produce oestrogen. Causes a follicle to develop Causes ovulation and corpus luteum to develop Pituitary Gland Causes ovulation to occur, stimulates the corpus luteum to produce progesterone FSH LH Causes the rebuilding of the uterus lining after menstruation and stimulates the pituitary gland to produce LH. Oestrogen Progesterone Ovary Repairs the lining of the uterus Maintains the uterus lining, ready for a fertilised egg Maintains the lining of the uterus and inhibits production of FSH from the pituitary gland.

43. Explain the human menstrual cycle The menstrual cycle begins with the shedding of the uterus lining (Days 1-5) The pituitary gland releases FSH to stimulate follicle growth (Day 1 onwards) The follicles release low levels of oestrogen. This causes the build up of the uterus lining. Oestrogen also inhibits FSH and LH release from the pituitary gland (NEGATIVE FEEDBACK). More oestrogen is released from growing follicles. It reaches a peak where it now stimulates the pituitary gland to release more FSH and LH (POSITIVE FEEDBACK) (Day 10). This causes a surge in LH and FSH. The surge in LH causes a follicle to release its egg – ovulation. (Day 14). Chapter 13 – The Human Menstrual Cycle

44. LH now causes the empty follicle to develop into the corpus luteum. This secretes progesterone and a small amount of oestrogen. Progesterone maintains the thick uterus lining and inhibits FSH and LH release (NEGATIVE FEEDBACK). If fertilisation does not occur, the corpus luteum degenerates and stops producing progesterone. The drop in progesterone stops maintaining the uterus lining, so it breaks down - menstruation. FSH is also no longer inhibited. FSH can be released again and the cycle continues. Chapter 13 - The Human Menstrual Cycle

45. Chapter 14 - RNA • Give a brief overview of how the sequence of bases in DNA determines the structure of a protein • The sequence of DNA bases in a gene codes for the sequence of amino acids in a protein • Sections of the DNA code are transcribed onto a single stranded ribonucleic acid (RNA) molecule in the nucleus (TRANSCRIPTION) • In eukaryotic cells messenger RNA (mRNA) carries the genetic information from the nucleus to the cytoplasm • Proteins are synthesised in the cytoplasm (TRANSLATION) • The sequence of bases in mRNA (GENETIC CODE) determines the sequence of amino acids in a protein

46. Chapter 14 - RNA Label the structures of RNA: • Explain the main features of the genetic code • Each amino acid is coded for by a sequence of 3 bases on mRNA (codon) • A few amino acids have a single codon • Most amino acids have more than 1 codon (the code is degenerate) • 3 codons are stop codons to mark the end of the polypeptide chain • The code is non-overlapping (each base is read only once) • It is a universal code (same codon = same amino acid in almost all organisms) Phosphate group BASE: Adenine Ribose BASE: Cytosine BASE: Guanine BASE: Uracil

47. Chapter 14 - RNA Describe the structure and role of messenger RNA: Describe the structure and role of transfer RNA: Small molecule, single stranded chain folded into a clover-leaf shape. Has a point where the aa attaches and opposite to that is a sequence of 3 bases that make up the anticodon. This will pair with the codon on mRNA during protein synthesis. Long strand, single helix. Forms a mirror copy of DNA. Once formed mRNA leaves the nucleus via nuclear pores, enters the cytoplasm and associates with ribosomes. Single Single Double Largest Between DNA and tRNA Smallest Single-helix Clover-shape Double-helix ribose deoxyribose ribose A, T, C, G A, U, C, G A, U, C, G

48. Chapter 14 - Transcription • Explain the process of transcription (the process of making pre-mRNA) • The DNA is unwound at a certain point (the start of a gene) • DNA Helicase (an enzyme) breaks the hydrogen bonds between the DNA bases • RNA polymerase then moves along the coding (template) strand of the DNA attaching RNA nucleotides to it by complementary base pairing (G-C, C-G, T-A, A-U) • A new strand is formed - pre-mRNA. DNA reforms its normal double helix structure behind RNA polymerase as transcription progresses • When a stop codon is reached RNA polymerase detaches and the completed pre-mRNA is released

49. Chapter 14 - Splicing • Explain what splicing is and why it is important • Both the DNA gene sequence and the pre-mRNA consist of exons (code for proteins) andintrons (non-coding) • The pre-mRNA is therefore spliced to remove introns (non-protein coding sequences) and join exons together • The exons can be rejoined in a variety of ways – so a single section of DNA can code for up to a dozen polypeptides. • This forms mRNA. • Mutations can affect the splicing of the pre-mRNA and thus form non-functional proteins

50. Chapter 14 – Translation (polypeptide synthesis) Describe the process of translation • mRNA passes out of the nuclear pore, into the cytoplasm • A ribosome attaches to the starting codon on the mRNA • The tRNA with the complementary anticodon moves to the ribosome and pairs up with the codon on the mRNA. This tRNA carries an amino acid. • A tRNA with the complementary anticodon pairs with the next codon on the mRNA. This tRNA carries another amino acid. • The ribosome moves along the mRNA, bringing together 2 tRNA molecules at a time. • The 2 amino acids on the tRNA join by a peptide bond (requires an enzyme and ATP) • The ribosome moves onto a third codon and the first tRNA molecule is released and is free to collect another amino acid from the pool in the cell. • The process continues (with more ribosomes passing behind the first, allowing many identical proteins to be assembled simultaneously). • The synthesis of a polypeptide continues until the ribosome reaches a stop codon. Then, the ribosome, tRNA and mRNA molecules all separate and the polypeptide chain is complete.