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A2 And As Course Handbook


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A2 And As Course Handbook

  1. 1. AS and A2 Biology Course Handbook Haringey Sixth Form Centre Aiming For Excellence
  2. 2. Welcome to biology and the sixth form centre Welcome to Haringey Sixth Form Centre and the Faculty of Maths, Science & Sports. We hope that your experience here will be a positive and enjoyable one. This handbook is provided to give you the information that you need to start your course and will also be useful throughout the time that you are studying here at the Centre. Your Personal Tutor will spend time during induction helping you to find out everything you need to know and part of this time will be spent looking through the handbook. There are many people who are available to help you through your studies, their names and roles have been included in the handbook so that if the answers to your questions are not here you always have another way of finding them out. You will also find important dates and deadlines for you to note and include in your Student Diary. It is essential that you stick closely to all course deadlines if you are to succeed and achieve your goals. As with all organisations there are rules and procedures to follow. We hope that these have been made clear, but if you are not sure about any of them please refer to the relevant sections in the handbook or ask your Personal Tutor. Finally, in this handbook, you will find various sources of information and advice which are designed to help you achieve the best possible results and provide progression to careers and higher education. But, above all, this handbook is designed to help you to enjoy your time at Haringey Sixth Form Centre. Preparing food for testing
  3. 3. Table of Contents 1. Welcome Page two 2. Table of Contents Page three 3. Staff names and roles Page four 4. Course and assessment structure Pages five to six 4. Progression Page seven 5. Expectations Page seven 6. Calendar Pages eight, nine and ten 8. Study skills and Study centres Page eleven 9. Health and safety Page twelve 10. Equipment and Resources Page thirteen 11. Reading list and websites Page fourteen 12. Glossary Pages fifteen and sixteen 3
  4. 4. Maths Sport and Science Staff names, roles and contact details Role Name Extension Head of Faculty Mike O’Brien 5940 Programme area Olu Kubweza 5943 manager - Maths Programme area Richard Jones 5885 manager - Sports Faculty Administrator Kamila 5941 Babayeva Learning Mentor Michael Debrah 5949 Teachers - Biology Teresa Greer 5944 Laura Nicholls 5944 Chemistry Michelle 5949 Blenheim- Aning Zabed Ahmed 5943 Physics Mike O’Brien 5940 Maths Olu Kubweza 5943 Daniel Oladejo 5943 Marios 5943 Americanos Sports Richard Jones 5885 Shane Bell- 5884 Nevin Jennifer 5887 Maysmor-Gee Technicians Anjna Vara 5946 Julie Nicholson 5946 4
  5. 5. Course structure units and assessment detail Unit One: Biology and Disease (AS) Topics include: How digestive and gas exchange systems may be affected by communicable and non-communicable diseases How a knowledge of basic biology allows us to understand the symptoms of disease and interpret data relating to risk factors. Assessment Written Paper: 1 Hour 15 Minutes Five - seven short answer questions plus two longer questions (a short comprehension and a short structure essay) Weighting: 33% of total AS Level marks 16% of total A Level marks Unit Two: The Variety of Living Organisms (AS) Topics Include The influence of genetic and environmental factors on intraspecific variation How the variety of life is reflected in similarities and differences in its biochemical basis and cellular organisation How size and metabolic rate affect an organism's requirements and give rise to adaptations. Assessment Written Paper: 1 hour 45 minutes Five - seven short answer questions plus two longer questions (one will emphasise data handling and include a section requiring continuous prose. The other will assess How Science Works). Weighting: 46% of total AS Level marks 23% of total A Level marks Unit three: Practical and Investigative Skills (AS) Topics include: Practical work in the contexts of units one and two Assessment of implementing skills on practical work as a whole Assessment Internal Assessment Weighting: 20% of total AS Level marks 10% of total A Level marks 5
  6. 6. Unit four: Populations and Environment (A2) Topics include: How living things form ecosystems through which energy is transferred How human activity affects ecological balance in a variety of ways Assessment Written Paper: 1 Hour 30 Minutes Six - nine short answer questions plus two longer questions (a short comprehension and a short structured essay) Weighting: 17% of total A Level marks Unit five: Control in Cells and in Organisms (A2) Topics Include Stimulus and responses - the biology of nervous and endocrine systems Homeostasis and the maintenance of a constant internal environment Genes and genetic expression Assessment Written Paper: 2 hours 15 minutes Eight - ten short answer questions plus two longer questions a data handling question and a synoptic essay - choice of one out of two) Weighting: 23% of total A Level marks Unit six: Practical and Investigative Skills (A2) Topics include: Practical work in the contexts of units four and five Assessment of implementing skills on practical work as a whole Assessment Internal Assessment Weighting: 10% of total A Level marks 6
  7. 7. Progression and Possibilities where biology can take you Biology is one of the most popular A Level subjects in the country, attracting students studying a wide range of other subjects. Many of these students enjoy the subject so much they eventually choose a biologically related degree course. Others go on to careers in law, computing, accounting or teaching. So, whatever field you will eventually work in, you will find biology a very rewarding and challenging course which will develop many of the skills essential for a successful career. Biology is a great choice of subject for people who want a career in health and clinical professions, such as medicine, dentistry, veterinary science, physiotherapy, pharmacy, optometry, nursing, zoology, marine biology or forensic science. Students who take Biology often also study from a wide range of subjects, including Psychology, Sociology, English, PE, Chemistry, Physics and Critical Thinking Course Expectations what is expected from you Throughout the course there will be set independent assignments. These will be varied in format, from individual to group work, from essays to investigative reports and even past papers. This independent work is vital to success on the course. The guideline number of hours a week that should be spent on independent assignments is the same as the number of teaching hours a week. Here you will have five teacher guided hours of Biology and be set independent work equivalent to another five hours work. Submission of homework is monitored. You can expect two fixed deadlines a week and to have your marked work returned to you within a week of submission with feedback to help you in future assignments. In the classroom we will be undertaking a range of different types of learning, there will be plenty of practical investigations and a lot of the teaching will be student driven, so not a lot of lecturing from the front! Hopefully you will find biology an interesting and varied course to follow! 7
  8. 8. Calendar basic year outline This calendar provides the basic year outline, there will be a dedicated live google calendar with specific deadlines and key course dates included for you to access online. Feel free to scribble on this calendar to remind yourself of dates. Autumn Term Week Number Week Beginning Content 1 7 Sep 2009 Induction Week 2 14 Sep 2009 Start of Teaching and Induction Assignment 3 21 Sep 2009 4 5 Oct 2009 5 12 Oct 2009 6 19 Oct 2009 7 26 Oct 2009 Half Term 8 2 Nov 2009 9 9 Nov 2009 10 16 Nov 2009 11 23 Nov 2009 12 30 Nov 2009 13 7 Dec 2009 14 14 Dec 2009 Christmas Break End of Week 8
  9. 9. Calendar basic year outline Spring Term Week Number Week Beginning Content 15 4 Jan 2010 Start of Term 16 11 Jan 2010 17 18 Jan 2010 18 25 Jan 2010 19 1 Feb 2010 20 8 Feb 2010 21 15 Feb 2010 Half Term 22 22 Feb 2010 23 1 Mar 2010 24 8 Mar 2010 25 15 Mar 2010 26 22 Mar 2010 27 29 Mar 2010 9
  10. 10. Summer Term Week Number Week Beginning Content 28 19 Apr 2010 29 26 Apr 2010 30 3 May 2010 31 10 May 2010 32 17 May 2010 33 24 May 2010 34 31 May 2010 Half Term 35 7 Jun 2010 36 14 Jun 2010 37 21 Jun 2010 38 28 Jun 2010 12 5 Jul 2010 10
  11. 11. Study Skills Advice In order to study biology successfully there are a number of basic skills you will need to be able to use. There is a significant amount of manipulating data and calculations involved in biology so good basic numeracy skills are essential. From rates of reaction to specific statistical analysis to reading and analysing ECG’s! A great deal of information will be provided to you electronically, through websites, virtual learning environment and email so good basic ICT skills are needed. You will be writing investigative reports as part of your course and essays as part of your assessment and in exams so ability to present information clearly and logically is important. Don’t panic, you will be provided with help in all of these areas through the course wiki and classroom activities. Additional help is available too, all you need to do is Study Centre Monday to Thursday 9am-5pm Friday 9am - 4pm The Science and Maths study centre is an area dedicated to learning in these subjects. There are a number of tables available for study, a selection of PC’s available and textbooks from across the courses. The PC’s all have maths and science specific applications and internet and email access. There are bookshelves dedicated to subjects and a whole host of other reading material. Subject relevant journals like New Scientist are also available. Also in the study centre staff will at most times be available to help you with your studies, whether it is in a subject specific workshop, or you just want to drop in and ask a question, there will generally be someone available to help you. Please note: the study centre is not a social area, no card games, eating or drinking is allowed. 11
  12. 12. Health and Safety All students are required to strictly follow all Centre Health and Safety policies procedures and rules. You can read about these in your Student Diary or on the Centre Virtual Learning Environment. Very often you will be given instructions by your Personal Tutor or subject teachers about safety issues within particular areas. You should pay particular attention to the location of fire escapes and fire escape routes which are signposted around the site. Students on certain courses or subjects will have certain specific health & safety rules and procedures to learn. Science Health & Safety Policy For all students and staff working within the Science Programme Area there is a specific health & safety policy which includes a set specific health & safety policy which includes a set of rules for students in science lessons, procedures for laboratory working and access to in- formation about safety issues. All students will be provided with a copy of these rules and be expected to agree to follow them. Any student who is unable to do so will be excluded from any science practical sessions and laboratories. You will be provided with necessary safety equipment such as laboratory coats, safety spectacles and gloves when needed. You will be expected to become familiar with the location of important emergency equipment such as first aid boxes and eye wash stations but should refer all incidents to your science teacher without delay. In addition to this: In the Biology lab students should never wear open-toe sandals – full shoes required at all times Students will be expected to wash their hands before and after every practical experiment Students should always assume that nothing in the lab is edible! Students should always adhere to all instructions, failure to do so will cause an end of the practical activity Students should be aware of the biohazards involved with the materials used in the lab Students should take due care with any sharp objects utilised in the lab, for example in the process of dissection Students should always behave in a professional manner during practical experiments; any deviation from this will bring an end to the practical session Specific health and safety guidance will be provided for all practicals, students should take due care to make sure they understand these before the practical starts 12
  13. 13. Equipment and Resources the essentials The Centre provides a certain amount of essential equipment, like lab coats and computer software, but there are certain things you need to provide yourself. These include: Basic Stationary (Pen, pencil (2H), ruler etc) A4 folder to keep your work in A4 ruled paper Scientific calculator £25 deposit to cover your textbooks These are the basic essentials for the day to day requirements of your classroom sessions 13
  14. 14. Reading List books and websites Course texts You will be provided with a textbook for the course and the book we are using is Nelson Thornes AQA AS Biology. There are however a range of books produced specifically for this course that will be available in the study centre and LRC. These include: Textbooks AS Biology for AQA produced by Michael Kent Oxford University Press Collins AS Biology for AQA - Collins AS and A2 Science by Keith Hirst, et al Revision Guides AS Level Biology AQA A Revision Guide by Richard Parsons CGP AQA Biology: Biology and Disease Unit 1 (Collins Student Support Materials) AS Biology for AQA Revision Guide Oxford University Press Other Practical Skills in Biology by Dr Allan Jones, Prof Rob Reed, and Dr Jonathan Weyers Websites This website is a wiki that is specific to your course, as in it is designed for sixth form centre students only. You will find past paper questions, study skills details and useful links here as well as pages on all the topics that you cover over the two years. This website is designed to be collaborative, which means you are expected to edit is as well. online biology service integrated with the chosen course textbook contains practice tests and course materials www. Sixth form centres virtual learning environment 14
  15. 15. Glossary of Terms Useful Biological Words This is only a small sample of the very science specific terms that you will come across regularly. A more comprehensive glossary including an exams glossary can be found at Term Definition Activation Energy Before a chemical reaction can take place, bonds must be broken. This requires energy. This activation energy is normally provided by heating the substances involved in the reaction. Enzymes reduce the amount of activation energy necessary, so reactions in living organisms can take place at relatively low temperatures. Active Transport A process that involves the movement of substances from where they are in a low concentration to where they are in a higher concentration; in other words, it involves the movement of substances against a concentration gradient. Active transport involves the use of specific carrier proteins in cell membranes. It also requires energy in the form of ATP. Antibody There are several different types of white blood cell. One of these is the lymphocyte. An antibody is a molecule produced by a B lymphocyte cell when it encounters a particular antigen. For example, the virus that causes measles has antigens on its surface. If a person gets measles, these antigens cause the lymphocytes to produce measles antibodies. The antibodies help the person to overcome the infection and recover. Apoptosis The process in which healthy animal cells die during the normal development of an organism. For example, there are millions of cells in the brain of a human embryo. During development apoptosis results in many of these cells dying even though they are quite healthy. The result of the death of these cells is the pattern of cells found in the adult brain. Asexual Reproduction Reproduction by any means which does not involve the fusion of gametes or sex cells. Asexual reproduction occurs when a bacterial cell divides into two new cells. The growth of new plants from tubers and bulbs also involves asexual reproduction. Bactericidal A term used to describe substances which kill bacteria. Killing is essential for sterilisation so antiseptics and disinfectants are bactericidal. Although some of the antibiotics used to combat disease are bactericidal, many are bacteriostatic. Bacteriostatic antibiotics prevent bacteria from multiplying and enable the body's natural defences to destroy those already present. Bronchiole One of the small airways in the lung that goes from the larger bronchi to the alveoli. 15
  16. 16. Term Definition Carcinogen A substance which will cause cancer. Many organic substances, such as those found in the tar in cigarette smoke, are carcinogens. They damage DNA. Cells in which the DNA is damaged may become cancerous. This is more likely when there is an inherited tendency to develop cancer. Cardiovascular Disease A disease that affects the heart or blood vessels. Cardiovascular diseases are the main causes of death in the UK and account for more than one in three deaths. The main forms of cardiovascular disease are coronary heart disease and stroke. Diastole The stage in the cardiac or heart cycle when the heart muscle relaxes. During this stage the heart is filling with blood. Discontinuous Variation Variation in which individuals fall into distinct categories. In peas, for example, plants are either tall or short. There are no intermediates. Discontinuous variation results from the genes that an organism inherits. Environment has little or no effect. DNA The molecule that forms the genetic material of all living organisms. Chemically, DNA consists of two polynucleotide chains forming a double helix. Each chain consists of a sugar-phosphate backbone. One of four nucleotide bases is attached to each sugar in this backbone. These bases are joined, adenine to thymine and cytosine to guanine, by hydrogen bonds. In an animal or plant cell, DNA is found in the chromosomes in the nucleus. There are also small amounts of DNA in the mitochondria and chloroplasts. Genes are sections of DNA that code for particular proteins. Endocytosis A process which involves the transport of large particles or fluids into cells. The cell surface membrane surrounds the particles concerned. A vesicle is pinched off from the membrane and moves into the cytoplasm of the cell. Enzyme Enzymes are proteins that speed up chemical reactions in living organisms. For instance, inside a typical cell, many different reactions are taking place. Each of these is catalysed by a specific enzyme. Without these enzymes, the reactions would take place very slowly at the temperatures inside cells. Epithelium Tissue which forms the outer surface of many animals. Epithelial cells also line the cavities of organs such as the gut and lungs. The epithelium consists of one or more layers of cells sitting on a basement membrane. These cells may be flat (squamous or pavement epithelium) or tall in shape (columnar epithelium). Fatty Acid Molecules containing a COOH group and a hydrocarbon chain. Some fatty acids have double bonds present between some of the carbon atoms in the hydrocarbon chain. These are known as unsaturated fatty acids. Saturated fatty acids have no double bonds in their hydrocarbon chain. Fatty acids with long hydrocarbon chains are important constituents of triglycerides and phospholipids. 16
  17. 17. Term Definition Fibrinogen Fibrinogen is a soluble protein found in the blood plasma. When an injury occurs, fibrinogen is converted to insoluble fibrin. This fibrin forms a mesh over the surface of the wound, which traps red blood cells forming a blood clot. Fickʼs Law A law which relates some of the factors affecting the rate of diffusion across a gas exchange surface. Fick's law states that rate of diffusion is proportional to: surface area x difference in concentration / thickness of gas exchange surface Surfaces over which there is a rapid rate of diffusion have adaptations which provide a large surface area and maintain a large difference in concentration. They are also very thin. Gamete A sex cell, e.g. ova and sperm. In animals and plants, gametes are haploid and each contains a single set of chromosomes. A special form of cell division, meiosis, takes place in their formation. Meiosis results in the production of gametes with half the number of chromosomes found in a body cell. Reproduction that involves the fusion of gametes is called sexual reproduction. Gene A piece of DNA which has a specific sequence of nucleotide bases. Each gene codes for a specific protein. An example of this in humans is the CF (cystic fibrosis) gene, which codes for the CFTR protein; this helps to transport chloride ions across cell membranes. An individual gene may have more than one form. These forms or alleles differ from each other in the sequences of their nucleotide bases and, as a result, produce slightly different proteins. Germ Cell A gamete, or a cell which is able to develop into a gamete. Germ-line gene therapy involves inserting genes into germ cells. Any change which resulted from the insertion of such a gene would be passed on to subsequent generations. Because of the possible effects of this, germ-line gene therapy is not permitted in humans. Glycolipid A molecule consisting of a lipid and a carbohydrate. They form part of the cell-surface membrane. They have similar functions to glycoproteins; for example, they help cells to bind to each other to form tissues. Guanine One of the nucleotide bases found in nucleic acids. When the two strands of nucleotides which make up a molecule of DNA come together, guanine always pairs with cytosine. The atoms of these two bases are arranged in such a way that three hydrogen bonds form between them. Heterotroph A method of nutrition in which an organism gains its nutrients by feeding on other organisms. The complex organic molecules in its food are broken down by enzymes into simpler soluble substances before being built up again to form the complex organic substances which the organism requires. Heterotrophs are the consumers in food chains. 17
  18. 18. Term Definition Homeostasis The maintenance of stable internal conditions. In a mammal, the concentration of glucose, ions and carbon dioxide in the blood, and body temperature are all maintained within a narrow range of optimum conditions. Each condition has a norm or set value. The mechanisms involved in maintaining this set value rely on negative feedback. This is a process where a departure from the set value is detected by receptors. These relay information to effectors which bring about a return to the norm value. Homologous Chromosomes Similar chromosomes, i.e. a paternal chromosome and maternal chromosome form a pair of chromosomes. Human diploid cells are made up of 22 pairs of homologous chromosomes and one pair of sex chromosomes. Homologous chromosomes will have the same sequence of genes and are capable of pairing with each other when a cell divides by meiosis. Homozygote An organism in which the alleles of a particular gene are identical to each other. For example, in cystic fibrosis, F represents the normal allele and f represents the mutant allele. There are two possible homozygotes. A person with the genotype FF does not have cystic fibrosis while a person with the genotype ff has cystic fibrosis. Hydrolysis A chemical reaction where larger molecules are broken down into smaller ones by a reaction with water. Hydrolysis is very important in digesting biologically important polymers. The reactions where proteins break down to form amino acids, and starch molecules break down to form maltose and glucose are examples of hydrolysis. Hydrolysis is splitting larger molecules by a reaction with water; condensation involves joining smaller molecules with the removal of water. Hydrophobic Water-repelling. In a phospholipid molecule the fatty acid part of the molecule repels water molecules, and the phosphate group attracts water molecules. These properties are important in the arrangement of phospholipids in cell membranes. The molecules form a double layer or bilayer, with the water-repelling fatty acids facing towards the inside, away from contact with water. The hydrophilic phosphate groups are on the outside, and in contact with watery fluids in the cytoplasm or outside the membrane. Try to use the terms hydrophobic and hydrophilic rather than 'water-hating' and 'water-loving'. Induced Fit Theory Model to explain the way in which an enzyme enables a substrate to participate in a chemical reaction. When the substrate enters the active site, the enzyme changes shape, fitting more closely around the substrate and speeding up the rate of reaction. Interspecific Between different species. Interspecific competition is competition between different species of organism. Weeds compete interspecifically with crop plants for resources such as water, light and mineral ions. Interspecific hybrids are made by crossing two species. For example, wheat and rye are two different species of cereal. Plant breeders have crossed these two species to produce an interspecific hybrid called Triticale. Triticale combines the high quality and yield of wheat with the resistance to fungal infection of rye. 18
  19. 19. Term Definition Lactose The main sugar found in milk. Lactose is a disaccharide. It is made up from the two monosaccharides, glucose and galactose, joined to each other by condensation. Lipid A group of substances found in all cells. Lipids do not dissolve in water but they are soluble in organic solvents such as ethanol. A number of different substances are classified as lipids. They include triglycerides, phospholipids and steroids. Lock and Key Theory Model to explain the way in which an enzyme (the lock) helps a substrate (the key) to participate in a chemical reaction. Lysosome An organelle containing digestive enzymes surrounded by a membrane. This membrane prevents the enzymes digesting the proteins and lipids in the cell. Lysosomes are involved in the breakdown of unwanted structures and in the destruction of old cells when they are replaced during development. Meoisis A type of nuclear division important in the formation of the gametes or sex cells in animals and plants. Meiosis results in the production of haploid cells each of which contains half the number of chromosomes found in a body cell, one chromosome from each homologous pair. The processes of independent assortment and crossing over which occur during meiosis contribute to genetic variation. Microorganism An organism that is too small to see without the aid of a microscope. (The fruiting bodies of fungi are an exception to this.) There are three main groups of microorganism. These are viruses, bacteria and fungi. Many microorganisms cause diseases but bacteria and fungi also play a very important role in decomposition and the recycling of substances such as carbon. Microorganisms are also called microbes. Monohybrid Inheritance Mode of genetic transmission in which a characteristic is determined by a single gene. Monosaccharide A carbohydrate that is made up of a single sugar unit. Monosccharides are small molecules which dissolve readily in water. They are classified according to how many carbon atoms they contain. Hexoses such as glucose contain six carbon atoms. Pentoses such as ribose contain five carbon atoms. Monosaccharides join together to form disaccharides and polysaccharides. Negative Feedback Many substances and systems in living organisms have a set level. This is true of the concentration of glucose in the blood and of body temperature. Negative feedback is the process whereby a departure from this set level sets in motion changes which return it to the original level. Nucleotide Nucleic acids are polymers made up from a number of nucleotides joined to each other by condensation. Each nucleotide has three components: a five-carbon or pentose sugar ribose in RNA and deoxyribose in DNA; a phosphate group; a nucleotide base. 19
  20. 20. Term Definition Organism A living thing. Organisms are divided into five kingdoms. Animals, plants, fungi, prokaryotes such as bacteria, and protoctists. Osmosis Osmosis is a special sort of diffusion. It is the movement of water from a weak solution with a low concentration of solute molecules, to a solution with a higher concentration of solute molecules, through a partially permeable membrane. Osmosis involves the movement of water molecules not solutions. Parasite An organism that lives in or on a host organism. The parasite gains an advantage from this relationship while the host suffers a disadvantage. Parasites of humans include malarial parasites and tapeworms, bacteria such as those which cause food poisoning, and viruses such the genital wart virus. Witchweed is a parasitic weed that infects crops such as maize in Africa. Pathogen A general term used to describe a microorganism that causes disease. Campylobacter and Helicobacter are examples of pathogenic bacteria. Viruses are also pathogens, as are the fungi which cause diseases such as athletes' foot and thrush. Phospholipid A phospholipid molecule is a lipid with two distinct sections. It has a head region consisting of glycerol and a phosphate group. This part of the molecule is attracted to water. The other end consists of two fatty acid tails. This end of the molecule repels water. Phospholipids are important components of cell membranes where they are arranged in a bilayer with the heads pointing outwards and the tails pointing towards each other. Protein A polypeptide is a molecule made from a large number of amino acids joined by condensation. This polypeptide, sometimes on its own, sometimes with others, is folded to form a protein. The shape of a particular protein is very important in determining its function. There are twenty different amino acids, and they can be combined in different ways to produce many different proteins. Reflex Arc The nerve pathway associated with a reflex. Some reflexes, such as the knee-jerk reflex, only involve two neurones, a sensory neurone and a motor neurone. Others, such as that involving the withdrawal of the hand after touching a hot object, involve three neurones. These are a sensory neurone, a relay neurone and a motor neurone. Surface Area The area (measured in units such as mm2 or m2) of a surface. E.g. the outside of an alveolus. Substrate In biochemical reactions, a substrate is the molecule on which an enzyme acts. The substrate of the enzyme amylase, for example, is starch while that of maltase is maltose. Enzymes are very specific in their actions. Only a substrate molecule with a particular shape will fit the active site of a particular enzyme. Transcription The process in which the genetic information contained in a DNA molecule is copied to produce messenger RNA. This is the first stage in protein synthesis. A molecule of DNA unwinds. The sequence of nucleotides on one of the strands, called the template strand, is used to produce a mRNA 20 molecule by complementary base-pairing.