AS and A2 Biology
Sixth Form Centre
Aiming For Excellence
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
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
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
Preparing food for
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
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
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
Learning Mentor Michael Debrah 5949
Teachers - Biology Teresa Greer 5944
Laura Nicholls 5944
Chemistry Michelle 5949
Zabed Ahmed 5943
Physics Mike O’Brien 5940
Maths Olu Kubweza 5943
Daniel Oladejo 5943
Sports Richard Jones 5885
Shane Bell- 5884
Technicians Anjna Vara 5946
Julie Nicholson 5946
units and assessment detail
Unit One: Biology and Disease (AS)
How digestive and gas exchange systems may be affected by communicable and
How a knowledge of basic biology allows us to understand the symptoms of
disease and interpret data relating to risk factors.
Written Paper: 1 Hour 15 Minutes
Five - seven short answer questions plus two longer questions (a short comprehension and
a short structure essay)
33% of total AS Level marks
16% of total A Level marks
Unit Two: The Variety of Living Organisms (AS)
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
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
46% of total AS Level marks
23% of total A Level marks
Unit three: Practical and Investigative Skills (AS)
Practical work in the contexts of units one and two
Assessment of implementing skills on practical work as a whole
20% of total AS Level marks
10% of total A Level marks
Unit four: Populations and Environment (A2)
How living things form ecosystems through which energy is transferred
How human activity affects ecological balance in a variety of ways
Written Paper: 1 Hour 30 Minutes
Six - nine short answer questions plus two longer questions (a short comprehension and a
short structured essay)
17% of total A Level marks
Unit five: Control in Cells and in Organisms (A2)
Stimulus and responses - the biology of nervous and endocrine systems
Homeostasis and the maintenance of a constant internal environment
Genes and genetic expression
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)
23% of total A Level marks
Unit six: Practical and Investigative Skills (A2)
Practical work in the contexts of units four and ﬁve
Assessment of implementing skills on practical work as a whole
10% of total A Level marks
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
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
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!
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.
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
basic year outline
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
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
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
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
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
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
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
Students will be expected to wash their hands before and after every practical
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
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
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
Equipment and Resources
The Centre provides a certain amount of essential equipment, like lab coats
software, but there are certain things you need to provide yourself.
Basic Stationary (Pen, pencil (2H), ruler etc)
A4 folder to keep your work in
A4 ruled paper
£25 deposit to cover your textbooks
These are the basic essentials for the day to day requirements of your
books and websites
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:
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
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
Practical Skills in Biology by Dr Allan Jones, Prof Rob Reed, and Dr Jonathan
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
Sixth form centres virtual learning environment
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
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
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
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
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
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
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
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
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
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.
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
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
Try to use the terms hydrophobic and hydrophilic rather than 'water-hating'
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
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
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
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
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
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.
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
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
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
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.