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Genetic Screening:

Genetic Screening:

  • Genetic screening is used to:
  • Identify carriers: heterozygotes with normal phenotypes. This can be followed up with counselling to help potential parents make a decision.
  • Embryo testing: a sample of cells from a developing foetus can be analysed. The sample is obtained either by amniocentesis (withdrawing amniotic fluid around 15-17 weeks of pregnancy) or by chorionic villus sampling (cells removed from the placenta at 8-12 weeks).
  • Both techniques carry a risk of miscarriage.
  • Pre-implantation genetic diagnosis: used to test an embryo created by IVF.
  • Pros and Cons of genetic screening:


Advantages of genetic testingDisadvantages of genetic testing
§  Can opt for termination.

§  Can get counselling.

§  Can buy special medical equipment / care in preparation for birth.

§  Can opt not to have children (if parents are tested.)

§  Utilitarian argument.

§  Abortion is morally wrong.

§  Tests can be inaccurate.

§  Small chance of test resulting in miscarriage.

§  Unnatural procedure.

§  Embryos have a right to life.

§  Embryos cannot give informed consent.


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  • DNA is a type of nucleic acid called deoxyribonucleic acid.
  • It is a long chain molecule made up of nucleotides.
  • One nucleotide is made up of:
  • A 5 carbon sugar
  • A phosphate group
  • An organic base
  • Nucleotides link together by condensation reactions between the sugar of one and the phosphate group of the other.
  • Each nucleotide in DNA has 1 of 4 different bases: Adenine, Guanine, Cytosine, or Thymine.
  • Two long polynucleotide strands, running in opposite directions, are held together by hydrogen bonds between the bases.
  • This ladder-like structure, with alternating sugar and phosphate molecules forming the uprights and pairs of bases forming the rungs, is then twisted in a helix.
  • The bases pair in a particular way, based on their shape and chemical structure:
  • A & T pair forming 2 hydrogen bonds
  • C & G pair forming 3 hydrogen bonds


  • RNA (ribonucleic acid) is made up a single strand of nucleotides. In these, the sugar is called ribose and the bases are adenine, guanine, cytosine, and uracil.
  • There are 3 types of RNA:
  • Messenger RNA (mRNA)
  • Transfer RNA (tRNA)
  • Ribosomal RNA (rRNA)


  • Protein Synthesis occurs in two stages
  • Transcription:
  • Takes place in nucleus
  • A complementary copy of the gene is made using RNA


  • Gene opens up. Hydrogen bonds break between bases.
  • RNA nucleotides attracted to complementary bases and form hydrogen bonds.
  • RNA nucleotides joined together by RNA Polymerase.
  • Complementary RNA copy of gene now made. It is called mRNA (messenger RNA).
  • mRNA molecule leaves nucleus through nuclear pore


  • Translation:
  • Occurs on the ribosomes of the rough endoplasmic reticulum
  • The beginning of the sequence is always marked with the start codon AUG which codes for the amino acid methionine
  • A transfer RNA molecule (tRNA) with 3 bases exposed (an anticodon) pairs with a specific codon on the mRNA
  • Attached to the tRNA molecule is a specific amino acid
  • The amino acids, arranged in the order dictated by the mRNA codons, are joined with peptide bonds to form a polypeptide
  • A stop codon signals the last amino acid in the polypeptide chain


Base triplets in DNA

Transcription (in the nucleus)

Codons in mRNA

Translation (on the ribosomes)

Amino acid sequence in polypeptide chain


  • The genetic code in the DNA making up the chromosomes acts as a code for protein synthesis.
  • It dictates the amino acids required to make the protein and the order in which they should be bonded together.
  • 3 bases code for 1 amino acid and these base triplets are non-overlapping.
  • The code is degenerate: there is more than 1 triplet for each amino acid.
  • A gene is a sequence of bases on a DNA molecule (a short section of a chromosome) coding for a sequence of amino acids in a polypeptide chain.


  • DNA copying or replication must occur before a cell divides to ensure that daughter cells receive a copy of the genetic code.
  • DNA double helix unwinds
  • Hydrogen bonds between the base pairs break
  • Free DNA nucleotides line up along side each strand
  • Hydrogen bonds form between complementary bases
  • DNA polymerase links adjacent nucleotides
  • 2 identical DNA double helices are formed by this semi-conservative replication
  • Original DNA, all heavy ∴ DNA band at bottom of centrifuge
  • 1st generation DNA, ½ old, ½ new ∴ DNA band in middle of centrifuge
  • 2nd generation DNA, some ½ and ½ (forms one band at top) & some all new ∴ second band in the middle of centrifuge.


  • Sometimes, the DNA replication does not work perfectly – an incorrect base may slip into place. This is called a gene mutation.
  • If this occurs in a sperm or ovum, which ultimately forms a zygote, every cell in the new organism will carry the mutation.
  • If the mutation occurs in non-coding DNA, it will have no effect.
  • In a gene, it will cause an error in the mRNA and an incorrect amino acid may be included in the polypeptide chain causing a genetic disorder e.g. sickle cell anaemia.
  • A number of different mutations can affect the gene coding for the cystic fibrosis transmembrane regulatory (CFTR) protein channels, which allow chloride ions to pass through the membrane.
  • The most common mutation is a deletion of 3 nucleotides.
  • The altered protein may not open, or may reduce the flow of chloride ions through the channel.


  • Human cells contain 23 pairs of homologous chromosomes. At a particular position/locus on each of the pair is found a gene for a particular characteristic.
  • Different forms of the same gene are called alleles. If a cell contains two copies of an allele, their genotype is described as homozygous. Different alleles at a locus result in a heterozygous
  • The characteristic resulting from the genotype is the organism’s phenotype.
  • A recessive allele (represented by a small case letter) is only expressed in the homozygous condition.
  • A dominant allele (represented by the same letter in the upper case) will be expressed in the phenotype in either the homozygous or heterozygous condition.
  • In humans, recessive mutations of single genes result in:
  • Cystic fibrosis: mucus that is too viscous.

It affects…

  • Lungs:
  • The amount of water in the mucus produced must be regulated:
  • Too runny and it floods the airway
  • Too viscous (sticky) and it can’t be cleared by the cilia
  • This is controlled by the transport of sodium and chloride ions across the epithelial cells.
  • Water follows the ions because of osmosis.
  • Summary:
  • The CFTR channel is non-functional, so chloride ions cannot pass out of the cell towards the lumen.
  • The sodium ion channels are open and sodium ions are continually absorbed from the mucus.
  • Water is drawn out of the mucus by osmosis and it becomes much too viscous.
  • The cilia cannot move the viscous mucus – it builds up in the airway and becomes infected.
  • Because of low oxygen levels in the mucus, anaerobic bacteria thrive.
  • White blood cells invade the mucus, then die and release DNA making it even more viscous.
  • Mucus blocks the bronchioles, reducing the number of ventilated alveoli. This reduces the efficiency of gas exchange.


  • Digestive System:
  • The viscous mucus blocks the pancreatic duct.
  • Enzymes are not released into the small intestine and food is therefore not digested effectively. Undigested food cannot be absorbed and energy is lost in the faeces (mal-absorption syndrome).


  • Reproductive System:
  • In females, a mucus plug blocks the cervix
  • In males, the vas deferens leading from the testes is either blocked or missing
  • Thalassemia: abnormal haemoglobin formation.
  • Albinism: lack of pigment production.
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  • Are globular proteins which act as catalysts. They speed up chemical reactions by lowering the activation energy, and remain unchanged at the end of the reaction.
  • They provide an alternate reaction pathway, which requires less energy to start.
  • Part of the molecule is a specifically shaped active site, into which a substrate fits to form an enzyme-substrate complex.
  • The lock and key hypothesis suggested an exact match between the shapes of the substrate and active site. How does it occur?
  • Substrate diffuses into the active site
  • Substrate binds to the active site
  • Bonds in the substrate are broken as a result
  • Products form and unbind from the active site and diffuse out of the active site
  • The induced fit hypothesis describes the active site moulding around the substrate once it is in place.
  • Limitations of enzymes:
  • An increase in temperature (and thus an increase in the kinetic energy of the molecules) increases the chances of a collision between enzyme and substrate molecules. The rate of reaction increases.
  • Beyond the optimum temperature, the increased vibrations of the atoms in the protein molecule break the bonds maintaining the tertiary structure. The active site of the enzyme is irreversibly denatured.
  • pH changes around the enzymes optimum pH alter the charge distribution in the active site, reducing the compatibility of enzyme and substrate. Tertiary structure bonds are again affected and extreme changes will denature the enzyme.
  • An increase in either substrate or enzyme concentration will increase the rate of reaction until the other acts as a limiting factor.
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  • Structure of an amino acid:
  • The amino acid monomers join together in a condensation reaction to form peptide bonds. The polymer formed is called a polypeptide.
  • How do they form?
  • Primary structure: the sequence of amino acids in the polypeptide chain. Amino acids are connected by peptide bonds. Most proteins do not function in their primary form.
  • Secondary structure: the shape the molecule folds because of hydrogen bonding between the C=O of one amino acid and the N-H of the amine group of another – an  helix or a  pleated sheet.
  • Tertiary structure: the final 3D shape of the molecule, held together by ionic bonds, interactions between hydrophilic R groups and strong disulphide bridges between R groups containing sulphur.
  • Quaternary structure: if the protein contains more than one polypeptide chain.
  • Fibrous proteins remain as long chains, often with several polypeptides cross-linked for extra strength. They are insoluble and are important structural molecules e.g. keratin, collagen.
  • Globular proteins are folded into a compact spherical shape. They are soluble and are important metabolic moleculesg. enzymes, antibodies, and some hormones.
  • The specific sequence of specific amino acids determines the shape of the protein and, therefore, its function.
  • Test for proteins:
  • Biuret solution turns blue to purple/lilac in the presence of protein.


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Movement across the plasma membrane:

Movement across the plasma membrane:

  • Osmosis: the movement of water molecules from high concentration to low concentration through a partially permeable membrane.
  • Diffusion: the movement of molecules or ions from an area of their high concentration to an area of their low concentration.
  • It will continue until the substance is evenly distributed throughout the whole volume.
  • Small-uncharged molecules e.g. oxygen and carbon dioxide can diffuse across the cell membrane.
  • Hydrophilic molecules and ions cannot penetrate the hydrophobic phospholipid tails.
  • Diffusion is made easier, or facilitated, by proteins:
  • Channel proteins span the membrane and have a specific shape to transport specific particles. Some are gated – they can be open or closed.
  • Carrier proteins bind with the molecule or ion, change shape and transport the particle across the membrane. Movement can occur in either direction, depending on the concentration gradient.
  • Diffusion, facilitated diffusion, and osmosis are passive – they do not require energy.
  • Active transport: ATP supplies energy to change the shape of a carrier protein molecule when substances are moved against the concentration gradient i.e. from low to high concentration.
  • Exocytosis involves the bulk transport of substances out of the cell e.g. insulin into the blood.
  • Vesicles (little membrane sacs) fuse with the cell surface membrane and the contents are released.
  • Endocytosis is the reverse: substances are taken into a cell by the creation of a vesicle.


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Cell Membrane:

Cell Membrane:

  • Cell membrane is made up of a phospholipid bilayer.
  • The phosphate head of the phospholipid is a polar molecule and attracts water – it is hydrophilic.
  • The fatty acid tails are non-polar and hydrophobic.
  • In the cell membrane, the hydrophobic tails face inwards to avoid water, while the hydrophilic heads point outwards.
  • In the phospholipid bilayer are other molecules:
  • Proteins: some are fixed, while others move around. May be enzymes, carriers, or channels.
  • Cholesterol: reduces the fluidity of membrane by preventing movement of phospholipids.
  • Glycoproteins: (polysaccharide + protein) cell recognition and receptors
  • Glycolipids: (polysaccharide + lipid) cell recognition and receptors
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Heart and Circulation:

Heart and Circulation:

  • In small unicellular organisms, substances move around slowly by diffusion.
  • Diffusion is too slow to move substances round the larger bodies of multicellular organisms.
  • Thus has a circulatory system: substances are carried in blood pumped by a heart.
  • There are 3 types of circulatory system:
  1. Closed circulatory system (e.g. in vertebrates) blood is enclosed in narrow blood vessels. This increases efficiency: blood travels faster as a higher pressure is generated.
  2. Single circulation: (e.g. fish) heart pumps blood to gills for gas exchange, then to tissues and back to the heart.
  3. Double circulation: (e.g. birds and mammals) right ventricle pumps blood to lungs. Blood returns to the left atrium and then the left ventricle pumps it to the rest of the body. Blood travels round the body faster, delivering nutrients faster, so the animals have a higher metabolic rate.
  • Valves ensure blood flows in one direction.



  • The heart:




  • The cardiac cycle:


Phase Detail
Atrial systole§  Pressure in the atria increases as they fill with blood returning from the veins.

§  Increased pressure opens the atrioventricular valves allowing blood to enter the ventricles.

§  The atria contract to force remaining blood into ventricles.

Ventricular systole§  Ventricles contract from the base up, increasing the pressure and closing the atrioventricular valves.

§  The semilunar valves open and blood is forced into the arteries.

Diastole§  As the atria and ventricles relax, pressure falls.

§  In the ventricle, this causes closure of the semilunar valves.

§  In the atria, blood is drawn into the heart from the veins.



  • The initiation and conduction pathways of the heartbeat:
  • Cardiac muscle contracts without being stimulated by a nerve impulse.
  • The electrical charge in the heart muscle cells changes – depolarisation. This spreads from cell to cell (like a wave) causing them to contract.
  • Depolarisation starts in the Sino atrial node or SAN (pacemaker) in the right atrium and spreads across the left and right atria causing them to contract.
  • The atria are electrically insulated from the ventricles so the wave of depolarisation converges on the atrioventricular node (AVN).
  • It then travels down the Bundle of His in the septum and into the Purkyne fibres, which then make the ventricles contract from the bottom upwards pushing blood into the aorta and pulmonary artery.
  • When the cells are depolarised, there is a small electrical current detectable on the skin.
  • This is measured in an electrocardiogram or ECG, which can be used to diagnose cardiovascular disease, problems with the conducting system or irregular heartbeat rhythms (arrhythmias).






  • Blood Vessels:
 ImageStructure & Function

Carry high-pressure blood away from the heart.

 – Thick muscle layer to withstand high-pressure blood.

– Elastic tissue allows artery to stretch when blood is forced into it. The elastic layer recoils during diastole, so there is a continuous blood flow.

– They have a protective collagen layer.

– They have a round shape.

– They have a relatively small lumen.

– They have no valves.


Carry low-pressure blood towards the heart.

 – Thin muscle layer (low pressure blood).

– They have valves to stop backflow.

– Have a protective collagen layer.

– Not a round shape (wall not thick enough to hold shape).

– Large lumen (decreases effect of friction).


Adapted for exchange of substances.

 – Walls are one cell thick (cells are called endothelial cells).

– Lumen is the same width as one RBC (therefore more of RBC in contact with wall, therefore smaller diffusion distance).

– No muscle or elastic tissue.

– They are very small.



  • Atherosclerosis:
  • A disease where fatty deposits block an artery or increase its chances of being blocked by a blood clot (thrombosis).
  • It occurs…

  • After atherosclerosis has developed there is a chance that a blood clot might form in the damaged area.



  • Clot Formation:

It occurs when…

  • The damaged tissue in the blood vessel wall releases thromboplastin.
  • Blood contain platelets. When they contact the damaged tissue, they are activated. They become sticky and release calcium ions.
  • Blood plasma contains prothrombin. Thromboplastin causes prothrombin to change to an enzyme – thrombin.
  • Blood plasma also contains fibrinogen. In the presence of calcium ions, thrombin causes fibrinogen to change to fibrin.
  • Fibrin precipitates to form long fibres. Platelets and red blood cells get tangled in the fibres.


  • Risk Factors of Cardiovascular Disease:


GeneticRisk is increased if your parents have CVD.


– Some vitamins act as antioxidants, reducing the damaging effects of free radicals.

– High salt levels cause the kidneys to retain water, increasing blood pressure.

AgeMore likely as you get older.
GenderIncidence is much higher for men than women.
High Blood Pressure 


– Carbon monoxide prevents haemoglobin from carrying sufficient O2 – heart rate increases.

– Nicotine stimulates adrenaline release, increasing heart rate and blood pressure.

– Chemicals damage endothelium, triggering atherosclerosis.

– Decreased levels of HDLs.



– Most common risk factor.

– Exercise can halve the risk of developing CHD.

– Reduces blood pressure.

StressLeads to increased blood pressure, poor diet and increased alcohol consumption.


– Heavy drinkers have an increased risk of CHD as alcohol raises blood pressure, contributes to obesity, and causes irregular heartbeat.

– It also increases levels of LDLs.

– Moderate amounts of alcohol may increase HDL levels.


  • Treatments for Cardiovascular Disease:
TreatmentWhat it does?BenefitsRisks
Antihypertensive– Reduce blood pressure by affecting the nervous system.

– Stop the muscles in blood vessel walls to contract.

– Blood pressure is reduced.– If dosage is not correct, blood pressure may become too low.
Plant statins– Inhibit an enzyme in the liver that catalyzes a reaction involved in producing cholesterol.– Levels of cholesterol are lowered.– Liver failure
Anticoagulants– Inhibit blood clotting.– Reduced risk of forming a blood clot in the circulatory system.– Blood clots slowly or not when it should.
Platelet inhibitory– Reduce the activation of platelets (become less sticky).– Reduced risk of forming a blood clot in the circulatory system.– Aspirin affects the stomach wall, thus the risk of stomach ulcers developing increases (for people with a tendency to this condition).





  • Cholesterol:
  • Insoluble cholesterol is transported combined with proteins to form soluble lipoproteins.


High-density lipoproteins or HDLsContain more protein and transport unsaturated fats to the liver where they are broken down.– Reduce blood cholesterol deposition



Low-density lipoproteins or LDLs

(The main blood cholesterol carriers)

Associated with saturated fats.


– Overload membrane receptors and reduce cholesterol absorption from the blood.

– Associated with the formation of atherosclerotic plaques.


  • Saturated fats also reduce the activity of LDL membrane receptors and therefore increase blood cholesterol levels.
  • Eating both monounsaturated and polyunsaturated fats reduces the level of LDLs in the blood.




  • Determining Risks:
  • Risk is the probability of occurrence of some unwanted event or outcome.
  • A time period is always quoted.
  • Not all individuals are at risk to the same degree.
  • Risk factors increase the chance of the harmful outcome.
  • Factors that contribute to health risks include:
  • Heredity
  • Physical environment
  • Social environment
  • Lifestyle and behaviour choices
  • Two factors are positively correlated if an increase in one is accompanied by an increase in the other.
  • A positive correlation does not necessarily mean that the two are causally linked!


  • People’s behaviour is affected by the perception of risk.
  • They overestimate the risk of something happening if the risk is not under their control, unnatural, unfamiliar, dreaded, unfair, or very small.
  • There is a tendency to underestimate the risk if it has an effect in the long-term future.



  • Body Mass Index:

BMI = Mass / Height2

  • Your energy budget balances the number of calories you require with those that you consume. They should be the same.
  • Energy consumed > energy expended -> mass gain.
  • Energy consumed < energy expended -> mass lost.



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  • Triglycerides are either fats or oils. They are made from the elements carbon, hydrogen, and oxygen only.
  • They are insoluble in water.
  • Triglycerides are used for:
  1. Long term energy storage molecules
  2. Insulation
  3. Protection (pericardium)
  4. Buoyancy
  5. Synthesis of specific hormones (steroids)


  • Triglycerides are formed in condensation reactions between 1 glycerol and 3 fatty acids.
  • An ester bond forms between the fatty acid and the glycerol.
  • Saturated fatty acids contain the maximum number of hydrogen atoms and no carbon-carbon double bonds. They are found in animal fats and dairy products.
  • Monounsaturated fats contain 1 double bond, for example olive oil.
  • Polyunsaturated fats contain a larger number of double bonds, for example, vegetables and fish oils.
  • If one of the fatty acids in a triglyceride is replaced with a phosphate group, a phospholipid is formed. These molecules make up part of the cell membrane.
  • Cholesterol is a short lipid molecule with a structure very different to a triglyceride. It is important for cell membranes, sex hormones, and bile salts. Found in food, associated with saturated fats.
  • The C=C bonds form ‘kinks’ in the fatty acid chains, pushing adjacent triglycerides away from each other. This lowers the effect of intermolecular forces, lowering the boiling and melting temperatures.


  • Tests for triglycerides:
  1. Add ethanol (dissolves fat)
  2. Add water.
  3. A white precipitate indicates a positive result.
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  • They have a general formula: Cx(H2O)n
  • Saccharides are made from sugar molecules that are made from combinations of the elements carbon, hydrogen, and oxygen only.
  • They are used for:
  1. Fuels for respiration (glucose).
  2. Energy storage molecules (starch and glycogen).
  3. Structural molecules (cellulose).



– Single sugar units–  glucose

NB: there are 2 types of glucose: alpha and beta

– Used in respiration
– Fructose– Found in fruit & honey
– Galactose– Found in lactose
(All the above are hexose sugars: C6H12O6)
Disaccharides– 2 single sugar units joined by a glycosidic bond.– Maltose

(2 glucose molecules)

– Found in germinating seeds e.g. barley
– Sucrose

(Glucose and fructose)

– Crystals used in cooking
– Lactose

(Glucose and galactose)

– Sugar found in milk
Oligosaccharides– 3-10 sugar units.Found in vegetables e.g. leeks, lentils, beans

Polysaccharides (polymers) are long chains of glucose molecules.

  • Glycogen:
  • Found in muscle and liver cells for energy storage.
  • Made of poly alpha glucose linked together.
  • Insoluble because it has 1,4 and some 1,6 links, which form branches in the chain.
  • Very compacted, thus good for storage.
  • Starch:
  • Found in amyloplasts (starch grains) inside plant cells for energy storage.
  • Made of 2 types of molecules: amylose and amylopectin.
  • Amylose molecule is a very long chain of glucose molecules with 1,4 links.
  • Amylopectin is similar to glycogen.
  • Insoluble and very compact.
  • Cellulose:
  • Made from poly beta glucose
  • Main component of cell walls as it is a very strong structural molecule.
  • Cellulose has no branches, so adjacent cellulose chains line up close.
  • Hydrogen bonds between adjacent chains, creating very strong cellulose fibrils.


  • Saccharides link together by condensation reactions, producing water. A glycosidic bond forms between the saccharide molecules.
  • The opposite of a condensation reaction is hydrolysis. It requires water.
  • Tests for saccharides:
  • Iodine solution turns brown to blue/black in the presence of starch.
  • Benedict’s solution turns blue to brick red in the presence of a reducing sugar.
  • Non-reducing sugars (disaccharides and polysaccharides) will give a positive result to Benedict’s if heated in acid first.
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  • Water molecules are non-linear and polar molecules because the H and O atoms are different in size and electronegativity.
  • They form hydrogen bonds with other water molecules.
  • Some properties include…



Excellent solventEssential role in transport of molecules in biological systems.
Less dense as a solidIce insulates water beneath it, protecting marine life from freezing.
High SHCCells do not heat up or cool down easily, thus has a homeostatic/stable temperature.
TransparentAllows marine life to exist as photosynthesis can occur underwater as well as gas exchange.
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