SNAB Topic 2 - Genes and Health


Gas Exchange

  • Gas exchange surfaces are surfaces which allow the changing of gases to be used for chemical reactions e.g. Respiration
  • Gas Exchange Surfaces have:
  • Large SA:V ratios
  • Thin Exchange Surface
  • Steep Concentration Gradients maintained
  • The rate of diffusion can be expressed in Fick's Law

Rate of Diffusion is proportional to the Surface Area x Difference in Concentration / Thickness of Gas Exchange Surface

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The Lungs

  • The Lungs are where gas exchange occurs in mammals and other animals
  • They form part of the Respiratory System

Image result for the lungs a level biology

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  • Rapid diffusion occurs due to:
  • Thin Exchange Surface (1 Cell thick)
  • Constant Air Supply maintains concentration gradient
  • Rich Capillary Network increases Surface Area

Image result for the lungs a level biology

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Cell Membrane Structure

  • The Cell Membrane contains the cell, maintains the osmotic balance, controls the movement of substances across the membrane and allows cell recognition to occur
  • The Cell Membrane is described as a fluid mosaic model as it is freely moving with proteins interdispersed within the phospholipid bilayer
  • Cell Membranes are: Flexible, fluid and selectively permeable
  • The Cell Membrane consists of:
  • Phospholipid Bilayer: Lipids which have two fatty acid tails and a phosphate head resulting in hydrophilic heads and hydrophobic tails meaning the phospholipids align in a bilayer
  • Channel and Carrier Proteins: Transporting of substances across the membrane
  • Glycolipids: Carbohydrates attached to the phospholipid bilayer for cell recognition
  • Glycoproteins: Carbohydrates attached to the proteins in the membrane for cell recognition
  • Cholesterol: Regulates membrane fluidity
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Fluid Mosaic Model

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Evidence for Fluid Mosaic Model

  • Phospholipids are both hydrophilic and hydrophobic so form bilayers in aqueous solutions
  • A monolayer film of phospholipids is twice as large as the cell surface area
  • Microscope Images of cell surfaces show proteins sticking out
  • When lectins, which react with carbohydrates, are added to a membrane they are found only on the outside
  • Some water-soluble substances pass into and out of cells
  • Ionic and Polar Molecules do not pass easily through membranes, but lipid soluble substances do
  • Techniques such as Electron Microscopy and the use of Radioactive Isotopes were used to develope the model
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  • Diffusion is the movement of particles along a concentration gradient (from a high concentration to a low concentration) across a membrane passively
  • Form of Passive Movement
  • Diffusion occurs until an equilibrium is reached
  • Particles move across the membrane in both directions but if the concentration is higher on one side of the membrane there will be a net movement along the concentration gradient until the net diffusion reaches zero.
  • Small uncharged molecules e.g. Oxygen can pass through the membrane
  • Larger or Charged Particles have to be transported through proteins
  • Diffusion through these proteins is called Facillitated Diffusion
  • It can occur in Channel Proteins and Carrier Proteins
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  • The movement of water molecules from a concentration of high water concentration to a low water concentration (Along a concentration gradient) across a partially permeable membrane, passively

Image result for osmosis

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Active Transport

  • Active Transport is the movement of particles from a low concentration to a high concentration across a membrane, actively using a carrier protein (Against a concentration gradient)
  • The molecules require energy in the form of ATP
  • Energy is released during a hydrolysis reaction breaking the bond between the second and third phosphate on a molecule of ATP

Image result for active transport diagramImage result for atp

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  • Endocytosis is the movement of large molecules into a cell using vesicles e.g. Phagocytosis
  • Exocyotis is the movement of large molecules out of a cell using vesicles e.g. secretion of proteins, hormones and neurotransmitters

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  • Place 5 test tubes of equal volumes of water in 8 water baths set at different temperatures (0C, 10C, 20C, 30C, 40C, 50C, 60C and 70C) for 5 minutes
  • Cut equal sized pieces of beetroot from the same plant
  • Rinse under running water to remove the excess pigment on the surface of the pieces
  • Place the pieces in the 8 tubes of distilled water
  • Leave the beetroot pieces in the tube for 5 minutes
  • Remove the pieces of beetroot carefully avoiding releasing any pigment
  • Pipette the liquid from each tube into cuvettes
  • Use a colorimeter to determine light absorption of the water
  • Plot the results on a graph: (X - temp, Y - transmission of light)


  • Little light absorption below 40C as the membrane is intact
  • Increase in leakage as the membrane becomes more fluid around 55C and proteins denature
  • Similar results for concentration of alcohol or solvents
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DNA Structure

  • Deoxyribonucleic Acid (DNA) consists of a Phosphate, Deoxyribose Sugar and a Nitrogenous Organic Base (A, T, C or G)
  • These components join together in condensation reactions to form a DNA Nucleotide
  • DNA Nucleotides join together in a condesation reaction which forms a Phosphodiester Bond. Multiple nuclotides form the Sugar-Phosphate Backbone

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RNA Structure

  • Ribonucleic Acid (RNA) is a single stranded nucleic acid
  • Unlike DNA it has Ribose Sugar and Uracil replaces Thymine
  • There are three types of RNA
  • Messenger RNA (mRNA) transports the genetic information to the ribosomes
  • Transfer RNA (tRNA) carry specific amino acids for use in translation
  • Ribosomal RNA (rRNA) joins amino acids and forms part of the ribosome

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  • Transcription produces an mRNA Strand from the genetic code on DNA
  • DNA Helicase breaks the Hydrogen Bonds between the DNA strands, unwinding the Double Helix
  • RNA Nucleotides align alongside their complementary bases on the Template Strand
  • Condensation reactions bind the RNA nucleotides together with Phosphodiester Bonds
  • The Reaction is catalysed by the enzyme RNA Polymerase
  • Once the stop codon is transcibed, the mRNA produced disociates from the DNA Template Strand
  • The mRNA leaves the nucleus via a nuclear pore and enters the cytoplasm
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Transcription Diagram

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  • Translation is the formation of a polypeptide chain from the mRNA strand
  • mRNA associates with a ribsome
  • tRNA carries specific amino acids to the ribosomes
  • The anticodon on each tRNA complimentary binds to the codon on the mRNA
  • A second tRNA complimentary binds to the next codon
  • A peptide bond forms between the two specific amino acids on the tRNA during a condensation reaction
  • The ribosome shifts along and the first tRNA disosociates from the mRNA strand
  • The process continues until a stop codon is coded for
  • The polypeptide chain then can be folded into a protein
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Translation Diagram

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Genetic Code

  • The genetic code carried by DNA is a codon or 3 bases which code for an amino acid
  • The genetic code also consists of start and stop codons
  • The genetic code is non-overlapping (codons are only coded for once)
  • There are 64 possible three base combinations
  • Many amino acids are coded for by multiple codes (Degenerate)
  • This means that mutations in the codons aren't as severe as it reduces the likelihood of an incorrect amino acid being coded for
  • 11 amino acids can be synthesised by the body whilst 9 (Essential Amino Acids) can't be produced so to be used in proteins these have to be consumed
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Amino Acids

  • Monomers of Proteins
  • Consist of: an amine group (NH2), carboxyl group (COOH) and a variable group (R)
  • Hydrogen atoms can disosociate from the carboxyl group and can bind to amine group to create a dipole
  • Amino Acids can bind together in condensation reactions to form peptide bonds
  • Multiple amino acids bonded together form polypeptide chains which can form proteins

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  • Proteins are formed from polypeptide chains of amino acids specifically folded
  • Primary Structure: The specific sequence of amino acids in a polypeptide chain
  • Secondary Structure: The formation of α-helixes and β-pleated sheets by the use of hydrogen bonds to coil/fold the polypeptide chain
  • Tertiary Structure: The formation of a specific 3D shape by the use of ionic, hydrogen and disulfide bonds as well as hydrophilic/hydrophobic interactions
  • Quaternary Structure: The addition of multiple proteins to form a more complex 3D structure
  • Conjugated Protein: A protein that has inorganic (non-protein based) components in its structure
  • Two types: Globular and Fibrous

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  • Haemoglobin is a globular protein made up of four polypeptides and four haem groups (Prosthetic groups) which form a conjugated protein
  • Haemoglobin is used to transport oxygen around the body by allowing it to bind to the haem groups
  • Haemoglobin is specialised for its function as the four haem groups allow four times the amount of oxygen to be transported around the body

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  • Haemoglobin is a globular protein made up of four polypeptides and four haem groups (Prosthetic groups) which form a conjugated protein
  • Haemoglobin is used to transport oxygen around the body by allowing it to bind to the haem groups
  • Haemoglobin is specialised for its function as the four haem groups allow four times the amount of oxygen to be transported around the body

Image result for haemoglobin

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  • Collagen is an insoluble fibrous protein formed of coiled proteins held together by hydrogen bonds
  • The triple helix structure creates a strong structure due to cross linking of the chains at staggered points so there are no weak points
  • This creates a strong structural protein for the body
  • Another fibrous protein is Keratin which forms nails, hair and skin

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  • Enzymes are globular proteins which act as biological catalysts by reducing the activation energy of a reaction
  • They have a specific active site which allows certain substrates to bind to them
  • Their specific 3D shape is due to the folding in the Tertiary Structure
  • Intracellular enzymes catalyse reactions Inside the cell e.g. DNA Polymerase
  • Extracellular enzymes catalyse reaction outside the cell e.g. Amylase
  • The Lock and Key Model suggests that substrates fit into the active site (like a key into a lock) due to different charges in the active sites which aid the bonding/splitting of the substrates
  • The Induced Fit Model suggests that the enzyme slightly changes its shape to allow the substrate to fit into the active site
  • Enzymes lower the activation energy by providing an alternative pathway for the reactions to occur along, as well as providing a more favourable pH in the active site and putting strain/bring reactants closer together to speed up the rate of reaction
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Rates of Reaction

  • The activation energy is the minimum amount of energy required to start a reaction
  • Reactions can be sped up by increasing the energy (which isn't as efficient) or by lowering the activation energy.
  • Increasing the amount of enzymes, increases the initial rate of reaction (The rate of the reaction before the concentration of substrates becomes the limiting factor) because there are more active sites for the substrates to bind to
  • The Initial Rate of Reaction is calculated by dividing the change in y values by the change in the x values (the Gradient)

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  • Set up a conical flask containing a controlled amount of hydrogen peroxide and buffer solution and place in a water bath set up at 20C to control the temperature
  • Set up a gas syringe with a bung that fits the conical flask
  • Cut a piece of potato (or measure a volume of catalase)
  • Place the potato/catalase in the conical flask and quickly attach the bung
  • Record the gas produced at regular intervals
  • Repeat with the same amount of catalase for 3 times
  • Repeat with 5 different amounts of catalase for 3 times each
  • Plot the results on a graph and determine the initial rates of reaction
  • The results should show an increase in initial rate of reaction, the more enzyme is added
  • Ensure Goggles and Gloves are worn as Hydrogen Peroxide is dangerous
  • The experiment can also be repeated by controlling the amount of enzyme and changing either the temperature, pH or substrate concentrations
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DNA Replication

  • DNA is able to replicate to produce identical copies of DNA by Semi-conservative Replication
  • DNA Helicase causes the DNA strands unwind and split apart as the hydrogen bonds break
  • DNA nucleotides pair up with their complimentary bases
  • DNA Polymerase catalyses the condensation reactions which link the adjacent DNA nucleotides together
  • DNA Ligase joins partly formed strands together
  • Hydrogen bonds reform between the bases on the newly formed strands of DNA
  • Two identical daughter strands are created
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DNA Replication Diagram

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Meselson and Stahl

  • Conservative Replication: When one new double helix is made and the old one remains intact
  • Dispersive Replication: When the new molecules are a mix of new and old parts
  • Semi-Conservative Replication: When each molecule has one new strand and one old strand
  • Meselson and Stahl proved that DNA replication is Semi-Conservative
  • Bacteria were cultured in a medum containing N15 before being moved into a medium of N14
  • The bacteria were allowed to replicate once before being extracted and centrifuged
  • The results produced a band that sat in the middle of the test tube
  • This removed the possibility of Conservative Replication as the results should have shown one heavy and one light band instead of one band in the middle
  • The bacteria were allowed to grow again in the N14 for one replication before being centrifuged
  • The results showed a medium and a light band
  • This removed the possibility of Dispersive Replication as the DNA would be expected to sit in the middle as there would be a mixture of both old and new DNA
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Meselson and Stahl Experiment

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DNA Mutations

  • Mutations are mistakes in the new base sequence of DNA
  • Addition Mutations are where extra bases are added into the base sequence
  • Deletition Mutations are where bases are removed from the base sequence
  • Substitution Mutations are where bases are changed from their original form
  • Mutations during DNA replication for meiosis results in mutations being passed onto offspring
  • Transcription creates new mRNA. In body cells, this can result in cancers
  • Translation creates proteins which may become faulty if the amino acids are translated incorrectly resulting in functioning not occuring properly
  • Mutations during DNA replication have the biggest effect as they are passed onto new cells
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Mucus Regulation

  • Mucus regulation maintains the salinity and thus fluidity of the mucus in body
  • Cl- is pumped out into the cell across the basal membrane
  • Cl- diffuses through the open CFTR channels into the mucus via the apical membrane
  • Na+ diffuses down the electrical gradient into the mucus
  • Elevated salt concentration in the mucus draws water out of the cell by osmosis
  • Water is drawn into the cell by osmosis to restore the osmotic balance
  • This helps to prevent the mucus becoming to viscous when there is too little water in the mucus
  • In Cystic Fibrosis Sufferers, the CFTR channel is non-functional
  • This means the ENaC (Sodium) Channel is permanently open
  • This causes Na+ to be continually removed from the mucus
  • This causes Cl- to diffuse down an electrical gradient into the tissue fluid
  • Water is drawn out of the cell from the basal membrane via osmosis
  • Water is then removed from the mucus to restore the water lost via osmosis
  • This results in sticky thick mucus that can't be removed easily
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Cystic Fibrosis

Respiratory System

  • Mucus accumulates in the lungs
  • bacteria trapped in the mucus increase the risk of infection
  • Mucus blocks bronchioles reducing the flow of air to the alveol which reduces gas exchange

Digestive System

  • Mucus blocks the pancreatic duct so digestive enzymes can't be secreted into the duodenum
  • Food is therefore not properly digested which leads to tiredness and difficulty gaining weight
  • Enzymes trapped within the pancrease cause fibrosed cysts and can digest pancreatic cells
  • This results in damage to insulin producing cells leading to diabetes

Reproductive System

  • In men the Vas Deferens (Sperm Duct) is blocked or missing so sperm can't leave the testes
  • In women mucus can block the cervix so sperm can't reach the ovum
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Genetic Terminology

  • Gene: A sequence of bases on a DNA molecule that codes for a sequence of amino acids in a polypeptide chain
  • Allele: A different version of the same gene
  • Recessive Allele: An allele which is only expressed when both alleles are present
  • Dominant Allele: An allele which is always expressed when at least one allele is present
  • Incomplete Dominance: Neither allele is dominant and the resultant phenotype is a mix e.g. combining a red plant with a white plant results in a pink plant
  • Heterozygous: One of each allele
  • Homozygous: Both alleles are the same
  • Carrier: A heterozygote which carries an allele for a disease but doesn't suffer from it
  • Genotype: The genetic constitution of an organism
  • Phenotype: The physical characteristic demonstrated by an organism
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Monohybrid Inheritance

  • Monohybrid Inheritance is the inheritance of one phenotype
  • Gregor Mendel deduced his theories of inheritance by cross breeding Pea Plants
  • Punnett Squares can be used to predict the outcome of crossbreeding two organisms

Image result for punnett square

This results 25% being Homozygous Dominant (AA), 50% being Heterozygous/Carriers (Aa) and 25% being Homozygous Recessive (aa)

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Prenatal DNA Tests


  • Amniocentesis involves extracting amniotic fluid containing foetal cells
  • The cells are grown and are tested (screened) for certain genes
  • This process occurs at 14-16 weeks
  • There is a 1% risk of miscarriage

Chorionic Villus Sampling

  • Chorionic Villus Sampling involves extracting embryonic tissue from the placenta
  • The DNA is then tested (screened) for certain genes
  • This process occurs at 8-10 weeks
  • There is a 1-2% risk of miscarriage


  • Parents can be tested for specific known genes that can cause a disease to determine the likelihood of their baby suffering from that disease
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Pre-Implantation Genetic Diagnosis

  • Embryos created through In Vetro Fertilisation (IVF) are tested for a faulty allele
  • Only those which have a normal allele are implanted into the woman
  • This is a very expensive procedure and can be quite unreliable
  • There are also ethical concerns regarding the destruction of the spare embryos

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Issues with Genetic Screening

  • Ethical Frameworks can determine what to do regarding whether it is right to pass on a genetic disorder. The four frameworks are:
  • Rights and Duties (Parents have the duty to keep the rights of their children i.e. right to life)
  • Utilitarianism (Maximising the good)
  • Making decisions for yourself (Foetus can't choose its own fate)
  • Leading a virtuous life
  • There are other decision in deciding the best course of action:
  • Risk of Miscarriage or harm to the foetus from the tests
  • Religious Beliefs - right to life of the foetus
  • Potential Abortion in the event of a prenatal diagnosis
  • Development of a 'pure/perfect' human
  • The cost of bring up a baby that suffers from the genetic issues
  • Psychological and emotional issues surrounding the birth of a 'disabled' baby
  • Dealing with the risk/consequences of false negatives and false positives
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Harry Clarke


This is a really good resource, used this for my revision, got an A**



Jumeriah College

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