Topic 2: Genes and Health


Gas exchange surfaces

The lungs allow fro rapid gas exchange. This is due to alveoli creating a large surface area. The blood supply is excellent due to the high number of capillaries.

A high surface area to volume ratio means a fast rate of diffusion. As the surface area to volume ratio decreases, the rate of diffusion slows down.

Large animals would have a slower dffusion pathway as the distance needed for the gas to diffuse in and out would be greater.

We can state that:

Rate of diffusion is directly proportional to Surface area X Concentration

                                                                    Thickness of gas exchange

This relationship is known as Fick's law

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Cell membrane

Carbohydrate - makes glycoprotein and aid in cell recognition

Proteins - Specific to cell for recognition and transport. Can be in two forms peripheral and integral

Phospholipid - Hydrophillic polar head made of a phospahte group, hydrophobic non-polar tail made up of lipid chains.

This forms a layer at water surfaces due to the two solubility properties and can create a barrier between two water surfaces - a bilayer.

Cholestrol - provides stability and limits the fluidity

Before the fluid mosaic model, it was believed that a cell membrane was a protein-lipid sandwich.  The dark masses under a microscape were thought to be proteins, but this did not allow for hydrophillic heads to be near water and hydrophobic tails to avoid water. A bilayer was more appropriate due to the orientation of the heads and the tails. Phosphate heads are more electron dense which explians the darker region under the microscope. 

The use of flourescence with mouse cells indicted that cytoplasm can be shared amongst cells.

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Investigating membrane strucutre and membrane perm

The effect of temperature and alcohol on membrane permeabilty

Control variables - source of the beetroot, age of beetroot, volume of water used, time left in the water solution, pH

Independent variable - temperature of the water and alcohol concentration of the water solution

Dependent variable - % light through the solution

Using a cork borer, cut the beetroot into equal size pieces. Rinse the beetroot in water and pat dry being careful not to squeeze the cylinders. Place on in each test tube with 1cm3 of water in each one. Place each tube in an ice bath of temperatures 15, ,20, 25, 30,35. Leave for 15 minutes. Remove the beetroot and shake the tubes to disperse the dye. Transfer solution to a cuvette and calibrate the colorimeter usig distilled water as a control. Take readings of the absorbance.

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The net movement of water molecules from a solution with a low concentration of solute o a solution with a higher concentration of solute trough a patially permeable membrane

Solute - a soluble substance e.g glucose can be dissolved

Solvent - A liquid which can dissovle a solute e.g water

Solution - a mixture of a dissolved solute in  solvent

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Passive transport

Passive transport is the movement from an area of high concentration to an area of low concentration. These particles will never reach equilibrium as they are constantly moving to the lower concentration. This can be in the form of diffusion or facilitated diffusion which uses carrier proteins to transport molecules along the electrochemical gradient.

Active transport is the movement of molecule against the concentration gradient so requires energy in the form of ATP. This is done through the use of carrrier proteins. The breaking of ATP using ATPase and water to create ADP and Pi relaeses energy through the bonds to move the substance through the membrane. The carrier protien acts as a pump and changes shape to squeeze it through.

Exocytosis - the process of exiting the cell. A vesicle surrounds the protein and transports it towards the membrane so it doesnt get released in the cell. The lipid flows into the bilayer and the vesicle binds so the protein can exit. The vesicle then becomes part of the bilayer.

Endocytosis - The reverse process where proteins are taken into the cell. They are taken in by the creation of a vesicle. Part of the cell surface membrane engulfs the material and brings it into the cell.

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DNA - Deoxyribonucleic acid

Consists of a phosphate group, deoxyribose sugar and a nitrogenous organic base.

The bases can be guanine (g), cytosine (c), Uracil (u) and adenine (a)

Phosphodiester bonds link nucleotides together in chains through condensation reactions between the phosphate and sugar. More than one polynucleotide can be joined by hydrogen bonds between complementary bases. one of the polynucleotides will be inverted to keep hydrophillic areas on the outside and to allow phoshodiester bonds with the right carbons.

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Protein synthesis

DNA is copied into RNA for protein synthesis as DNA is too large to leave the nucleus.

Transcription - RNA polymerase attaches onto the beginning of the double helix and begins to break the hydrogen bonds, seperating the strands. One strand is then used as a template to make an mRNA copy of the DNA. RNA Polymerase lines up free RNA nucleotides matching them with the complementary bases. These new RNA nulceotides then join by RNA Polymerase forming an mRNA molecule. The hydrogen bonds then reform between the DNA strands as the polymerase moves along. When the RNA polymerase reaches the stop codon it detaches an the mRNA can move ou of the nucleus.

Translation - mRNA attaches itself onto a ribosome within the cytoplasm where tRNA molecules carry amino acids to the ribosome. A tRNA molecule with complementary anticodon attaches to the start codon. Two amino acids are joined together by a peptide bond. The first tRNA then detaches leaving its amino acid behind. This process continues until the tRNA attaches onto a stop-codon. The peptide chain then moves away and translation is complete.

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Genetic code

Degenerate - Several triplets can code for the same amino acid.

Non-overlapping - successive triplets read in order from the start triplet. After three bases, a new triplet begins.

Triplet code - A set of three bases code for an amino acid.

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Sequences of bases

A gene is a sequences of mononucleotide bases on a DNA molecule that codes for the sequence of amino acids in a polypeptide.

The sequence of amino acids in a polypeptide forms the primary structure of a protein.

Different proteins have a different number and order of amino acids.

It's the order of mononucleotide bases in a gene that determines the order of amino acids in a particular protein.

Each amino aicd is coded for by a sequence of of three bases (called a triplet) in a gene.

Different sequences of bases code for different amino acids. So the sequence of bases in a section of DNA is a template that's used to make proteins during protein synthesis.

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Amino acid

Two amino acids join together by dipeptide bonds formed in a condensation reaction. The bond forms between the -OH and -H of the adjacent amino aicd.

Structure of a protein:

Primary structure - Amino acids joined by a peptide bond into a long chain. R groups change the shape and bends.

Secondary Structure - R groups bend and twist to change the shape to create an a-helix (loops) or b-pleated sheet (parallel foldings). These are held together by hydrogen bonds.

Tertiary structure  - Polypeptide chain folds further to form a 3D shape. Bonds between R groups maintain the structure. Polar R groups are hydrophyllic and non-polar are hydrophobic. If the structure is functional, it is then known as a protein

Quaternary structure - more than one polypeptide chain held tightly together into a new structure. Single chain proteins do not have a quaternary structure.

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Enzymes are a biological catalyst that speed up metabolic reactions. They are proteins with a active site which has a specific shape where the substrate can bind to. They are specific to each substrate due to the highly folded tertiary structure.

When an enzyme and substrate bind, they form an enzyme-substrate complex. The lock and key model shows this as they fit together like a key and a lock. However, the lock and key model doesnt register that the active site changes shape to get an induced fit. This prevents alien substrates tryin to bind as only certain substrates change the shapes.

Each enzyme has a different active site and tertiary structure that allows only complementary shaped substrate to fit. If the tertiary structure is altered in any way, the shape of the active site will change preventing the induced fit. This can be changed by temperature or pH

The more enzymes present in a solution, the more substrates to collide with the active sites and form an enzyme-substrate complex. This increases the rate of reaction. The amount of substrate is limited and becomes a limiting factor.

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Investigate enzymes and initial rates of reaction

Control variables - pH and temperature of the solution, time left to acclimatize and for the reaction to take place. 

Independent variable - enzyme concentration

Dependent variable - volume of oxygen produced

Add 4cm3 of hydrogen peroxide 1mol to a boiling tube with 1cm3 of buffer solution. Set the tube up with a gas syringe and delivery tube. Use a pipette to add 1cm3 of catalase and quickly add the bung and delivery tube. Record the volume of gas produced every 10 seconds for the first 60 seconds to collect initial rate of reaction. Repeat another 5 times with different concentrations of catalase

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DNA replication

DNA is always copied when cells divide. The way that DNA copys itself is called semi-conservative replication. Half of the DNA stayed the same, the other half is new.

Step 1 - Break the hydrogen bonds between complementary base pairs in double stranded DNA

Step 2 - One strand of DNA remains unchanged (conserved). New complementary DNA nuleotides are using the enzyme DNA polymerase added to the original strand.

Meselson and Stahl - 

Dense 15N nucleotides grow on an agar plate and allowed to divide once. They did the same for 14N nulceotides and they replicated once. They then extracted the DNA from the bacteria and placed it into a centrifuge. This spins round rapidly and the denser nucleotides would sink to the bottom. Medium density DNA disproves conservative replication as it suggests it was made of both 15N and 14N. They then regrew the bacteria in a 14N medium. They achieved some light DNA and some medium density. One band was at the top of the centrifuge and one in the middle. This showed  that DNA was made up of one original strand and a new strand proving semi-conservative replication.

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A mutation is the change of seqeunce of bases within the genetic code can result in a different amino acid being included in the final protein translated. 

Gene locus - Position of a gene on a chromosome.

eg. Cystic fibrosis. This is a genetic disorder aused by a mutation in a gene. The protein the gene codes for is important for mucus production

Types of mutation:

-Substitution one base is substituted with another e.g ATGCCT becomes ATTCCT

-Deletion one base is deleted e.g ATGCCT becomes ATCCT

-Insertion an extra base is added e.g ATGCCT becomes ATGACCT

-Duplication one or more bases are repeated e.g ATGCCT becomes ATGCCCCT

-Inversion a sequence of bases is reversed e.g ATGCCT becomes ATGTCC

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Gene - A sequence of bases on a DNA molecule that codes for a protein, which results in a characteristic

Allele - A different version of a gene. Different versions have slightly different base sequences which code for different versions of the same charateristic.

Genotype - The alleles a person has e.g BB, Bb

Phenotype - The characteristics displayed by an organism e.g brown eyes.

Dominant - An allele whose characteristics appears in the phenotype by only one copy (Bb/BB)

Recessive - An allele whose characteristics appear if there are two copies in the phenotype (bb)

Incomplete dominance - When the trait for a dominant allele isnt completely shown over the recessive allele. So both alleles have an influence on the phenotype.

Homozygote - An organsim that carries two copies of the same allele for certain characteristics.

Heterozygote - An orgnaism that carries two different copies of the same allele.

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Cystic fibrosis

A cystic fibrosis sufferer has mucus stickier and thicker than a non CF sufferer. It is caused by a mutation in the gene that codes for the CFTR protein (Cystic fibrosis Transmembrane Conductance Regulator). CFTR is a channel protein that transports chloride ions out of cells and into mucus - this causes water to move by osmosis (making the mucus slippery). Mutated CFTR is less efficient at transporting chloride ions out of the cell, so less water moves out by osmosis.

Respiritory system - Cillia are unable to move up and down meaning mucus builds up in airways. This can cause complete blockages and prevents gas exchange from taking place and surface area is reduced. They are also more prone to lung infections

Digestive system - Tube that connects small intestine to the pancreas becomes blocked - stopping digestive enzymes reaching the small intestine and break down food for digestion. Mucus can cause cysts in the pancreas that prevent enzymes being created. Absorbtion of nutrients is blocked by mucus.

Reproductive system - Tubes in the testes become blocked stopping sperm produced from being released. For women, cervical mucus prevents the sperm reaching the egg and reduces the mobility of the sperm.

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Genetic screening

Gene therapy- Use of a section of DNA containing the normal allele as a way to treat genetic disorders. This is done by inserting the DNA into cells where they are expressed as proteins and can interfere with expression and correct mutations.

Genetic testing - Used to identify the presence of abnormal genes in the DNA, confirm diagnosis, identify carriers. A sample is collected by removing cells, fluid or blood containing cells, tissue, blood from the umbilical chord, piece of hair or swab of the cheek.

Preimplantation techniques occur before the zygote has settled in the uterus, whereas prenatal tests occur once the embryo is developed.

Chorionic Villus Sampling - sample of cells from the placenta between the 11th and 14th week of pregnancy. Can be done through the abdomen or through the cervix. Can lead to miscarriage (1-2%), higher rates of abortion, infection

Amniocentesis - Sample from amniotic fluid between the 15th and 18th week of pregnancy. A needle is used to extract a sample. Can lead to miscarriage (>1%), infection, puncture of the placenta, late abortion

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Social and ethical issues - genetic screening

Prenatal tests increase the chance of miscarriage slightly

False results can provide incorrect information being given

Some find it unethical to abort due to a genetic condition.

Can be abused and used for other things e.g. designer babies

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Types of protein

Globular proteins - 

Round and compact made up of many polypeptie chains.

Chains are coiled so that the hydrophillic parts are on hte outside and hydrophobic face outwards.

Proteins are soluble so can be transported in water.

e.g Haemoglobin carries oxygen round the blood

Fibrous proteins - 

Long insoluble polypeptide chains

Held togehter by hydrogen and disulfide bonds which add strength.

Fibrous proteins are often found in supportive tissue

e.g collagen is a strong fibrous protein that forms connective tissue in animals.

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