Chapter 2 - Genes and Health

What are the properties of gas exchange surfaces in living organisms?

Large surface area to volume ratio

Thickness of surface - one cell thick - reduces diffusion distance

Maintaining concentration gradient - so diffusion happens faster - done by having a rich blood supply in the many capillaries.

How is the rate of diffusion dependent on these properties?

A large surface area increases the rate.

A steep concentration gradient enhances the driving force for diffusion.

A short diffusion distance minimizes the time for diffusion.

What is Fick’s Law of Diffusion?

Rate of diffusion = surface are x difference in concentration / thickness of the gas exchange surface

How is the structure of the mammalian lung adapted for rapid gas exchange?

Large surface area: Millions of alveoli provide a vast surface for diffusion.

Thin exchange surface: Alveoli and capillary walls are one cell thick, minimizing diffusion distance.

Rich blood supply: Dense capillary networks maintain steep concentration gradients.

Ventilation mechanism: Breathing maintains high oxygen and low carbon dioxide levels in alveoli.

Moist surface: Alveoli are lined with moisture to dissolve gases for diffusion.

Elastic fibres: Allow alveoli to stretch and recoil, aiding ventilation.

What is the structure of a cell membrane (fluid mosaic model) ?

Phospholipid Bilayer:

- Made up of two layers of phospholipids.

- Hydrophilic heads (phosphate groups) face outward, interacting with water.

-Hydrophobic tails (fatty acids) face inward, avoiding water

• Proteins:

  • -Integral proteins: Span the bilayer, forming channels or carriers for transport.

    -Peripheral proteins: Sit on the membrane surface for structural support or cell signalling.

  Cholesterol:

  -Found between phospholipids.

  -Regulates membrane fluidity, providing stability and flexibility.

  Glycoproteins and Glycolipids:

  - Carbohydrate chains attached to proteins or lipids.

  -Involved in cell signalling, recognition, and adhesion.

What are the properties of a cell membrane?

• Partially Permeable:

  Allows selective movement of molecules (e.g., small non-polar molecules like oxygen diffuse freely).

  Larger or polar molecules require protein channels or carriers.

  Fluidity:

  Phospholipids can move laterally, giving the membrane flexibility and allowing it to change shape.

  Cholesterol prevents the membrane from becoming too rigid or too fluid.

  Dynamic Structure:

  Proteins and lipids are not fixed; they move within the bilayer, contributing to the "mosaic" nature.

  Transport Functions:

  Passive Transport: Diffusion, facilitated diffusion, and osmosis.

  Active Transport: Protein pumps move molecules against the gradient using ATP.

  Cell Communication:

  Glycoproteins and glycolipids detect and respond to signaling molecules (e.g., hormones).

  Barrier:

  Separates the intracellular and extracellular environments, maintaining homeostasis.

Explain the effect of temperature on membrane permeability

• Low temperature: Membrane becomes less fluid and more rigid, decreasing permeability.

• High temperature: Increased fluidity and permeability; extreme heat denatures proteins and disrupts the bilayer, causing membrane breakdown.

Explain the effect of alcohol concentration on membrane permeability

• Low concentration: Slightly increases fluidity and permeability.

• Moderate concentration: Increases permeability, causing leakage of cellular contents.

• High concentration: Destroys the bilayer, denatures proteins, and leads to loss of membrane integrity, causing cell death.

Osmosis

A type of diffusion involving the movement of free water molecules. From high to low concentration of free water molecules until equilibrium is reached (free water molecules move from region of low solute concentration to one of high concentration of solute).

Through phospholipid bilayer.

Passive, no energy required.

Diffusion

Down a concentration gradient, from high to low concentration of the substance until equilibrium is reached.

Hydrophobic (lipid-soluble) or small uncharged molecules through a phospholipid bilayers.

Passive, no energy required.

Facilitated diffusion

Down a concentration gradient, from high to low concentration of the substance until equilibrium reached.

Hydrophilic molecules or ions, through channel proteins or via carrier proteins that change shape.

Passive, no energy required.

Active transport

Against concentration gradient, low to high concentration through carrier proteins that change shape.

Requires energy, supplied by ATP.

Endocytosis

Used for bulk transport of substances into the cell.

Vesicles are created from the cell surface membrane, bringing their contents into the cell.

Exocytosis

Used for bulk transport of substances out of the cell.

Vesicles fuse with the cell surface membrane, releasing their contents.

What is the role of carrier proteins in membrane transport?

Transport large or polar molecules via facilitated diffusion (passive) or active transport (ATP-dependent).

What is the role of channel proteins in membrane transport?

Provide hydrophilic pores for ions to pass via facilitated diffusion. Some are gated, opening in response to stimuli.

What is the structure of mononucleotides?

Nitrogenous Base: A purine (adenine or guanine) or pyrimidine (cytosine, thymine in DNA, or uracil in RNA).

Pentose Sugar: Ribose (in RNA) or deoxyribose (in DNA), differing by an -OH or hydrogen at the 2' carbon.

Phosphate Group: Attached to the 5' carbon of the sugar, forming phosphodiester bonds in nucleic acids.

Structure of DNA and how is it formed?

Structure: DNA is a double helix made of two strands of nucleotides. Each nucleotide consists of a deoxyribose sugar, phosphate group, and nitrogenous base (A, T, C, G).

Base Pairing: A pairs with T, and C pairs with G via hydrogen bonds.

Formation: Nucleotides polymerize into long chains via phosphodiester bonds, and two complementary strands twist into a double helix. This structure allows for efficient information storage and replication.

Structure of RNA and how is it formed?

Structure: RNA is a single-stranded molecule made of ribose sugar, phosphate groups, and nitrogenous bases (A, U, C, G).

Formation: RNA is synthesized during transcription by RNA polymerase using a DNA template, forming a complementary RNA strand in the 5' to 3' direction.

What bonds are individual mononucleotides in a molecule of mRNA joined together by?

phosphodiester bonds

Describe the process of transcription

- the enzyme RNA polymerase attaches to the DNA coding for a particular protein, causing the hydrogen bonds between the bases to break.

- as a result the DNA unwinds.

- the sequence of the template (also called the anti-sense) strand is transcribed to make a messenger RNA molecule.

- the base sequence on the mRNA is complementary to the template strand. This means that the sequence is the same as that on the coding (also called the sense) strand of the DNA.

- phosphodiester bonds form between the RNA nucleotides.

- the mRNA then leaves the nucleus through a pore in the nuclear envelope.

The DNA 'zips' back up.

Describe the process of translation

- The mRNA, which was synthesized during transcription, attaches to a ribosome in the cytoplasm.

- Transfer RNA (tRNA) molecules, each carrying a specific amino acid, bind to the mRNA at the ribosome. Each tRNA has an anticodon that is complementary to a codon on the mRNA.

- The ribosome facilitates the binding of the tRNA anticodon to the mRNA codon, ensuring the correct sequence of amino acids.

- As the ribosome moves along the mRNA, it reads the codons and continues to match them with the appropriate tRNA molecules.

- Peptide bonds form between the amino acids, creating a growing polypeptide chain.

- This process continues until the ribosome reaches a stop codon on the mRNA, signalling the end of translation.

- The newly synthesized polypeptide chain is released, and it folds into its functional three-dimensional shape.

Non-overlapping meaning in the genetic code?

The codons are separate

Triplet meaning in the genetic code?

codon is made of 3 bases

Degenerate meaning in the genetic code?

there is more than one codon for each amino acid

20 amino acids - 64 codons

What is the structure of a gene?

Promoter: Initiates transcription.

Exons and Introns: Exons are coding regions; introns are spliced out.

5' and 3' UTRs (untranslated regions): Regulate mRNA stability and translation.

Start/Stop Codons: Define the coding region.

Enhancers/Silencers: Regulate transcription levels.

What is the basic structure of an amino acid?

• Central alpha carbon bonded to:

  • Amine group (-NH₂): Basic.

  • Carboxyl group (-COOH): Acidic.

  • Hydrogen atom (H).

  • R group: Variable side chain, determines properties.

• General formula: H₂N-CHR-COOH.

How are polypeptides and proteins formed?

• Amino Acids as Monomers:

  • Amino acids are the building blocks of proteins.

  • Each contains an amino group (-NH₂), carboxyl group (-COOH), hydrogen atom, and a variable R group.

• Formation of Peptide Bonds:

  • Amino acids join via condensation reactions, releasing water (H₂O).

  • A peptide bond (-C-N-) forms between the carboxyl group of one amino acid and the amino group of another.

• Polypeptide Formation:

  • Repeated condensation reactions produce a polypeptide chain, which is a sequence of amino acids linked by peptide bonds.

Explain primary protein structure

Primary structure of a protein is the sequenec of amino acids in a protein

Explain secondary protein structure

The 2D arrangement of the chain of amino acids- either alpha helix or beta pleated sheet.

Explain tertiary protein structure

the 3D folding of the secondary structure into a complex shape. The shape is determined by the type of bonding present, namely: hydrogen bonding (forces of attraction between partially charged atoms in R groups), ionic bonds (salt bridges, form between oppositely charged groups on the R groups), and disulphide bridges (covalent bonds between sulphur atoms in cysteine).

Explain quaternary protein structure

3D arrangement of more than one polypeptide. Not all proteins have all levels of structure.

Explain the structure of fibrous proteins

Structure:

• long parallel polypeptides

• very little tertiary/quaternary structure

• occasional cross-linkages which form microfibres for tensile strength

• insoluble

What is the function of fibrous proteins

Function:

• used for structural purposes - such as collagen

• collagen has high tensile strength due to presence of both hydrogen and covalent bonds in the structure. Collagen molecules are made up of three polypeptides which form an alpha triple helix which forms fibrils and strong collagen fibres.

• Collagen forms the structure of bones, cartilage and connective tissue and is a main component of tendons which connect muscles to bones.

Explain the structure of globular proteins

Structure:

• complex tertiary/quaternary structure

• form colloids in water

What is the function of globular proteins

Function:

• hormones, antibodies, carrier proteins e.g. haemoglobin

• haemoglobin is a water soluble globular protein which consists of 4 beta polypeptide chains and a haem group.

• It carries oxygen in the blood as oxygen can bind to the haem (Fe2+) group and oxygen is then released when required.

What are enzymes?

biological catalysts that increase the rate of reaction by lowering the activation energy of the reactions they catalyse, including both anabolic and catabolic, intracellular and extracellular reactions.

anabolic

Anabolic processes build complex molecules from simpler ones (e.g., protein synthesis). They require energy (endergonic).

catabolic

Catabolic processes break down complex molecules into simpler ones (e.g., glucose breakdown). They release energy (exergonic).

intracellular

Intracellular refers to processes occurring inside the cell (e.g., DNA replication by DNA polymerase).

extracellular

Extracellular refers to processes occurring outside the cell (e.g., digestion by amylase in saliva).

Lock and key theory

the active site is the area of the enzyme where the substrate binds.

enzymes are specific to substrates they bund to, as only one type of substrate fits into the active site of the enzyme.

Induced fit theory

When the enzyme and substrate form a complex, the structure of the enzyme is distorted so that the active site of the enzyme fits around the substrate.

How does enzyme concentration affect the initial rate of reaction?

the rate of reaction increases as enzyme concentration increases as there are more active sites for substrates to bind to.

however, increasing the enzyme concentration beyond a certain point has no effect on the RoR as there are more active sites than substrates so substrate concentration becomes the limiting factor.

How does substrate concentration affect the initial rate of reaction?

as concentration of substrate increases, RoR increases as more enzyme-substrate complexes are formed.

however, beyond a certain point the RoR will no longer increase as enzyme concentration becomes the limiting factor.

How does temperature affect the initial rate of reaction?

RoR increases up to the optimum temp which is the temp that enzymes work best at

RoR decreases beyond the optimum temp because enzymes denature.

How does pH affect the initial rate of reaction?

enzymes work within a narrow range of a specific pH value, values above or below this alter the bonds within its structure, hence the shape of its active site.

Describe the process of semi-conservative DNA replication

The double helix unwinds and the hydrogen bonds between the complementary bases break, catalysed by DNA helicase, thus separating the two strands of DNA.

One of the strands is used as a template and complementary base pairing occurs between the template strand and free nucleotides.

Adjacent nucleotides are joined by phosphodiester bonds formed in condensation reactions, catalysed by DNA polymerase.

How did Meselson ad Stahl prove semi-conservative replication (instead of conservative or dispersive)?

Originally grew DNA in a culture containing N15 - (an isotope of nitrogen for several generations, so all the bases contained this isotope.

They then grew the DNA in a culture of N14 for one generation. After this generation, The DNA contained one strand containing 15-N and one strand containing 14-N.

After another generation, half of the DNA molecules were the same as in generation one, and the other half contained entirely 14-N (where the 14-N stand from generation one had been used as a template)

Providing evidence for the semi-conservative model.

Mutations are permanent changes in the DNA of an organism. Gene mutations are changes in the arrangement of bases by: (5 main ways, name them)

Substitution (change in one base)

Insertion (adding another base in)

Deletion (taking a base out)

Duplication (adding the same base more than once)

Inversion (swapping the order of bases around)

How can a mutation lead to a defective enzyme being produced

What do these changes to the base sequence result in?

a change to the mRNA, tRNA and therefore the primary structure of the protein.

Mutations may also occur in the formation of mRNA and tRNA themselves.

What happens if the mucus is too runny?

1. Na+ pump activates and removes Na+ ions from cells.
2. concentration in the cell of Na+ ions decreases.
3. Na+ ions move down the concentration gradient through the channel on the apical membrane.
4. Cl- ions move down the electrochemical gradien caused by the movement of Na+ ions.
5. Water will follow by osmosis
6. Reduce water content of mucus.

What happens if mucus is too thick?

1. Cl- pump on basal membrane activates and moves Cl- moves into the cell.
2. Cl- concentration in cells increases.
3. Cl- ions move down the concentration gradient through CFTR protein on apical membrane into mucus.
4. Na+ ions move down electrochemical gradient.
5. Water will follow by osmosis.

What happens in people with cystic fibrosis?

- CFTR protein doesn't function
- Na+ is always being removed from cell + mucus
- Cl- ions move down electrochemical gradient
- Water is constantly removed from mucus - making it very thick and sticky

Effects of CF on the respiratory system

Build-up of mucus in the lungs traps bacteria, thus increasing the risk of infection.

Build up of mucus in the airways decreases the SA of alveoli involved exposed to fresh air, reducing the SA for gas exchange.

Effect of CF on the reproductive system

Women - cervical mucus prevents the sperm from reaching the egg

Men - the sperm duct is blocked with mucus, meaning that the sperm produced cant leave the testes

Effect of CF on the digestive system

the pancreatic duct which connects the pancreas to the small intestine can become blocked with mucus, so the digestive enzymes dont reach the small intestine. as a result food isn’t properly digested, so fewer nutrients are absorbed.

the mucus lining in the duodenum is very thick, reducing the absorption of nutrients.

mucus can cause cysts to form in the pancreas and damage the insulin producing cells, leading to diabetes.

Gene

a piece of DNA which has a specific sequence of bases. Each gene codes for a specific protein.

Allele

a different form of a particular gene

Genotype

all the alleles of an organism

Phenotype

the set of observable characteristics of an individual resulting from the interaction of its genotype with the environment

Recessive

an allele that produces a feature only if two copies are present.

Dominant

an allele that produces a feature even if only one copy of the allele is present

Incomplete dominance

a form of intermediate inheritance in which only one allele for a specific trait is not completely expressed over its paired allele.

this results in a third phenotype in which the expressed physical trait is a combination of the dominant and recessive phenotypes.

Homozygote

an individual having two identical alleles of a particular gene

Heterozygote

an individual having two different alleles of a particular gene

Monohybrid inheritance

the inheritance of just one characteristic.

What is the purpose of genetic screening?

to determine if the DNA of an individual contains alleles for genetic disorders.

e.g. it can be used to identify carriers and for preimplantation genetic diagnosis and prenatal testing such as CVS.

Pre-implantation on genetic diagnosis

embryos created through IVF are tested for genetic disorders before they are implanted into the woman’s uterus.

Chorionic villus sampling (CVS)

•  A small sample of placental tissue (that included cells of the embryo or fetus) is removed, either through the wall of the abdomen or through the vagina. 

 

• It can be carried out earlier in the pregnancy, between 8 and 12 weeks, since there isn't a need to wait for amniotic fluid to develop.

 

•  Estimated risk of about 1-2% of inducing miscarriage.

 

• Used to test for Used to test for chromosome abnormalities (trisomy 21,18,14 or xy chromosomes)

DEFINITIVE (YES/NO)

Amniocentesis

•   Involves inserting a needle into the amniotic fluid to collect fetal cells that have fallen off the placenta and fetus.

 

•   Gets carried out at around 15-17 weeks of pregnancy

 

•   There is a 1% risk of amniocentesis causing a miscarriage

 

• Used to test for chromosome abnormalities (trisomy 21,18,14 or xy chromosomes) & spina bifida

DEFINITIVE (YES/NO)

Non-invasive prenatal diagnosis (NIPD)

•   Analysing DNA fragments in the mothers blood plasma during pregnancy. Most of this 'cell-free DNA' is from the mother herself, about 10-20% is from the embryo.

 

• cffDNA becomes detectable in the mother at around 4-5 weeks of pregnancy, but at this stage the levels are too low to be analysed.

 

•  Samples are likely to be analysed after 7-9 weeks of pregnancy

 

• Used to test for Used to test for chromosome abnormalities (trisomy 21,18,14 or xy chromosomes)

NOT DEFINITIVE

Ethical framework 1 - Rights and Duties

• There are certain human rights that should always be permitted.

• Because you have rights you have duties to people. E.g. A baby has the right to life and as a mother to that baby you have the duty to care for them.

Ethical framework 2 - Maximising the amount of good in the world

• Suggests that everyone should act in a way that maximises the amount of good in the world

• Also known as utilitarianism

• Believe that nothing is every right or wrong ( it changes in situations that you are in)

• E.g. There might be circumstances where something normally right (keeping a promise) would be wrong and there might be times when something normally wrong (killing someone) is right.

Ethical framework 3 - Making decisions for yourself

• We should be allowed to make decisions for yourself about your life.

• E.g. If it has been suggested by a doctor that you have surgery, you have to give informed consent before they are allowed to operate on you

Ethical framework 4 - leading a virtuous life

• Suggests that the good life consists of acting virtuously

• What virtues to you expect from a teacher (understanding, kind).

There are some social and ethical issues surrounding genetic testing, what are they?

There is a risk of harm to foetus or miscarriage

the outcome of testing might lead to an abortion - right to life

the cost of bringing up a baby with a genetic disorder

emotional and mental issues surrounding caring for a baby with a genetic disorder.