SNAB Topic 2: Genes and Health

• Describe how the primary structure of leptin enables it to be soluble in water.

  (remember, globular proteins are soluble in water, fibrous proteins are not)

  • primary structure determines folding of polypeptide
  • leads to formation of globular tertiary structure
  • hydrophobic groups located in centre and hydrophilic groups located on outside
  • water forms H bonds with hydrophilic groups
• (ii) The thick filament in a myofibril contains myosin. The myosin head contains the enzyme ATPase. Explain the importance of the primary structure for the functioning of this enzyme.

  • primary structure determines interaction between R groups
  • determines tertiary structure
  • affects shape of active site
  • active site is complimentary to ATP
• (ii) Explain how human genome sequencing can be used to identify the mutations associated with MPS I.

  • sequence the genome of people with MPS1
  • sequence the genome of a number of people without the condition
  • compare base sequences to identify mutations found only in individuals with condition
• Mucopolysaccharides are complex molecules found in the human body. Mucopolysaccharides can be broken down by enzymes. Describe how an enzyme could break down the polysaccharide component of mucopolysaccharides.

  Hydrolysis of glycosidic bonds
• The scientists proposed a null hypothesis for the rate of breakdown of protein in the muscle cells of the two groups. The data from this investigation showed that there was no significant difference at the 0.05 probability level. (i) Explain what is meant by no significant difference at the 0.05 probability level for the rate of breakdown of protein.

  More than a 5% likelihood, therefore the rate of breakdown of protein is the same for both groups. Calculated value less than critical value at p=0.05
• Describe how the tertiary structure of myosin is related to its function.

  • Folded into a specific shape with a globular head.
  • that can bind to actin
  • myosin has a site that can bind to ATP
  • one part is straight to form a bundle with other myosin molecules
• Muscle cells contain globular and fibrous proteins. Compare and contrast the molecular structures of globular and fibrous proteins.

  • both are chains of amino acids joined by peptide bonds
  • both contain hydrogen bonds, ionic bonds and disulphide bridges holding them together
  • globular proteins have hydrophilic groups on outside, fibrous have hydrophobic
  • globular folded into compact shapes, fibrous are long chains
• Describe how a peptide bond is formed.

  Formed by a condensation reaction between amine group and carboxyl group of adjacent amino acids.
• Describe how the structure of phospholipase allows it to hydrolyse phospholipids.

  • structure of enzyme determined by polypeptide sequence
  • tertiary structure provides active site
  • active site complimentary to part of phospholipid
  • to break ester bonds
• (ii) The R group differs between the amino acids. The R group may contain elements that are not found in a carbohydrate. Name one of these elements.

  Nitrogen or Sulphur
• Multiple genetic crosses were carried out between individuals homozygous for a recessive allele and individuals heterozygous for the same gene. Describe how the outcome of these crosses would be affected if a gene drive was used with the recessive allele.

  • Without gene drive, expected outcome would be 50% heterozygous and 50% homozygous recessive
  • with gene drive, proportion of HR increases
  • the stronger the gene drive, the greater the proportion of HR
• The synthesis of mRNA occurs in a process called transcription. Compare and contrast the process of transcription with the process of DNA replication.

  • Similarities: involve formation of polynucleotide, involve DNA helicase
  • Differences: transcription uses RNA nucleotides, replication uses DNA nucleotides / transcription uses RNA polymerase, replication uses DNA polymerase / transcription only copies one template strand, replication copies both strands of DNA
• DNA is a polymer made from monomers called nucleotides. Describe how nucleotides join together to form DNA.

  Condensation reaction forming phosphodiester bonds, catalysed by DNA polymerase
• Analyse the data to explain why Meselson and Stahl accepted one of the models for DNA replication and rejected the other

  • Semi-conservative model accepted
  • Generation 1 has a single band halfway between 15N and 14N
  • because DNA has one strand of 14 and one strand of 15
  • in semiconservative, further gens would have no band at 15N
• Penicillin is an antibiotic. It was discovered in 1928. Since then many antibiotics have been identified and are widely used in the treatment of bacterial infections. Scientists have recently discovered a new class of antibiotics that bind to ribosomes. (i) Explain why these antibiotics could affect the production of proteins in bacteria.

  • Ribosome shape is altered
  • mRNA prevented from binding to ribosome
  • Translation cannot occur
  • Protein not synthesised
• (ii) These new antibiotics attach to a site on the ribosome not affected by any known antibiotics. Deduce why these new antibiotics might be used to treat bacteria that are resistant to other antibiotics.

  • bacteria have not been exposed to these antibiotics before so do not have mechanisms to make them resistant to them
  • bacteria have developed resistance to other antibiotics via natural selection
• (iii) Explain why the experiment would be improved if all the bases were provided but only the thymine was radioactive.

  • thymine only found in DNA
  • other radioactive bases taken up by all nucleic acids
  • only DNA would be measured
• (i) Describe the role of the CFTR protein in ensuring that the mucus produced in the lungs has the right consistency.

  • Chloride ions leave cells through CFTR channel protein
  • sodium ions leave cells (following chloride ions)
  • increasing solute concentration in the mucus
  • water moves out of the cells into the mucus by osmosis
• Compare and contrast the structure of ATP and a DNA nucleotide.

  Similarity: both contain phosphate, pentose sugar and a base

  
  

  Differences:

  • DNA nucleotide contains deoxyribose whereas ATP contains ribose
  • DNA nucleotide can contain other bases but ATP only contains adenine
  • DNA nucleotide only contains one phosphate, ATP contains three
• Cystic fibrosis is a condition that affects breathing. Explain why cystic fibrosis affects the rate of oxygen uptake in the lungs.

  • thick mucus
  • accumulation of mucus cannot be moved by cilia
  • air flow is restricted through bronchi
• Explain why thicker mucus is produced if the functioning of the CFTR channel protein is impaired.

  • Chloride ions cannot leave the cell through CFTR protein
  • sodium ions do not move out of cells
  • water moves out of mucus by osmosis
• (ii) Explain why different mutations in the CFTR gene can lead to differences in the severity of the symptoms of cystic fibrosis.

  • Different mutations will have different effects on the protein produced
  • Chloride ion transport effected by extent of changes to protein
  • Varying thickness of mucus
• (c) Explain why this couple may choose not to have this test. (chorionic villus sampling)

  • test result may be inaccurate
  • increased risk of miscarriage
  • false positive could lead to incorrect termination of fetus
  • could be against religious/cultural beliefs
• Explain what is meant by an inherited recessive disorder.

  A disorder caused by a faulty allele that is only expressed if genotype is homozygous recessive.
• (i) State what is meant by the term allele.

  An alternative form of a gene found at the same locus on a chromosome.
• Betalains are large polar molecules. Explain why betalain molecules cannot move through intact cell membranes.

  • betalain molecules are too large to fit through cell membrane
  • there are no carrier or channel proteins for betalain
  • polar so repelled by hydrophobic fatty acid tails in membrane
• Which of the molecules found in cell membranes have hydrophilic regions?

  phospholipids, channel proteins and cholesterol
• Diffusion and active transport are mechanisms by which molecules can enter cells. Compare and contrast these two mechanisms.

  Sim: both move molecules through cell surface membrane, and in both molecules can move through proteins

  Diff: diffusion occurs down conc. grad whereas active transport occurs against, diffusion is passive and doesn't require ATP
• Compare and contrast the processes of endocytosis and exocytosis.

  Sim: both involve vesicles and energy from ATP

  Diff: exocytosis involves leaving, endo is entering / in exo, vesicles fuse with csm; in endo, formation of vesicles from csm
• Describe how glucose molecules move into the cell.

  By facilitated diffusion through carrier proteins.
• Describe how glucose moves into cells by facilitated diffusion.


  Via a carrier protein from a high to low concentration.
• Explain how the structure of glycogen allows it to be an energy store.

  • Branched polymer of glucose that can be rapidly hydrolysed to provide glucose for respiration.
  • Compact to allow large amount of energy to be stored in a small space.
  • Insoluble so no osmotic effect on cells.
• A cell surface membrane is partially permeable. The phospholipid bilayer is important in controlling the movement of molecules through the membrane. Explain how the structure of a phospholipid molecule contributes to the partial permeability of a cell surface membrane.

  • polar and antipolar components allow non-polar and fat-soluble molecules to pass through the membrane.
  • polar molecules cannot pass through
• Give reasons for the variation in the lung volumes of healthy individuals.

  Different height, weight, gender, age
• Describe the function of carrier proteins in a cell surface membrane.

  • involved in facilitated diffusion
  • movement of large/polar molecules
  • from a high concentration to low concentration
  • needs ATP
• Describe the movement of water by osmosis.

  Against a solute concentration gradient through a partially permeable membrane.
• How many times does an oxygen molecule cross a cell surface membrane to move from the centre of an alveolus to the centre of a red blood cell?

  5 - both sides of alveolus cell wall, both sides of capillary wall, across csm of red blood cell
• Explain how the structure of the human lungs enables rapid gas exchange.

  • many alveoli provide large surface area
  • alveoli and capillaries have one cell thick walls for short diffusion distance
  • high concentration gradient maintained by ventilation and circulation
  • extensive capillary network provides large surface area for gas exchange