Question
Cellular processes at the molecular level are regulated by enzymes.
a. Outline the process of DNA profiling. [4]
b. Outline the role of DNA polymerase III in DNA replication. [4]
c. Explain the factors that affect the rate of enzyme-controlled reactions in cells. [7]
▶️Answer/Explanation
a.
- A DNA sample is obtained from a person (e.g. from hair, blood, or saliva).
- The DNA is amplified using polymerase chain reaction (PCR).
- Short tandem repeats (STRs) in the DNA are amplified.
- Gel electrophoresis is used to separate the DNA fragments based on size.
- The pattern of bands (the DNA profile) is unique to the individual and can be used for identification (e.g. in forensics or paternity testing).
b.
- DNA polymerase III binds to the template strand near the RNA primer.
- It adds nucleotides to the growing DNA strand using complementary base pairing (A with T, C with G).
- It synthesizes the new strand in the 5′ to 3′ direction.
- It forms phosphodiester bonds between nucleotides to build the sugar-phosphate backbone.
- On the lagging strand, it creates Okazaki fragments as synthesis is discontinuous.
c.
Temperature: Increasing temperature speeds up the reaction to an optimum point due to faster molecular movement and more frequent enzyme-substrate collisions.
Beyond the optimum, high temperatures cause denaturation of the enzyme, changing the shape of the active site and reducing activity.
pH: Enzymes have an optimum pH; too high or too low can alter the charge and shape of the active site, reducing enzyme function.
Substrate concentration: Increasing substrate concentration increases the reaction rate until all active sites are saturated.
Enzyme concentration: More enzymes provide more active sites, increasing reaction rate.
Inhibitors:
Competitive inhibitors block the active site.
Non-competitive inhibitors bind elsewhere and change the enzyme’s shape.
End-product inhibition: When the final product of a metabolic pathway inhibits an early enzyme via an allosteric site, preventing overproduction.
—————————————————————————————————–Markscheme————————————————————————————————
a. sample of DNA obtained from person/hair/blood/mouth/crime scene ✔
b. PCR used to amplify/make copies of DNA (in sample) ✔
c. using Taq DNA polymerase / using DNA polymerase from thermophilic bacteria ✔
d. tandem repeats amplified/used ✔
e. gel electrophoresis used to separate DNA (into bands) ✔
f. separation according to length of fragments/number of repeats
OR
fragments of same length/number of repeats travel same distance ✔
g. pattern of bands/numbers of repeats is the profile/is unique to the individual ✔
h. example of application/forensics/crime investigation/paternity ✔
Do not accept ‘determine ancestry’ for mph. Other genes/chromosomes are more often used for that.
Accept STR for (short) tandem repeat in mpd.
a. binds to template strand adjacent to a primer/at the primer ✔
b. adds nucleotides to template strand/to single stranded DNA ✔
c. using complementary base pairing ✔
d. links nucleotides with sugar-phosphate/phosphodiester bonds ✔
e. adds nucleotides/builds new strand in 5′
3′ direction ✔
f. lagging strand is built in short segments/Okazaki fragments/synthesis is discontinuous ✔
Accept A to T and G to C instead of ‘complementary’ in mpc.
For mpb it must be clear that nucleotides, not bases, are added to an existing strand of DNA. Do not accept ‘to replication fork’ for this.
a. temperature increases rate up to optimum and higher temperatures decrease rate / graph ✔
b. faster molecular movement as temperature rises (so more substrate-active site collisions) ✔
c. high temperature/heat causes denaturation/irreversible change to active site (so rate reduces) ✔
d. rate decreased if pH is above and below optimum/if pH is too high or low / graph ✔
e. pH affects shape/structure of enzyme/active site /affects ionization (of amino acids) ✔
f. increases in substrate concentration cause rate to rise towards a plateau/WTTE / graph ✔
g. greater chance of substrate-active site collisions with higher substrate concentration
OR
active sites saturated/all full at high substrate concentrations ✔
h. higher enzyme concentration increases rate (as there are more active sites) ✔
i. enzyme inhibitors/competitive inhibitors/non-competitive inhibitors reduce the rate ✔
j. end-product inhibitors switch off metabolic pathway / act on enzyme at start of pathway rate ✔
OR
allosteric site used to control enzyme activity by binding of (non-competitive) inhibitor
Graphs can be used for mpa, mpd mpf and mph but x-axis must have the variable indicated.
For mpa there must be exponential rise to optimum then faster drop.
For mpd there must be a bellshaped curve but it need not be exactly symmetrical.
For mpf and mph there must be decreasing increases in rate towards a plateau.
Question
Proteins carry out many important functions in cells and in organisms.
a. Outline the process of protein denaturation. [4]
b. Explain the production of antibodies in humans. [7]
c. Distinguish between competitive and non-competitive enzyme inhibition. [4]
▶️Answer/Explanation
a.
- Denaturation is the loss of a protein’s three-dimensional structure (tertiary and sometimes secondary structure).
- It is caused by extreme conditions, such as high temperature or extreme pH.
- These conditions disrupt the intramolecular bonds, including hydrogen bonds and ionic bonds.
- As a result, the protein loses its function because the shape of the active site changes, preventing substrate binding in enzymes.
- Denaturation is typically irreversible.
- Soluble proteins may precipitate out when denatured.
b.
- Antigens (foreign molecules, often on pathogens) enter the body and stimulate an immune response.
- Macrophages (phagocytes) ingest pathogens and present their antigens on their surface.
- These antigens activate helper T-cells, which in turn activate specific B-cells.
- The activated B-cells undergo clonal selection and divide by mitosis to form many identical cells.
- Some of these B-cells differentiate into plasma cells, which produce and secrete antibodies specific to the antigen.
- Antibodies bind to the antigen, marking the pathogen for destruction or neutralizing it.
- Some B-cells become memory cells, providing immunity by allowing a faster response if the antigen is encountered again.
c.
- Competitive inhibition: the inhibitor resembles the substrate and binds to the active site, preventing substrate binding.
- Non-competitive inhibition: the inhibitor binds to a site other than the active site (an allosteric site), changing the enzyme’s shape.
- In competitive inhibition, the effect can be reduced by increasing substrate concentration.
- In non-competitive inhibition, increasing substrate concentration has no effect because the active site is distorted.
——————————————————————————————————Markscheme————————————————————————————————–
a. change to conformation/shape/tertiary structure/3-D shape;
b. bonds within the protein/intramolecular bonds broken/changed;
c. pH and temperature (outside tolerated ranges) can cause denaturation;
d. vibrations/heat at high temperatures breaks bonds;
e. high pH/low pH/extreme pH alters ionization/charges (of amino acids and breaks ionic bonds);
f. protein cannot carry out its function
OR
active site of enzymes cannot bind substrates/catalyze reaction/no enzyme-substate complex;
g. permanent/irreversible change (usually)
OR
soluble proteins become insoluble/precipitate;
Allow any mark points if made clearly on an annotated graph or diagram.
a. antigens stimulate antibody production;
b. antibodies produced by lymphocytes;
c. macrophages/phagocytes ingest/engulf pathogens and display antigens from them;
d. T-cells activated by binding antigen/by macrophage displaying antigen;
e. activated T-cells cause activation of B-cells;
f. mitosis/division of (activated) B-cells (to produce a clone of cells)
g. plasma cells formed from divided/activated/growing/differentiating B-cells;
h. plasma cells/plasma B-cells secrete antibodies;
i. clonal selection / plasma cells make same type of antibody/antibody specific to same antigen;
j. some activated B-cells become memory cells;
c.