Formative exam questions Flashcards
An 18 year old woman, with a family history of cystic fibrosis (CF), has recently discovered she is pregnant. The woman’s twin brother has cystic fibrosis, an autosomal recessive disorder.
The woman’s older sister is expecting non-identical twins.
i) What is the probability that both of these twins have cystic fibrosis? (1 mark)
Individual no. 1 is the older sister (oldest sibling always furthest to the left)
If both parents are heterozygous there is 1/4 one child would have CF, therefore 1/4 x 1/4 (1/16) chance that both would have CF
Would this probability increase, decrease or stay the same if these twins had been identical? Explain your answer. (2 marks)
Identical twins have identical DNA so could be considered as one individual, so 25% chance that they are both affected
The woman’s partner (individual II.5) has no family history of cystic fibrosis. The prevalence of CF mutant alleles in the general population is 1 in 25.
What is the probability that their unborn child (III.3) will have cystic fibrosis?
1%
(Explanation: the woman is a carrier, the chance that her partner is a carrier is 1/25; chance that two carriers have an affected child is 1⁄4; Therefore 1 x 1/25 x 1⁄4 = 1/100
Approximately 90% of patients with cystic fibrosis have pancreatic insufficiency, which means that the exocrine pancreas no longer functions at a level needed to digest food.
i) The transmission electron micrograph below shows part of a pancreatic exocrine acinus. Add accurately-positioned arrows, labelled with the given letters, to indicate the following:
Heterochromatin (H)
A zymogen granule (Z)
Nuclear envelope (NE)
Rough endoplasmic reticulum (RER) (2 marks)
Name one enzyme typically secreted by pancreatic exocrine cells. (1 mark)
Accept any of the following for 1 mark:
Trypsinogen (accept trypsin)
Chymotrypsinogen (accept chymotrypsin)
Lipase
Amylase
Ribonuclease
Deoxyribonuclease
Gelatinase
Elastase
The pancreas is both an endocrine and an exocrine organ.
i) Define endocrine secretion.
Hormones or other products are secreted directly into the blood (1/2 mark) to act on a distant tissue or organ
ii) Which specific tissue within the pancreas is responsible for endocrine secretion?
Islet of langerhans
Name one endocrine product from the pancreas.
Insulin
Glucagon
Somatostatin
Pancreatic polypeptide
Gastrin (mostly secreted by G cells in stomach and by
duodenum, but also secreted by pancreas)
Ghrelin (major source is stomach, but also secreted by
small intestine, pancreas and many other body tissues)
A 40 year old woman has recurrent headaches. After excluding any significant pathology, the GP recommends that she takes either aspirin or ibuprofen for symptomatic relief. These are non-steroidal anti-inflammatory drugs (NSAIDs) that reduce prostaglandin production by inhibiting the enzyme cyclooxygenase (COX).
Aspirin is a non-competitive COX inhibitor, whereas ibuprofen is a competitive COX inhibitor.
The graph below shows the kinetic data for the reaction in the presence and absence of aspirin and ibuprofen.
Identify the kinetic curve that corresponds to the reaction:
i) in the absence of aspirin or ibuprofen
curve 1
Identify the kinetic curve that corresponds to the reaction:
ii) in the presence of aspirin.
Curve 3
On the graph, draw dotted lines to indicate the Vmax and KM values for both enzymes. Clearly label each line that you draw. (2 marks)
The Vmax lines should be horizontal asymptopes that do not touch the curves. (1 mark). (Only award 1⁄2 mark if the Vmax line touches the curves)
The KM line(s) should be half of Vmax in both cases and should be identical or almost identical (1 mark). (Allow small increase in KM for COX-2 (curve A) because COX-2 is a ‘slightly’ better enzyme than COX-1.)
Which other type of plot could be used to obtain more accurate estimates of Vmax and KM for these two isoenzymes?
Lineweaver-Burk plot
State one type of ‘protein motion’ that can take place when the COX-2 protein is associated with the ER membrane.
Rotational
Conformational
Lateral
State two factors that restrain the motion of membrane proteins
Association with other membrane proteins
Association with extra-membranous proteins (peripheral
proteins) e.g. the cytoskeleton
Pain sensation is transmitted through both myelinated and unmelinated fibres.
The micrograph below shows nerve fibres that have been carefully separated by microdissection.
On the nerve fibre indicated with the black arrow, use accurately positioned arrows, labelled with the given letters, to identify:
i) The myelin sheath (M) ii) A node of Ranvier (R)
A 62 year old man has a chronic cough. He has smoked 40 cigarettes a day for the past 40 years. A biopsy shows that he has developed squamous epithelium within his bronchi, most likely as a response to the irritant effect of cigarette smoke.
Which of the attached images is representative of this type of epithelium?
Correct answer:
(c) C
A 55 year old man undergoes a renal biopsy. The specimen is stained with H&E and the renal corpuscles appear normal (see attached image).
What type of epithelial cells lining the Bowman’s capsule are indicated by the arrows?
Correct answer:
(c) Simple squamous
Justification for correct answer:
Simple cuboidal epithelium forms the walls of the renal tubules surrounding each renal corpuscle, whilst the lining of Bowman’s capsule is made of simple squamous epithelial cells, which are indicated by the arrows.
A 32 year old man who has had a previous myocardial infarction (MI) is prescribed a beta-blocker as secondary prevention against a future MI.
What effect will this drug have on components of signalling cascades in cardiomyocytes?
Correct answer:
A. Decreases cAMP and PKA and has no effect on PLC or IP3/DAG
Justification for correct answer:
Inhibition of β-1 receptors will lead to reduced production of cAMP production and its downstream signalling molecule PKA and therefore reduced force of myocyte contraction. IP3 and PLC are not involved.
A 40 year woman is diagnosed with epilepsy and commences treatment with phenytoin, an inducer of the enzyme CYP3A4. The GP advises her to not eat grapefruit or drink grapefruit juice because that would non-competitively inhibit the activity of the enzyme.
Which Lineweaver-Burk plot best depicts the kinetics of the enzyme CYP3A4 in the absence and presence [+I] of grapefruit juice? (See attachment for answer options; v = reaction velocity, [S] = substrate concentration).
Correct Answer:
(d) D
Justification for correct answer:
Non-competitive inhibitors do not affect binding of substrate to the enzyme so Km is unaffected. They only cause a decrease in Vmax (maximum velocity achievable).
A 61 year woman is given carbachol eyedrops to induce pupillary construction following cataract surgery. Carbachol stimulates the muscarinic M2 receptor on the sphincter muscle cells of the iris.
What effect will this drug have on intracellular second messenger production?
(a) Decreased cAMP
(b) Decreased IP3
(c) Increased cAMP
(d) Increased DAG
(e) Increased IP3
Correct answer:
(a) Decreased cAMP
Justification for correct answer:
M2 receptors are coupled to a Gαi subunit. Therefore, adenylyl cyclase will be inhibited, resulting in a reduced intracellular concentration of cAMP.
