Biochem FEEDs (2023) Flashcards
The enzyme HGPRT is found to be deficient. This will result in:
a. Increased synthesis of Krebs cycle intermediates
b. Activation of xanthine oxidase
c. Increased purine synthesis
d. Hyperuricemia
d. Hyperuricemia
Rationale: HGPRT (hypoxanthine-guanine phosphoribosyltransferase) is crucial for the salvage pathway of purines. A deficiency in HGPRT results in the accumulation of hypoxanthine and guanine, which are subsequently converted to uric acid, leading to hyperuricemia. This condition is associated with Lesch-Nyhan syndrome.
Deoxyribonucleotides are formed by reduction of:
a. Ribonucleosides
b. Ribonucleoside monophosphates
c. Ribonucleoside diphosphates
d. Ribonucleoside triphosphates
c. Ribonucleoside diphosphates
Rationale: Deoxyribonucleotides are synthesized from ribonucleoside diphosphates by the enzyme ribonucleotide reductase, which reduces the 2’-hydroxyl group of the ribose ring to a hydrogen atom, producing deoxyribonucleotides.
The action of allopurinol is best described as:
a. Competitive inhibition of xanthine oxidase
b. Inhibiting PRPP synthesis
c. Increasing the solubility of uric acid in plasma
d. Inhibiting leukocyte movement by affecting microtubule formation
a. Competitive inhibition of xanthine oxidase
Rationale: Allopurinol is a structural analog of hypoxanthine. It acts as a competitive inhibitor of xanthine oxidase, the enzyme responsible for converting hypoxanthine and xanthine to uric acid. This inhibition reduces uric acid production, helping to manage conditions like gout.
Which of the following is not a cause of gout?
a. Inefficient purine base salvage reactions
b. Increased PRPP synthetase activity
c. Overproduction of pyrimidine bases
d. von Gierke’s disease
c. Overproduction of pyrimidine bases
Rationale: Gout is caused by the accumulation of uric acid, which is a product of purine metabolism, not pyrimidine metabolism. The overproduction of pyrimidine bases does not contribute to gout.
Which of the following is absent in deoxyribonucleosides?
a. Phosphate group
b. 5-carbon sugar in pentose ring form
c. Purine or pyrimidine base
d. Glycosidic bond
a. Phosphate group
Rationale: Deoxyribonucleosides consist of a deoxyribose sugar attached to a purine or pyrimidine base via a glycosidic bond. They do not contain a phosphate group, which differentiates them from deoxyribonucleotides.
True of nitrogenous bases:
a. In nucleotides, they are attached to the phosphate by a phosphodiester bond
b. A prime (‘) symbol is used to define the carbon atoms in its ring
c. The N-9 atom of purines is linked to carbon 1 of the sugar
d. Nitrogenous base pairs are linked together by N-glycosidic bonds
c. The N-9 atom of purines is linked to carbon 1 of the sugar
Rationale: In nucleotides, the N-9 nitrogen of purine bases (adenine and guanine) is linked to the 1’ carbon of the sugar (ribose or deoxyribose) via an N-glycosidic bond. The other options are incorrect because (a) nitrogenous bases are attached to the sugar, not the phosphate, by a glycosidic bond, (b) the prime (‘) symbol is used to define carbon atoms in the sugar, not in the nitrogenous base, and (d) base pairs are linked together by hydrogen bonds, not N-glycosidic bonds.
PRPP acts as a substrate for:
a. Purine synthesis
b. Pyrimidine synthesis
c. Both Purine and pyrimidine synthesis
d. Neither purine nor pyrimidine synthesis
c. Both purine and pyrimidine synthesis
Rationale: PRPP (phosphoribosyl pyrophosphate) is a crucial substrate for the synthesis of both purine and pyrimidine nucleotides. It donates the ribose-phosphate group that is required for the formation of nucleotides.
True of dihydrofolate reductase:
a. It is inhibited by methotrexate
b. It catalyzes an ATP-requiring reaction
c. Its absence results in an accumulation of thymidine nucleotides
d. All of the above are true
a. It is inhibited by methotrexate
Rationale: Dihydrofolate reductase is an enzyme that catalyzes the reduction of dihydrofolate to tetrahydrofolate, a cofactor required for the synthesis of thymidine nucleotides. Methotrexate is a competitive inhibitor of this enzyme. The other options are incorrect as the reaction catalyzed by dihydrofolate reductase does not require ATP, and its absence results in a deficiency of thymidine nucleotides, not an accumulation.
Why are humans not affected by sulfonamides and trimethoprim?
a. Mammalian cells do not synthesize folate
b. Folate is not essential for DNA synthesis in humans
c. Humans can synthesize folate from PABA unlike microorganisms
d. Humans have enzymes that can digest sulfonamides and trimethoprim and render them inactive
a. Mammalian cells do not synthesize folate
Rationale: Sulfonamides and trimethoprim target bacterial folate synthesis. Humans obtain folate through their diet and do not synthesize it, rendering these drugs ineffective against human cells. Folate is essential for DNA synthesis in humans (b), humans cannot synthesize folate from PABA (c), and humans do not have enzymes that digest sulfonamides and trimethoprim (d).
A 50-pair DNA chain has 30 bases made up of thymine residues. How many guanine bases would you expect to see in this chain?
a. 10
b. 20
c. 30
d. 40
b. 20
Rationale: In a 50-pair DNA chain, there are 100 bases in total. Given that there are 30 thymine residues, there must be 30 adenine residues (as thymine pairs with adenine). This leaves 40 bases that must be cytosine and guanine. Since cytosine and guanine pair together in equal numbers, there must be 20 cytosine and 20 guanine bases.
Which of the following is an intracellular messenger?
a. cGMP
b. 6-mercaptopurine
c. ATP
d. UDP-glucose
a. cGMP
Rationale: cGMP (cyclic guanosine monophosphate) is a second messenger important in many biological processes. It is used for signal transduction in cells, particularly in processes such as vasodilation. The other options, such as 6-mercaptopurine, ATP, and UDP-glucose, have roles in metabolism and other cellular processes but are not typically classified as intracellular messengers.
What is the rate-limiting enzyme of pyrimidine synthesis?
a. PRPP synthetase
b. Carbamoyl phosphatase
c. Aspartate transcarbamoylase
d. Adenosine deaminase
b. Carbamoyl phosphatase
Rationale: The correct enzyme is actually carbamoyl phosphate synthetase II (CPS II), which catalyzes the formation of carbamoyl phosphate from glutamine and bicarbonate. This enzyme is the rate-limiting step in pyrimidine synthesis. The other enzymes listed are involved in different pathways.
The origin of replication is an area in the DNA strand where replication begins. Which statement regarding the origin of replication is true?
a. Prokaryotes have multiple origins of replication in one DNA strand
b. When the 2 strands separate from the origin of replication, a single replication fork is formed
c. The origin of replication is made up mostly of guanine-cytosine base pairs
d. The separation or melting of the strands is an ATP-dependent process
d. The separation or melting of the strands is an ATP-dependent process
Rationale: The process of strand separation at the origin of replication requires energy, which is provided by ATP hydrolysis. Prokaryotes typically have a single origin of replication per DNA molecule (a), two replication forks form at the origin (b), and the origin of replication is often rich in adenine-thymine base pairs, not guanine-cytosine (c), because A-T pairs are easier to separate.
True of the lagging strand:
a. Chain elongation occurs in the 3’→ 5’ direction
b. It is the strand that is being copied in the direction of the advancing replication fork
c. It is synthesized continuously as the replication fork advances
d. The enzyme responsible for joining its strands is DNA ligase
d. The enzyme responsible for joining its strands is DNA ligase
Rationale: The lagging strand is synthesized discontinuously in short fragments called Okazaki fragments. These fragments are later joined together by DNA ligase. Chain elongation on the lagging strand occurs in the 5’→ 3’ direction (a is incorrect). The lagging strand is synthesized in the direction opposite to the replication fork (b and c are incorrect).
This catalyzes DNA chain elongation:
a. DNA Polymerase I
b. DNA Polymerase III
c. Helicase
d. DNA Topoisomerase
b. DNA Polymerase III
Rationale: DNA Polymerase III is the primary enzyme responsible for DNA chain elongation during replication in prokaryotes. DNA Polymerase I is mainly involved in removing RNA primers and filling in the gaps, Helicase unwinds the DNA helix, and DNA Topoisomerase relieves the torsional strain ahead of the replication fork.
This type of DNA repair resolves pyrimidine dimers:
a. Nucleotide excision repair
b. Mismatch repair
c. Base excision repair
d. Double strand break repair
a. Nucleotide excision repair
Rationale: Nucleotide excision repair (NER) is the primary mechanism for repairing bulky lesions such as pyrimidine dimers caused by UV radiation. This process removes a short single-stranded segment containing the lesion and uses the complementary strand as a template for repair.
This is non-densely packed and transcriptionally active:
a. Euchromatin
b. Intron
c. Solenoid
d. Metaphase chromatid
a. Euchromatin
Rationale: Euchromatin is less densely packed than heterochromatin and is generally transcriptionally active, allowing for gene expression. The other options, such as introns, solenoids, and metaphase chromatids, do not describe a state of chromatin that is transcriptionally active.
Which of the following is responsible for the unwinding of double-stranded DNA?
a. DNA Helicase
b. DNA Ligase
c. DNA Topoisomerase
d. DNA Primase
a. DNA Helicase
Rationale: DNA helicase unwinds the double-stranded DNA during replication, creating the replication fork. DNA ligase joins Okazaki fragments on the lagging strand, DNA topoisomerase relieves torsional strain, and DNA primase synthesizes RNA primers.
How many percent of human DNA is made up of repetitive sequences?
a. 10%
b. 30%
c. 50%
d. 70%
b. 30%
Rationale: More than 50% of DNA in eukaryotic organisms is unique or non-repetitive sequences, while approximately 30% consists of repetitive sequences, such as centromeres, telomeres, and other repetitive elements.
In epigenetics, which of the following will give rise to a transcriptionally active chromosome?
a. DNA methylation
b. Histone acetylation
c. snRNA pairing
d. Telomerase signaling
b. Histone acetylation
Rationale: Histone acetylation is an epigenetic modification that relaxes chromatin structure, making it more accessible to transcription factors and thereby promoting gene expression. DNA methylation typically silences gene expression, snRNA pairing and telomerase signaling are not directly related to transcriptional activation.
A person ate mushrooms picked in a wooded area. Shortly thereafter, he was rushed to the hospital, where he died. He had no previous medical problems. Which stage of transcription was probably affected?
a. Initiation
b. Elongation
c. Termination
d. Post-transcriptional modification
a. Initiation
Rationale: Certain toxic mushrooms contain alpha-amanitin, a potent inhibitor of RNA polymerase II, which is essential for the initiation of transcription in eukaryotic cells. Inhibition of RNA polymerase II prevents the synthesis of mRNA, which is critical for protein production and cell survival.
This is required for transcription initiation:
a. Sigma factor
b. Rho factor
c. Spliceosome
d. RNA polymerase
d. RNA polymerase
Rationale: RNA polymerase is the enzyme responsible for synthesizing RNA from a DNA template. In prokaryotes, the sigma factor (a) is also essential for initiation, but RNA polymerase itself is required for the process in both prokaryotes and eukaryotes. The other options, Rho factor (b) and spliceosome (c), are involved in termination and RNA processing, respectively.
What is the metabolic fate of exons?
a. They are excised upon processing of heterogenous nuclear RNA (hnRNA) to messenger RNA (mRNA)
b. They are retained upon processing of ribosomal RNA and are converted to miRNA and siRNA
c. They are translated into mRNA sequences and participate in the synthesis of proteins
d. They are added to mRNA during the splicing reactions by the spliceosome to prevent mRNA digestion by exonucleases
c. They are translated into mRNA sequences and participate in the synthesis of proteins
Rationale: Exons are the coding sequences of a gene that are retained in the final mRNA product after RNA splicing. They are translated into protein sequences during translation. The other options are incorrect as exons are not excised (a), converted into miRNA or siRNA (b), or added to mRNA to prevent digestion (d).
Which of the following is a component of the 5’ cap?
a. 7-methylguanosine
b. Ribonuclease P
c. The sequence AAUAAA in the intron
d. Small nuclear ribonucleic acid (snRNA)
a. 7-methylguanosine
Rationale: The 5’ cap of eukaryotic mRNA consists of a 7-methylguanosine residue linked to the first nucleotide of the mRNA via a 5’-5’ triphosphate bridge. This modification is crucial for mRNA stability, nuclear export, and translation initiation. The other options are not components of the 5’ cap.
What is the difference between DNA synthesis and RNA synthesis?
a. The whole DNA strand is transcribed to form RNA but not in DNA synthesis
b. The base pairing sequence is different for Guanine in DNA and RNA
c. RNA polymerase does not require a primer, unlike DNA synthesis
d. ATP is required as an energy source for DNA synthesis but not for RNA synthesis
c. RNA polymerase does not require a primer, unlike DNA synthesis
Rationale: RNA polymerase can initiate RNA synthesis de novo without the need for a primer, whereas DNA polymerase requires a short RNA primer synthesized by primase to initiate DNA synthesis. This is a key difference between the two processes. The other options are incorrect because only specific regions of DNA are transcribed into RNA (a), base pairing is the same for guanine in both DNA and RNA (b), and ATP is required as an energy source for both DNA and RNA synthesis (d).
The DNA template segment has this sequence: 3’ AAGCTTCCGC 5’. What would be the RNA product?
a. 5’ UUCGAAGGCG 3’
b. 3’ UUCGAAGGCG 5’
c. 5’ GCGGAAGCUU 3’
d. 3’ TTCGTTGGCG 5’
a. 5’ UUCGAAGGCG 3’
Rationale: RNA is synthesized in the 5’ to 3’ direction using the DNA template strand, which is read in the 3’ to 5’ direction. The complementary RNA sequence to the DNA template strand (3’ AAGCTTCCGC 5’) would be 5’ UUCGAAGGCG 3’.
In Rho-independent RNA transcription, the termination sequence is signaled by:
a. The presence of the sigma factor which causes RNA polymerase to detach from the DNA template strand
b. The rho factor itself
c. A hairpin turn complementary to a region of the DNA template near the termination region (inverted repeats)
d. Topoisomerases inhibiting the formation of negative supercoils during the process of elongation
c. A hairpin turn complementary to a region of the DNA template near the termination region (inverted repeats)
Rationale: Rho-independent termination involves the formation of a hairpin structure in the RNA transcript followed by a series of uracil residues. This structure destabilizes the RNA-DNA hybrid, causing the RNA polymerase to dissociate from the DNA template.
The post-transcriptional processing of rRNA involves which of the following events?
a. Hydrolytic cleavage of portions of the primary transcript
b. Addition of a methylguanosine cap
c. Addition of a Poly-A-tail
d. Insertion of a CCA nucleotide to the 3’ terminus
a. Hydrolytic cleavage of portions of the primary transcript
Rationale: The primary transcript of rRNA undergoes hydrolytic cleavage to produce the mature rRNA species. The other options, such as the addition of a methylguanosine cap, a Poly-A tail, or a CCA nucleotide, are related to the processing of other types of RNA.
When the structure of the TATA box is altered, which phase of transcription is affected?
a. Initiation
b. Elongation
c. Termination
d. Splicing
a. Initiation
Rationale: The TATA box is a promoter element crucial for the initiation of transcription. Alterations in its structure can affect the binding of transcription factors and RNA polymerase, thereby impacting the initiation phase of transcription.
Which drug inhibits the elongation phase of transcription?
a. AZT
b. ddI
c. Rifampin
d. Actinomycin D
d. Actinomycin D
Rationale: Actinomycin D inhibits the elongation phase of transcription by intercalating into DNA and preventing the movement of RNA polymerase along the DNA template. The other drugs, such as AZT, ddI, and Rifampin, have different mechanisms of action related to DNA synthesis or other processes.
Inadequate intake of iodine may cause the following, EXCEPT:
a. Goiter
b. Hypothyroidism
c. Cretinism
d. Wilson’s disease
d. Wilson’s disease
Rationale: Wilson’s disease is a genetic disorder related to copper metabolism, not iodine deficiency. Iodine deficiency can lead to goiter (a), hypothyroidism (b), and cretinism (c).
Principal cation in the intracellular fluid compartment:
a. Potassium
b. Sodium
c. Copper
d. Phosphorus
a. Potassium
Rationale: Potassium is the principal cation in the intracellular fluid compartment. Sodium (b) is the main extracellular cation, and copper (c) and phosphorus (d) have other roles in the body but are not the primary intracellular cations.
In children, a severe deficiency of vitamin D causes:
a. Skin cancer
b. Osteomalacia
c. Rickets
d. Night blindness
c. Rickets
Rationale: Rickets is a disease in children caused by a severe deficiency of vitamin D, leading to impaired bone mineralization. Osteomalacia (b) is the adult equivalent, and vitamin D deficiency is not linked to skin cancer (a) or night blindness (d).
Which of the following vitamins would most likely be deficient in a person with scurvy?
a. Thiamine
b. Cobalamin
c. Pantothenic acid
d. Vitamin C
d. Vitamin C (Ascorbic Acid)
Rationale: Scurvy is caused by a deficiency of vitamin C, which is essential for collagen synthesis. The other vitamins, such as thiamine (a), cobalamin (b), and pantothenic acid (c), are not related to scurvy.
True of vitamin D, EXCEPT:
a. D2 is the form which is vegetable in origin
b. D3 is the form which is animal in origin
c. Rickets manifests as brittle bones due to demineralization
d. Induces intestinal and renal absorption of phosphates
c. Rickets manifests as brittle bones due to demineralization
Rationale: While rickets does involve bone demineralization, it typically results in soft, weak bones rather than brittle bones. Brittle bones are more characteristic of osteoporosis. Vitamin D2 (a) is of vegetable origin, vitamin D3 (b) is of animal origin, and vitamin D induces the absorption of phosphates (d).
Pernicious anemia is the classic consequence of:
a. Thiamine deficiency
b. Cobalamin deficiency
c. Pantothenic acid deficiency
d. Folic acid deficiency
b. Cobalamin deficiency
Rationale: Pernicious anemia is caused by a deficiency in vitamin B12 (cobalamin), often due to a lack of intrinsic factor needed for its absorption. This deficiency leads to impaired red blood cell production and neurological symptoms. The other vitamins are not typically associated with pernicious anemia.
A patient presents with dermatitis, diarrhea, dementia, and stomatitis. He is probably deficient in:
a. Thiamine
b. Niacin
c. Vitamin B12
d. Pantothenic acid
b. Niacin
Rationale: These symptoms are characteristic of pellagra, which is caused by a deficiency in niacin (vitamin B3). The other vitamins listed do not typically cause this combination of symptoms.
All of the following vitamins are soluble in organic solvents used to extract lipids from tissues, EXCEPT:
a. Vitamin K
b. Vitamin A
c. Vitamin C
d. Vitamin E
c. Vitamin C
Rationale: Vitamin C is water-soluble, whereas vitamins A, K, and E are fat-soluble and can be extracted using organic solvents that dissolve lipids.
Specific dynamic action of food:
a. Energy used during digestion, absorption, and metabolism of food
b. Energy needed to maintain basic physiologic functions under standard conditions
c. Energy used during movement and muscle contraction
d. Energy used for muscles and growth of body organs
a. Energy used during digestion, absorption, and metabolism of food
Rationale: The specific dynamic action (also known as the thermic effect) refers to the energy expended by the body to process food, including digestion, absorption, and metabolism.
Basal metabolic rate:
a. Energy used during digestion, absorption, and metabolism of food
b. Energy needed to maintain basic physiologic functions under standard conditions
c. Energy used during movement and muscle contraction
d. Energy used for muscles and growth of body organs
b. Energy needed to maintain basic physiologic functions under standard conditions
Rationale: Basal metabolic rate (BMR) is the amount of energy required to maintain basic physiological functions, such as breathing, circulation, and cell production, while at rest in a neutrally temperate environment.
A deficiency of vitamin B12 causes:
a. Cheilosis
b. Beriberi
c. Scurvy
d. Pernicious anemia
d. Pernicious anemia
Rationale: Vitamin B12 deficiency leads to pernicious anemia due to impaired red blood cell production. Cheilosis (a) is caused by riboflavin (vitamin B2) deficiency, beriberi (b) is caused by thiamine (vitamin B1) deficiency, and scurvy (c) is caused by vitamin C deficiency.
Compounds having the same structural formula but differing in spatial configuration are known as:
a. Anomers
b. Stereoisomers
c. Optical Isomers
d. Epimers
b. Stereoisomers
Rationale: Stereoisomers are molecules that have the same molecular formula and sequence of bonded atoms (structural formula), but differ in the three-dimensional orientations of their atoms in space. Examples include enantiomers and diastereomers.
Table sugar or sucrose is made up of which two single sugar units?
a. Maltose and glucose
b. Glucose and fructose
c. Lactose and glucose
d. Fructose and galactose
b. Glucose and fructose
Rationale: Sucrose is a disaccharide composed of one glucose molecule and one fructose molecule linked by a glycosidic bond.
Which is an example of a triose?
a. Erythrose
b. Glyceraldehyde
c. Xylulose
d. Glucose
b. Glyceraldehyde
Rationale: Glyceraldehyde is a three-carbon sugar (triose). Erythrose (a) is a tetrose, xylulose (c) is a pentose, and glucose (d) is a hexose.
Which of the following is a C-4 epimer of glucose?
a. Mannose
b. Xylose
c. Erythrose
d. Galactose
d. Galactose
Rationale: Galactose is a C-4 epimer of glucose, meaning they differ in configuration around the fourth carbon atom.
Carbohydrates are compounds that contain:
a. Hydroxyl groups
b. Aldehyde or ketone group
c. At least 3 carbons
d. All of the above
d. All of the above
Rationale: Carbohydrates contain hydroxyl groups (a), an aldehyde or ketone group (b), and at least three carbons (c). These functional groups define the structure and reactivity of carbohydrates.
Which process is slow in responding to a falling blood glucose but can provide sustained synthesis of glucose?
a. Aerobic Glycolysis
b. Anaerobic Glycolysis
c. Glycogenolysis
d. Gluconeogenesis
d. Gluconeogenesis
Rationale: Gluconeogenesis is the process of synthesizing glucose from non-carbohydrate precursors. It is a slow process but provides a sustained source of glucose, especially during prolonged fasting or starvation. Glycogenolysis (c) is faster in response to falling blood glucose but is limited by glycogen stores.
Both beriberi and Wernicke-Korsakoff syndrome can be reversed by administering:
a. Vitamin B12
b. Vitamin B1
c. Vitamin B3
d. Vitamin B5
b. Vitamin B1
Rationale: Vitamin B1 (thiamine) deficiency causes both beriberi and Wernicke-Korsakoff syndrome. Administering thiamine can help reverse these conditions.
What is the bond that makes up the primary level of protein structure?
A. peptide
B. hydrogen
C. ionic
D. van der Waals
A. peptide
Rationale: The primary structure of a protein is composed of a sequence of amino acids linked together by peptide bonds. These bonds form between the carboxyl group of one amino acid and the amino group of another.
