Chapter: Molecular Biology
Topic: Gene Regulation
Subtopic: Sirtuins
Keyword Definitions:
• Sirtuins: A family of NAD⁺-dependent deacetylases regulating metabolism, aging, and stress response.
• Metabolism: The chemical processes by which cells generate energy and synthesize molecules.
• Memory: Brain process of storing and retrieving information.
• Vision: The sense that allows organisms to perceive light and images.
• Olfaction: The sensory system for detecting odors.
Lead Question - 2013
Sirtuins are associated with ?
a) Memory
b) Metabolism
c) Vision
d) Olfaction
Explanation: Sirtuins are NAD⁺-dependent enzymes involved in metabolism, aging, and stress adaptation. They regulate mitochondrial function, energy balance, DNA repair, and insulin sensitivity. Their role in promoting longevity is well established. The correct answer is Metabolism. Understanding sirtuins provides therapeutic implications in diabetes, neurodegeneration, and aging-related disorders.
1) Mutation in MSUD leads to deficiency of which enzyme?
a) Branched chain alpha-keto acid dehydrogenase
b) Pyruvate kinase
c) Glucose-6-phosphatase
d) Aldolase B
Explanation: Maple syrup urine disease (MSUD) results from mutations in the branched-chain alpha-keto acid dehydrogenase complex. This prevents breakdown of leucine, isoleucine, and valine, leading to neurotoxicity. The answer is Branched chain alpha-keto acid dehydrogenase. Early diagnosis and dietary management are essential for prevention of neurological complications.
2) A neonate with irritability, poor feeding, and sweet-smelling urine likely has defect in?
a) Urea cycle
b) Branched chain amino acid metabolism
c) Glycogenolysis
d) Fatty acid oxidation
Explanation: Clinical features of poor feeding, lethargy, seizures, and characteristic maple syrup odor in urine indicate MSUD. The underlying defect is in branched chain amino acid metabolism. Without treatment, it leads to severe neurological damage. Dietary restriction of branched-chain amino acids is the cornerstone of therapy.
3) Which vitamin-derived cofactor is essential for branched-chain ketoacid dehydrogenase complex?
a) Thiamine pyrophosphate
b) Pyridoxal phosphate
c) Biotin
d) Niacin
Explanation: The branched-chain ketoacid dehydrogenase requires several cofactors, including thiamine pyrophosphate, lipoic acid, FAD, NAD, and CoA. Thiamine deficiency worsens MSUD. The correct answer is Thiamine pyrophosphate. Some patients respond to high-dose thiamine therapy, known as thiamine-responsive MSUD.
4) A child with MSUD presents in metabolic crisis. Best immediate management is?
a) High protein diet
b) IV glucose with insulin
c) Branched chain amino acid supplementation
d) Ketogenic diet
Explanation: During metabolic crisis in MSUD, excess branched-chain amino acids accumulate, leading to encephalopathy. Immediate treatment includes IV glucose with insulin to reduce catabolism and promote protein synthesis. Dialysis may be required in severe cases. Long-term management involves dietary restriction of branched-chain amino acids.
5) Which metabolite accumulates in urine in MSUD, giving characteristic odor?
a) Homogentisic acid
b) Isovaleric acid
c) Alpha-ketoisocaproic acid
d) Phenylpyruvic acid
Explanation: In MSUD, alpha-ketoacids of branched chain amino acids accumulate, especially alpha-ketoisocaproic acid from leucine. This leads to neurotoxicity and distinctive maple syrup odor in urine. Early newborn screening by tandem mass spectrometry helps in timely detection and management.
6) Assertion-Reason: Assertion: Sirtuins promote longevity. Reason: They activate glycolysis and inhibit oxidative phosphorylation.
a) Both true, Reason correct explanation
b) Both true, Reason not explanation
c) Assertion true, Reason false
d) Both false
Explanation: Sirtuins are linked to longevity through improved DNA repair, stress resistance, and metabolic adaptation. They do not activate glycolysis but instead enhance mitochondrial oxidative metabolism. Thus, the Assertion is true, Reason is false. Calorie restriction upregulates sirtuins, contributing to lifespan extension.
7) Which sirtuin specifically regulates mitochondrial biogenesis?
a) SIRT1
b) SIRT2
c) SIRT3
d) SIRT6
Explanation: Sirtuins act at different cellular sites. SIRT1 activates PGC-1α, promoting mitochondrial biogenesis. SIRT3 acts within mitochondria, regulating enzymes of oxidative phosphorylation. This highlights their importance in energy metabolism and aging biology.
8) A patient with obesity and insulin resistance is found to have reduced sirtuin activity. This indicates disturbance in?
a) Glucose metabolism
b) Protein synthesis
c) Hemoglobin breakdown
d) Lipid absorption
Explanation: Sirtuins regulate insulin sensitivity, adipogenesis, and glucose-lipid metabolism. Reduced activity contributes to metabolic syndrome. Thus, the defect is in Glucose metabolism. Pharmacological activation of sirtuins (e.g., resveratrol) is being explored as therapy for diabetes and obesity.
9) Fill in the blank: In MSUD, the defective enzyme complex normally catalyzes __________.
a) Decarboxylation of branched chain alpha-keto acids
b) Carboxylation of pyruvate
c) Transamination of amino acids
d) Beta oxidation of fatty acids
Explanation: Branched chain alpha-keto acid dehydrogenase catalyzes decarboxylation of branched chain alpha-keto acids. Deficiency leads to accumulation of toxic intermediates in MSUD. This biochemical hallmark is used in newborn screening programs worldwide.
10) Choose the correct statements about sirtuins:
1. They are NAD⁺ dependent
2. They regulate stress responses
3. They inhibit apoptosis
4. They are DNA polymerases
a) 1,2,3 correct
b) 1,3,4 correct
c) 2,3,4 correct
d) 1,2,4 correct
Explanation: Sirtuins are NAD⁺ dependent enzymes that regulate metabolism, stress resistance, and apoptosis. They are not DNA polymerases. Thus, the correct option is 1,2,3. Their modulation is a therapeutic target in aging and metabolic disorders.
Topic: Inborn Errors of Metabolism
Subtopic: Maple Syrup Urine Disease
Keyword Definitions:
• Maple Syrup Urine Disease (MSUD): Genetic disorder due to defect in branched-chain α-keto acid dehydrogenase complex.
• Type Ia MSUD: Subtype linked to mutation in E1α subunit gene (BCKDHA).
• Gene Mutation: Permanent alteration in DNA sequence affecting protein function.
• Branched-chain Amino Acids: Leucine, isoleucine, and valine metabolized via BCKD complex.
• Inheritance: MSUD is transmitted as an autosomal recessive condition.
• Leucine Toxicity: Excess leucine causes severe neurological dysfunction in MSUD.
• Tandem Mass Spectrometry: Diagnostic tool for detecting amino acid disorders in neonates.
• Thiamine: Cofactor of E1 activity in the BCKD complex.
• Burnt Sugar Odor: Characteristic maple syrup-like smell in urine of MSUD patients.
• Dialysis: Rapid detoxification method for metabolic crisis in MSUD.
Lead Question - 2013
In type Ia maple syrup urine disease, gene mutation seen is ?
a) E1α
b) E1β
c) E2
d) E3
Explanation: Type Ia MSUD results from mutation in the E1α subunit of branched-chain α-keto acid dehydrogenase, encoded by the BCKDHA gene.
This defect blocks oxidative decarboxylation of branched-chain amino acids, leading to accumulation and toxicity.
Therefore, the correct answer is a) E1α.
1) The enzyme complex defective in maple syrup urine disease is ?
a) Pyruvate dehydrogenase
b) Branched-chain α-keto acid dehydrogenase
c) Alpha-ketoglutarate dehydrogenase
d) Glucose-6-phosphatase
Explanation: MSUD occurs due to defective branched-chain α-keto acid dehydrogenase.
This enzyme normally metabolizes leucine, isoleucine, and valine.
When absent or deficient, these amino acids and their keto acids accumulate, producing neurotoxicity.
Clinical features include seizures and developmental delay.
Correct answer is b) Branched-chain α-keto acid dehydrogenase.
2) A 10-day-old neonate presents with vomiting, poor feeding, seizures, and urine with burnt sugar odor. Most likely diagnosis is ?
a) Phenylketonuria
b) Maple syrup urine disease
c) Alkaptonuria
d) Tyrosinemia
Explanation: A neonate with encephalopathy, seizures, poor feeding, and urine with burnt sugar odor is strongly suggestive of MSUD.
The disease presents in early life and may be fatal without rapid treatment.
Early diagnosis improves outcomes significantly.
Correct answer is b) Maple syrup urine disease.
3) Which branched-chain amino acid is most neurotoxic in maple syrup urine disease?
a) Leucine
b) Valine
c) Isoleucine
d) All equally toxic
Explanation: Although all branched-chain amino acids accumulate in MSUD, leucine is the most neurotoxic.
Elevated leucine levels cause cerebral edema, seizures, and brain damage.
Clinical management often focuses on rapid reduction of leucine concentration.
Therefore, the correct answer is a) Leucine.
4) A 6-month-old child with developmental delay is suspected of MSUD. Which confirmatory test is most appropriate?
a) Benedict’s test
b) Tandem mass spectrometry
c) Rothera’s test
d) Seliwanoff’s test
Explanation: Tandem mass spectrometry detects elevated plasma leucine, isoleucine, and valine levels, confirming MSUD.
It is also used in neonatal screening programs, ensuring early diagnosis and timely treatment.
This makes it the investigation of choice for suspected cases.
Correct answer is b) Tandem mass spectrometry.
5) Inheritance pattern of maple syrup urine disease is ?
a) Autosomal dominant
b) Autosomal recessive
c) X-linked dominant
d) Mitochondrial
Explanation: MSUD follows autosomal recessive inheritance, requiring mutations in both alleles for disease manifestation.
Parents are usually carriers without clinical features.
Risk of disease recurrence in siblings is 25% for carrier couples.
Correct answer is b) Autosomal recessive.
6) A neonate with MSUD develops acute encephalopathy. Immediate management includes ?
a) Vitamin C supplementation
b) Dialysis and intravenous glucose
c) High protein diet
d) Iron therapy
Explanation: Acute metabolic crises in MSUD are treated with dialysis to remove toxic metabolites and IV glucose to suppress catabolism.
This rapidly lowers amino acid levels and prevents further neurological injury.
Correct answer is b) Dialysis and intravenous glucose.
7) Which cofactor is essential for branched-chain α-keto acid dehydrogenase activity?
a) Thiamine pyrophosphate
b) Biotin
c) Vitamin C
d) Folic acid
Explanation: Branched-chain α-keto acid dehydrogenase requires several cofactors for function, including thiamine pyrophosphate, FAD, NAD+, and lipoic acid.
Thiamine is essential for E1 enzyme activity, and supplementation may benefit thiamine-responsive MSUD patients.
Correct answer is a) Thiamine pyrophosphate.
8) A 2-year-old child with intermittent ataxia and urine odor is diagnosed with intermittent MSUD. Which therapy may be beneficial?
a) Riboflavin
b) Thiamine
c) Pyridoxine
d) Vitamin K
Explanation: Intermittent MSUD cases may respond to high-dose thiamine supplementation, improving enzyme activity and reducing clinical episodes.
This response defines thiamine-responsive MSUD.
Correct answer is b) Thiamine.
9) A newborn screening program detects elevated leucine, isoleucine, and valine. Which disease should be suspected?
a) Alkaptonuria
b) Phenylketonuria
c) Maple syrup urine disease
d) Homocystinuria
Explanation: Detection of elevated branched-chain amino acids in newborn screening strongly suggests maple syrup urine disease.
Early detection allows initiation of dietary restriction, preventing irreversible neurological damage.
Correct answer is c) Maple syrup urine disease.
10) Dietary management of MSUD primarily involves restriction of ?
a) Methionine
b) Phenylalanine
c) Leucine, isoleucine, valine
d) Tyrosine
Explanation: The cornerstone of MSUD therapy is dietary restriction of branched-chain amino acids (leucine, isoleucine, valine).
Controlled supplementation ensures normal growth while avoiding toxic accumulation.
Careful monitoring is essential to maintain metabolic balance.
Correct answer is c) Leucine, isoleucine, valine.
Subtopic: Human Sex Chromosomes
Keywords & Definitions:
Y chromosome: One of the two human sex chromosomes, typically present in males.
Metacentric chromosome: Chromosome with centromere in the middle, arms roughly equal length.
Submetacentric chromosome: Chromosome with centromere slightly off-center, arms of unequal length.
Acrocentric chromosome: Chromosome with centromere near one end, producing a very short p arm.
Centromere: The constricted region of a chromosome where sister chromatids attach.
Chromosome morphology: The physical structure and shape of a chromosome.
Sex chromosomes: Chromosomes determining sex (X and Y in humans).
Chromosome arm: The segment of chromosome on either side of centromere (p - short arm, q - long arm).
Karyotype: The number and appearance of chromosomes in a cell.
Genetics: The study of genes and inheritance.
Lead Question - 2012:
Y chromosome is ?
a) Metacentric
b) Submetacentric
c) Acrocentric
d) None
Explanation & Answer:
The correct answer is c) Acrocentric. The human Y chromosome is acrocentric, meaning its centromere is located near one end, resulting in a very short p arm and a long q arm. This morphology differentiates it from metacentric and submetacentric chromosomes.
Q2. Which human chromosome pair is acrocentric besides the Y chromosome?
a) Chromosome 1
b) Chromosomes 13, 14, 15, 21, 22
c) Chromosome 16
d) Chromosome 18
Explanation & Answer:
Besides the Y chromosome, chromosomes 13, 14, 15, 21, and 22 are acrocentric. These have centromeres close to one end and carry satellite stalks important in ribosomal RNA gene clusters.
Q3. What is the typical shape of the X chromosome?
a) Metacentric
b) Submetacentric
c) Acrocentric
d) Telocentric
Explanation & Answer:
The X chromosome is typically submetacentric with the centromere off-center, giving unequal arm lengths, which contrasts with the acrocentric Y chromosome.
Q4 (Clinical). Turner syndrome is caused by which karyotype abnormality?
a) 45,X (monosomy X)
b) 47,XXY
c) 46,XY
d) 47,XYY
Explanation & Answer:
Turner syndrome results from monosomy X (45,X), where a female has only one X chromosome, leading to characteristic physical and reproductive abnormalities.
Q5. Which region on the Y chromosome is responsible for male sex determination?
a) SRY gene
b) SHOX gene
c) TDF gene
d) Both a and c
Explanation & Answer:
The SRY (Sex-determining Region Y) gene, also called TDF (Testis Determining Factor), is located on the Y chromosome and initiates male sex determination by triggering testis development.
Q6. What is the approximate size of the human Y chromosome?
a) 50 million base pairs
b) 20 million base pairs
c) 10 million base pairs
d) 100 million base pairs
Explanation & Answer:
The Y chromosome is small, approximately 57 million base pairs, roughly 2% of the total human genome, reflecting its limited gene content compared to autosomes and the X chromosome.
Q7. Which chromosome abnormality involves presence of an extra Y chromosome?
a) Klinefelter syndrome
b) XYY syndrome
c) Turner syndrome
d) Down syndrome
Explanation & Answer:
XYY syndrome results from an extra Y chromosome in males (47,XYY), often causing taller stature and sometimes behavioral issues, but usually normal fertility.
Q8. What type of chromosomal anomaly is a Robertsonian translocation?
a) Fusion of two acrocentric chromosomes
b) Duplication of chromosome segment
c) Deletion of chromosome arm
d) Inversion of chromosome segment
Explanation & Answer:
Robertsonian translocation is a chromosomal rearrangement where two acrocentric chromosomes fuse at their centromeres, reducing chromosome number but often maintaining genetic material.
Q9. The short arm of acrocentric chromosomes often contains which genes?
a) Ribosomal RNA (rRNA) genes
b) Hemoglobin genes
c) Immunoglobulin genes
d) Mitochondrial genes
Explanation & Answer:
The short arms of acrocentric chromosomes contain nucleolus organizer regions rich in ribosomal RNA genes essential for ribosome biogenesis.
Q10 (Clinical). Which syndrome is characterized by 47,XXY karyotype?
a) Klinefelter syndrome
b) Turner syndrome
c) Down syndrome
d) Edwards syndrome
Explanation & Answer:
Klinefelter syndrome is a male sex chromosome aneuploidy characterized by an extra X chromosome (47,XXY), causing hypogonadism, infertility, and tall stature.