In patients with Retinitis pigmentosa, which of the following substances is known to have decreased levels?
In case of LPL deficiency, which of the following will increase after a fat rich diet?
True regarding mitochondrial DNA is:
Retinitis pigmentosa is associated with deficiency of:
Amino acid required for conversion of norepinephrine to epinephrine:-
Type-I hyperlipoproteinemia is caused by deficiency of:-
Mark the false statement regarding mitochondrial DNA:
Which amino acid is involved in the synthesis of creatinine, NO, and urea?
Which of the following is not a dietary fiber?
In Zellweger syndrome, which of the following is absent?
NEET-PG 2019 - Biochemistry NEET-PG Practice Questions and MCQs
Question 11: In patients with Retinitis pigmentosa, which of the following substances is known to have decreased levels?
- A. Thromboxane
- B. Arachidonic acid
- C. Docosahexaenoic acid (DHA) (Correct Answer)
- D. Linoleic acid
Explanation: ***Docosahexaenoic acid (DHA)*** - **Docosahexaenoic acid (DHA)** is a crucial **omega-3 fatty acid** abundantly found in the **photoreceptor cell membranes**, particularly in the retina. - Reduced levels of DHA are frequently observed in patients with **retinitis pigmentosa**, suggesting its role in disease pathogenesis and retinal health. *Arachidonic acid* - **Arachidonic acid** is an **omega-6 fatty acid** and a precursor to many signaling molecules, but its levels are **not typically decreased** in retinitis pigmentosa. - It plays a **pro-inflammatory role** and is involved in various physiological processes, distinct from the primary metabolic defects in retinitis pigmentosa. *Linoleic acid* - **Linoleic acid** is an essential **omega-6 fatty acid** and a precursor to arachidonic acid, but its deficiencies are not characteristic of retinitis pigmentosa. - It is crucial for **skin barrier function** and overall health, but its metabolic pathways are distinct from those primarily affected in retinal degenerations. *Thromboxane* - **Thromboxane** is a **lipid mediator** primarily involved in **platelet aggregation** and vasoconstriction. - It is not directly associated with the metabolic pathways or structural integrity of the retina, and its levels are not typically altered in retinitis pigmentosa.
Question 12: In case of LPL deficiency, which of the following will increase after a fat rich diet?
- A. LDL
- B. HDL
- C. Lipoprotein (a)
- D. Chylomicron (Correct Answer)
Explanation: ***Chylomicron*** - **LPL (lipoprotein lipase)** is crucial for the breakdown of **chylomicrons** and VLDL. A deficiency leads to an accumulation of undigested chylomicrons in the bloodstream after a fat-rich meal. - **Chylomicrons** transport dietary triglycerides from the intestines to tissues. Without LPL, these triglycerides remain packaged in chylomicrons. *LDL* - **LDL (low-density lipoprotein)** levels are not directly increased by a short-term fat-rich diet in the context of LPL deficiency. LDL primarily carries cholesterol and is formed from VLDL remnants, a process that is also impaired by LPL deficiency indirectly. - While chronic LPL deficiency can affect overall lipid metabolism, the immediate post-meal increase is not in LDL but in triglyceride-rich lipoproteins. *HDL* - **HDL (high-density lipoprotein)** is involved in reverse cholesterol transport and is generally not directly increased after a fat-rich diet, especially in LPL deficiency. - In fact, severe hypertriglyceridemia, often seen in LPL deficiency, can sometimes lead to lower HDL levels due to altered lipid exchange. *Lipoprotein (a)* - **Lipoprotein (a)**, or Lp(a), is a genetically determined lipoprotein similar to LDL but with an added apolipoprotein (a) and its levels are not acutely affected by dietary fat intake or LPL deficiency. - Lp(a) levels are determined primarily by genetic factors and do not participate in the post-prandial handling of dietary fats.
Question 13: True regarding mitochondrial DNA is:
- A. Linear double stranded
- B. All respiratory proteins are synthesized within mitochondria itself
- C. Inherited from mother (Correct Answer)
- D. Low mutation rate
Explanation: ***Inherited from mother*** - **Mitochondrial DNA (mtDNA)** is exclusively inherited from the mother because the sperm's mitochondria are typically destroyed after fertilization or do not enter the oocyte. - This **maternal inheritance pattern** makes mtDNA useful for tracing lineage and studying human population movements. *Linear double stranded* - **Mitochondrial DNA** is typically **circular**, not linear, and double-stranded, similar to a plasmid in bacteria. - **Linear DNA** is characteristic of nuclear chromosomes in eukaryotes. *All respiratory proteins are synthesized within mitochondria itself* - While mitochondria contain their own ribosomes and synthesize some proteins, the majority of **respiratory chain proteins** are encoded by **nuclear DNA** and imported into the mitochondria. - The **mitochondrial genome** encodes only a small fraction of the proteins necessary for mitochondrial function, primarily components of the electron transport chain. *Low mutation rate* - **Mitochondrial DNA** has a **higher mutation rate** compared to nuclear DNA due to a less robust DNA repair system and exposure to reactive oxygen species generated during oxidative phosphorylation. - The high mutation rate can contribute to mitochondrial diseases and can also be used in evolutionary studies.
Question 14: Retinitis pigmentosa is associated with deficiency of:
- A. Timnodonic acid
- B. DHA (Correct Answer)
- C. Eicosa pentaenoic acid
- D. Arachidonic acid
Explanation: ***DHA*** - **Docosahexaenoic acid (DHA)** is the major polyunsaturated fatty acid in the **retinal photoreceptor outer segments** and is crucial for their function. - Deficiency in DHA has been linked to several retinal degeneration disorders, including **retinitis pigmentosa**, suggesting its importance in maintaining retinal health. *Timnodonic acid* - This is an older term for **eicosapentaenoic acid (EPA)**, which is an omega-3 fatty acid. - While EPA is beneficial for overall health, it is **not the primary fatty acid** associated with the direct structural and functional health of retinal photoreceptors as DHA is. *Eicosa pentaenoic acid* - **Eicosapentaenoic acid (EPA)** is an omega-3 fatty acid found in fish oil, known for its anti-inflammatory properties. - While important for general health, EPA is **not as abundant or critical for retinal structure and function** as DHA. *Arachidonic acid* - **Arachidonic acid (AA)** is an omega-6 fatty acid found in cell membranes and is a precursor to pro-inflammatory mediators. - While present in the retina, AA is generally **not associated with a protective or causative role in retinitis pigmentosa** in the same way DHA is.
Question 15: Amino acid required for conversion of norepinephrine to epinephrine:-
- A. Lysine
- B. Tryptophan
- C. Methionine (Correct Answer)
- D. Phenylalanine
Explanation: ***Methionine*** - **Norepinephrine** is converted to **epinephrine** by the enzyme **phenylethanolamine N-methyltransferase (PNMT)**. - This enzyme uses **S-adenosylmethionine (SAM)** as a **methyl donor**, which is derived from methionine. *Lysine* - **Lysine** is an essential amino acid primarily involved in **protein synthesis**, **calcium absorption**, and the production of **carnitine**. - It does not directly participate in the methylation reaction converting norepinephrine to epinephrine. *Tryptophan* - **Tryptophan** is a precursor for **serotonin** and **niacin** synthesis. - It is not involved in the catecholamine synthesis pathway from norepinephrine to epinephrine. *Phenylalanine* - **Phenylalanine** is the initial amino acid in the **catecholamine synthesis pathway**, being converted to **tyrosine**, then to DOPA, dopamine, and norepinephrine. - While it's crucial for the synthesis *up to* norepinephrine, it is not directly involved in the *conversion of norepinephrine to epinephrine*.
Question 16: Type-I hyperlipoproteinemia is caused by deficiency of:-
- A. Lipoprotein lipase (Correct Answer)
- B. Elevated triglycerides in plasma
- C. Elevated LDL
- D. Elevated cholesterol
Explanation: ***Lipoprotein lipase*** - **Type I hyperlipoproteinemia**, also known as **familial lipoprotein lipase deficiency**, is caused by a genetic defect leading to **deficiency or defect in lipoprotein lipase (LPL)** or its cofactor **apolipoprotein C-II**. - LPL is crucial for the **hydrolysis of triglycerides** in chylomicrons and VLDL at the capillary endothelium. - This enzymatic deficiency leads to **massive accumulation of chylomicrons** and severe hypertriglyceridemia (often >1000 mg/dL). - Clinical features include **eruptive xanthomas, lipemia retinalis, hepatosplenomegaly**, and **recurrent pancreatitis**. *Elevated triglycerides in plasma* - This is indeed the **most prominent laboratory finding** in Type I hyperlipoproteinemia, with triglyceride levels often exceeding 1000-2000 mg/dL. - However, this is the **consequence/manifestation** of the LPL deficiency, not the underlying cause. - The question asks what causes Type I hyperlipoproteinemia, which is the enzyme deficiency itself. *Elevated LDL* - Type I hyperlipoproteinemia typically has **normal or even reduced LDL levels**. - **Elevated LDL** is characteristic of **Type IIa hyperlipoproteinemia (familial hypercholesterolemia)**, which involves defects in LDL receptor or ApoB-100. - Type I primarily affects **chylomicron metabolism**, not LDL. *Elevated cholesterol* - Cholesterol levels are typically **normal or only mildly elevated** in Type I hyperlipoproteinemia. - The triglyceride elevation is disproportionately massive compared to any cholesterol elevation. - Significant isolated cholesterol elevation points to Type IIa or IIb dyslipidemias.
Question 17: Mark the false statement regarding mitochondrial DNA:
- A. AGA and AGG are stop codons in mitochondrial DNA
- B. Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA
- C. Does not show heteroplasmy (Correct Answer)
- D. 1% of cellular DNA, 13 proteins of respiratory chain
Explanation: ***Does not show heteroplasmy*** - This statement is false because **mitochondrial DNA (mtDNA)** commonly exhibits **heteroplasmy**, meaning the presence of more than one type of mitochondrial genome within a cell or individual. - **Heteroplasmy** arises due to the presence of both normal and mutated mtDNA, which can be passed down from the mother. *AGA and AGG are stop codons in mitochondrial DNA* - This statement is true; in the **universal genetic code**, AGA and AGG code for **arginine**, but in **human mitochondrial DNA**, they serve as **stop codons**. - This is an example of the **differences** in genetic code interpretation between the nuclear genome and the mitochondrial genome. *Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA* - This statement is true; **Kearns-Sayre Syndrome** is a well-known mitochondrial disorder caused by a **large single deletion** in the mitochondrial DNA. - This deletion often leads to chronic progressive **external ophthalmoplegia**, **retinal pigmentary degeneration**, and **cardiac conduction defects**. *1% of cellular DNA, 13 proteins of respiratory chain* - This statement is true; **mitochondrial DNA constitutes** approximately **1% of the total cellular DNA** by mass. - It codes for **13 essential proteins** that are part of the **electron transport chain** (respiratory chain) complexes in the mitochondrion, along with ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs).
Question 18: Which amino acid is involved in the synthesis of creatinine, NO, and urea?
- A. Arginine (Correct Answer)
- B. Glycine
- C. Aspartate
- D. Citrulline
Explanation: ***Arginine*** - **Arginine** is a precursor for **Nitric Oxide (NO)** synthesis via **nitric oxide synthase**. - It is also a substrate for **creatinine** synthesis (along with glycine and methionine) and plays a key role in the **urea cycle**. *Glycine* - **Glycine** is a precursor for **creatinine** synthesis, but not directly involved in NO or urea production as the primary amino acid. - It is also a component of glutathione and purine synthesis. *Aspartate* - **Aspartate** is a key intermediate in the **urea cycle**, contributing one nitrogen atom to urea. - It is not directly a precursor for NO or creatinine. *Citrulline* - **Citrulline** is an intermediate in the **urea cycle** and is converted to arginine. - It is not directly involved in the synthesis of creatinine or as the primary precursor for NO.
Question 19: Which of the following is not a dietary fiber?
- A. Cellulose
- B. Starch (Correct Answer)
- C. Pectin
- D. Inulin
Explanation: ***Starch*** - **Starch** is a **complex carbohydrate** that serves as a major energy source for humans and can be digested into glucose. - Unlike dietary fiber, starch is broken down by enzymes (like **amylase**) in the digestive tract, absorbed, and used for energy. *Cellulose* - **Cellulose** is a **polysaccharide** found in the cell walls of plants and is a major component of dietary fiber. - Humans lack the enzymes to digest cellulose, so it passes through the digestive system largely intact, adding **bulk to stool**. *Pectin* - **Pectin** is a **soluble dietary fiber** found in fruits, particularly apples and citrus. - It forms a gel-like substance when mixed with water and is known for its ability to lower cholesterol and regulate blood sugar. *Inulin* - **Inulin** is a **soluble dietary fiber** and a type of fructan, found in many plants like chicory root, onions, and garlic. - It acts as a **prebiotic**, promoting the growth of beneficial gut bacteria in the colon.
Question 20: In Zellweger syndrome, which of the following is absent?
- A. Golgi apparatus
- B. Peroxisomes (Correct Answer)
- C. Mitochondria
- D. ER
Explanation: ***Peroxisomes*** - **Zellweger syndrome** is an **autosomal recessive disorder** characterized by a severe reduction or absence of functional peroxisomes. - Peroxisomes are essential organelles involved in **lipid metabolism**, particularly the breakdown of very long-chain fatty acids (VLCFAs) and branched-chain fatty acids, leading to their accumulation in the blood and tissues. *Golgi apparatus* - The **Golgi apparatus** is an intact and functional organelle in Zellweger syndrome. - It plays a crucial role in modifying, sorting, and packaging proteins and lipids for secretion or delivery to other organelles, functions that remain unaffected in this condition. *Mitochondria* - **Mitochondria**, responsible for cellular respiration and ATP production, are present and functional in Zellweger syndrome. - While metabolic disturbances occur, they are not due to primary mitochondrial dysfunction. *ER* - The **endoplasmic reticulum (ER)**, a network of membranes involved in protein and lipid synthesis, is also intact and functional. - It is not directly implicated in the pathogenesis of Zellweger syndrome.