Which amino acid is used by the liver in the urea cycle?
Anaplerotic reaction is catalyzed by?
Which macronutrient has the highest thermogenic effect?
Pantothenic acid is needed for donating the following moiety?
Beta-alanine is derived from ?
Which of the following is not a precursor in the synthesis of pyrimidines?
What is attached to the 3' end of mRNA after transcription?
Which glycogen storage disease also presents as a lysosomal storage disease?
The anticodon region is an important part of which type of RNA?
What is the classification of the Y chromosome?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 151: Which amino acid is used by the liver in the urea cycle?
- A. Glutamine
- B. Glutamate
- C. Aspartate (Correct Answer)
- D. Ornithine
Explanation: ***Aspartate*** - **Aspartate** provides the second nitrogen atom to the urea cycle, directly contributing to the formation of **argininosuccinate** through condensation with citrulline. - It is crucial for the efficient removal of **ammonia** in the form of urea. *Glutamine* - **Glutamine** transports ammonia from peripheral tissues to the liver and kidneys, but it is typically deamidated to **glutamate** before its nitrogen can enter the urea cycle. - While it's a major ammonia carrier, it's not directly incorporated into urea as an intact amino acid. *Glutamate* - **Glutamate** can donate its amino group to form **aspartate** (via transamination with oxaloacetate) or release ammonia directly (via glutamate dehydrogenase), both of which then enter the urea cycle. - However, glutamate itself is not directly incorporated into the urea molecule in the same way aspartate is. *Ornithine* - **Ornithine** is an amino acid that participates in the urea cycle as a carrier molecule, being regenerated at the end of each cycle. - While essential for the cycle to function, it is not "used" in the sense of being consumed or providing nitrogen for urea formation - rather it acts as a catalytic intermediate that is recycled.
Question 152: Anaplerotic reaction is catalyzed by?
- A. Enolase
- B. Pyruvate kinase
- C. G6PD
- D. Pyruvate carboxylase (Correct Answer)
Explanation: ***Pyruvate carboxylase*** - **Pyruvate carboxylase** catalyzes the ATP-dependent carboxylation of **pyruvate** to **oxaloacetate**. - This reaction is crucial for replenishing intermediates of the **citric acid cycle**, making it an anaplerotic reaction. *Enolase* - **Enolase** catalyzes the conversion of **2-phosphoglycerate** to **phosphoenolpyruvate** in **glycolysis**. - This reaction is part of catabolism and does not replenish citric acid cycle intermediates. *Pyruvate kinase* - **Pyruvate kinase** catalyzes the final step of **glycolysis**, converting **phosphoenolpyruvate** to **pyruvate**. - This enzyme is involved in ATP production and the overall catabolic pathway of glucose. *G6PD* - **Glucose-6-phosphate dehydrogenase (G6PD)** is the rate-limiting enzyme in the **pentose phosphate pathway**. - It produces **NADPH** and precursors for nucleotide synthesis, but not directly involved in anaplerotic reactions for the citric acid cycle.
Question 153: Which macronutrient has the highest thermogenic effect?
- A. Fat
- B. Proteins (Correct Answer)
- C. Carbohydrate
- D. Alcohol
Explanation: ***Proteins*** - Proteins have the **highest thermogenic effect** (also known as the **thermic effect of food** or TEF) among macronutrients, typically ranging from **20-30%** of their caloric content. - This high TEF is due to the energy required for their **digestion**, **absorption**, and **metabolism** (e.g., deamination, protein synthesis), making them more metabolically "expensive" to process than fats or carbohydrates. *Fat* - Fat has the **lowest thermogenic effect**, accounting for approximately **0-3%** of its caloric content. - This low TEF is because fats are **easily absorbed** and stored, requiring minimal energy for their processing. *Carbohydrate* - Carbohydrates have an intermediate thermogenic effect, typically ranging from **5-10%** of their caloric content. - The energy expenditure for carbohydrate metabolism involves processes like **digestion**, **absorption**, and **glucose oxidation** or conversion to glycogen. *Alcohol* - While alcohol has a relatively high thermogenic effect (**20-25%**), it is **not classified as a macronutrient** in traditional nutritional science. - The three primary macronutrients are proteins, carbohydrates, and fats. Among these, **proteins** have the highest TEF.
Question 154: Pantothenic acid is needed for donating the following moiety?
- A. Carboxyl
- B. Hydroxyl
- C. Amino
- D. Acetyl group (Correct Answer)
Explanation: ***Acetyl group*** - **Pantothenic acid** (vitamin B5) is a crucial component of **coenzyme A (CoA)**, which plays a central role in transferring **acetyl groups**. - This transfer is vital in numerous metabolic pathways, including the **Krebs cycle**, fatty acid synthesis, and fatty acid oxidation. *Carboxyl* - The transfer of **carboxyl groups** is typically mediated by a different coenzyme, **biotin** (vitamin B7), not pantothenic acid. - Biotin-dependent enzymes are involved in carboxylation reactions like those in gluconeogenesis and fatty acid synthesis. *Hydroxyl* - While hydroxyl groups are common in organic chemistry, pantothenic acid's primary role is not the donation of isolated **hydroxyl groups**. - Hydroxyl groups are often donated or accepted during redox reactions or hydrolysis, often by specific enzymes directly. *Amino* - The transfer of **amino groups** is primarily associated with **pyridoxal phosphate (PLP)**, the active form of vitamin B6. - PLP-dependent enzymes, such as **transaminases**, are critical for amino acid metabolism.
Question 155: Beta-alanine is derived from ?
- A. Adenosine
- B. Uracil (Correct Answer)
- C. Guanosine
- D. Thymine
Explanation: ***Correct Option: Uracil*** - **Uracil**, a pyrimidine base found in RNA, is the primary source of **β-alanine** through its catabolic pathway. - The degradation sequence: **Uracil** → **Dihydrouracil** → **β-Ureidopropionate** → **β-Alanine** + CO₂ + NH₃ - This pathway is catalyzed by enzymes including dihydropyrimidine dehydrogenase and β-ureidopropionase. - **β-Alanine** is also obtained from dietary sources and is a component of carnosine and pantothenic acid (Vitamin B5). *Incorrect Option: Thymine* - **Thymine** is a pyrimidine base in DNA with a similar catabolic pathway to uracil. - However, thymine produces **β-aminoisobutyrate** (NOT β-alanine) as its final product. - The pathway: **Thymine** → **Dihydrothymine** → **β-Ureidoisobutyrate** → **β-Aminoisobutyrate** - The extra methyl group on thymine (compared to uracil) results in a different end product. *Incorrect Option: Adenosine* - **Adenosine** is a purine nucleoside composed of adenine and ribose. - Purine catabolism leads to **uric acid** formation, not β-alanine. - It follows the pathway: Adenosine → Inosine → Hypoxanthine → Xanthine → Uric acid. *Incorrect Option: Guanosine* - **Guanosine** is a purine nucleoside consisting of guanine and ribose. - Like adenosine, it is catabolized to **uric acid** via xanthine. - It does not participate in β-alanine synthesis.
Question 156: Which of the following is not a precursor in the synthesis of pyrimidines?
- A. Glutamine
- B. Carbon dioxide (CO2)
- C. Aspartic acid
- D. Thymidine (Correct Answer)
Explanation: ***Thymidine*** - **Thymidine** is a *nucleoside* consisting of deoxyribose and thymine. It is a *product* and a component of DNA, not a precursor in the *de novo synthesis* of pyrimidine bases. - While it can be incorporated into DNA via the *salvage pathway*, it does not serve as an initial building block for the pyrimidine ring itself. *Glutamine* - **Glutamine** provides the **nitrogen atoms** crucial for the formation of the pyrimidine ring, specifically N3 in the pyrimidine base. - It is a key donor of *amino groups* in various anabolic pathways, including nucleotide synthesis. *Carbon dioxide (CO2)* - **Carbon dioxide (CO2)** contributes one of the carbon atoms (C2) to the pyrimidine ring. - It combines with **ammonia** (derived from glutamine) to form **carbamoyl phosphate**, an essential intermediate. *Aspartic acid* - **Aspartic acid** provides four atoms (N1, C4, C5, C6) of the pyrimidine ring. - Its carbon skeleton and amino group are directly incorporated into the pyrimidine structure during the *de novo synthesis* pathway.
Question 157: What is attached to the 3' end of mRNA after transcription?
- A. CCA
- B. Intron
- C. 7-methylguanosine
- D. Poly-A tail (Correct Answer)
Explanation: ***Poly-A tail*** - A **poly-A tail**, consisting of multiple adenosine monophosphates, is added to the **3' end of mRNA** after transcription to protect it from degradation. - This modification aids in the **transport of mRNA from the nucleus to the cytoplasm** and in its translation. *CCA* - The **CCA sequence** is found at the **3' end of tRNA**, not mRNA, and is critical for amino acid attachment. - It is added post-transcriptionally to tRNA molecules by the enzyme **tRNA nucleotidyltransferase**. *Intron* - **Introns** are non-coding regions within a gene that are transcribed into mRNA but are subsequently removed during **RNA splicing**, not added to the 3' end. - Their removal ensures that only the **coding regions (exons)** are translated into protein. *7-methylguanosine* - **7-methylguanosine** forms the **5' cap** of mRNA, which is added to the 5' end, not the 3' end. - This cap is important for **mRNA stability**, ribosome binding, and protection against degradation.
Question 158: Which glycogen storage disease also presents as a lysosomal storage disease?
- A. Von Gierke's disease
- B. McArdle's disease
- C. Andersen's disease
- D. Pompe's disease (Correct Answer)
Explanation: ***Pompe's disease*** - Also known as **glycogen storage disease type II**, it is caused by a deficiency of **acid alpha-glucosidase (GAA)**, a *lysosomal enzyme*. - This deficiency leads to the accumulation of **glycogen in lysosomes**, particularly affecting muscle tissue, thereby earning its classification as both a glycogen storage disease and a lysosomal storage disease. *Von Gierke's disease* - This is **glycogen storage disease type I** and is due to a deficiency in **glucose-6-phosphatase**. - It primarily affects the **liver and kidneys**, causing severe **hypoglycemia** and **lactic acidosis**, but it is not classified as a lysosomal storage disease. *McArdle's disease* - This is **glycogen storage disease type V**, caused by a deficiency in **muscle glycogen phosphorylase (myophosphorylase)**. - It manifests as **exercise intolerance** and muscle pain, but it does not involve lysosomal enzyme defects or glycogen accumulation in lysosomes. *Andersen's disease* - This is **glycogen storage disease type IV**, caused by a deficiency in the **glycogen branching enzyme**. - It leads to the formation of **abnormal glycogen structures**, primarily affecting the liver and causing early liver failure, but it is not a lysosomal storage disorder.
Question 159: The anticodon region is an important part of which type of RNA?
- A. r-RNA
- B. m-RNA
- C. t-RNA (Correct Answer)
- D. hn-RNA
Explanation: **t-RNA** - The **anticodon region** is a critical component of **transfer RNA (tRNA)**, responsible for recognizing and binding to the complementary codon on mRNA during protein synthesis. - This interaction ensures that the correct **amino acid** is delivered to the growing polypeptide chain according to the genetic code. *r-RNA* - **Ribosomal RNA (rRNA)** is a structural and enzymatic component of **ribosomes**, which are the cellular machinery for protein synthesis. - While rRNA plays a crucial role in forming **peptide bonds** and facilitating translation, it does not possess an anticodon region. *m-RNA* - **Messenger RNA (mRNA)** carries the **genetic code** from DNA to the ribosomes in the form of codons, which specify the sequence of amino acids for protein synthesis. - mRNA molecules have codons, but they do not have an **anticodon region**; instead, they are read by the anticodons of tRNA. *hn-RNA* - **Heterogeneous nuclear RNA (hnRNA)** is a precursor to mRNA in eukaryotic cells, containing both exons and introns. - It undergoes extensive processing, including **splicing**, to become mature mRNA, but it does not have an **anticodon region**.
Question 160: What is the classification of the Y chromosome?
- A. Metacentric
- B. Submetacentric (Correct Answer)
- C. Acrocentric
- D. None of the options
Explanation: ***Submetacentric*** - The **Y chromosome** is classified as submetacentric because its **centromere** is located off-center, resulting in two arms of unequal length. - The short arm (Yp) is smaller than the long arm (Yq), but not as disproportionate as in acrocentric chromosomes. - The **X chromosome** is also submetacentric, making both sex chromosomes belong to this category. *Metacentric* - A **metacentric chromosome** has its **centromere** located in the middle, resulting in two arms of approximately equal length. - Examples include chromosomes 1, 3, 16, 19, and 20, which have nearly equal arm ratios unlike the Y chromosome. *Acrocentric* - An **acrocentric chromosome** has its **centromere** located very close to one end, creating one very short arm and one very long arm. - The five acrocentric human chromosomes are **13, 14, 15, 21, and 22**, which possess satellite DNA and nucleolar organizing regions (NORs) on their short arms. - The **Y chromosome is NOT acrocentric** despite historical confusion; it has a more centrally positioned centromere than true acrocentric chromosomes. *None of the options* - This option is incorrect because the Y chromosome has a specific and well-established classification as **submetacentric** based on its centromere position and arm ratio.