Which of the following is not a precursor in the synthesis of pyrimidines?
What is attached to the 3' end of mRNA after transcription?
What does Chargaff's rule state regarding the base pairing in DNA?
Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
What is the rate-limiting enzyme in heme synthesis?
The primary defect which leads to sickle cell anemia is:
The cofactor vitamin B12 is required for the following conversion:
Which amino acids are involved in the formation of purines?
The Watson-Crick model is for which type of DNA?
Which of the following is a ribozyme?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 131: 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 132: 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 133: What does Chargaff's rule state regarding the base pairing in DNA?
- A. A=T, G=C (Correct Answer)
- B. A=G, T=C
- C. A=C, G=T
- D. Any combination possible
Explanation: ***A=T, G=C*** - **Chargaff's rules** state that in any double-stranded DNA, the amount of **adenine (A)** is approximately equal to the amount of **thymine (T)**, and the amount of **guanine (G)** is approximately equal to the amount of **cytosine (C)**. - This equivalency reflects the specific **base pairing** in the DNA double helix, where A always pairs with T, and G always pairs with C. *A=G, T=C* - This statement is incorrect as it proposes an atypical and biologically inaccurate pairing between a **purine (A)** and another **purine (G)**, and a **pyrimidine (T)** with a **pyrimidine (C)**. - This combination would disrupt the uniform diameter of the DNA double helix required for its structural stability. *A=C, G=T* - This option is incorrect because it suggests pairing a purine (A) with a pyrimidine (C) and a purine (G) with a pyrimidine (T) in a way that is not observed in natural DNA. - Such pairings would also lead to an irregular width of the DNA molecule, destabilizing its structure. *Any combination possible* - This statement is false; base pairing in DNA is **highly specific** and not random due to chemical and structural constraints. - The specific pairing rules (**A with T, G with C**) are crucial for maintaining the consistent structure of the DNA double helix and for accurate DNA replication and transcription.
Question 134: Which of the following statements is most specifically characteristic of mature cytoplasmic messenger RNA (mRNA) compared to its precursor?
- A. Transcribed from nuclear DNA.
- B. Has a lower molecular weight than hn-RNA. (Correct Answer)
- C. Contains uracil instead of thymine.
- D. Sugar is ribose.
Explanation: ***Has a lower molecular weight than hn-RNA.*** - **Mature mRNA** undergoes **splicing**, which removes **introns** (non-coding regions) from the heterogeneous nuclear RNA (hnRNA) precursor. - The removal of these introns results in a **shorter, more compact molecule** with a lower molecular weight compared to the original hnRNA. *Transcribed from nuclear DNA.* - While mRNA is indeed **transcribed from DNA**, this statement is true for **all types of RNA (rRNA, tRNA, and mRNA)**, not just mature cytoplasmic mRNA specifically, and does not differentiate it. - The initial transcript is **hnRNA**, which is then processed into mature mRNA. *Contains uracil instead of thymine.* - This is a characteristic of **all RNA molecules**, not just mature cytoplasmic mRNA, and is a fundamental difference between RNA and DNA. - DNA contains **thymine**, while RNA contains **uracil**. *Sugar is ribose.* - This is a distinguishing feature of **all RNA molecules**, indicating that the sugar component of its nucleotides is **ribose**, whereas DNA contains **deoxyribose**. - This statement is not unique to mature cytoplasmic mRNA.
Question 135: What is the rate-limiting enzyme in heme synthesis?
- A. ALA synthase (Correct Answer)
- B. HMG CoA reductase
- C. ALA dehydratase
- D. Uroporphyrinogen 1 synthase
Explanation: ***ALA synthase*** - **Aminolevulinate synthase** (ALA synthase) is the first and **rate-limiting enzyme** in the heme synthesis pathway. - Its activity is tightly regulated, and its overexpression or deficiency can lead to disorders like **acute intermittent porphyria**. *Hmg coa reductase* - **HMG-CoA reductase** is the **rate-limiting enzyme** in the **cholesterol biosynthesis pathway**, not heme synthesis. - It is the target enzyme for statin medications, which lower cholesterol levels. *ALA dehydratase* - **ALA dehydratase** (also known as porphobilinogen synthase) is the second enzyme in the heme synthesis pathway, responsible for converting two molecules of **ALA to porphobilinogen**. - While critical, it is not the rate-limiting step; inhibition of this enzyme can lead to **lead poisoning**. *Uroporphyrinogen 1 synthase* - **Uroporphyrinogen I synthase** (also called hydroxymethylbilane synthase or porphobilinogen deaminase) catalyzes the formation of **hydroxymethylbilane** from four molecules of **porphobilinogen**. - A deficiency in this enzyme is associated with **acute intermittent porphyria**, but it is not the rate-limiting enzyme of the overall pathway.
Question 136: The primary defect which leads to sickle cell anemia is:
- A. An abnormality in porphyrin part of hemoglobin
- B. A nonsense mutation in the β-chain of HbA
- C. Substitution of valine by glutamate in the α-chain of HbA
- D. Replacement of glutamate by valine in β-chain of HbA (Correct Answer)
Explanation: ***Replacement of glutamate by valine in β-chain of HbA*** - The primary defect in sickle cell anemia is a **point mutation** that leads to the replacement of **glutamic acid** with **valine** in the **sixth position** of the β-globin chain [1]. - This mutation causes the hemoglobin (HbS) to polymerize under low oxygen conditions, resulting in the characteristic **sickle-shaped red blood cells** [1]. *A nonsence mutation in the I3-chain of HbA* - A nonsense mutation leads to a **premature stop codon**, which is not the mechanism behind sickle cell anemia. - This mutation does not involve the β-globin chain, which is critical in this specific disorder. *Substitution of valine by glutamate in the a-chain of HbA* - This statement is incorrect as sickle cell anemia specifically involves the **β-chain**, not the **α-chain**. - Substituting valine with glutamate does not cause sickling but rather the opposite of the actual defect observed in this condition. *An abnormality in porphyrin part of hemoglobin* - Sickle cell anemia does not arise from **porphyrin metabolism issues**, which are related to conditions like **porphyrias**. - The primary defect is a change in the amino acid sequence, not in the porphyrin structure of hemoglobin. **References:** [1] Cross SS. Underwood's Pathology: A Clinical Approach. 6th ed. Common Clinical Problems From Blood And Bone Marrow Disease, pp. 598-599.
Question 137: The cofactor vitamin B12 is required for the following conversion:
- A. Dopamine to Norepinephrine
- B. Propionyl CoA to methyl malonyl CoA
- C. Methyl malonyl CoA to succinyl CoA (Correct Answer)
- D. Homocysteine to cysteine
Explanation: ***Methyl malonyl CoA to succinyl CoA*** - **Vitamin B12**, in its active form **adenosylcobalamin**, is a crucial cofactor for the enzyme **methylmalonyl-CoA mutase**, which catalyzes the isomerization of **methylmalonyl-CoA to succinyl-CoA**. - This conversion is vital for the metabolism of **odd-chain fatty acids** and certain **amino acids**, allowing their entry into the **Krebs cycle**. *Dopamine to Norepinephrine* - This conversion is catalyzed by **dopamine beta-hydroxylase**, which requires **vitamin C** (ascorbate) and **copper** as cofactors, not vitamin B12. - It is a key step in the synthesis of **catecholamines** within the nervous system. *Propionyl CoA to methyl malonyl CoA* - This conversion is catalyzed by **propionyl-CoA carboxylase** and requires **biotin** as a cofactor, not vitamin B12. - This reaction is the first step in the metabolic pathway that leads to succinyl-CoA from odd-chain fatty acids. *Homocysteine to cysteine* - This conversion occurs via the **transsulfuration pathway** and requires **vitamin B6** (pyridoxal phosphate) as a cofactor, not vitamin B12. - The enzymes involved are **cystathionine β-synthase** and **cystathionine γ-lyase**, both B6-dependent. - Vitamin B12 is involved in the **remethylation** of homocysteine to methionine (not in transsulfuration to cysteine).
Question 138: Which amino acids are involved in the formation of purines?
- A. Aspartate, glycine
- B. Aspartate, glutamate
- C. Aspartate, glycine, glutamine (Correct Answer)
- D. Aspartic acid, glycine, uric acid
Explanation: ***Aspartate, glycine, glutamine*** - **Glycine** provides the largest single contribution to the purine ring structure: carbons at positions 4 and 5, and nitrogen at position 7 (the entire glycine molecule is incorporated). - **Aspartate** contributes the nitrogen atom at position 1 of the purine ring. - **Glutamine** donates two nitrogen atoms to the purine ring, specifically at positions 3 and 9. *Aspartate, glycine* - While both **aspartate** and **glycine** are crucial, **glutamine** is also required for the complete synthesis of the purine ring, contributing two nitrogen atoms. - Omission of glutamine makes this option incomplete for the full complement of amino acids involved. *Aspartate, glutamate* - **Glutamate** is not directly involved as an amino acid precursor contributing atoms to the purine ring structure. - **Glutamine**, an amide of glutamate, is the active donor of nitrogen atoms for purine synthesis. *Aspartic acid, glycine, uric acid* - **Aspartic acid** is another name for aspartate, and **glycine** is correctly identified as a precursor. - However, **uric acid** is the end product of purine catabolism, not an amino acid involved in its synthesis.
Question 139: The Watson-Crick model is for which type of DNA?
- A. B DNA (Correct Answer)
- B. A DNA
- C. C DNA
- D. Z DNA
Explanation: ***B DNA*** - The **Watson-Crick model** describes the most common and stable form of DNA found in living organisms under physiological conditions. - **B-DNA** is a right-handed double helix, characterized by a **major and minor groove**, with approximately 10-10.5 base pairs per turn. *A DNA* - **A-DNA** is a **right-handed double helix** that forms under dehydrating conditions and is shorter and wider than B-DNA. - It has a more tilted base pair arrangement and is not the primary form described by the Watson-Crick model. *C DNA* - **C-DNA** is a less common **right-handed double helix** that forms under even lower hydration conditions and in the presence of certain ions. - It has fewer base pairs per turn than B-DNA, typically around 9.3 base pairs. *Z DNA* - **Z-DNA** is unique because it is a **left-handed double helix**, unlike the right-handed forms of A, B, and C DNA. - It is transiently formed in regions with a high concentration of **GC base pairs** and has a zigzag backbone, hence its name.
Question 140: Which of the following is a ribozyme?
- A. Peptidyl transferase (Correct Answer)
- B. Elongation factor 2
- C. Primase
- D. RNA polymerase
Explanation: ***Peptidyl transferase*** - This enzyme is an integral part of the **large ribosomal subunit** and is responsible for catalyzing the formation of peptide bonds during protein synthesis. - While historically thought to be purely proteinaceous, it is now known that the **catalytic activity** of peptidyl transferase comes from its **rRNA component**, specifically the 23S rRNA in prokaryotes and 28S rRNA in eukaryotes, making it a ribozyme. *Elongation factor 2* - **Elongation Factor 2 (EF2)** is a **GTPase** that facilitates the translocation of the ribosome along the mRNA during protein synthesis. - It is a **protein**, not an RNA molecule, and thus does not possess catalytic activity as a ribozyme. *Primase* - **Primase** is an **RNA polymerase** that synthesizes short RNA primers required for the initiation of DNA replication. - It is a **protein enzyme** and not an RNA molecule with catalytic activity. *RNA polymerase* - **RNA polymerase** is a **protein enzyme** responsible for synthesizing RNA from a DNA template during transcription. - It uses a DNA template to produce an RNA strand, but its own catalytic activity is derived from its **protein structure**, not from an RNA component.