Which of the following is a ribozyme?
What is the primary function of exonuclease in DNA replication?
Where does oxidative deamination primarily occur in the human body?
Which of the following is a termination codon?
Protein refolding is carried out by?
What is the primary function of reverse transcription?
Which of the following micronutrient deficiencies can lead to anemia?
Which of the following requires vitamin B12?
What is the classification of the Y chromosome?
What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 51: 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.
Question 52: What is the primary function of exonuclease in DNA replication?
- A. Polymerization
- B. Proofreading (Correct Answer)
- C. Chain elongation
- D. Termination
Explanation: ***Proofreading*** - Exonucleases, particularly those associated with **DNA polymerases**, are crucial for **proofreading** during DNA replication. - They remove incorrectly paired nucleotides from the 3' end of the growing DNA strand, ensuring high fidelity of replication. *Polymerization* - **DNA polymerase** is primarily responsible for the **polymerization** of new DNA strands by adding nucleotides. - While exonucleases can be part of the polymerase complex, their main function is not polymerization itself. *Chain elongation* - **Chain elongation** refers to the process of adding nucleotides to the growing DNA strand, which is performed by **DNA polymerase**. - Exonucleases act as a quality control mechanism during this elongation process, rather than carrying out the elongation. *Termination* - **Termination** of DNA replication involves specific sequences and proteins that signal the end of replication, not the primary function of exonucleases. - Exonucleases are active throughout the replication process to maintain accuracy.
Question 53: Where does oxidative deamination primarily occur in the human body?
- A. Cytoplasm of all cells
- B. Mitochondria of all cells
- C. Cytoplasm of liver cells
- D. Mitochondria of liver cells (Correct Answer)
Explanation: ***Mitochondria of liver cells*** - **Oxidative deamination**, particularly of glutamate, is a central process in **amino acid catabolism** and occurs predominantly in the **mitochondria of liver cells**. - This process is crucial for removing the **amino group (NH3)** from amino acids, forming ammonia, which is then detoxified into urea. *Cytoplasm of all cells* - While cells have cytoplasmic metabolic pathways, the primary enzyme for oxidative deamination, **glutamate dehydrogenase**, is located in the mitochondria. - The cytoplasm primarily handles glycolysis and various synthetic pathways, but not the bulk of oxidative deamination. *Mitochondria of all cells* - Although mitochondria are the site of oxidative metabolism in most cells, the **liver** is the main organ responsible for processing exogenous amino acids and their subsequent comprehensive deamination. - Other cells perform some amino acid metabolism, but not the large-scale oxidative deamination seen in the liver. *Cytoplasm of liver cells* - The cytoplasm of liver cells is involved in various metabolic processes, including gluconeogenesis and fatty acid synthesis. - However, the key enzymes for oxidative deamination are specifically compartmentalized within the **mitochondria** of these cells, not the cytoplasm.
Question 54: Which of the following is a termination codon?
- A. AUG
- B. UAA (Correct Answer)
- C. AUA
- D. AGG
Explanation: ***UAA*** - **UAA** is one of the three **stop codons** (UAA, UAG, UGA) that signals the termination of protein synthesis during translation. - When the ribosome encounters a UAA codon, no corresponding tRNA with an anticodon binds, and release factors bind instead, leading to the dissociation of the polypeptide chain. *AUG* - **AUG** is the universal **start codon** in most organisms, encoding for methionine in eukaryotes and N-formylmethionine in prokaryotes. - Its presence signals the initiation of protein synthesis, not its termination. *AUA* - **AUA** is a codon that codes for the amino acid **Isoleucine**. - It is a **sense codon** and does not act as a signal for termination. *AGG* - **AGG** is a codon that codes for the amino acid **Arginine**. - Similar to AUA, it is a **sense codon** and participates in elongating the polypeptide chain, rather than terminating it.
Question 55: Protein refolding is carried out by?
- A. Valine
- B. Threonine
- C. Chaperone (Correct Answer)
- D. Aspartate
Explanation: ***Chaperone*** - **Chaperone proteins** assist in the proper folding of other proteins, particularly during stress conditions like heat shock, by preventing **aggregation** and promoting correct conformation. - They do not become part of the final functional protein but transiently bind during the folding process, thus facilitating **protein refolding** and assembly. *Valine* - **Valine** is an **essential amino acid** and a building block for proteins, but it does not play a direct role in protein refolding. - It contributes to the **hydrophobic core** of proteins due to its non-polar side chain, influencing protein structure but not managing the folding process. *Threonine* - **Threonine** is an **essential amino acid** with a polar side chain, often involved in **glycosylation** and phosphorylation, but not in the complex process of protein refolding. - Its hydroxyl group can participate in **hydrogen bonding**, influencing protein stability and interactions, but not acting as a folding catalyst. *Aspartate* - **Aspartate** is a **non-essential acidic amino acid** that can be involved in various metabolic pathways and is a component of proteins. - Its acidic side chain can form **salt bridges** and hydrogen bonds, contributing to the protein's overall charge and structure, but it does not actively oversee protein refolding.
Question 56: What is the primary function of reverse transcription?
- A. Synthesis of DNA from an RNA template (Correct Answer)
- B. Synthesis of RNA from a DNA template
- C. Synthesis of DNA from a DNA template
- D. Synthesis of RNA from an RNA template
Explanation: ***Synthesis of DNA from an RNA template*** - **Reverse transcription** is catalyzed by the enzyme **reverse transcriptase**, which uses an **RNA template** to synthesize a complementary DNA (cDNA) strand. - This process is fundamental in the life cycle of **retroviruses** like HIV, allowing them to integrate their genetic material into the host genome. *Synthesis of RNA from a DNA template* - This process is known as **transcription**, where genetic information is copied from **DNA to RNA**, not the reverse. - It is a key step in gene expression, leading to the production of various types of RNA molecules. *Synthesis of DNA from a DNA template* - This describes **DNA replication**, the process by which DNA makes copies of itself, ensuring genetic continuity during cell division. - It involves enzymes like **DNA polymerase** and creates two identical DNA molecules from one original DNA molecule. *Synthesis of RNA from an RNA template* - This process is known as **RNA replication** and is characteristic of certain **RNA viruses** (e.g., influenza virus), where RNA serves as both the template and the genetic material. - It involves an enzyme called **RNA-dependent RNA polymerase**.
Question 57: Which of the following micronutrient deficiencies can lead to anemia?
- A. Molybdenum
- B. Copper (Correct Answer)
- C. Fluorine
- D. Selenium
Explanation: ***Copper*** - **Copper** is essential for **iron metabolism** and red blood cell formation; its deficiency can lead to **sideroblastic anemia** (often with microcytic or normocytic features) that may be accompanied by neutropenia. - Copper is required for **ceruloplasmin** function, which is necessary for iron mobilization from stores and incorporation into hemoglobin. - It also plays a role in the function of **superoxide dismutase** and **cytochrome c oxidase**, enzymes involved in antioxidant defense and energy production. *Molybdenum* - **Molybdenum** is a cofactor for several enzymes, including **xanthine oxidase** and **sulfite oxidase**, crucial for purine metabolism and detoxification. - While essential, its deficiency does not typically lead to **anemia** in humans. *Selenium* - **Selenium** is a component of selenoproteins, such as **glutathione peroxidase**, which protect cells from oxidative damage. - Deficiency is associated with conditions like **Keshan disease** (cardiomyopathy) but not primary anemia. *Fluorine* - **Fluorine** (as fluoride) is primarily known for its role in **bone and tooth mineralization**, protecting against dental caries. - It does not directly participate in **hematopoiesis** or iron metabolism, and its deficiency is not linked to anemia.
Question 58: Which of the following requires vitamin B12?
- A. Conversion of serine to lysine
- B. Conversion of serine to glycine
- C. Conversion of glutamine to glutamate
- D. Conversion of homocysteine to methionine (Correct Answer)
Explanation: ***Homocysteine to methionine*** - The conversion of **homocysteine to methionine** is catalyzed by **methionine synthase**, an enzyme that requires **vitamin B12** (cobalamin) as a cofactor. - **Vitamin B12** facilitates the transfer of a methyl group from **methyltetrahydrofolate** to homocysteine, forming methionine. *Conversion of serine to lysine* - The metabolism of **serine to lysine** involves multiple steps and different enzymes, but it does not directly require **vitamin B12**. - Lysine is an **essential amino acid** and is primarily obtained from dietary sources or synthesized through complex pathways. *Conversion of serine to glycine* - The conversion of **serine to glycine** is catalyzed by **serine hydroxymethyltransferase**, which requires **tetrahydrofolate (THF)** as a cofactor, not vitamin B12. - This reaction generates **5,10-methylenetetrahydrofolate**, an important one-carbon donor. *Conversion of glutamine to glutamate* - The conversion of **glutamine to glutamate** is primarily catalyzed by **glutaminase**, an enzyme that does not require **vitamin B12**. - This reaction involves the removal of an **ammonia group** from glutamine to form glutamate.
Question 59: 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.
Question 60: What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
- A. NAD
- B. NADP (Correct Answer)
- C. FAD
- D. FMN
Explanation: ***NADP*** - **NADP+** (nicotinamide adenine dinucleotide phosphate) acts as the **electron acceptor** in the **glucose-6-phosphate dehydrogenase (G6PD)** reaction, becoming **NADPH**. - **NADPH** is crucial for maintaining the **redox balance** in cells, particularly in red blood cells, by reducing **oxidative stress**. *NAD* - **NAD+** (nicotinamide adenine dinucleotide) is a primary cofactor for many **dehydrogenase reactions** in catabolic pathways like **glycolysis** and the **Krebs cycle**. - It primarily functions as an electron acceptor in pathways that generate **ATP**, distinct from the role of **NADPH** in reductive biosynthesis and antioxidant defense. *FAD* - **FAD** (flavin adenine dinucleotide) is a coenzyme derived from **riboflavin (vitamin B2)** that is involved in various redox reactions, often in the form of **flavoproteins**. - Enzymes like **succinate dehydrogenase** in the electron transport chain utilize **FAD** as an electron acceptor, which is not the case for G6PD. *FMN* - **FMN** (flavin mononucleotide) is another coenzyme derived from **riboflavin**, structurally similar to FAD but lacking the additional adenosine monophosphate. - It participates in electron transfer reactions, particularly within **complex I** of the **electron transport chain**, but is not a cofactor for G6PD.