What is the function of DNA ligase?
What is the primary function of reverse transcription?
Which of the following usually require a RNA intermediate for cloning/replication?
Which of the following is a termination codon?
Protein refolding is carried out by?
Where does oxidative deamination primarily occur in the human body?
Which of the following is a plasma protein involved in blood clotting?
Which of the following pairs of compounds has the highest standard reduction potential?
Which of the following statements BEST describes the net ATP production in glycolysis?
What cofactor is required for the proper functioning of glucose-6-phosphate dehydrogenase?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 81: What is the function of DNA ligase?
- A. Unwinding (denaturation) of dsDNA to provide an ssDNA template
- B. Initiation of DNA synthesis and elongation
- C. Initiates synthesis of RNA primers
- D. Joins the Okazaki fragments on the lagging strand by sealing the nicks between them. (Correct Answer)
Explanation: ***Joins the Okazaki fragments on the lagging strand by sealing the nicks between them.*** - **DNA ligase** catalyzes the formation of a **phosphodiester bond** between adjacent nucleotides, specifically joining the 3'-hydroxyl of one fragment to the 5'-phosphate of another. - This enzyme is crucial for completing **DNA replication** on the lagging strand by connecting the discontinuous **Okazaki fragments**. *Unwinding (denaturation) of dsDNA to provide an ssDNA template* - This function is primarily carried out by **DNA helicase**, which unwinds the double helix, separating the two strands. - DNA ligase plays no direct role in the initial unwinding of the DNA molecule. *Initiation of DNA synthesis and elongation* - The initiation of DNA synthesis is performed by **DNA primase** (laying down RNA primers) and then extended by **DNA polymerase**. - DNA ligase's role is to seal gaps, not to initiate or elongate new DNA strands. *Initiates synthesis of RNA primers* - The synthesis of **RNA primers** is the specific function of **DNA primase**. - These primers provide a free 3'-hydroxyl group for DNA polymerase to begin synthesizing new DNA.
Question 82: 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 83: Which of the following usually require a RNA intermediate for cloning/replication?
- A. Cosmids
- B. Retroviruses (Correct Answer)
- C. Plasmids
- D. Transposons
Explanation: ***Retroviruses*** - **All retroviruses require an RNA intermediate** for their replication cycle, making this the correct answer. - Retroviruses have an **RNA genome** that must be **reverse transcribed into DNA** by reverse transcriptase enzyme before integration into the host genome. - The integrated DNA (provirus) is then transcribed back to RNA, which serves both as mRNA for viral proteins and as genomic RNA for new virions. - Examples include **HIV, HTLV**, and other retroviruses that definitively use this RNA → DNA → RNA replication strategy. *Transposons* - This option is **too broad** to be correct. Only **retrotransposons** (Class I transposons) use RNA intermediates via a "copy-and-paste" mechanism involving reverse transcription. - However, **DNA transposons** (Class II) move by a "cut-and-paste" DNA mechanism **without any RNA intermediate**. - Since the question asks what "usually requires" RNA intermediate, and many common transposons (like bacterial Tn5, Tn10) are DNA transposons, this answer is imprecise. *Cosmids* - Cosmids are **hybrid cloning vectors** containing cos sites from bacteriophage lambda combined with plasmid sequences. - They replicate as **DNA plasmids** in bacteria using DNA-dependent DNA polymerase. - No RNA intermediate is involved in their replication mechanism. *Plasmids* - Plasmids are **extrachromosomal circular DNA molecules** that replicate independently within bacterial or yeast cells. - Replication occurs via **DNA-to-DNA synthesis** using DNA polymerase. - No RNA intermediate is required for plasmid propagation.
Question 84: 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 85: 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 86: 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 87: Which of the following is a plasma protein involved in blood clotting?
- A. Fibrinogen (Correct Answer)
- B. Lactate dehydrogenase (LDH)
- C. Aspartate aminotransferase (SGOT)
- D. Alanine aminotransferase (SGPT)
Explanation: ***Fibrinogen*** - **Fibrinogen** is a crucial plasma protein that is converted into **fibrin** during the coagulation cascade. - **Fibrin** then forms a meshwork, which is the structural basis of a **blood clot**. *Lactate dehydrogenase (LDH)* - **LDH** is an enzyme found in many tissues throughout the body and is involved in **cellular metabolism**, specifically the conversion of pyruvate to lactate. - Elevated levels of **LDH** can indicate tissue damage or disease but are not directly involved in blood clotting. *Aspartate aminotransferase (SGOT)* - **SGOT** (now commonly referred to as **AST**) is an enzyme primarily found in the **liver, heart, skeletal muscle, kidneys, brain, and red blood cells**. - High levels of **AST** are often indicative of **liver damage** or other organ injury, but it does not play a direct role in blood coagulation. *Alanine aminotransferase (SGPT)* - **SGPT** (now commonly referred to as **ALT**) is an enzyme predominantly found in the **liver**. - Elevated **ALT** levels are a sensitive marker for **liver cell damage** but are not involved in the blood clotting process.
Question 88: Which of the following pairs of compounds has the highest standard reduction potential?
- A. NADH/NAD+
- B. Succinate/Fumarate
- C. Ubiquinone/Ubiquinol
- D. Fe³⁺/Fe²⁺ (Correct Answer)
Explanation: ***Fe³⁺/Fe²⁺*** - The **Fe³⁺/Fe²⁺ couple** has a **standard reduction potential (E'0)** of **+0.77 V**, making it the highest among the given options. - A higher positive E'0 indicates a stronger tendency for the oxidized form to accept electrons and be reduced. *NADH/NAD+* - The **NADH/NAD+ couple** has a **standard reduction potential** of **-0.32 V**, indicating it is a strong reducing agent. - Its negative reduction potential means it readily donates electrons during metabolic processes. *Succinate/Fumarate* - The **succinate/fumarate couple** has a **standard reduction potential** of **+0.03 V**. - This pair is involved in the **TCA cycle**, where succinate is oxidized to fumarate, releasing electrons. *Ubiquinone/Ubiquinol* - The **ubiquinone/ubiquinol couple** has a **standard reduction potential** varying around **+0.05 to +0.10 V**, depending on the specific state. - It acts as a mobile electron carrier in the **electron transport chain**, accepting electrons from NADH and FADH2.
Question 89: Which of the following statements BEST describes the net ATP production in glycolysis?
- A. Glycolysis produces 2 molecules of pyruvate
- B. Glycolysis produces a net gain of 2 ATP per glucose molecule (Correct Answer)
- C. Hexokinase consumes ATP during glycolysis
- D. Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules
Explanation: ***Glycolysis produces a net gain of 2 ATP per glucose molecule*** - In the initial "investment" phase of glycolysis, **2 ATP molecules are consumed** to phosphorylate glucose. - In the subsequent "payoff" phase, **4 ATP molecules are produced** through substrate-level phosphorylation, resulting in a net gain of 2 ATP. *Glycolysis produces 2 molecules of pyruvate* - While glycolysis does produce **2 molecules of pyruvate** from one glucose molecule, this statement describes the end product of the pathway, not the net ATP production. - Pyruvate is a crucial product that can be further metabolized in aerobic or anaerobic conditions, but it does not directly represent the energy yield in terms of ATP. *Hexokinase consumes ATP during glycolysis* - **Hexokinase** is indeed the enzyme that catalyzes the first ATP-consuming step in glycolysis, phosphorylating glucose to glucose-6-phosphate. - However, this statement describes only one aspect of ATP utilization within the pathway and does not account for the total ATP produced or the overall net gain. *Aldolase catalyzes the conversion of fructose-1,6-bisphosphate into two three-carbon molecules* - **Aldolase** is a key enzyme in glycolysis responsible for cleaving **fructose-1,6-bisphosphate** into dihydroxyacetone phosphate and glyceraldehyde-3-phosphate. - This step is part of the preparatory phase of glycolysis but does not directly describe the net ATP production.
Question 90: 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.