NEET-PG 2012 - Biochemistry Practice Questions

Q: In which condition is the utilization of pyruvate in tissues decreased?

Beriberi. ***Beriberi*** - Beriberi is caused by **thiamine (vitamin B1) deficiency**, which is a crucial cofactor for the **pyruvate dehydrogenase complex (PDH)**. - A dysfunctional PDH enzyme leads to a decreased conversion of **pyruvate to acetyl-CoA**, thus **decreasing pyruvate utilization** and causing its accumulation. *Pernicious anemia* - This condition is caused by a deficiency in **vitamin B12 (cobalamin)**, typically due to a lack of intrinsic factor, leading to **megaloblastic anemia**. - While vitamin B12 is essential for various metabolic pathways, it does not directly impair the utilization of **pyruvate** by PDH. *Scurvy* - Scurvy results from **vitamin C (ascorbic acid) deficiency**, which is essential for collagen synthesis and acts as an antioxidant. - Vitamin C deficiency does not directly impact the activity of the **pyruvate dehydrogenase complex** or the utilization of pyruvate. *Pellagra* - Pellagra is caused by a deficiency in **niacin (vitamin B3)**, or its precursor, tryptophan. - Niacin is a component of **NAD+ and NADP+**, which are crucial coenzymes in many metabolic reactions, but its deficiency does not primarily manifest as decreased **pyruvate utilization**.

Q: Which of the following is not a component of a nucleotide?

Fatty acid. ***Fatty acid*** - A **fatty acid** is a component of **lipids**, such as triglycerides and phospholipids, which are structurally and functionally distinct from **nucleotides**. - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), whereas fatty acids are essential for cell membranes and energy storage. *Sugar* - A **pentose sugar** (either **deoxyribose** in DNA or **ribose** in RNA) is a fundamental component of every nucleotide. - This sugar forms the backbone of the nucleic acid strand, covalently linked to the phosphate group and the nitrogenous base. *Phosphate* - A **phosphate group** is a crucial component of a nucleotide, providing the negative charge and forming the phosphodiester bonds that link nucleotides together into a nucleic acid chain. - The number of phosphate groups (mono-, di-, or triphosphate) determines the nucleotide's energy state and function. *Base* - A **nitrogenous base** (adenine, guanine, cytosine, thymine, or uracil) is an essential component of a nucleotide, responsible for genetic information storage and pairing. - This base is attached to the pentose sugar and determines the specific identity of the nucleotide within the DNA or RNA sequence.

Q: The Central Dogma of molecular biology includes all except?

Reverse transcription. ***Reverse transcription*** - The **original Central Dogma** as proposed by Francis Crick described the unidirectional flow of genetic information: **DNA → RNA → Protein**. - **Reverse transcription** (RNA → DNA) was **not part of the original Central Dogma** and was only discovered later with the identification of **reverse transcriptase enzyme** in retroviruses by Baltimore and Temin (1970). - While reverse transcription is now recognized as an important biological process, it represents an **exception to the classical Central Dogma** rather than a core component. *Transcription* - **Transcription** (DNA → RNA) is a **fundamental process** within the Central Dogma. - It involves synthesis of RNA from a DNA template and is the first step in gene expression in all living organisms. *Translation* - **Translation** (RNA → Protein) is the **second major step** of the Central Dogma. - This process occurs on ribosomes where mRNA is decoded into a specific sequence of amino acids to form proteins. *RNA replication* - **RNA replication** (RNA → RNA), while primarily seen in RNA viruses, is considered part of the **expanded Central Dogma**. - It represents one of the possible flows of genetic information in biological systems, though not universal to all organisms.

Q: At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?

Glyceraldehyde-3-phosphate dehydrogenase. ***Glyceraldehyde-3-phosphate dehydrogenase*** - This enzyme catalyzes the oxidation and **phosphorylation** of glyceraldehyde-3-phosphate, producing **1,3-bisphosphoglycerate**. - During this reaction, **NAD+ is reduced to NADH**, which is a crucial step for energy production. *Pyruvate kinase* - This enzyme catalyzes the final step of glycolysis, transferring a phosphate group from **phosphoenolpyruvate** to ADP, forming ATP and pyruvate. - This step involves **substrate-level phosphorylation** for ATP production, not NADH. *Enolase* - This enzyme catalyzes the dehydration of **2-phosphoglycerate** to form **phosphoenolpyruvate (PEP)**. - This reaction involves the removal of a water molecule and does not produce NADH. *PFK-1* - **Phosphofructokinase-1 (PFK-1)** catalyzes the phosphorylation of fructose-6-phosphate to **fructose-1,6-bisphosphate**. - This is an ATP-consuming and a crucial regulatory step in glycolysis, but it does not involve NADH production.

Q: Which enzyme is required for cutting the DNA strand during synthesis?

Topoisomerase. ***Topoisomerase*** - **Topoisomerases** are enzymes essential for DNA replication; they induce temporary **single- or double-strand breaks** in DNA to relieve **supercoiling** ahead of the replication fork. - This cutting and rejoining activity prevents the DNA from becoming excessively tangled and facilitates the unwinding process required for synthesis. *DNA polymerase* - **DNA polymerase** is responsible for **synthesizing new DNA strands** by adding nucleotides, not for cutting the DNA backbone. - It works by moving along the template strand, reading the bases, and then adding complementary nucleotides to the growing DNA strand. *DNA ligase* - **DNA ligase** functions to **join DNA fragments** together by forming phosphodiester bonds, especially in sealing Okazaki fragments during lagging strand synthesis. - Its role is to ligate (join) cut strands, not to initiate cuts in the DNA. *Helicase* - **Helicase** unwinds the DNA double helix into single strands using ATP hydrolysis; it **separates the two strands** but does not cut the phosphodiester backbone. - This enzyme creates the replication fork by disrupting hydrogen bonds between base pairs, making the DNA accessible for replication machinery.

Q: DNA microarrays allow detection of gene mutations through which process?

Hybridization. ***Hybridization*** - DNA microarrays detect mutations by **hybridizing labeled patient DNA** to **thousands of oligonucleotide probes** containing known DNA sequences immobilized on a solid surface. - A mismatch between the patient's DNA and the probe results in **reduced or absent hybridization**, indicating a **mutation or genetic variation**. - This principle allows **high-throughput screening** of multiple genes simultaneously. *Polymerase Chain Reaction* - PCR is used to **amplify specific DNA sequences exponentially**, creating millions of copies from minimal starting material. - While PCR may be used to **prepare DNA samples** before microarray analysis, it is **not the detection mechanism** on the chip itself. *Cloning* - Cloning involves creating **identical copies of DNA fragments, cells, or organisms** using vectors and host cells. - It's a method for **producing large quantities** of specific DNA sequences but **not a detection technique** for mutations on microarrays. *Southern Blotting* - Southern blotting detects specific DNA sequences through **gel electrophoresis, membrane transfer, and probe hybridization**. - While it also uses hybridization, it is a **low-throughput technique** analyzing one sample at a time, unlike the **high-throughput parallel analysis** of DNA microarrays.

Q: Protein glycosylation occurs in:

Endoplasmic reticulum (ER). ***ER*** - **N-linked glycosylation**, the most common type of protein glycosylation, initiates in the **endoplasmic reticulum (ER)**, where an oligosaccharide precursor is transferred to asparagine residues of newly synthesized proteins. - The ER environment facilitates protein folding and quality control, ensuring correctly folded glycoproteins are transported further. *Golgi bodies* - While **further modification and processing** of glycosylated proteins occur in the Golgi apparatus, the initial step of N-linked glycosylation begins in the ER. - The Golgi is responsible for trimming and adding different sugar residues to complete the **glycan chains** and for sorting the glycoproteins to their final destinations. *Mitochondria* - Mitochondria are primarily involved in **cellular respiration** and **ATP production**. - They do not play a significant role in protein glycosylation; most mitochondrial proteins are imported from the cytoplasm in an unglycosylated state. *Peroxisomes* - Peroxisomes are involved in various **metabolic processes**, including fatty acid oxidation and detoxification. - They are not known to be sites of protein glycosylation.

Q: Which isoform of LDH is raised in Anemia ?

LDH 2. ***Correct: LDH 2*** - **LDH 2 (H3M1)** is highly abundant in **red blood cells (RBCs)**, second only to LDH 1. - In **hemolytic anemia**, there is increased destruction of red blood cells, leading to the release of intracellular LDH isoforms into the bloodstream. - Both **LDH 1 and LDH 2 are significantly elevated** in hemolytic anemia, as RBCs contain predominantly these two isoforms. - The elevation of LDH 2 is particularly notable and diagnostically useful in anemia, especially hemolytic conditions. *Incorrect: LDH 1* - **LDH 1 (H4)** is the most abundant isoform in **heart muscle** and is also present in **red blood cells**. - While LDH 1 is indeed elevated in hemolytic anemia, this option is not the best answer in this context. - LDH 1 elevation is classically associated with **myocardial infarction**. *Incorrect: LDH 4* - **LDH 4 (HM3)** is present in various tissues, including **liver, skeletal muscle, and kidneys**, but in lower concentrations. - Not the primary isoform associated with anemia. *Incorrect: LDH 5* - **LDH 5 (M4)** is predominantly found in the **liver and skeletal muscles**. - Elevation of LDH 5 typically indicates **liver damage, muscle injury, or malignancies**, not primarily anemia.

Q: Which molecule serves as the ultimate source of acetyl groups for fatty acid synthesis?

Acetyl CoA. ***Acetyl CoA*** - **Acetyl CoA** is the ultimate source of all acetyl groups used in fatty acid synthesis - It serves as the substrate for **acetyl CoA carboxylase**, which converts it to **malonyl CoA** - After transport from mitochondria via **citrate**, acetyl CoA is the precursor for all two-carbon units incorporated into fatty acids - One molecule of acetyl CoA also serves as the primer for fatty acid synthesis *Malonyl CoA* - **Malonyl CoA** is the direct two-carbon donor to the growing fatty acid chain - However, it is derived from **acetyl CoA** through carboxylation by **acetyl CoA carboxylase** - It is an intermediate, not the ultimate source of acetyl groups *Palmitate* - **Palmitate** is a 16-carbon saturated fatty acid that is the end product of de novo fatty acid synthesis - It is the product of fatty acid synthesis, not a donor of acetyl groups *Citrate* - **Citrate** transports acetyl groups from the **mitochondria** to the **cytosol** where fatty acid synthesis occurs - In the cytosol, **ATP citrate lyase** cleaves citrate back into **acetyl CoA** and oxaloacetate - Citrate is a transport vehicle, not the ultimate source of acetyl groups

Q: What is the role of Apoprotein C-II in lipid metabolism?

Activates lipoprotein lipase. ***Activates lipoprotein lipase*** - **Apoprotein C-II (ApoC-II)** is a crucial **activator** of **lipoprotein lipase (LPL)**. - LPL is an enzyme responsible for **hydrolyzing triglycerides** from chylomicrons and VLDL, allowing fatty acids to be taken up by tissues. - **Deficiency of ApoC-II** leads to severe hypertriglyceridemia due to inability to activate LPL. *Inhibits lipoprotein lipase* - This is the function of **ApoC-III**, not ApoC-II. - ApoC-III **inhibits LPL activity**, which is the opposite role of ApoC-II. *Facilitates triglyceride transport* - While apoproteins are essential for **assembly and transport of lipoproteins** that carry triglycerides, this is not the specific primary role of ApoC-II. - ApoC-II's primary function is **regulating LPL enzyme activity**, not direct transport facilitation. *None of the options* - This is incorrect because ApoC-II clearly **activates lipoprotein lipase**, which is one of the given options.

Question 1

In which condition is the utilization of pyruvate in tissues decreased?

Accepted Answer

Beriberi

Answer

Pernicious anemia

Answer

Scurvy

Answer

Pellagra

Question 2

Which of the following is not a component of a nucleotide?

Accepted Answer

Fatty acid

Answer

Sugar

Answer

Base

Answer

Phosphate

Question 3

The Central Dogma of molecular biology includes all except?

Accepted Answer

Reverse transcription

Answer

Transcription

Answer

RNA replication

Answer

Translation

Question 4

At which step in glycolysis is NADH produced during the oxidation of glyceraldehyde-3-phosphate?

Accepted Answer

Glyceraldehyde-3-phosphate dehydrogenase

Answer

Pyruvate kinase

Answer

Enolase

Answer

PFK-1

Question 5

Which enzyme is required for cutting the DNA strand during synthesis?

Accepted Answer

Topoisomerase

Answer

DNA polymerase

Answer

DNA ligase

Answer

Helicase

Question 6

DNA microarrays allow detection of gene mutations through which process?

Accepted Answer

Hybridization

Answer

Polymerase Chain Reaction

Answer

Cloning

Answer

Southern Blotting

Question 7

Protein glycosylation occurs in:

Accepted Answer

Endoplasmic reticulum (ER)

Answer

Peroxisomes

Answer

Mitochondria

Answer

Golgi bodies

Question 8

Which isoform of LDH is raised in Anemia ?

Accepted Answer

LDH 2

Answer

LDH 5

Answer

LDH 4

Answer

LDH 1

Question 9

Which molecule serves as the ultimate source of acetyl groups for fatty acid synthesis?

Accepted Answer

Acetyl CoA

Answer

Malonyl CoA

Answer

Palmitate

Answer

Citrate

Question 10

What is the role of Apoprotein C-II in lipid metabolism?

Accepted Answer

Activates lipoprotein lipase

Answer

None of the options

Answer

Inhibits lipoprotein lipase

Answer

Facilitates triglyceride transport

NEET-PG 2012 — Biochemistry