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Biochemistry is an important unit of the CSIR NET Life Science examination. It includes topics such as metabolic pathways, protein synthesis, histone modifications, the citric acid cycle, and fatty acid oxidation.
Here are five important questions based on the December 2024 Shift 1 question paper. These questions will help students revise important concepts .
Question 1: Glyoxylate Cycle
Which of the following statements about the glyoxylate cycle is INCORRECT?
(A) The glyoxylate cycle is absent in humans.
(B) The glyoxylate cycle occurs in special cell structures called glyoxysomes.
(C) This cycle helps organisms make sugars from fatty acids.
(D) This cycle involves enzymes present in both glyoxysomes and mitochondria.
Correct Answer: (D)
Solution and Explanation
The glyoxylate cycle is a modified form of the citric acid cycle (TCA cycle). It helps certain organisms convert the carbon present in acetyl-CoA into compounds that can be used to produce sugars.
This pathway is particularly important in plants, fungi, and many bacteria.
Let's understand the options:
Option (A): Correct. Humans do not have a functional glyoxylate cycle because they lack the key enzymes isocitrate lyase and malate synthase.
Option (B): Correct. In plants, the glyoxylate cycle takes place mainly in specialized organelles called glyoxysomes, particularly in germinating oil-rich seeds.
Option (C): Correct. The cycle helps plants convert carbon from stored fatty acids into compounds that can support sugar production.
Option (D): Incorrect The glyoxylate cycle in plants involves enzymes associated mainly with glyoxysomes and the cytosol, not both glyoxysomes and mitochondria as stated in the option.
Question 2: Protein Synthesis and Peptide Bond Formation
A newly discovered organism makes proteins in the reverse direction, from the C-terminal end to the N-terminal end. Which atom of the main protein chain would act as the nucleophile during peptide bond formation?
(A) Nitrogen of the amino group
(B) Carbon of the carboxyl group
(C) Oxygen of the carboxyl group
(D) Alpha-carbon (Cα)
Correct Answer: (C) Oxygen of the carboxyl group
Solution and Explanation
In normal protein synthesis, the ribosome adds new amino acids to the C-terminal end of a growing protein chain. Therefore, the protein is synthesized in the direction:
N-terminus → C-terminus
During normal peptide bond formation, the amino group of the incoming aminoacyl-tRNA attacks the carbonyl carbon of the peptidyl-tRNA. The amino group acts as the nucleophile.
However, the question describes a hypothetical organism that synthesizes proteins in the reverse direction.
If the reaction mechanism is reversed, the atom corresponding to the attacking nucleophile would be the oxygen of the carboxyl group.
Let's understand the options:
Option (A): Incorrect. The amino nitrogen acts as the nucleophile during normal peptide bond formation.
Option (B): Incorrect. The carbon of the carboxyl group acts as the electrophilic center during normal peptide bond formation.
Option (C): Correct according to the intended reverse-reaction interpretation. The carboxyl oxygen is the expected answer when the reaction is considered in reverse.
Option (D): Incorrect. The alpha-carbon is not the nucleophile responsible for peptide bond formation.
Question 3: Histone Modifications
Where do most histone modifications take place?
(A) N-terminal tail
(B) C-terminal tail
(C) Globular core
(D) Equally throughout the polypeptide chain.
Correct Answer: (A) N-terminal tail
Solution and Explanation
Histones are proteins that help DNA remain organized inside the nucleus. DNA wraps around histone proteins to form a structure called the nucleosome.
Histone proteins can undergo chemical changes known as post-translational modifications (PTMs). These modifications influence how tightly DNA is packed and how genes are expressed.
Most commonly studied histone modifications occur on the flexible N-terminal tails of histones.
Examples include:
Acetylation: Addition of an acetyl group.
Methylation: Addition of a methyl group.
Phosphorylation: Addition of a phosphate group.
Ubiquitination: Addition of ubiquitin to a protein.
These modifications help regulate gene expression, DNA repair, and chromosome organization.
Let's understand the options:
Option (A): Correct. The N-terminal tails of histones are major sites of many histone modifications.
Option (B): Incorrect. The C-terminal region is not the main location for most commonly studied histone-tail modifications.
Option (C): Incorrect. The globular core also contains important modification sites, but it is not the principal answer to this question.
Option (D): Incorrect. Histone modifications do not occur equally throughout the entire polypeptide chain.
Question 4: Citric Acid Cycle (TCA Cycle)
The citric acid cycle mainly operates when oxygen is available. What is the main reason for this?
(A) Oxygen directly activates the enzymes of the TCA cycle.
(B) Oxygen acts as the final electron acceptor in the electron transport chain, allowing NADH to be converted back into NAD⁺.
(C) Oxygen removes toxic by-products of the TCA cycle.
(D) Oxygen directly activates ATP synthase.
Correct Answer: (B)
Solution and Explanation
The citric acid cycle, also called the TCA cycle or Krebs cycle, is an important metabolic pathway. It oxidizes acetyl-CoA and produces reduced coenzymes such as NADH and FADH₂.
These coenzymes carry high-energy electrons to the electron transport chain.
In aerobic respiration, oxygen acts as the final electron acceptor in the electron transport chain. It accepts electrons and combines with protons to form water.
This process allows NADH and FADH₂ to be oxidized back into NAD⁺ and FAD.
The regenerated NAD⁺ and FAD are needed for the TCA cycle to continue operating efficiently.
Let's understand the options:
Option (A): Incorrect. Oxygen does not directly activate the enzymes of the TCA cycle.
Option (B): Correct. Oxygen supports electron transport, allowing NADH to be reoxidized to NAD⁺.
Option (C): Incorrect. Removing toxic by-products is not the main reason the TCA cycle depends on aerobic respiration.
Option (D): Incorrect. Oxygen does not directly activate ATP synthase. ATP synthesis is driven by the proton gradient across the inner mitochondrial membrane.
Question 5: Beta-Oxidation of Fatty Acids
Which of the following statements about beta-oxidation of fatty acids is TRUE?
(A) It takes place in the intermembrane space of mitochondria.
(B) All reactions are the same for saturated and unsaturated fatty acids.
(C) It mainly takes place in the mitochondrial matrix for many fatty acids.
(D) It produces glucose directly as its main product.
Correct Answer: (C)
Solution and Explanation
Beta-oxidation is a metabolic pathway that breaks down fatty acids into smaller molecules of acetyl-CoA.
It is an important process for energy production, especially during fasting, prolonged exercise, and periods of low carbohydrate availability.
In eukaryotic cells, beta-oxidation of many fatty acids takes place mainly in the mitochondrial matrix. Very-long-chain fatty acids often begin their breakdown in peroxisomes.
During beta-oxidation, a fatty acid is shortened by two carbon atoms in each cycle.
Each cycle generally involves four steps:
1. Oxidation: FAD is reduced to FADH₂.
2. Hydration: Water is added to the intermediate.
3. Oxidation: NAD⁺ is reduced to NADH.
4. Thiolysis: Acetyl-CoA is released, and the fatty acyl chain becomes shorter.
Let's understand the options:
Option (A): Incorrect. The mitochondrial intermembrane space is not the main site of beta-oxidation.
Option (B): Incorrect. Unsaturated fatty acids may require additional enzymes, such as enoyl-CoA isomerase, and sometimes 2,4-dienoyl-CoA reductase.
Option (C): Correct. The mitochondrial matrix is the major site of beta-oxidation for many fatty acids.
Option (D): Incorrect. Beta-oxidation produces acetyl-CoA, NADH, and FADH₂. It does not directly produce glucose as its main product.
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