Biological Sciences

Nitrogenase is the enzyme responsible for biological nitrogen fixation, that is converting atmospheric nitrogen (N2) into ammonia (NH3). It is extremely sensitive to oxygen, meaning it is irreversibly inactivated by it. The nitrogenase requires anaerobic conditions to function effectively. Many nitrogen fixing organisms have special adaptations to create an environment of low oxygen partial pressure for the functioning of nitrogenase. For example, in legumes, leghemoglobin in the root nodules creates an oxygen-free environment. This is because oxygen is an oxidizing agent that interferes with the redox reactions of the nitrogenase enzyme. Therefore, nitrogen fixation is favored in oxygen free conditions.

The Ti (tumor-inducing) plasmid of Agrobacterium tumefaciens contains a segment of DNA called T DNA (Transfer DNA), which is transferred from the bacteria to the plant genome during infection. The T-DNA is a specific DNA that contains the genes required for tumor growth in plants. Options R, cDNA and A DNA are not related to the Ti plasmid.

Type I restriction enzymes have both endonuclease (cleaving DNA) and methyl-transferase (modifying DNA by adding a methyl group) activities. They:
• Recognize specific DNA sequences but cleave DNA at random locations far away from the recognition site.
• Methylate specific bases within the recognition sequence to protect the host DNA from cleavage.
This dual functionality is characteristic of Type I restriction enzymes, differentiating them from other types of restriction enzymes that mainly perform cleavage. Type II enzymes cleave DNA at specific site. Type III enzymes cleave DNA at a specific sequence but not at the recognition site.

Germline gene therapy involves inserting a functional gene into germ cells (sperms or eggs) or very early-stage embryos. This ensures that the genetic modification is heritable, meaning the change will be passed down to future generations. This method aims to correct genetic defects not just in an individual but also in their future progeny. Options A, B and C (Hepatic, fibroblast and endothelium cells) are somatic cells, which are not passed down to future generations.

Helper T cells (TH cells) are a type of white blood cells that plays a crucial role in adaptive immune response. Their main functions involve:
• Stimulating B cell division and antibody production: TH cells help B cells proliferate and differentiate into plasma cells, which produce antibodies.
• Activating Cytotoxic T cells: TH cells stimulate the activation and expansion of cytotoxic T cells (Tc) cells using cytokines. Cytotoxic T cells are the cells responsible for direct killing of infected cells or tumor cells.
• Stimulating macrophage activity: TH cells enhance the phagocytic and microbicidal activity of macrophages through cytokines.
TH cells themselves are not cytotoxic. Cytotoxicity is a primary function of cytotoxic T cells, not helper T cells. Cytotoxic T cells have CD8+ receptors. TH cells has CD4+ receptors.

Antigen specificity, the ability of a B cell to recognize and bind to a specific antigen, is primarily determined by the variable regions of its antibodies. These regions:
• Form the antigen-binding site, which is highly variable to bind to a wide range of antigens
• Undergo somatic recombination, a process that generates a diverse repertoire of antibody binding sites.
• Involve both the heavy and light chain variable regions, which together form the antigen-binding pocket.
Options A, B and C are incorrect because they do not fully explain how specificity arises. While interaction with antigen is important for activation, it does not determine specificity. The heavy and light chain contribute to specificity and is due to both not just individual chain.

Cysteine is a sulfur-containing amino acid featuring a thiol (-SH) group in its side chain. This group plays critical roles in protein chemistry:
• Forms disulfide bonds (-S-S-) during oxidation, stabilizing protein structures.
• Participates in catalytic activity in enzyme active sites.
Cysteine’s unique properties make it indispensable in maintaining protein structure and function.

Inulin is a polysaccharide composed of fructose units linked by β(2 → 1) glycosidic bonds. It is a storage carbohydrate found in plants like chicory and Jerusalem artichokes.
• Unlike starch (a glucose polymer), inulin is not digested by human enzymes and functions as dietary fiber.
• It acts as a prebiotic, promoting the growth of beneficial gut bacteria.
Inulin’s unique structure makes it valuable in health and nutrition research.

Gram-positive bacteria retain the purple stain in Gram staining due to their thick peptidoglycan layer, which has low lipid content. When treated with alcohol:
• The alcohol dehydrates the peptidoglycan layer, causing it to shrink and trap the crystal violet-iodine complex.
• Gram-negative bacteria, with higher lipid content, lose the outer membrane and the stain during alcohol treatment.
This property differentiates Gram-positive and Gram-negative bacteria in microbiological analysis.

Succinate dehydrogenase is an enzyme that is involved both in the Krebs cycle (citric acid cycle) and the electron transport chain (ETC). In Krebs cycle, succinate dehydrogenase catalyzes the oxidation of succinate to fumarate and at the same time in the ETC, it transfers the electrons to FADH2 to ubiquinone (coenzyme Q). This is the only enzyme that is embedded in the inner mitochondrial membrane, linking two major energy producing metabolic processes.

PCR (Polymerase Chain Reaction) is a technique used to amplify DNA by three
main steps:
• Denaturation: The DNA is heated to 95°C, causing the double stranded DNA molecule to separate into two single strands.
• Annealing: The temperature is lowered to 50-65°C to allow the primers to bind to the complementary sequences of the DNA strands.
• Extension: The temperature is raised to 72°C (optimum temperature for Taq DNA Polymerase) to allow DNA polymerase to synthesize new DNA strands by adding nucleotides to the primers.
These steps are repeated in cycles to amplify specific regions of the DNA. The correct order is
therefore Denaturation >Annealing > Extension.

Sucrose is a non-reducing sugar because it does not have a free aldehyde or ketone group. In sucrose, the glucose and fructose units are linked by their anomeric carbons, preventing the molecule from reacting with Benedict’s or Tollens’ solutions. Other points:
• Sucrose is commonly referred to as table sugar.
• It is a disaccharide with the formula .
• Non-reducing sugars do not participate in redox reactions typical of reducing sugars.

Vitamins and their metabolic coenzyme roles:
• Thiamine (Vitamin B1) is essential for carbohydrate metabolism, as it’s a precursor for co-carboxylase.
• Riboflavin forms important coenzymes, like flavin adenine dinucleotide (FAD) and riboflavin adenine nucleotide, which are vital for redox reactions.
• Niacin is converted into nicotinamide adenine dinucleotide (NAD) and NADP, both crucial for many metabolic processes, especially in redox reactions.
• Folic acid plays a role in nucleotide synthesis, and it is metabolized to tetrahydrofolic acid.
The correct matching is: (A) - (iv), (B) - (i), (C) - (iii), (D) - (ii).

Immunoglobulins with their specific roles:
• IgA is in mucosal secretions and is associated with secretory components.
• IgE activate basophils and mast cells in allergic reactions.
• IgG crosses the placenta, providing immunity to fetus.
• IgM is the first antibody in an immune response and exists as a pentamer.
Therefore, the correct answer is: (A) - (II), (B) - (I), (C) - (IV), (D) - (III)

Enzymes with their functions:
• Alkaline phosphatase removes 5’ phosphate groups from DNA and RNA
• DNA polymerase is involved in nick translation.
• S1 Nuclease cleaves single stranded DNA
• DNase I cleaves DNA randomly.
Therefore, the correct matching is: (A) - (IV), (B) - (III), (C) - (II), (D) - (I)

Prophase I of meiosis is divided into 5 stages that are characterized by specific events:
• Leptotene: Chromosomes condense and become visible under microscope.
• Zygotene: Homologous chromosomes pair up forming synaptonemal complex.
• Pachytene: Crossing over occurs between homologous chromosomes.
• Diplotene: Synaptonemal complex disassembles, and chiasmata (cross over points) becomes visible.
• Diakinesis: Chromosomes condense further and chiasmata move to terminal positions, preparing for separation.
The correct sequence is therefore Leptotene, Zygotene, Pachytene, Diplotene and Diakinesis.