A. Replication requires an RNA primer because DNA polymerases cannot initiate DNA synthesis. The chromatin of eukaryotes relaxes for replication, achieved through acetylation via histone acetyltransferases. Prokaryotes possess multiple DNA pol, with pol III extending the RNA primer with DNA, and pol I removing and replacing the primer with DNA. Replication starts at specific locations recognized by proteins like DnaA. Deoxynucleoside monophosphates (dNMP) join through phosphodiester bonds, forming a 3′→5′-phosphodiester bond and releasing pyrophosphate. Proofreading, involving the 3′→5′ exonuclease activity of some DNA pol, occurs during the S phase of the cell cycle. Repair can happen independently of replication and outside the S phase.

B. Glutathione in its reduced form (G-SH) acts as an antioxidant. Glutathione peroxidase, a selenium-containing enzyme, converts hydrogen peroxide (H2O2) to water while oxidizing glutathione (G-S-S-G). Glutathione reductase, requiring reduced nicotinamide adenine dinucleotide phosphate (NADPH), regenerates G-SH from G-S-S-G. NADPH is supplied by the oxidative reactions of the pentose phosphate pathway, regulated by NADPH availability at the glucose 6-phosphate dehydrogenase (G6PD)-catalyzed step. G6PD deficiency affects red blood cells, where the pentose phosphate pathway is the sole NADPH source. The pathway involves two irreversible oxidative reactions that generate NADPH, utilized in reductive processes like fatty acid, steroid hormone, and cholesterol synthesis.

A. Jaundice (icterus) manifests as yellow skin, nail beds, and sclerae due to elevated bilirubin levels (hyperbilirubinemia). Bilirubin's solubility in aqueous solutions increases when conjugated with uridine diphosphate–glucuronic acid in the liver, forming bilirubin diglucuronide or conjugated bilirubin (CB). In hemolytic conditions like G6PD deficiency, both CB and unconjugated bilirubin (UCB) increase, but UCB is present in the blood. CB is excreted in the intestine. Phototherapy treats unconjugated hyperbilirubinemia by converting bilirubin to more water-soluble isomeric forms. Bilirubin results from heme degradation in cells of the mononuclear phagocyte system, especially in the liver and spleen. Porphyrias, pathologies of heme synthesis, do not involve hyperbilirubinemia.
Answer = A. All replication requires an RNA primer because DNA polymerases (pol) cannot initiate DNA synthesis. The chromatin of eukaryotes gets decondensed (relaxed) for replication. Relaxation can be accomplished, for example, by acetylation via histone acetyltransferases. Prokaryotes have more than one DNA pol. For example, pol III extends the RNA primer with DNA, and pol I removes the primer and replaces it with DNA. Replication is initiated at specific locations (one in prokaryotes, many in eukaryotes) that are recognized by proteins (for example, DnaA in prokaryotes). Deoxynucleoside monophosphates (dNMP) are joined by a phosphodiester bond that links the 3′hydroxyl group of the last dNMP added with the 5′-phosphate group of the incoming nucleotide, thereby forming a 3′→5′-phosphodiester bond as pyrophosphate is released. Proofreading occurs during replication in the S (synthesis of DNA) phase of the cell cycle and involves the 3′→5′ exonuclease activity possessed by some DNA pol (see figure below). Because repair can occur independently of replication, it can be performed outside of the S phase.
Case 6: Dark Urine and Yellow Sclerae with
Answer = B. Glutathione in its reduced form (G-SH) is an important antioxidant. The selenium-containing enzyme glutathione peroxidase reduces hydrogen peroxide (H2O2, a reactive oxygen species) to water as glutathionine is oxidized (G-S-S-G). Reduced nicotinamide adenine dinucleotide phosphate (NADPH)requiring glutathionine reductase regenerates G-SH from G-S-S-G (see Figure A). The NADPH is supplied by the oxidative reactions of the pentose phosphate pathway (see Figure B), which is regulated by the availability of NADPH at the glucose 6-phosphate dehydrogenase (G6PD)-catalyzed step (the first step). Deficiency of G6PD occurs in all cells, but the effects are seen in red blood cells where the pentose phosphate pathway is the only source of NADPH. The pathway involves two irreversible oxidative reactions, each of which generates NADPH. The NADPH is used in reductive processes such as fatty acid synthesis (not oxidation) as well as steroid hormone and cholesterol synthesis.
Answer = A. Jaundice (icterus) refers to the yellow color of the skin, nail beds, and sclerae that results from bilirubin deposition when the bilirubin level in the blood is elevated (hyperbilirubinemia; see Image C). Bilirubin has low solubility in aqueous solutions, and its solubility is increased by conjugation with uridine diphosphate–glucuronic acid in the liver, forming bilirubin diglucuronide or conjugated bilirubin (CB). In hemolytic conditions, such as G6PD deficiency, both CB and unconjugated bilirubin (UCB) are increased, but it is UCB that is found in the blood. CB is sent into the intestine. Phototherapy is used to treat unconjugated hyperbilirubinemia because it converts bilirubin to isomeric forms that are more water soluble. Bilirubin is the product of heme degradation in cells of the mononuclear phagocyte system, particularly in the liver and the spleen. The porphyrias are pathologies of heme synthesis and, therefore, are not characterized by hyperbilirubinemia.