What is the function of primase?
Which RNA is used in RNA splicing?
Which of the following does not play a role in protein synthesis?
Which of the following GAG is not sulfated?
Which of the following types of bonds is considered the weakest?
Male to male transmission is seen in -
Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?
All are activated by insulin except?
ATP is consumed at which of the following steps of glycolysis?
Glycogen synthase is activated by?
NEET-PG 2012 - Biochemistry NEET-PG Practice Questions and MCQs
Question 111: What is the function of primase?
- A. Joining DNA fragments
- B. Synthesizing small RNA fragments during translation
- C. Unwinding of DNA
- D. Synthesizing small RNA fragments during DNA synthesis (Correct Answer)
Explanation: ***Synthesizing small RNA fragments during DNA synthesis*** - **Primase** is an enzyme that synthesises short **RNA primers** which are crucial for initiating DNA replication. - These **RNA primers** provide a free 3'-hydroxyl group, which **DNA polymerase** requires to start adding deoxyribonucleotides. *Joining DNA fragments* - This function is primarily carried out by **DNA ligase**, which forms phosphodiester bonds between adjacent nucleotides to join DNA fragments. - **DNA ligase** is essential for repairing DNA breaks and joining **Okazaki fragments** on the lagging strand during replication. *Synthesising small RNA fragments during translation* - Small RNA fragments are generally involved in **gene regulation** (e.g., microRNAs) or structural components of ribosomes (e.g., ribosomal RNA) during translation, but primase is not involved in their synthesis for this purpose. - The synthesis of **mRNA**, **tRNA**, and **rRNA** during translation is carried out by **RNA polymerases**, not **primase**. *Unwinding of DNA* - The **unwinding of the DNA double helix** is primarily performed by an enzyme called **DNA helicase**. - **DNA helicase** breaks the hydrogen bonds between complementary base pairs, separating the two strands to allow replication or transcription to proceed.
Question 112: Which RNA is used in RNA splicing?
- A. mRNA
- B. tRNA
- C. rRNA
- D. Small nuclear RNA (snRNA) (Correct Answer)
Explanation: ***Small nuclear RNA (snRNA)*** - **snRNAs** are key components of **spliceosomes**, the molecular machines that catalyze the removal of introns from pre-mRNA. - They bind to specific sequences within the pre-mRNA and facilitate the splicing reactions. *mRNA* - **mRNA (messenger RNA)** carries the genetic code from DNA to the ribosomes for **protein synthesis**. - While it is the molecule that gets spliced, it does not directly participate in the splicing machinery itself. *rRNA* - **rRNA (ribosomal RNA)** is a structural and catalytic component of **ribosomes**, where protein synthesis occurs. - It plays no direct role in the process of RNA splicing. *tRNA* - **tRNA (transfer RNA)** molecules are responsible for carrying specific **amino acids** to the ribosome during protein synthesis. - They are involved in translation, not in the processing of RNA by splicing.
Question 113: Which of the following does not play a role in protein synthesis?
- A. m-RNA
- B. ATP
- C. Intron (Correct Answer)
- D. Exon
Explanation: ***Intron*** - Introns are **non-coding regions** within a gene that are transcribed into RNA but are subsequently **spliced out** before translation. - They do not carry genetic information for protein synthesis; their removal ensures the correct sequence of amino acids is produced. *Exon* - Exons are the **coding regions** of a gene that contain the genetic information for protein synthesis. - After introns are removed, exons are ligated together to form the **mature mRNA** that is translated into protein. *m-RNA* - **Messenger RNA (mRNA)** carries the genetic code from DNA in the nucleus to the ribosomes in the cytoplasm. - It serves as the **template** for protein synthesis through the process of translation. *ATP* - **Adenosine triphosphate (ATP)** provides the **energy** required for various steps in protein synthesis, including mRNA transcription, amino acid activation, and ribosome movement. - It is a crucial energy currency that fuels the process of forming peptide bonds and assembling the polypeptide chain.
Question 114: Which of the following GAG is not sulfated?
- A. Keratan sulfate
- B. Dermatan sulfate
- C. Chondroitin sulfate
- D. Hyaluronic acid (Correct Answer)
Explanation: ***Hyaluronic acid*** - **Hyaluronic acid** is unique among glycosaminoglycans (GAGs) because it is the only one that is **not sulfated**. - It also distinguishes itself by being the only GAG that does **not form proteoglycans** and is not synthesized in the Golgi apparatus. *Chondroitin sulfate* - **Chondroitin sulfate** is a sulfated glycosaminoglycan that is a major component of the **extracellular matrix**, particularly in cartilage. - Its sulfate groups contribute to its **negative charge**, allowing it to attract water and provide resistance to compression. *Dermatan sulfate* - **Dermatan sulfate** is another sulfated GAG, found predominantly in the skin, blood vessels, and heart valves. - It contains **sulfate groups**, which are crucial for its interactions with various proteins and its role in tissue structure. *Keratan sulfate* - **Keratan sulfate** is a sulfated GAG found in the cornea, cartilage, and bone. - It is distinct from other GAGs due to its **lack of uronic acid** and the presence of sulfate groups.
Question 115: Which of the following types of bonds is considered the weakest?
- A. Covalent
- B. Hydrogen
- C. Electrostatic
- D. Van der Waals (Correct Answer)
Explanation: ***Van der Waals*** - **Van der Waals forces** are very **weak, short-range attractive forces** that arise from transient fluctuations in electron distribution, creating fleeting dipoles. - They are crucial for phenomena like **protein folding** and **molecular recognition**, but are easily overcome. *Covalent* - **Covalent bonds** involve the **sharing of electron pairs** between atoms, resulting in very strong and stable connections. - They require a significant amount of energy to break, making them fundamental to the structure of most organic and biological molecules. *Hydrogen* - **Hydrogen bonds** are **intermolecular forces** that occur when a hydrogen atom covalently bonded to a highly electronegative atom (like **oxygen** or **nitrogen**) is attracted to another electronegative atom. - While weaker than covalent bonds, they are significantly stronger than Van der Waals forces and play critical roles in **DNA structure** and **water properties**. *Electrostatic* - **Electrostatic interactions** (also known as **ionic bonds** or salt bridges) occur between oppositely charged ions or polar molecules. - These forces can be quite strong, especially in a non-polar environment, and are important for **protein stability** and **enzyme-substrate binding**.
Question 116: Male to male transmission is seen in -
- A. Autosomal dominant diseases (Correct Answer)
- B. Autosomal recessive
- C. X-linked dominant
- D. Mitochondrial disease
Explanation: ***Autosomal dominant diseases*** - **Autosomal dominant** inheritance patterns involve a gene located on one of the **autosomes**, meaning it is not sex-linked. - Therefore, a father carrying an autosomal dominant gene can pass it to both sons and daughters with a **50% probability** for each child. - **Male-to-male transmission** is a hallmark feature that helps distinguish autosomal dominant from X-linked inheritance patterns. *Autosomal recessive* - **Autosomal recessive** diseases require **two copies** of the mutated gene (one from each parent) for the disease to manifest. - While a father can pass a recessive allele to his son, male-to-male transmission of the **disease phenotype** requires the mother to also be at least a carrier, making it not a defining feature of this inheritance pattern. - The key characteristic is horizontal pattern (affected siblings) rather than vertical transmission. *X-linked dominant* - In **X-linked dominant** inheritance, affected fathers **cannot** transmit the trait to their sons because sons inherit their **X chromosome** from their mother and their Y chromosome from their father. - All daughters of an affected father will inherit the affected X chromosome and thus the disease. - **Absence of male-to-male transmission** is a key distinguishing feature. *Mitochondrial disease* - **Mitochondrial diseases** are inherited exclusively from the **mother** to all her children, regardless of their sex. - Fathers with mitochondrial disease cannot transmit the condition to any of their children. - This shows **maternal inheritance only**, with no paternal transmission possible.
Question 117: Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?
- A. Isocitrate dehydrogenase
- B. Malate dehydrogenase
- C. Aconitase
- D. Succinate thiokinase (Correct Answer)
Explanation: ***Succinate thiokinase*** - This enzyme (also known as **succinyl-CoA synthetase**) catalyzes the conversion of **succinyl-CoA** to **succinate**. - During this reaction, the energy released from breaking the **thioester bond** in succinyl-CoA is directly used to synthesize **GTP** (or ATP in some organisms) from GDP (or ADP) and inorganic phosphate, which is a classic example of **substrate-level phosphorylation**. *Isocitrate dehydrogenase* - This enzyme catalyzes the **oxidative decarboxylation** of isocitrate to $\alpha$-ketoglutarate. - This step produces **NADH** and **CO2** but does not involve substrate-level phosphorylation. *Malate dehydrogenase* - This enzyme catalyzes the oxidation of **L-malate** to **oxaloacetate** in the final step of the TCA cycle. - It produces **NADH** but does not involve the direct synthesis of ATP or GTP. *Aconitase* - This enzyme catalyzes the **isomerization** of **citrate** to **isocitrate** via an aconitate intermediate. - No energy is generated or consumed in the form of ATP/GTP during this rearrangement.
Question 118: All are activated by insulin except?
- A. Lipoprotein lipase
- B. Pyruvate kinase
- C. Acetyl-CoA carboxylase
- D. Hormone sensitive lipase (Correct Answer)
Explanation: ***Hormone sensitive lipase*** - **Insulin** is an **anabolic hormone** that promotes energy storage; it **inhibits** hormone-sensitive lipase (HSL) activity which is responsible for **fat breakdown (lipolysis)**. - When insulin levels are high, the body stores fat rather than breaks it down, thus **decreasing** HSL activity. *Lipoprotein lipase* - **Insulin activates lipoprotein lipase (LPL)**, an enzyme that breaks down triglycerides in **chylomicrons** and **VLDL** into fatty acids for storage in adipose tissue. - This activation promotes the uptake of fatty acids into fat cells, aligning with insulin's role in **energy storage**. *Pyruvate kinase* - **Insulin activates pyruvate kinase** in glycolysis, promoting the conversion of **phosphoenolpyruvate to pyruvate**. - This enzyme's activation enhances glucose utilization and energy production following a meal when insulin levels are high. *Acetyl-CoA carboxylase* - **Insulin activates acetyl-CoA carboxylase (ACC)**, the **rate-limiting enzyme in fatty acid synthesis**. - Activation of ACC leads to the production of **malonyl-CoA**, which commits acetyl-CoA to fatty acid synthesis, storing excess energy as fat.
Question 119: ATP is consumed at which of the following steps of glycolysis?
- A. Pyruvate kinase
- B. Isomerase
- C. Hexokinase (Correct Answer)
- D. Enolase
Explanation: ***Hexokinase*** - This enzyme catalyzes the **first step of glycolysis**, the phosphorylation of glucose to **glucose-6-phosphate**, which requires the consumption of one molecule of **ATP**. - ATP is hydrolyzed to **ADP**, providing the necessary phosphate group and energy for this irreversible reaction. - Note: Hexokinase is one of **two ATP-consuming steps** in glycolysis (the other being phosphofructokinase in step 3). *Pyruvate kinase* - This enzyme catalyzes the **final step of glycolysis**, converting **phosphoenolpyruvate (PEP)** to pyruvate. - This reaction involves the **production of ATP** from ADP, not its consumption, as it's one of the substrate-level phosphorylation steps. *Isomerase* - Isomerase enzymes, like phosphoglucose isomerase, convert one isomer to another (e.g., glucose-6-phosphate to fructose-6-phosphate). - These reactions generally involve an **internal rearrangement of atoms** and do not directly consume or produce ATP. *Enolase* - Enolase catalyzes the reversible conversion of **2-phosphoglycerate to phosphoenolpyruvate (PEP)**, releasing a molecule of water. - This step occurs before the ATP-generating step catalyzed by pyruvate kinase and **does not consume or produce ATP**.
Question 120: Glycogen synthase is activated by?
- A. Insulin (Correct Answer)
- B. Glucagon
- C. AMP
- D. Epinephrine
Explanation: **Insulin** - Insulin activates **glycogen synthase** through a signaling cascade that dephosphorylates the enzyme, shifting it to its active form (glycogen synthase a). - This activation promotes **glycogen synthesis** in the liver and muscles, lowering blood glucose levels. *Glucagon* - **Glucagon** primarily acts to increase blood glucose levels by activating **glycogen phosphorylase** and inhibiting glycogen synthase. - It promotes the breakdown of glycogen (glycogenolysis) rather than its synthesis. *Epinephrine* - **Epinephrine** (adrenaline) also promotes **glycogenolysis** (glycogen breakdown) by activating glycogen phosphorylase. - Its main role is to provide rapid energy during stress, not to store glucose as glycogen. *AMP* - **AMP** (adenosine monophosphate) is a key allosteric activator of **AMP-activated protein kinase (AMPK)**, which phosphorylates and inactivates glycogen synthase. - High AMP levels signal a low energy state, prompting ATP-generating pathways like glycogenolysis, not glycogen synthesis.