NEET-PG 2012 - Biochemistry Practice Questions

Q: What is the function of primase?

Synthesizing small RNA fragments during DNA synthesis. ***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.

Q: Which RNA is used in RNA splicing?

Small nuclear RNA (snRNA). ***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.

Q: Which of the following does not play a role in protein synthesis?

Intron. ***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.

Q: Which of the following GAG is not sulfated?

Hyaluronic acid. ***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.

Q: Which of the following types of bonds is considered the weakest?

Van der Waals. ***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**.

Q: Male to male transmission is seen in -

Autosomal dominant diseases. ***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.

Q: Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

Succinate thiokinase. ***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.

Q: All are activated by insulin except?

Hormone sensitive lipase. ***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.

Q: ATP is consumed at which of the following steps of glycolysis?

Hexokinase. ***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**.

Q: Glycogen synthase is activated by?

Insulin. **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.

Question 1

What is the function of primase?

Accepted Answer

Synthesizing small RNA fragments during DNA synthesis

Answer

Joining DNA fragments

Answer

Synthesizing small RNA fragments during translation

Answer

Unwinding of DNA

Question 2

Which RNA is used in RNA splicing?

Accepted Answer

Small nuclear RNA (snRNA)

Answer

mRNA

Answer

tRNA

Answer

rRNA

Question 3

Which of the following does not play a role in protein synthesis?

Accepted Answer

Intron

Answer

m-RNA

Answer

ATP

Answer

Exon

Question 4

Which of the following GAG is not sulfated?

Accepted Answer

Hyaluronic acid

Answer

Keratan sulfate

Answer

Dermatan sulfate

Answer

Chondroitin sulfate

Question 5

Which of the following types of bonds is considered the weakest?

Accepted Answer

Van der Waals

Answer

Covalent

Answer

Hydrogen

Answer

Electrostatic

Question 6

Male to male transmission is seen in -

Accepted Answer

Autosomal dominant diseases

Answer

Autosomal recessive

Answer

X-linked dominant

Answer

Mitochondrial disease

Question 7

Which enzyme in the TCA cycle catalyzes the step where substrate-level phosphorylation occurs?

Accepted Answer

Succinate thiokinase

Answer

Isocitrate dehydrogenase

Answer

Malate dehydrogenase

Answer

Aconitase

Question 8

All are activated by insulin except?

Accepted Answer

Hormone sensitive lipase

Answer

Lipoprotein lipase

Answer

Pyruvate kinase

Answer

Acetyl-CoA carboxylase

Question 9

ATP is consumed at which of the following steps of glycolysis?

Accepted Answer

Hexokinase

Answer

Pyruvate kinase

Answer

Isomerase

Answer

Enolase

Question 10

Glycogen synthase is activated by?

Accepted Answer

Insulin

Answer

Glucagon

Answer

AMP

Answer

Epinephrine

NEET-PG 2012 — Biochemistry