NEET-PG 2013 - Biochemistry Practice Questions

Q: Which type of RNA is most commonly associated with pseudouridine?

transfer RNA (tRNA). ***Transfer RNA (tRNA)*** - **Pseudouridine (ψ)** is one of the most abundant modified nucleosides in RNA, and **tRNA contains the highest proportion** of pseudouridine modifications among all RNA types. - **tRNA molecules can contain up to 10-15% modified bases**, with pseudouridine being particularly abundant in the **TψC arm** (thymine-pseudouridine-cytosine loop). - These modifications are critical for **tRNA stability, proper folding, and accurate codon-anticodon recognition** during translation. - Pseudouridine enhances base stacking and stabilizes RNA structure through additional hydrogen bonding capability. *Ribosomal RNA (rRNA)* - While rRNA does contain pseudouridine modifications, they are present in **lower proportions compared to tRNA**. - rRNA pseudouridine modifications do play important roles in **ribosomal assembly and function**, but tRNA remains the RNA type most commonly associated with this modification. *Messenger RNA (mRNA)* - **mRNA is generally much less modified** than tRNA or rRNA. - Pseudouridine modifications in mRNA are relatively rare in prokaryotes and were only recently discovered to be more common in eukaryotic mRNA. - When present, they may affect **mRNA stability and translation efficiency**. *DNA* - **DNA does not contain pseudouridine** as this is an RNA-specific modification. - Pseudouridine is formed by **post-transcriptional isomerization** of uridine residues in RNA.

Q: In eukaryotic cells, where does the majority of functional RNA activity occur?

Cytoplasm. ***Cytoplasm*** - The **cytoplasm** is the cellular compartment where the **majority of functional RNA activity** occurs, including **translation** (protein synthesis) involving mRNA, tRNA, and rRNA. - **Ribosomes** (the sites of translation) are located in the cytoplasm, either free-floating or bound to the endoplasmic reticulum. - Many types of **regulatory RNAs** such as microRNAs (miRNAs) and small interfering RNAs (siRNAs) exert their functions in the cytoplasm by targeting mRNAs for degradation or translational repression. - **mRNA degradation** and **RNA interference pathways** primarily operate in the cytoplasm. - The question asks for the broader location rather than the specific molecular machinery, making cytoplasm the most comprehensive answer. *Nucleus* - While RNA is **transcribed** from DNA and **processed** (capping, polyadenylation, splicing) in the nucleus, these are preparatory steps. - The nucleus is primarily the site of **RNA synthesis**, not where most RNA performs its functional roles. - Only a small fraction of functional RNA activity (like rRNA processing in the nucleolus) occurs here compared to the cytoplasm. *Ribosome* - While **ribosomes are the specific sites of translation** and are composed of rRNA and proteins, they represent molecular machinery rather than a cellular location. - Ribosomes themselves are located **within the cytoplasm**, making cytoplasm the more inclusive answer for where RNA activity occurs. - The question asks "where" in terms of cellular compartment, not which molecular complex. *None of the options* - This is incorrect as the cytoplasm is indeed the primary site where the majority of functional RNA activities occur in eukaryotic cells.

Q: Which of the following is NOT a characteristic of the genetic code?

Overlapping. ***Overlapping*** - The genetic code is generally **non-overlapping**, meaning each nucleotide is part of only one codon, and codons are read sequentially. - An overlapping code would mean that a single nucleotide could be part of multiple codons, which is not how protein synthesis typically occurs. *Nonambiguous* - This statement IS a characteristic; each codon specifies **only one amino acid**, meaning there is no ambiguity about which amino acid will be added. - While multiple codons can specify the same amino acid, a single codon never specifies more than one different amino acid. *Universal* - This statement IS a characteristic; the genetic code is largely **universal** across almost all organisms, from bacteria to humans. - The same codons typically specify the same amino acids in different species, which supports the idea of common ancestry. *Degeneracy* - This statement IS a characteristic; the genetic code is **degenerate**, meaning that most amino acids are specified by more than one codon. - This redundancy helps protect against the effects of single-nucleotide mutations.

Q: A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

3. ***3*** - A **frameshift mutation** occurs when nucleotides are inserted or deleted in a number not divisible by three, altering the **reading frame** of the codons. - If insertions or deletions occur in multiples of **three**, the reading frame is restored after the mutation, largely preserving the downstream amino acid sequence. *1* - An insertion or deletion of a single nucleotide (1) definitively causes a **frameshift mutation**. - This alters all subsequent **codons**, leading to a completely different amino acid sequence downstream from the mutation. *2* - An insertion or deletion of two nucleotides (2) also results in a **frameshift mutation**. - This change shifts the **reading frame**, leading to the production of an altered protein or a premature stop codon. *None of the options* - This option is incorrect because a specific number, **three**, can allow for a frameshift mutation to not affect the complete amino acid sequence. - Multiples of three maintain the original **reading frame** (although potentially adding or removing a specific amino acid), whereas other numbers guarantee a frameshift.

Q: Chemical process involved in conversion of progesterone to glucocorticoids is

Hydroxylation. ***Hydroxylation*** - The conversion of progesterone to glucocorticoids involves several enzymatic steps, with **hydroxylation reactions** being critical for adding hydroxyl groups at specific carbon positions (e.g., C-17, C-21, C-11). - These hydroxylation steps are catalyzed by various **cytochrome P450 enzymes** (e.g., 17α-hydroxylase, 21-hydroxylase, 11β-hydroxylase) within the adrenal cortex, leading to the formation of active glucocorticoids like **cortisol**. *Methylation* - **Methylation** involves the addition of a methyl group (-CH₃) to a molecule, a process more commonly associated with modifying DNA, proteins, or certain neurotransmitters. - While methylation is a vital biological process, it is not the primary chemical reaction involved in the **steroidogenesis pathway** converting progesterone to glucocorticoids. *Carboxylation* - **Carboxylation** is the addition of a carboxyl group (-COOH) to a molecule, a reaction crucial in processes like photosynthesis (carbon fixation) or the synthesis of certain proteins (e.g., clotting factors). - This chemical modification is not directly involved in the series of transformations that convert progesterone into **glucocorticoids**. *None of the options* - This option is incorrect because **hydroxylation** is indeed a fundamental chemical process in the conversion of progesterone to glucocorticoids.

Q: Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?

Pregnenolone. ***Pregnenolone*** - **Pregnenolone** is the **first steroid intermediate** formed from **cholesterol** in steroidogenesis - The conversion occurs in mitochondria via the **cholesterol side-chain cleavage enzyme (P450scc/CYP11A1)** - This is the **rate-limiting step** in steroid hormone biosynthesis - From pregnenolone, all other steroid hormones are subsequently synthesized *Progesterone* - Progesterone is the **second intermediate**, formed from pregnenolone - It serves as a precursor for glucocorticoids, mineralocorticoids, and androgens - Not the first intermediate from cholesterol *Glucocorticoid* - Glucocorticoids (e.g., cortisol) are **end products**, not intermediates - Formed several steps downstream from cholesterol via pregnenolone and progesterone *Mineralocorticoid* - Mineralocorticoids (e.g., aldosterone) are **end products**, not intermediates - Synthesized from progesterone through multiple enzymatic steps *Estradiol* - Estradiol is a **late-stage product** synthesized from androgens - Requires aromatase enzyme for conversion from testosterone - Multiple steps removed from the initial cholesterol conversion

Q: Which of the following coenzymes is directly derived from riboflavin?

FMN. ***FMN (Flavin Mononucleotide)*** - **FMN is the direct derivative** of riboflavin (vitamin B2), formed by phosphorylation of riboflavin - Serves as a prosthetic group in various **flavoproteins** involved in electron transfer reactions - Functions as a redox cofactor in multiple metabolic pathways including the electron transport chain *NAD (Nicotinamide Adenine Dinucleotide)* - Derived from **niacin (vitamin B3)**, not riboflavin - Key coenzyme in redox reactions, particularly in glycolysis and the citric acid cycle *THF (Tetrahydrofolate)* - Active form of **folate (vitamin B9)**, not riboflavin - Essential for one-carbon metabolism, DNA synthesis, and amino acid conversions *FAD (Flavin Adenine Dinucleotide)* - While FAD is also derived from riboflavin, it is a **secondary derivative** formed from FMN + ATP - The conversion pathway is: Riboflavin → FMN → FAD - FMN is the more direct answer to this question

Q: The main function of Vitamin C in the body is

Cofactor for hydroxylation reactions in collagen synthesis. ***Cofactor for hydroxylation reactions in collagen synthesis*** - Vitamin C (ascorbic acid) serves as an essential **cofactor** for **prolyl hydroxylase** and **lysyl hydroxylase** enzymes. - These enzymes catalyze the **hydroxylation of proline and lysine** residues in collagen, forming **hydroxyproline** and **hydroxylysine**. - This hydroxylation is crucial for the **stability and cross-linking** of collagen triple helix structure. - Deficiency leads to **scurvy**, characterized by defective collagen synthesis, bleeding gums, poor wound healing, and bone abnormalities. - This is the **primary and main function** of Vitamin C in the human body. *Involvement as antioxidant* - While Vitamin C does act as a **water-soluble antioxidant**, protecting cells from oxidative damage by free radicals, this is a **secondary function**. - It can donate electrons to neutralize reactive oxygen species and regenerate other antioxidants like Vitamin E. - This protective role is important but not the main function compared to its role in collagen synthesis. *Regulation of lipid synthesis* - Vitamin C is **not directly involved** in the primary pathways of lipid synthesis or metabolism. - It may play a minor role in **carnitine synthesis** (needed for fatty acid oxidation), but this is not a major function. - Other nutrients like B vitamins play more significant roles in lipid metabolism regulation. *Inhibition of cell growth* - Vitamin C does **not inhibit normal cell growth**; it is essential for cell health, differentiation, and tissue repair. - While high doses may have some anti-proliferative effects in certain cancer cell lines in vitro, this is not a physiological function in the healthy body.

Q: Which of the following trace elements has vitamin E-like action?

Selenium. ***Selenium*** - Selenium is an essential component of **glutathione peroxidase**, an enzyme that works alongside vitamin E to protect cells from **oxidative damage**. - Its antioxidant properties are similar to **vitamin E**, as both scavenge free radicals and prevent lipid peroxidation. *Iron* - Iron is vital for **oxygen transport** in hemoglobin and myoglobin, and for cellular respiration as a component of cytochromes. - While essential, iron does not have direct **antioxidant properties** akin to vitamin E; in excess, it can even promote oxidative stress. *Copper* - Copper is a cofactor for several enzymes, including **superoxide dismutase (SOD)**, an antioxidant enzyme, but its primary role is not directly analogous to vitamin E's lipid-soluble antioxidant function. - It also plays a role in **energy production**, iron metabolism, and neurotransmission. *Zinc* - Zinc is crucial for **immune function**, wound healing, and DNA synthesis, acting as a cofactor for over 300 enzymes. - Although it has indirect antioxidant effects by stabilizing cell membranes and reducing oxidative damage, its mechanism and direct action are not considered "vitamin E-like."

Q: Which vitamin is primarily associated with the antioxidant properties of glutathione?

Niacin. ***Niacin*** - **Niacin** (Vitamin B3) is the vitamin most directly associated with glutathione's antioxidant properties - Niacin is a precursor to **NAD+** and **NADP+**, which are converted to **NADPH** - **NADPH is the essential cofactor** for **glutathione reductase**, the primary enzyme that reduces oxidized glutathione (GSSG) back to its active reduced form (GSH) - This NADPH-dependent enzymatic pathway is the **main mechanism** for maintaining the body's glutathione antioxidant system - Without adequate niacin → NADPH, glutathione cannot be efficiently regenerated *Vitamin C* - **Vitamin C** can non-enzymatically reduce GSSG to GSH, providing a **secondary backup mechanism** - While vitamin C does support glutathione regeneration, this is an **indirect, non-enzymatic process** - It acts as an antioxidant itself but is not the primary vitamin associated with glutathione's antioxidant function *Vitamin E* - **Vitamin E** is a **lipid-soluble antioxidant** that primarily protects cell membranes from oxidative damage - Works synergistically with other antioxidants but has **no direct role** in glutathione synthesis or regeneration *Vitamin A* - **Vitamin A** (retinol) is crucial for vision, immune function, and cell differentiation - Has some antioxidant properties as a carotenoid derivative but **no direct involvement** in glutathione metabolism

Question 1

Which type of RNA is most commonly associated with pseudouridine?

Accepted Answer

transfer RNA (tRNA)

Answer

messenger RNA (mRNA)

Answer

ribosomal RNA (rRNA)

Answer

DNA

Question 2

In eukaryotic cells, where does the majority of functional RNA activity occur?

Accepted Answer

Cytoplasm

Answer

Nucleus

Answer

Ribosome

Answer

None of the options

Question 3

Which of the following is NOT a characteristic of the genetic code?

Accepted Answer

Overlapping

Answer

Universal

Answer

Degeneracy

Answer

Nonambiguous

Question 4

A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?

Accepted Answer

3

Answer

1

Answer

2

Answer

None of the options

Question 5

Chemical process involved in conversion of progesterone to glucocorticoids is

Accepted Answer

Hydroxylation

Answer

Methylation

Answer

Carboxylation

Answer

None of the options

Question 6

Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?

Accepted Answer

Pregnenolone

Answer

Glucocorticoid

Answer

Mineralocorticoid

Answer

Estradiol

Question 7

Which of the following coenzymes is directly derived from riboflavin?

Accepted Answer

FMN

Answer

NAD

Answer

THF

Answer

FAD

Question 8

The main function of Vitamin C in the body is

Accepted Answer

Cofactor for hydroxylation reactions in collagen synthesis

Answer

Regulation of lipid synthesis

Answer

Involvement as antioxidant

Answer

Inhibition of cell growth

Question 9

Which of the following trace elements has vitamin E-like action?

Accepted Answer

Selenium

Answer

Iron

Answer

Copper

Answer

Zinc

Question 10

Which vitamin is primarily associated with the antioxidant properties of glutathione?

Accepted Answer

Niacin

Answer

Vitamin E

Answer

Vitamin C

Answer

Vitamin A

NEET-PG 2013 — Biochemistry