Which enzyme polymerises Okazaki fragments?
Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?
Which type of RNA contains codons for specific amino acids?
Which type of RNA is most commonly associated with pseudouridine?
In eukaryotic cells, where does the majority of functional RNA activity occur?
Which of the following is NOT a characteristic of the genetic code?
A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?
Chemical process involved in conversion of progesterone to glucocorticoids is
Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?
Which of the following coenzymes is directly derived from riboflavin?
NEET-PG 2013 - Biochemistry NEET-PG Practice Questions and MCQs
Question 71: Which enzyme polymerises Okazaki fragments?
- A. DNA polymerase I
- B. DNA polymerase II
- C. DNA polymerase III (Correct Answer)
- D. RNA polymerase
Explanation: ***DNA polymerase III*** - **DNA polymerase III** is the primary replicative enzyme in **prokaryotes (bacteria)** responsible for synthesizing new DNA strands, including the **polymerization of Okazaki fragments** on the lagging strand. - It possesses high processivity (can add ~500 nucleotides without dissociating), essential for rapid and efficient DNA synthesis during replication, adding nucleotides in a **5' to 3' direction**. - In **eukaryotes**, DNA polymerase δ (delta) performs the analogous function of polymerizing Okazaki fragments. *DNA polymerase I* - **DNA polymerase I** in prokaryotes primarily functions in **removing RNA primers** left by primase and **filling the resulting gaps** with DNA nucleotides. - It has 5' to 3' exonuclease activity for primer removal and polymerase activity for gap filling, but is **not the main enzyme for elongating Okazaki fragments**. - Its role is in **DNA repair and finishing replication**, not the extensive synthesis of Okazaki fragments. *DNA polymerase II* - **DNA polymerase II** in prokaryotes is primarily involved in **DNA repair mechanisms**, particularly in **restarting stalled replication forks** and responding to DNA damage. - It is not the main enzyme responsible for the polymerization of **Okazaki fragments** during normal DNA replication. *RNA polymerase* - **RNA polymerase** (specifically **primase**, a specialized RNA polymerase) synthesizes short **RNA primers** (8-12 nucleotides) during DNA replication, which provide the 3'-OH group necessary to initiate DNA synthesis. - It does not synthesize DNA or polymerize **Okazaki fragments**; its function is to create RNA primers, not extend DNA strands.
Question 72: Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?
- A. DNA polymerase delta
- B. DNA polymerase alpha
- C. DNA polymerase gamma (Correct Answer)
- D. DNA polymerase beta
Explanation: ***DNA polymerase gamma*** - **DNA polymerase gamma** is the sole DNA polymerase responsible for replicating and repairing the mitochondrial DNA in eukaryotic cells. - It consists of a large catalytic subunit and two smaller accessory subunits that provide **proofreading** and processivity functions. *DNA polymerase alpha* - **DNA polymerase alpha** is primarily involved in initiating DNA replication on both the leading and lagging strands of nuclear DNA. - It forms a complex with **primase** to synthesize short RNA primers followed by a short stretch of DNA. *DNA polymerase delta* - **DNA polymerase delta** is a key enzyme in nuclear DNA replication, primarily responsible for the **elongation of the lagging strand**. - It also plays a significant role in various DNA repair pathways, including **nucleotide excision repair**. *DNA polymerase beta* - **DNA polymerase beta** is mainly involved in **DNA repair processes**, specifically **base excision repair (BER)** in the nucleus. - It has a low processivity and lacks **proofreading activity**, making it unsuitable for bulk DNA replication.
Question 73: Which type of RNA contains codons for specific amino acids?
- A. Transfer RNA (tRNA)
- B. Messenger RNA (mRNA) (Correct Answer)
- C. Small nuclear RNA (snRNA)
- D. Ribosomal RNA (rRNA)
Explanation: ***Messenger RNA (mRNA)*** - **mRNA** carries the genetic information from **DNA** in the nucleus to the **ribosomes** in the cytoplasm. - This information is encoded in sequences of three nucleotides called **codons**, each specifying a particular amino acid. *Transfer RNA (tRNA)* - **tRNA** molecules are responsible for **carrying specific amino acids** to the ribosome during protein synthesis. - Each **tRNA** has an **anticodon** that base-pairs with a complementary **codon** on the **mRNA** strand. *Small nuclear RNA (snRNA)* - **snRNA** is primarily involved in **RNA splicing**, a process that removes introns from pre-mRNA. - It forms part of the **spliceosome** complex, which is crucial for mature mRNA formation but does not contain codons itself. *Ribosomal RNA (rRNA)* - **rRNA** is a major component of **ribosomes**, the cellular machinery responsible for protein synthesis. - While it plays a critical structural and catalytic role in translation, it does not carry genetic code in the form of codons.
Question 74: Which type of RNA is most commonly associated with pseudouridine?
- A. messenger RNA (mRNA)
- B. ribosomal RNA (rRNA)
- C. transfer RNA (tRNA) (Correct Answer)
- D. DNA
Explanation: ***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.
Question 75: In eukaryotic cells, where does the majority of functional RNA activity occur?
- A. Nucleus
- B. Ribosome
- C. Cytoplasm (Correct Answer)
- D. None of the options
Explanation: ***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.
Question 76: Which of the following is NOT a characteristic of the genetic code?
- A. Overlapping (Correct Answer)
- B. Universal
- C. Degeneracy
- D. Nonambiguous
Explanation: ***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.
Question 77: A frameshift mutation does not affect the complete amino acid sequence if it occurs in multiples of what number?
- A. 1
- B. 2
- C. 3 (Correct Answer)
- D. None of the options
Explanation: ***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.
Question 78: Chemical process involved in conversion of progesterone to glucocorticoids is
- A. Methylation
- B. Hydroxylation (Correct Answer)
- C. Carboxylation
- D. None of the options
Explanation: ***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.
Question 79: Which is the first steroid intermediate formed in the conversion of cholesterol to steroid hormones?
- A. Glucocorticoid
- B. Mineralocorticoid
- C. Estradiol
- D. Pregnenolone (Correct Answer)
Explanation: ***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
Question 80: Which of the following coenzymes is directly derived from riboflavin?
- A. FMN (Correct Answer)
- B. NAD
- C. THF
- D. FAD
Explanation: ***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