NEET-PG 2013 - Biochemistry Practice Questions

Q: Which of the following molecular interactions are found in the structure of DNA?

All of the options. ***All of the options*** - All three types of molecular interactions listed are present in DNA structure, making this the correct answer. - **Hydrogen bonds** hold together the two strands of the DNA double helix, forming between complementary base pairs (A-T with 2 hydrogen bonds, G-C with 3 hydrogen bonds). - **Glycosidic bonds** (N-glycosidic bonds) link the nitrogenous bases to the C1' carbon of the deoxyribose sugar in each nucleotide. - **Covalent interactions** (phosphodiester bonds) form the strong, stable sugar-phosphate backbone by linking the 3' hydroxyl group of one sugar to the 5' phosphate group of the next. *Hydrogen bond* - This is a **true statement** - hydrogen bonds are essential structural components of DNA. - However, this option alone is **incomplete** as DNA structure also contains glycosidic bonds and covalent phosphodiester bonds. - If only hydrogen bonds were present, there would be no nucleotides or backbone structure. *Glycosidic bond* - This is a **true statement** - glycosidic bonds are present in every nucleotide of DNA. - However, this option alone is **incomplete** as DNA also requires hydrogen bonds for base pairing and phosphodiester bonds for the backbone. - Without other bonds, individual nucleotides could not form a functional double helix. *Covalent interactions* - This is a **true statement** - covalent phosphodiester bonds form the DNA backbone within each strand. - However, this option alone is **incomplete** as it doesn't account for glycosidic bonds (nucleotide formation) or hydrogen bonds (strand pairing). - While the strongest bonds in DNA, they alone cannot create the complete double helix structure.

Q: Which enzyme polymerises Okazaki fragments?

DNA polymerase III. ***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.

Q: Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?

DNA polymerase gamma. ***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.

Q: Which type of RNA contains codons for specific amino acids?

Messenger RNA (mRNA). ***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.

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

Question 1

Which of the following molecular interactions are found in the structure of DNA?

Accepted Answer

All of the options

Answer

Hydrogen bond

Answer

Glycosidic bond

Answer

Covalent interactions

Question 2

Which enzyme polymerises Okazaki fragments?

Accepted Answer

DNA polymerase III

Answer

DNA polymerase I

Answer

DNA polymerase II

Answer

RNA polymerase

Question 3

Which type of DNA polymerase is responsible for the replication of mitochondrial DNA?

Accepted Answer

DNA polymerase gamma

Answer

DNA polymerase delta

Answer

DNA polymerase alpha

Answer

DNA polymerase beta

Question 4

Which type of RNA contains codons for specific amino acids?

Accepted Answer

Messenger RNA (mRNA)

Answer

Transfer RNA (tRNA)

Answer

Small nuclear RNA (snRNA)

Answer

Ribosomal RNA (rRNA)

Question 5

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 6

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 7

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

Accepted Answer

Overlapping

Answer

Universal

Answer

Degeneracy

Answer

Nonambiguous

Question 8

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 9

Chemical process involved in conversion of progesterone to glucocorticoids is

Accepted Answer

Hydroxylation

Answer

Methylation

Answer

Carboxylation

Answer

None of the options

Question 10

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

Accepted Answer

Pregnenolone

Answer

Glucocorticoid

Answer

Mineralocorticoid

Answer

Estradiol

NEET-PG 2013 — Biochemistry