Nucleic Acid Biochemistry — MCQs

Nucleic Acid Biochemistry Indian Medical PG Practice Questions and MCQs

Question 261: Which component is part of the 50S ribosomal subunit?

A. 5.8S
B. 28S
C. 23S (Correct Answer)
D. 25S

Explanation: ***23S*** - The **23S ribosomal RNA** is a key structural and catalytic component of the **50S ribosomal subunit** in prokaryotes. - It forms the **peptidyl transferase center**, responsible for catalyzing the formation of peptide bonds during protein synthesis. *28S* - The **28S ribosomal RNA** is a component of the **large ribosomal subunit (60S)** in eukaryotes. - It is crucial for the structural integrity and catalytic activity of the eukaryotic ribosome. *5.8S* - The **5.8S ribosomal RNA** is another component of the **large ribosomal subunit (60S)** in eukaryotes. - It helps in the **assembly and stability** of the eukaryotic ribosomal complex. *25S* - The **25S ribosomal RNA** is found in the **large subunit (60S)** of ribosomes in **plants and some lower eukaryotes**. - It is functionally analogous to the 28S rRNA found in other eukaryotes.

Question 262: Which of the following statements is true about silent mutations in a gene?

A. No change in Amino acid sequence in protein (Correct Answer)
B. No change in mRNA sequence
C. No change in protein expression
D. None of the above

Explanation: ***No change in Amino acid sequence in protein*** - A **silent mutation** is a type of point mutation that results in a **substituted nucleotide** but does not alter the **amino acid sequence** of the protein. - This occurs due to the **redundancy (degeneracy) of the genetic code**, where multiple codons can specify the same amino acid. - Example: UUA → UUG both code for Leucine, making this a silent mutation. *No change in mRNA sequence* - A silent mutation *does* involve a **change in the DNA sequence**, which consequently leads to a **change in the mRNA sequence** (a different nucleotide at the mutation site). - The change in mRNA sequence is at the nucleotide level, but the amino acid encoded remains the same. *No change in protein expression* - While a silent mutation does not change the amino acid sequence, it *can* affect **protein expression** through several mechanisms: - **Codon usage bias**: Different codons are translated at different rates - **mRNA stability**: Sequence changes may affect secondary structure - **Translation efficiency**: Some synonymous codons are translated faster than others - Therefore, this statement is incorrect as silent mutations can affect protein expression levels. *None of the above* - This option is incorrect because there is one correct statement among the choices provided.

Question 263: DNA and RNA both contain which pyrimidine base?

A. Uracil
B. Thymine
C. Cytosine (Correct Answer)
D. Guanine

Explanation: ***Cytosine*** - **Cytosine** is a **pyrimidine** base found in both **DNA** and **RNA**. - In DNA, cytosine pairs with **guanine**; in RNA, it also pairs with guanine. *Uracil* - **Uracil** is a **pyrimidine** base found only in **RNA**, replacing thymine. - It pairs with **adenine** in RNA. *Thymine* - **Thymine** is a **pyrimidine** base found only in **DNA**. - It pairs with **adenine** in DNA and is replaced by uracil in RNA. *Guanine* - **Guanine** is a **purine** base, not a pyrimidine. - It is present in both **DNA** and **RNA**, and pairs with cytosine.

Question 264: Which of the following statements about base excision repair is correct?

A. Used for the repair of deaminated bases only
B. Uses the enzyme DNA glycosylase to remove damaged bases and create an apurinic site (Correct Answer)
C. Removes approximately 1 nucleotide at a time
D. Repairs bulky helix-distorting lesions caused by UV radiation

Explanation: ***Uses the enzyme DNA glycosylase to remove damaged bases and create an apurinic site*** - **Base excision repair (BER)** initiates with **DNA glycosylase**, which recognizes and excises a damaged nitrogenous base, leaving an **AP site** (apurinic or apyrimidinic site). - This AP site is then processed by AP endonuclease, which cleaves the phosphodiester backbone. - The repair is completed by DNA polymerase and DNA ligase. *Used for the repair of deaminated bases only* - While **deaminated bases** are a common substrate for BER (e.g., deamination of cytosine to uracil), BER also repairs other types of **non-bulky lesions**, such as **alkylated bases** and **oxidized bases**. - It is a versatile repair pathway for various forms of single base damage. *Removes approximately 1 nucleotide at a time* - BER involves removal of the damaged **base only** (not the nucleotide) by DNA glycosylase, creating an AP site. - The subsequent repair can be **short-patch BER** (1-2 nucleotides replaced) or **long-patch BER** (2-12 nucleotides replaced). - The initial action is base removal, not nucleotide removal. *Repairs bulky helix-distorting lesions caused by UV radiation* - This describes **nucleotide excision repair (NER)**, not BER. - BER handles **small, non-helix-distorting lesions** affecting individual bases. - UV-induced pyrimidine dimers and other bulky adducts are repaired by NER, which removes 24-32 nucleotide segments.

Question 265: Which one of the following statements about chromatin is not true?

A. DNA winds approximately 1.75 times around the nucleosomes
B. Covalent modification of histones influence chromatin compaction
C. Non-histone proteins are part of mitotic chromosomes
D. H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes (Correct Answer)

Explanation: ***H2A-H2B bind to both the entry and exit ends of DNA in nucleosomes*** - This statement is **not entirely true** as presented because while **H2A-H2B dimers** do make contacts with DNA near entry/exit regions, they do not bind **exclusively** at these ends. - In the nucleosome structure, two H2A-H2B dimers flank the central **(H3-H4)₂ tetramer** and interact with DNA throughout approximately **30 base pairs on each side**. - The **entry and exit points** of nucleosomal DNA are primarily stabilized by **linker histones (H1)**, which bind to the dyad axis and linker DNA regions. - The statement oversimplifies the complex three-dimensional interactions within the nucleosome core particle. *DNA winds approximately 1.75 times around the nucleosomes* - This statement is **true**; approximately **1.65 to 1.75 turns** of DNA (about 146-147 base pairs) wrap around the **histone octamer** to form the core nucleosome particle. - This precise winding is crucial for the compaction of DNA into eukaryotic chromatin and represents the fundamental repeating unit of chromatin structure. *Covalent modification of histones influence chromatin compaction* - This statement is **true**; **post-translational modifications** (PTMs) such as acetylation, methylation, phosphorylation, and ubiquitination on histone tails significantly impact **chromatin structure and accessibility**. - For example, **histone acetylation** generally leads to a more open chromatin conformation (euchromatin) by neutralizing positive charges, facilitating gene expression. - **Histone methylation** can lead to either open or compact chromatin depending on the specific residue modified (e.g., H3K4me3 for activation, H3K9me3 for repression). *Non-histone proteins are part of mitotic chromosomes* - This statement is **true**; mitotic chromosomes contain numerous **non-histone proteins** essential for chromosome structure and function. - Examples include **structural maintenance of chromosomes (SMC) proteins** like condensin and cohesin, topoisomerases (DNA topoisomerase II), and kinetochore proteins. - These non-histone proteins are crucial for chromosome condensation, sister chromatid cohesion, segregation, and proper mitotic progression.

Question 266: RNA with enzymatic activity is

A. Cytidine deaminase
B. Peptidase
C. Peptidyl transferase (Correct Answer)
D. Aminoacyl tRNA synthetase

Explanation: ***Peptidyl transferase*** - The **peptidyl transferase** activity, resident in the **large ribosomal subunit (50S in prokaryotes, 60S in eukaryotes)**, is actually carried out by **ribosomal RNA (rRNA)**, making it a ribozyme. - This enzymatic activity catalyzes the formation of a **peptide bond** between the amino acid at the P-site and the incoming amino acyl-tRNA at the A-site during protein synthesis. *Peptidase* - **Peptidases** are enzymes that catalyze the **hydrolysis of peptide bonds**, breaking down proteins into smaller peptides or amino acids. - While essential for protein turnover and digestion, peptidases are typically **protein enzymes**, not RNA molecules. *Cytidine deaminase* - **Cytidine deaminase** is an enzyme that catalyzes the **deamination of cytidine** to uridine, a reaction crucial for pyrimidine metabolism and often targeted in cancer therapy. - This enzyme is a **protein**, not an RNA molecule with enzymatic activity. *Aminoacyl tRNA synthetase* - **Aminoacyl-tRNA synthetases** are a family of enzymes responsible for **attaching the correct amino acid to its corresponding tRNA molecule**, a critical step in ensuring the accuracy of protein synthesis. - These are **protein enzymes** and do not possess RNA-based catalytic activity.

Question 267: Splicing is a process of what?

A. Removal of introns from pre-mRNA (Correct Answer)
B. Protein synthesis from mRNA
C. Activation of proteins during gene expression
D. DNA replication process

Explanation: ***Removal of introns from pre-mRNA*** - **Splicing** is a crucial step in **RNA processing** where non-coding sequences (**introns**) are excised from a newly synthesized **pre-mRNA** molecule. - This process ensures that only the protein-coding regions (**exons**) are joined together to form a mature mRNA. *Protein synthesis from mRNA* - This describes **translation**, the process where **ribosomes** read the genetic code in mRNA to synthesize a protein polypeptide chain. - Translation occurs after mRNA has been processed and exported from the nucleus. *Activation of proteins during gene expression* - This refers to **post-translational modifications** or **protein folding**, which are steps that occur after protein synthesis to make a protein functional. - Splicing is an upstream process, preceding protein synthesis. *DNA replication process* - **DNA replication** is the biological process of producing two identical replicas of DNA from one original DNA molecule. - This process is distinct from gene expression and RNA processing, focusing instead on the duplication of the entire genome.

Question 268: According to Chargaff's rules, which of the following base pairs shows equal numbers in double-stranded DNA?

A. A=C
B. A=G
C. G=C (Correct Answer)
D. A=T

Explanation: ***G=C*** - Chargaff's rules state that in double-stranded DNA, the amount of **guanine (G)** equals the amount of **cytosine (C)**. - This is due to **complementary base pairing** where G always pairs with C via three hydrogen bonds. - Both G=C and A=T are valid answers according to Chargaff's rules. *A=T* - According to Chargaff's rules, **adenine (A)** equals **thymine (T)** in double-stranded DNA. - A pairs with T via two hydrogen bonds in complementary base pairing. - This is also a correct statement of Chargaff's rules (equally valid as G=C). *A=G* - **Adenine (A)** and **guanine (G)** are both purines but do not pair with each other. - Their amounts are **not necessarily equal** in double-stranded DNA. *A=C* - **Adenine (A)** is a purine and **cytosine (C)** is a pyrimidine, but they do not form complementary base pairs. - Their amounts are **not equal** in double-stranded DNA.

Question 269: Action of Telomerase is:

A. DNA repair
B. Replication of DNA
C. Breakdown of telomere
D. Longevity of cell - Aging (Correct Answer)

Explanation: ***Longevity of cell - Aging*** - **Telomerase** is an enzyme that adds repetitive DNA sequences (telomeres) to the ends of eukaryotic chromosomes, protecting genetic information during cell division. - By restoring telomeres, telomerase allows cells to divide more times, thereby **increasing cellular longevity** and impacting the aging process. - This is the **primary function** of telomerase in cellular biology. *DNA repair* - **DNA repair mechanisms** are involved in correcting mutations and damage to DNA throughout the genome. - While telomerase maintains the ends of chromosomes, its primary function is not to repair damaged DNA sequences within the chromosome itself. - DNA repair enzymes include DNA polymerases, ligases, and nucleases that fix base mismatches and strand breaks. *Breakdown of telomere* - The **breakdown or shortening of telomeres** naturally occurs with each cell division in most somatic cells due to the \"end-replication problem.\" - **Telomerase** actively counteracts this shortening by adding to telomeres, rather than causing their breakdown. - This represents the opposite of telomerase action. *Replication of DNA* - While telomerase does synthesize DNA sequences, its action is **highly specific to telomeric repeats** at chromosome ends. - General DNA replication is carried out by **DNA polymerase**, not telomerase. - Telomerase is a specialized reverse transcriptase with a distinct function from standard replicative machinery.

Question 270: The term "modified base" refers to:

A. A purine or pyrimidine that has been altered (Correct Answer)
B. A purine or pyrimidine attached to a sugar by O-glycoside linkage
C. A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose
D. A nitrogen-containing base that is not a standard purine or pyrimidine

Explanation: ***A purine or pyrimidine that has been altered*** - A **modified base** refers to a purine (adenine, guanine) or pyrimidine (cytosine, thymine, uracil) that has undergone **post-transcriptional or post-replicative chemical alteration**. - These modifications can include **methylation**, deamination, or other structural changes, leading to changes in their properties and functions within nucleic acids. *A standard nucleotide consisting of a purine or pyrimidine attached to deoxyribose* - This describes a **standard deoxyribonucleotide** (the building block of DNA), which is composed of a nitrogenous base (purine or pyrimidine), a deoxyribose sugar, and one to three phosphate groups. - A **modified base** is distinct from a standard nucleotide because it involves an *alteration* to the base itself, not just its assembly into a nucleotide. *A purine or pyrimidine attached to a sugar by O-glycoside linkage* - In nucleic acids, **purine and pyrimidine bases** are attached to the C1' carbon of the sugar (deoxyribose or ribose) via an **N-glycosidic bond**, not an O-glycosidic linkage. - An **O-glycosidic linkage** is typically found in carbohydrates, connecting sugars to other molecules, but is not characteristic of the bond between a base and a sugar in nucleotides. *A nitrogen-containing base that is not a standard purine or pyrimidine* - While modified bases are indeed **nitrogen-containing bases** that often are not the *standard* A, T, C, G, or U, this definition is incomplete. - The key aspect of a **modified base** is that it *originated* from a standard purine or pyrimidine and was subsequently *altered*, differentiating it from completely novel nitrogenous bases.

Practice by Chapter

Nucleotide Structure and Function

Practice Questions

DNA Structure and Replication

Practice Questions

RNA Structure and Types

Practice Questions

Transcription: RNA Synthesis

Practice Questions

Post-Transcriptional Modifications

Practice Questions

Translation: Protein Synthesis

Practice Questions

Genetic Code and Codon Usage

Practice Questions

Regulation of Gene Expression

Practice Questions

Mutations and DNA Repair

Practice Questions

Purine Metabolism and Disorders

Practice Questions

Pyrimidine Metabolism and Disorders

Practice Questions

Nucleotide Degradation and Salvage Pathways

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