DNA Structure and Replication — MCQs
Which of the following molecular interactions are found in the structure of DNA?
Which enzyme polymerises Okazaki fragments?
Mutations are due to changes in:
Which of the following statements about gene therapy is false?
False statements are:
Which of the following statements about Taq DNA polymerase is correct?
Which one of the following statements about chromatin is not true?
Which type of bonds are represented by the dotted lines in the image? (AIIMS Nov 2017)
What is the most stabilizing force for nucleic acids?
What is meant by the melting of double-stranded DNA?
DNA Structure and Replication Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following molecular interactions are found in the structure of DNA?
- A. Hydrogen bond
- B. Glycosidic bond
- C. Covalent interactions
- D. All of the options (Correct Answer)
Explanation: ***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.
Question 2: 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 3: Mutations are due to changes in:
- A. DNA nucleotide sequence (Correct Answer)
- B. RNA nucleotide sequence
- C. Amino acid sequence of ribonuclease
- D. Cell membrane
Explanation: ***DNA nucleotide sequence*** - **Mutations** are defined as changes in the **genetic material**, which is primarily composed of **DNA**. - These changes in the **nucleotide sequence** of DNA can alter the genetic code, leading to changes in **protein structure and function**. *RNA nucleotide sequence* - While RNA can have its nucleotide sequence altered, these changes are generally not considered true **mutations** in the heritable sense for most organisms. - RNA is typically a temporary molecule, and changes to its sequence are usually not passed down to subsequent generations. *Amino acid sequence of ribonuclease* - An altered **amino acid sequence** in a protein like ribonuclease is a consequence of a **mutation in the DNA**, not the mutation itself. - **Ribonucleases** are enzymes that catalyze the degradation of RNA, and their structure is determined by the **DNA sequence**. *Cell membrane* - The cell membrane is a **lipid bilayer** with embedded proteins that regulates cellular transport and communication. - While its components can be affected by genetic mutations, alterations in the cell membrane itself do not constitute the primary definition of a **mutation**.
Question 4: Which of the following statements about gene therapy is false?
- A. Gene also considered as drug
- B. Gene therapy can be used to treat some cancers.
- C. Has been tried in cystic fibrosis
- D. Gene therapy is only used for genetic disorders. (Correct Answer)
Explanation: ***Gene therapy is only used for genetic disorders.*** - This statement is **false** because gene therapy has applications beyond just genetic disorders. It is also being explored and used in the treatment of acquired diseases such as **cancer** and **infectious diseases**. - While it's a prominent approach for correcting genetic defects, its scope is much broader, involving the introduction or modification of genes to achieve a therapeutic effect in various conditions. *Gene also considered as drug* - This statement is **true**. Gene therapy products are often regulated as **drugs** or **biological products** by regulatory bodies like the FDA. - This is because they involve the delivery of genetic material that acts to modify gene expression or cell function to produce a therapeutic effect, similar to how traditional drugs work. *Has been tried in cystic fibrosis* - This statement is **true**. Gene therapy has been extensively investigated as a potential treatment for **cystic fibrosis (CF)**. - CF is caused by mutations in the **CFTR gene**, and researchers have attempted to deliver functional copies of this gene to the affected cells, particularly in the lungs, to correct the underlying defect. *Gene therapy can be used to treat some cancers.* - This statement is **true**. Gene therapy is an active area of research and treatment for various **cancers** [1]. - Approaches include introducing genes that make cancer cells more susceptible to chemotherapy, enhancing the immune system's ability to fight cancer, or directly killing cancer cells through gene delivery [1].
Question 5: False statements are:
- A. DNA replication proceeds in one direction
- B. Lagging strand is synthesized by RNA primase
- C. All of the options (Correct Answer)
- D. Bacteria have multiple origins of replication
Explanation: ***All of the options*** - All statements are **false**. DNA replication proceeds **bidirectionally**, bacteria typically have a **single origin of replication**, and the lagging strand is synthesized by **DNA polymerase** after an RNA primer is laid down by **RNA primase**. *DNA replication proceeds in one direction* - This statement is **false** because **DNA replication** is a **bidirectional process**, meaning it proceeds in both directions from the origin of replication. - Replication forks move away from the **origin** on both sides, unraveling the DNA and synthesizing new strands. *Bacteria have multiple origins of replication* - This statement is **false**. Most **bacteria** (prokaryotes) have a **single origin of replication** (oriC) on their circular chromosome. - In contrast, **eukaryotes** have **multiple origins of replication** on their linear chromosomes to replicate their much larger genomes efficiently. - While rare exceptions exist in some bacterial species, the general rule for bacterial DNA replication is a single origin. *Lagging strand is synthesized by RNA primase* - This statement is **false**. The **lagging strand** is primarily synthesized by **DNA polymerase III** (in prokaryotes) or **DNA polymerase δ** (in eukaryotes). - **RNA primase** is responsible for synthesizing short **RNA primers** that provide a starting point for DNA polymerase, but it does not synthesize the entire lagging strand itself.
Question 6: Which of the following statements about Taq DNA polymerase is correct?
- A. Optimum temperature for chain elongation is 75°C (Correct Answer)
- B. Denatures at high temperatures
- C. Provides high fidelity during DNA synthesis
- D. Exhibits 3' to 5' exonuclease activity
Explanation: ***Optimum temperature for chain elongation is 75°C*** - **Taq polymerase** is a **thermostable enzyme** isolated from *Thermus aquaticus*, functioning optimally at high temperatures. - The optimal temperature for the **elongation step** in PCR, where Taq polymerase synthesizes new DNA strands, is typically around **72-78°C**, with 75°C falling within this optimal range. *Denatures at high temperatures* - While all proteins will eventually denature at extremely high temperatures, Taq polymerase is specifically known for its **thermostability** and **resistance to denaturation** at temperatures required for DNA strand separation in PCR (typically 94-98°C). - Its ability to withstand these high temperatures without significant loss of activity is its key advantage for use in **Polymerase Chain Reaction (PCR)**. *Provides high fidelity during DNA synthesis* - **Taq polymerase** is known for its relatively **low fidelity** due to the lack of 3' to 5' exonuclease activity (proofreading). - This low fidelity results in a higher error rate during DNA synthesis compared to other polymerases with proofreading capabilities, leading to more **mutations** during PCR. *Exhibits 3' to 5' exonuclease activity* - **Taq polymerase** typically **lacks 3' to 5' exonuclease activity**, meaning it does not have the ability to proofread and remove incorrectly incorporated nucleotides. - This absence of proofreading contributes to its relatively **lower fidelity** during DNA replication compared to other polymerases that possess this activity.
Question 7: 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 8: Which type of bonds are represented by the dotted lines in the image? (AIIMS Nov 2017)
- A. Hydrogen bond (Correct Answer)
- B. Covalent bond
- C. Ionic bond
- D. Phosphodiester
Explanation: ***Hydrogen bond*** - The dotted lines in the image represent the weak, non-covalent interactions between the **nitrogenous bases** on opposite strands of the DNA double helix. - Specifically, these are **hydrogen bonds** formed between complementary base pairs (Adenine with Thymine via two hydrogen bonds, and Guanine with Cytosine via three hydrogen bonds). *Covalent bond* - **Covalent bonds** involve the sharing of electron pairs between atoms and are much stronger than hydrogen bonds. - In DNA, covalent bonds form the **sugar-phosphate backbone** of each strand and link the nitrogenous bases to the deoxyribose sugars. *Ionic bond* - **Ionic bonds** involve the electrostatic attraction between oppositely charged ions, formed by the complete transfer of electrons. - While ions (like magnesium or sodium) interact with DNA, the dotted lines specifically represent the inter-strand forces between bases, which are not ionic. *Phosphodiester* - A **phosphodiester bond** is a specific type of covalent bond that links the 3' carbon of one deoxyribose sugar to the 5' carbon of the next deoxyribose sugar via a phosphate group, forming the backbone of a single DNA strand. - The dotted lines are between the two DNA strands, not within a single strand's backbone.
Question 9: What is the most stabilizing force for nucleic acids?
- A. Conformational entropy
- B. Hydrogen bonds (Correct Answer)
- C. Covalent bond
- D. Van der Waals interaction
Explanation: ***Hydrogen bonds*** - **Hydrogen bonds** between complementary base pairs (A-T/U and G-C) are the **primary stabilizing force** in nucleic acid double helix structures. - These bonds hold the two strands together and provide **specificity** in base pairing, which is fundamental to DNA/RNA structure and function. - The cumulative effect of multiple hydrogen bonds along the helix provides substantial stability to the overall structure. *Van der Waals interaction* - Van der Waals forces contribute to **base stacking interactions** within the double helix. - While base stacking is important for stability, it is considered a **secondary stabilizing force** compared to the hydrogen bonds that directly hold complementary strands together. - These interactions contribute to the hydrophobic core of the helix. *Covalent bond* - **Covalent bonds** form the **sugar-phosphate backbone** of each DNA or RNA strand, linking nucleotides together. - While essential for strand integrity, they do not stabilize the double-helical structure by holding two strands together. - They maintain the primary structure but not the secondary structure stability. *Conformational entropy* - **Conformational entropy** refers to the degree of disorder or freedom of movement within a molecule. - The formation of a stable, ordered double helix represents a **decrease** in conformational entropy. - This is an unfavorable thermodynamic factor that opposes helix formation, not a stabilizing force.
Question 10: What is meant by the melting of double-stranded DNA?
- A. Splitting of double strands into single strands (Correct Answer)
- B. Splitting of DNA into fragments
- C. Formation of triple helix
- D. Separation of double-stranded bases
Explanation: ***Splitting of double strands into single strands*** * **DNA melting**, also known as **DNA denaturation**, refers to the process where the two complementary strands of a **double-stranded DNA** molecule separate to form two individual single strands. * This process involves the breaking of the **hydrogen bonds** between the paired bases (**A-T and G-C**) due to increased temperature or changes in pH. * The temperature at which 50% of the DNA is denatured is called the **melting temperature (Tm)**, which depends on GC content (higher GC = higher Tm due to three hydrogen bonds vs. two in AT pairs). *Splitting of DNA into fragments* * The splitting of DNA into fragments is referred to as **DNA fragmentation**, which typically occurs due to processes like **restriction enzyme digestion**, mechanical shearing, or programmed cell death (apoptosis). * This process involves the breaking of the **phosphodiester bonds** within the DNA backbone, not just the hydrogen bonds between strands. *Formation of triple helix* * The formation of a **triple helix** (triplex DNA) is a less common DNA structure where a third oligonucleotide strand binds into the major groove of a **B-form DNA duplex**. * This process is distinct from DNA melting, which involves the *separation* of existing double strands rather than the *addition* of a third strand. *Separation of double-stranded bases* * The term "double-stranded bases" is imprecise terminology; bases are paired (e.g., A with T, G with C) within the double helix structure. * While the separation of base pairs does occur during melting, the more accurate description is the **separation of the entire double helix into two single strands**, not just the individual bases.
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