DNA Structure and Function — MCQs
Which of the following techniques can be used to detect single base pair substitutions?
Mutations are due to changes in:
Which of the following is not a component of a nucleotide?
DNA sequence is determined by?
Mark the false statement regarding mitochondrial DNA:
Which of the following is NOT a characteristic of the genetic code?
What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
Which of the following statements about Taq DNA polymerase is correct?
Best sample for DNA profiling in sexual assault after 72 hours?
Best site for DNA extraction from a 2-month-old decomposed body?
DNA Structure and Function Indian Medical PG Practice Questions and MCQs
Question 1: Which of the following techniques can be used to detect single base pair substitutions?
- A. FISH
- B. Southern blot
- C. PCR (Correct Answer)
- D. Restriction Fragment Length Polymorphism (RFLP)
Explanation: ***PCR (with sequencing or allele-specific methods)*** - **PCR-based techniques** are the most versatile methods for detecting single base pair substitutions (point mutations) - **Allele-specific PCR** can directly detect known point mutations by using primers specific to mutant or wild-type alleles - **PCR followed by Sanger sequencing** is the gold standard for identifying any single base pair substitution - **High-resolution melting (HRM) analysis** after PCR can detect mutations based on melting curve differences - PCR amplification is the foundation that enables these detection methods *FISH (Fluorescence in situ hybridization)* - FISH detects **large chromosomal abnormalities** such as aneuploidy, translocations, large deletions, and duplications - It visualizes chromosomal-level changes using fluorescent probes - **Not sensitive enough** to detect single base pair changes, as these are too small to visualize cytogenetically *Southern blot* - Southern blot detects **large DNA rearrangements**, insertions, deletions, or copy number variations - Analyzes restriction enzyme fragments separated by gel electrophoresis - **Generally cannot detect** single base pair substitutions unless they create or abolish a restriction enzyme recognition site - Even when applicable, PCR-based methods are more efficient and sensitive *Restriction Fragment Length Polymorphism (RFLP)* - RFLP can detect single base pair substitutions **only if** they create or abolish a **restriction enzyme recognition site** - Classic example: **Sickle cell mutation** (GAG→GTG in β-globin gene) abolishes an MstII restriction site - **Limited applicability** - can only detect the subset of point mutations that affect restriction sites - PCR-based methods are preferred as they can detect **any** single base pair substitution, not just those affecting restriction sites
Question 2: 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 3: Which of the following is not a component of a nucleotide?
- A. Sugar
- B. Fatty acid (Correct Answer)
- C. Base
- D. Phosphate
Explanation: ***Fatty acid*** - A **fatty acid** is a component of **lipids**, such as triglycerides and phospholipids, which are structurally and functionally distinct from **nucleotides**. - **Nucleotides** are the building blocks of nucleic acids (DNA and RNA), whereas fatty acids are essential for cell membranes and energy storage. *Sugar* - A **pentose sugar** (either **deoxyribose** in DNA or **ribose** in RNA) is a fundamental component of every nucleotide. - This sugar forms the backbone of the nucleic acid strand, covalently linked to the phosphate group and the nitrogenous base. *Phosphate* - A **phosphate group** is a crucial component of a nucleotide, providing the negative charge and forming the phosphodiester bonds that link nucleotides together into a nucleic acid chain. - The number of phosphate groups (mono-, di-, or triphosphate) determines the nucleotide's energy state and function. *Base* - A **nitrogenous base** (adenine, guanine, cytosine, thymine, or uracil) is an essential component of a nucleotide, responsible for genetic information storage and pairing. - This base is attached to the pentose sugar and determines the specific identity of the nucleotide within the DNA or RNA sequence.
Question 4: DNA sequence is determined by?
- A. Sanger sequencing (Correct Answer)
- B. PCR
- C. FISH
- D. Gel electrophoresis
Explanation: ***Correct: Sanger sequencing*** - **Sanger sequencing** (chain-termination method) is the gold standard technique used to determine the exact order of nucleotides within a DNA molecule - It uses dideoxynucleotides (ddNTPs) to terminate DNA strand elongation at specific bases, producing fragments of varying lengths - These fragments are separated by capillary electrophoresis and the sequence is read based on the terminal fluorescent label - Directly determines DNA sequence with high accuracy *Incorrect: PCR* - **Polymerase Chain Reaction (PCR)** amplifies specific DNA segments to create millions of copies - It does NOT determine the sequence itself - it only makes copies of DNA - PCR-amplified DNA can be used as a template for subsequent sequencing, but PCR itself doesn't reveal sequence information *Incorrect: FISH* - **Fluorescence in situ hybridization (FISH)** detects and localizes specific DNA sequences on chromosomes - Used for chromosomal mapping and detecting chromosomal abnormalities - Does not determine the nucleotide sequence *Incorrect: Gel electrophoresis* - Separates DNA fragments based on size and charge - Used to analyze DNA but cannot determine the specific nucleotide sequence - Useful for visualizing DNA after amplification or restriction digestion
Question 5: Mark the false statement regarding mitochondrial DNA:
- A. AGA and AGG are stop codons in mitochondrial DNA
- B. Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA
- C. Does not show heteroplasmy (Correct Answer)
- D. 1% of cellular DNA, 13 proteins of respiratory chain
Explanation: ***Does not show heteroplasmy*** - This statement is false because **mitochondrial DNA (mtDNA)** commonly exhibits **heteroplasmy**, meaning the presence of more than one type of mitochondrial genome within a cell or individual. - **Heteroplasmy** arises due to the presence of both normal and mutated mtDNA, which can be passed down from the mother. *AGA and AGG are stop codons in mitochondrial DNA* - This statement is true; in the **universal genetic code**, AGA and AGG code for **arginine**, but in **human mitochondrial DNA**, they serve as **stop codons**. - This is an example of the **differences** in genetic code interpretation between the nuclear genome and the mitochondrial genome. *Kearns-Sayre Syndrome is a large deletion in mitochondrial DNA* - This statement is true; **Kearns-Sayre Syndrome** is a well-known mitochondrial disorder caused by a **large single deletion** in the mitochondrial DNA. - This deletion often leads to chronic progressive **external ophthalmoplegia**, **retinal pigmentary degeneration**, and **cardiac conduction defects**. *1% of cellular DNA, 13 proteins of respiratory chain* - This statement is true; **mitochondrial DNA constitutes** approximately **1% of the total cellular DNA** by mass. - It codes for **13 essential proteins** that are part of the **electron transport chain** (respiratory chain) complexes in the mitochondrion, along with ribosomal RNAs (rRNAs) and transfer RNAs (tRNAs).
Question 6: 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 7: What sequence on the template strand of DNA corresponds to the first amino acid inserted into a protein?
- A. 3' TAC 5' (Correct Answer)
- B. 3' TAG 5'
- C. 3' TAA 5'
- D. 3' ATG 5'
Explanation: ***3' TAC 5'*** - The **start codon** for protein synthesis on **mRNA** is **5'-AUG-3'**, which codes for **methionine** (or N-formylmethionine in prokaryotes) and signals the initiation of translation. - To produce an mRNA codon of **5'-AUG-3'**, the complementary sequence on the **template DNA strand** must be **3'-TAC-5'** (adenine pairs with uracil/thymine, guanine pairs with cytosine, and the strands are antiparallel). - During transcription, RNA polymerase reads the template strand in the 3' to 5' direction and synthesizes mRNA in the 5' to 3' direction. *3' TAG 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUC-3'**, which codes for **isoleucine**, not methionine. - Therefore, this sequence does not correspond to the first amino acid inserted into a protein. *3' TAA 5'* - This template DNA sequence would be transcribed to produce the mRNA codon **5'-AUU-3'**, which also codes for **isoleucine**, not methionine. - This is not the initiation codon sequence. *3' ATG 5'* - While **ATG** appears in this sequence, when presented as the **template strand** in the 3' to 5' orientation, it would be transcribed to produce mRNA **5'-UAC-3'**, which codes for **tyrosine**, not methionine. - The sequence **ATG** on the **coding strand** (non-template strand) corresponds to the start codon, but this option incorrectly presents it as the template strand sequence.
Question 8: 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 9: Best sample for DNA profiling in sexual assault after 72 hours?
- A. Victim's clothes (Correct Answer)
- B. Cervical swab
- C. Vaginal swab
- D. Fingernail scrapings
Explanation: ***Victim's clothes*** - After 72 hours, **spermatozoa** in or on the skin are often degraded, but foreign DNA (from the perpetrator's skin cells, semen, or other bodily fluids) can persist on clothing, especially in protected areas. - Clothing acts as a **storage medium for trace evidence**, and DNA can remain viable for profiling much longer than on mucous membranes due to drying and lack of enzymatic degradation. *Cervical swab* - **Spermatozoa** typically become undetectable in the cervix within 24-48 hours, though some studies show persistence up to 72 hours. - Beyond 72 hours, the likelihood of obtaining viable **perpetrator DNA** from a cervical swab significantly decreases due to degradation and cellular turnover. *Vaginal swab* - **Spermatozoa** found in the vagina are subject to enzymatic degradation and expulsion, with viability for DNA profiling decreasing significantly after 24-48 hours. - While trace DNA might still be present, the quantity and quality for profiling are usually much lower than on clothing after such an extended period. *Fingernail scrapings* - While fingernail scrapings can yield **perpetrator DNA** if there was physical struggle, this is highly dependent on the act of scraping the assailant's skin. - It is not a guaranteed source in every sexual assault and is less likely to contain strong DNA evidence after 72 hours compared to clothing, which captures shed cells and fluids passively.
Question 10: Best site for DNA extraction from a 2-month-old decomposed body?
- A. Muscle
- B. Bone
- C. Teeth (Correct Answer)
- D. Hair
Explanation: ***Teeth*** - Teeth, particularly the **pulp and dentin**, provide a highly protected environment for DNA, making them ideal for DNA extraction from **decomposed remains** due to their robust structure. - The hard enamel casing shields the internal DNA from environmental degradation and microbial contamination, allowing for excellent preservation over extended periods. - **Dental pulp** is consistently reliable and easily accessible, making teeth the **preferred first choice** in forensic DNA extraction from decomposed bodies. *Bone* - **Bone**, particularly the **petrous portion of the temporal bone** and long bones, is also an **excellent source** of DNA in decomposed remains and is widely used in forensic practice. - However, DNA extraction from bone requires more extensive processing (demineralization, grinding) compared to teeth, making it a **second-line choice** when teeth are available. - The petrous temporal bone is notably resistant to degradation, but teeth remain more practically accessible. *Muscle* - **Muscle tissue** contains significant DNA when fresh, but is highly susceptible to **autolysis and bacterial degradation** in a decomposed body. - As decomposition progresses over 2 months, muscle tissue breaks down rapidly, reducing both the quantity and quality of recoverable DNA significantly. *Hair* - **Hair shafts** primarily contain mitochondrial DNA (mtDNA) with minimal nuclear DNA, which limits their use for individual identification. - Hair roots (if present) contain nuclear DNA, but in decomposed remains, hair is often shed or degraded, making it an unreliable source compared to teeth.
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