DNA Structure and Function Indian Medical PG Practice Questions and MCQs
Practice Indian Medical PG questions for DNA Structure and Function. These multiple choice questions (MCQs) cover important concepts and help you prepare for your exams.
DNA Structure and Function Indian Medical PG 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)
DNA Structure and Function 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
DNA Structure and Function Indian Medical PG 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
DNA Structure and Function 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**.
DNA Structure and Function Indian Medical PG Question 3: Which of the following is not a component of a nucleotide?
- A. Sugar
- B. Fatty acid (Correct Answer)
- C. Base
- D. Phosphate
DNA Structure and Function 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.
DNA Structure and Function Indian Medical PG Question 4: DNA sequence is determined by?
- A. Sanger sequencing (Correct Answer)
- B. PCR
- C. FISH
- D. Gel electrophoresis
DNA Structure and Function 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
DNA Structure and Function Indian Medical PG 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
DNA Structure and Function 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).
DNA Structure and Function Indian Medical PG Question 6: Which of the following is NOT a characteristic of the genetic code?
- A. Overlapping (Correct Answer)
- B. Universal
- C. Degeneracy
- D. Nonambiguous
DNA Structure and Function 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.
DNA Structure and Function Indian Medical PG 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'
DNA Structure and Function 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.
DNA Structure and Function Indian Medical PG 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
DNA Structure and Function 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.
DNA Structure and Function Indian Medical PG 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
DNA Structure and Function 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.
DNA Structure and Function Indian Medical PG Question 10: Best site for DNA extraction from a 2-month-old decomposed body?
- A. Muscle
- B. Bone
- C. Teeth (Correct Answer)
- D. Hair
DNA Structure and Function 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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