DNA Profiling and Forensic Biology — MCQs

DNA Profiling and Forensic Biology Indian Medical PG Practice Questions and MCQs

Question 11: Chimeric DNA is used for what purpose?

A. Paternity testing
B. Maternity testing
C. Personal identification
D. Organ transplantation (Correct Answer)

Explanation: ### Explanation **Correct Answer: D. Organ transplantation** **Understanding Chimerism in Forensic Medicine** A **chimera** is an individual who possesses two or more genetically distinct cell lines derived from different zygotes. In the context of modern medicine, **iatrogenic chimerism** most commonly occurs following **organ transplantation** or bone marrow/hematopoietic stem cell transplants. In these cases, the recipient’s blood or tissues contain a mix of their own DNA and the donor's DNA. This "Chimeric DNA" is crucial for monitoring **graft-versus-host disease (GVHD)** and **graft acceptance**. By quantifying the ratio of donor to recipient DNA (chimerism analysis), clinicians can determine if the transplant is successful or if the recipient's immune system is rejecting the organ. **Why other options are incorrect:** * **A & B (Paternity/Maternity Testing):** These tests rely on the inheritance of alleles from biological parents. Chimerism actually **complicates** these tests; a chimeric parent might have different DNA in their blood than in their germ cells (sperm/eggs), leading to false exclusions of parentage. * **C (Personal Identification):** Chimerism is a pitfall in forensic identification. If a suspect is a chimera (e.g., after a bone marrow transplant), their blood DNA profile will match the donor, while their skin or hair DNA will match themselves, leading to potential legal errors. **High-Yield Clinical Pearls for NEET-PG:** * **Microchimerism:** The most common natural form, where fetal cells persist in the mother’s body (or vice versa) for decades. * **Tetragametic Chimerism:** A rare natural condition where two non-identical twin embryos fuse in utero. * **Forensic Significance:** In a victim who has received a blood transfusion or bone marrow transplant, DNA profiling should ideally be done using **hair follicles or buccal swabs** rather than blood to avoid chimeric interference. * **DNA Profiling Technique:** The gold standard for detecting chimerism is **Short Tandem Repeat (STR) analysis** via PCR.

Question 12: Which of the following is not a potential source of DNA for forensic analysis?

A. Hair roots
B. Red blood cells (Correct Answer)
C. WBC
D. Muscle tissue

Explanation: **Explanation:** The fundamental principle of DNA profiling is that the sample must contain **nucleated cells**. DNA is primarily located within the cell nucleus (genomic DNA) and the mitochondria (mtDNA). **Why Red Blood Cells (RBCs) are the correct answer:** Mature human erythrocytes (RBCs) are **enucleated**; they lack a nucleus and mitochondria to maximize space for hemoglobin. Since they contain no nuclear material, they do not contain DNA. When blood is used for DNA profiling, the DNA is actually extracted from the **leukocytes (WBCs)**, not the RBCs. **Analysis of Incorrect Options:** * **WBC (White Blood Cells):** These are nucleated cells and serve as the primary source of DNA in a liquid blood sample or bloodstain. * **Hair Roots:** The hair shaft itself contains mostly keratin and degraded DNA, but the **hair bulb (root)** contains follicular cells with active nuclei, making it an excellent source of genomic DNA. * **Muscle Tissue:** Myocytes are nucleated (and often multinucleated) cells containing abundant genomic and mitochondrial DNA, frequently used in decomposing bodies or mass disaster identification. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial DNA (mtDNA):** Inherited only from the mother. It is useful for identifying skeletal remains or hair shafts where nuclear DNA is degraded. * **RFLP vs. PCR:** PCR (Polymerase Chain Reaction) is the modern gold standard as it can amplify DNA from minute samples (e.g., a single skin cell). * **Short Tandem Repeats (STR):** The most commonly used markers in forensic DNA profiling today. * **Alec Jeffreys:** Known as the "Father of DNA Fingerprinting."

Question 13: DNA fingerprinting is done by which of the following methods?

A. Splitting DNA
B. DNA from White Blood Cells (WBC)
C. DNA from nucleated cells
D. All of the above (Correct Answer)

Explanation: ### Explanation **DNA Fingerprinting (Profiling)** is a technique used to identify individuals by analyzing unique patterns in their DNA. The fundamental requirement for this process is the presence of **nucleated cells**, as the nucleus contains the genomic DNA necessary for analysis. #### Why "All of the Above" is Correct: 1. **DNA from Nucleated Cells (Option C):** This is the core principle. Any cell with a nucleus (e.g., skin cells, hair follicles, sperm, vaginal swabs) can be used. Mature Red Blood Cells (RBCs) lack a nucleus and thus cannot be used for DNA profiling. 2. **DNA from White Blood Cells (Option B):** In a blood sample, the DNA is extracted specifically from the **WBCs (Leukocytes)** because they are nucleated, unlike RBCs or platelets. 3. **Splitting DNA (Option A):** This refers to the laboratory process of **Restriction Digestion**. To analyze DNA, it must be "split" or cut into fragments of varying lengths using **Restriction Endonucleases** (Restriction Fragment Length Polymorphism - RFLP). Without splitting the DNA, the unique banding patterns cannot be visualized. #### High-Yield Clinical Pearls for NEET-PG: * **Father of DNA Fingerprinting:** Sir Alec Jeffreys (1984). In India, the pioneer was **Dr. Lalji Singh**. * **Target Sequences:** DNA profiling targets **VNTRs** (Variable Number Tandem Repeats) or **STRs** (Short Tandem Repeats). STR analysis is currently the gold standard. * **PCR (Polymerase Chain Reaction):** Used to amplify minute quantities of DNA (e.g., from a single hair root or a tiny bloodstain). * **Mitochondrial DNA (mtDNA):** Inherited only from the mother; used when nuclear DNA is degraded or missing (e.g., old skeletal remains). * **Specimen Choice:** EDTA blood is the preferred sample for DNA profiling in living individuals.

Question 14: DNA fingerprinting is performed using DNA obtained from which source?

A. Splitting DNA
B. DNA from white blood cells
C. DNA from nucleated cells
D. All of the above (Correct Answer)

Explanation: **Explanation:** DNA fingerprinting (DNA profiling) relies on the presence of genomic DNA within a sample. The fundamental principle is that **any nucleated cell** in the human body contains the same genetic blueprint, making it a viable source for analysis. 1. **DNA from Nucleated Cells (Option C):** This is the most accurate biological description. Mature red blood cells (RBCs) in humans are anucleated and lack a nucleus; therefore, they do not contain genomic DNA. Any cell that retains its nucleus (e.g., buccal cells, hair follicles, skin cells, spermatozoa) can be used for DNA profiling. 2. **DNA from White Blood Cells (Option B):** In a blood sample, the DNA is extracted specifically from the **leukocytes (WBCs)** because, unlike RBCs and platelets, they possess a nucleus. This is the most common source used in forensic laboratories. 3. **Splitting DNA (Option A):** This refers to the laboratory process of "splitting" or cutting DNA into fragments using **Restriction Endonucleases** (Restriction Fragment Length Polymorphism - RFLP) or denaturing double-stranded DNA into single strands during PCR. Since the technique involves manipulating and analyzing these "split" fragments, it is considered an integral part of the DNA fingerprinting process. **High-Yield Facts for NEET-PG:** * **Father of DNA Fingerprinting:** Sir Alec Jeffreys (World); Dr. Lalji Singh (India). * **VNTRs:** DNA profiling targets "Variable Number Tandem Repeats," which are highly polymorphic sequences. * **Mitochondrial DNA (mtDNA):** Used when genomic DNA is degraded or for maternal lineage (passed only from mother to offspring). * **Sample Stability:** DNA can be extracted from dried stains (blood, semen) even years later, provided they were not exposed to extreme heat or humidity.

Question 15: In which anticoagulant should blood samples for DNA fingerprinting be transported?

A. Normal saline
B. EDTA (Correct Answer)
C. NaF
D. Thymol

Explanation: **Explanation:** **Why EDTA is the Correct Answer:** Ethylenediaminetetraacetic acid (EDTA) is the gold standard anticoagulant for DNA profiling. The underlying medical concept involves **enzyme inhibition**. DNA is highly susceptible to degradation by **DNase enzymes**, which require divalent metal cations (like $Mg^{2+}$ and $Ca^{2+}$) as cofactors to function. EDTA acts as a potent **chelating agent**, binding these metal ions and effectively "switching off" the DNase enzymes. This preserves the structural integrity of the high-molecular-weight DNA required for techniques like RFLP or PCR-based STR analysis. **Analysis of Incorrect Options:** * **Normal Saline (A):** This is a physiological preservative used for transporting fresh tissues or rinsing stains, but it lacks anticoagulant and enzyme-inhibiting properties, leading to rapid DNA degradation. * **Sodium Fluoride (NaF) (C):** NaF is an antiglycolytic agent used for blood glucose estimation. While it inhibits glycolysis, it can interfere with the Taq polymerase enzyme used in PCR, making it unsuitable for DNA analysis. * **Thymol (D):** This is a preservative used for urine samples to prevent bacterial growth. It has no role in stabilizing DNA in blood samples. **High-Yield Clinical Pearls for NEET-PG:** * **Preferred Sample:** For DNA profiling, **5 ml of whole blood** collected in a **Purple/Lavender top (EDTA)** tube is ideal. * **Alternative Samples:** If blood is unavailable, the **buccal swab** is the next best non-invasive source. * **Storage:** If transport is delayed, samples should be refrigerated at **4°C**, not frozen, to prevent cell lysis before reaching the lab. * **Avoid Heparin:** Heparin (Green top) should be avoided as it acts as a potent **PCR inhibitor**.

Question 16: Species identification is done by which test?

A. Neutron activation analysis
B. Precipitin test (Correct Answer)
C. Benzidine test
D. Spectroscopy

Explanation: **Explanation:** The identification of the species of origin (determining whether a bloodstain is human or animal) is a crucial step in forensic serology. **1. Why Precipitin Test is correct:** The **Precipitin test** is the gold standard for species identification. It is based on the **antigen-antibody reaction**. When an extract of the suspected bloodstain (containing human serum proteins/antigens) is reacted against "Antihuman serum" (prepared in rabbits), a visible precipitate forms at the junction if the blood is of human origin. Modern variations include the **Crossover Electrophoresis** and the **Uhlenhuth test**. **2. Analysis of Incorrect Options:** * **Neutron Activation Analysis (NAA):** This is a highly sensitive nuclear process used to determine the concentration of trace elements in samples like hair, nails, or poisons (e.g., Arsenic). It does not identify species. * **Benzidine Test:** This is a **presumptive (preliminary) test** for the presence of blood. It detects the peroxidase-like activity of hemoglobin but cannot differentiate between human and animal blood. (Note: It is now largely replaced by the Phenolphthalein/Kastle-Meyer test due to carcinogenicity). * **Spectroscopy:** This is a **confirmatory test** for the presence of blood. It identifies specific absorption bands of hemoglobin derivatives (like hemochromogen) but is not used for species differentiation. **High-Yield Clinical Pearls for NEET-PG:** * **Sequence of Examination:** Preliminary test (Kastle-Meyer) → Confirmatory test (Teichmann/Takayama) → Species identification (Precipitin) → Individualization (DNA profiling). * **Teichmann Test:** Produces rhombic, dark brown crystals of Haemin. * **Takayama Test:** Produces pink, feathery crystals of Pyridine Haemochromogen (more reliable than Teichmann). * **Species Specificity:** The Precipitin test can remain positive even in dried bloodstains that are several years old.

Question 17: In a macerated fetus, which organ is typically used for DNA profiling?

A. Lungs (Correct Answer)
B. Spleen
C. Heart
D. Skeletal muscle

Explanation: ### Explanation In forensic pathology, a **macerated fetus** undergoes aseptic autolysis due to prolonged intrauterine death. This process leads to the rapid liquefaction of soft tissues, making DNA extraction challenging. **Why Lungs are the Correct Choice:** The **lungs** are considered the organ of choice for DNA profiling in a macerated fetus because they are relatively protected within the thoracic cage and are among the last organs to undergo complete liquefaction. The dense connective tissue framework and the collapsed state of the fetal lungs (since they have not expanded with air) help preserve cellular integrity and DNA longer than other visceral organs. **Analysis of Incorrect Options:** * **B. Spleen:** The spleen is one of the first organs to undergo autolysis (becoming "diffluent" or liquid). In a macerated fetus, it often loses its structural integrity very early, making it unsuitable for DNA sampling. * **C. Heart:** While relatively muscular, the heart undergoes endocardial staining and softening early in the maceration process, making it less reliable than the lungs. * **D. Skeletal Muscle:** In maceration, the skin slips and the underlying muscles become soft, friable, and waterlogged (soggy). They degrade faster than the protected internal thoracic organs. **High-Yield Clinical Pearls for NEET-PG:** * **Maceration vs. Putrefaction:** Maceration is **aseptic** autolysis (occurs in sterile liquor amnii), whereas putrefaction is **septic** (driven by bacteria). * **Signs of Maceration:** Skin slipping and bullae appear within 24 hours. **Spalding’s Sign** (overlapping of cranial bones) is a classic radiological finding. * **Alternative Samples:** If the fetus is severely decomposed, **long bones** or **tooth buds** (if developed) may be used as they provide the best protection for DNA.

Question 18: DNA fingerprinting was discovered by:

A. Southern
B. Galion
C. Crick
D. Jeffery (Correct Answer)

Explanation: **Explanation:** **Correct Option: D (Sir Alec Jeffreys)** DNA fingerprinting (DNA profiling) was developed in 1984 by **Sir Alec Jeffreys** at the University of Leicester. He discovered that certain sequences of highly variable DNA (called **VNTRs**—Variable Number Tandem Repeats) are unique to every individual (except monozygotic twins). This technique revolutionized forensic medicine by allowing the identification of individuals from biological samples like blood, semen, or hair roots. **Analysis of Incorrect Options:** * **A. Southern:** Edwin Southern developed the **Southern Blot**, a laboratory method used to detect specific DNA sequences. While Jeffreys used Southern blotting in his process, Southern did not invent DNA fingerprinting itself. * **B. Galton:** Sir Francis Galton was a pioneer in **dactylography** (fingerprints). He established the individuality and permanence of fingerprints and devised the first classification system, but he was not involved in DNA analysis. * **C. Crick:** Francis Crick, along with James Watson, discovered the **double-helix structure of DNA** in 1953. This was the foundational discovery of molecular biology, but it predated forensic DNA profiling by decades. **High-Yield Clinical Pearls for NEET-PG:** * **Father of DNA Fingerprinting in India:** Dr. Lalji Singh (developed indigenous probes at CCMB, Hyderabad). * **Specimen of Choice:** While any nucleated cell works, **EDTA blood** is the preferred sample for DNA profiling in living subjects. * **The "Gold Standard":** Currently, **STR (Short Tandem Repeat) analysis** is the most widely used method in forensics due to its high sensitivity. * **Mitochondrial DNA (mtDNA):** Useful for identifying skeletal remains or hair shafts (which lack nuclei) and tracing **maternal lineage**.

Question 19: For DNA testing, which tissue sample is optimal during an autopsy?

A. Liver
B. Spleen (Correct Answer)
C. Kidney
D. Brain

Explanation: **Explanation:** In forensic pathology, the **Spleen** is considered the optimal solid organ for DNA extraction during an autopsy. The primary reason is its high density of nucleated cells (specifically lymphocytes). Since DNA is contained within the nucleus, tissues with high cellularity yield a greater quantity and better quality of genomic DNA. Furthermore, the spleen is relatively resistant to rapid putrefaction compared to other abdominal viscera, preserving DNA integrity for a longer period post-mortem. **Analysis of Options:** * **Spleen (Correct):** It is the "gold standard" among solid organs due to the high concentration of white blood cells. In cases of advanced decomposition where blood cannot be collected, the spleen remains the preferred source. * **Liver:** While large, the liver contains high levels of enzymes and bile, which can act as PCR inhibitors or accelerate DNA degradation during the extraction process. * **Kidney:** Although it contains nucleated cells, the cellular density is significantly lower than that of the spleen, leading to a lower DNA yield. * **Brain:** The brain has a high lipid content (myelin), which can interfere with the chemical processes of DNA purification. It also undergoes liquefactive necrosis relatively quickly. **High-Yield Clinical Pearls for NEET-PG:** * **Best overall sample:** EDTA-preserved whole blood is the first choice in a fresh corpse. * **Best solid organ:** Spleen. * **Best sample in charred or highly decomposed bodies:** Tooth pulp (protected by enamel) or compact bone (femur). * **Preservative for DNA:** Samples should be collected in **EDTA** or simply frozen. **Never** use formalin, as it causes cross-linking and fragments DNA.

Question 20: Severe destruction of DNA can be tested by:

A. ELISA
B. PCR (Correct Answer)
C. STR
D. Western blot

Explanation: **Explanation:** **Why PCR is the Correct Answer:** Polymerase Chain Reaction (PCR) is the gold standard for analyzing samples with **severe DNA degradation** or minute quantities of biological material. The underlying medical concept is **amplification**: PCR can take a single, fragmented, or partially destroyed DNA template and replicate it millions of times to create a detectable signal. In forensic medicine, even if the DNA is significantly compromised (e.g., from old skeletal remains, charred bodies, or trace stains), PCR-based techniques can target specific intact regions to provide a profile. **Analysis of Incorrect Options:** * **ELISA (Enzyme-Linked Immunosorbent Assay):** This is a biochemical technique used to detect **antigens or antibodies**, not DNA. It is commonly used for viral screening (like HIV or Hepatitis) but cannot reconstruct or test destroyed genetic material. * **STR (Short Tandem Repeats):** While STR analysis is the *method* used for DNA profiling, it is actually a **type of marker** analyzed *via* PCR. STR itself is not the testing mechanism for destroyed DNA; rather, PCR is the technology that makes STR analysis possible in degraded samples. * **Western Blot:** This technique is used specifically to detect **proteins** based on molecular weight. It has no application in DNA recovery or genetic profiling. **High-Yield Clinical Pearls for NEET-PG:** * **Mitochondrial DNA (mtDNA):** In cases of extreme degradation where nuclear DNA is completely lost (e.g., hair shafts without roots or ancient bones), mtDNA is used because it is present in high copy numbers per cell. * **RFLP vs. PCR:** Older methods like RFLP (Restriction Fragment Length Polymorphism) require large amounts of high-quality, undegraded DNA. PCR replaced RFLP because of its ability to work with "low template" or damaged DNA. * **Kary Mullis:** The scientist credited with inventing PCR (frequently asked in basic science sections).

Practice by Chapter

DNA Structure and Function

Practice Questions

DNA Extraction Methods

Practice Questions

PCR Technology

Practice Questions

Short Tandem Repeat Analysis

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Y-STR and Mitochondrial DNA

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DNA Databases

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Population Genetics in Forensics

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Statistical Interpretation

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Body Fluid Identification

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Bloodstain Pattern Analysis

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Emerging DNA Technologies

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Quality Assurance in DNA Testing

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