DNA: Structure, Function, and Forensic Application

DNA Structure and Function:

• Definition: DNA (Deoxyribonucleic Acid) is the genetic material in the nuclei of cells in all living organisms.
• Cellular Presence: DNA is present in the nucleus of cells, forming a double helix structure. In humans, this structure is supercoiled into chromosomes, comprising proteins and DNA.
• Chromosomal Details: Humans have 23 pairs of chromosomes, with 23 inherited from the mother and 23 from the father.
• Genetic Coding: DNA encodes all information about the body’s internal organization, physical characteristics, and physiological functions using a sequence of four nucleotides (bases): Adenine (A), Guanine (G), Thymine (T), and Cytosine (C), linked by a sugar-phosphate backbone.

2. DNA Sources and Collection:

• Sources: DNA can be extracted from any biological material like saliva, semen, vaginal fluids, blood, body tissues, teeth, hair, and bones. The amount of DNA varies across these sources, with blood and saliva being richer sources compared to teeth and hair roots.
• Touch DNA: This refers to DNA left on objects during physical contact, containing very low amounts of DNA, making it less ideal for DNA profiling.
• Sample Collection and Transportation: Proper techniques are crucial to avoid contamination and ensure DNA sample quality during collection, packaging, storage, and transportation.

3. DNA Profiling:

• Human DNA Similarity: 99.9% of human DNA is identical. For forensic DNA profiling, a few locations in the remaining 0.1% of DNA are analyzed.
• Loci and STRs: DNA profiling focuses on specific locations called loci, which have repetitive sequences of DNA known as Short Tandem Repeats (STRs). The number of repetitions (alleles) at each locus varies among individuals, which forms the basis of a DNA profile.
• Process: DNA profiling involves several steps:
  1. Isolation, purification, and quantitation of DNA.
  2. Amplification of selected genetic markers.
  3. Visualization of DNA fragments and genotyping.
  4. Statistical analysis and interpretation of results.

4. Reliability and Limitations of DNA Profiling:

• Probabilistic Nature: DNA analysis is based on probability and is not always conclusive. Factors like sample contamination, delays in testing, or sample switching can lead to inconclusive results.
• Inconclusiveness: Convictions cannot rely solely on DNA evidence; corroborating evidence is necessary.
• Outcomes of DNA Comparison:
  1. Match: Indistinguishable DNA profiles indicate a match.
  2. Exclusion: Differences in profiles suggest different sources.
  3. Inconclusive: The data does not support a definitive conclusion.

5. Legal Perspective:

• Law Commission Report: The identity from DNA matches is not conclusive; it provides a probability (random occurrence ratio) rather than a certainty.
• Judicial View: DNA evidence’s probative value varies depending on the case’s context and other evidence. It is becoming more reliable with advancements in science, but it is not infallible.

Conclusion: While DNA profiling is a powerful tool in forensic science, its reliability and conclusiveness depend on various factors, including the quality of the sample and corroborating evidence. It serves as a significant investigative aid but is not definitive proof of identity in legal contexts.

Source: TH

Multiple-choice questions (MCQs) with answers and explanations  

1. What is the basic structural unit of DNA?

A) Amino acid

B) Nucleotide

C) Ribosome

D) Carbohydrate

Answer: B) Nucleotide
Explanation: DNA is composed of nucleotides, which consist of a sugar, a phosphate group, and a nitrogenous base (Adenine, Guanine, Thymine, or Cytosine).

2. In which part of the cell is DNA primarily located?

A) Cytoplasm

B) Ribosome

C) Nucleus

D) Mitochondria

Answer: C) Nucleus
Explanation: DNA is primarily located in the nucleus of the cell, where it is organized into chromosomes.

3. How many pairs of chromosomes are present in a typical human cell?

A) 22 pairs

B) 23 pairs

C) 24 pairs

D) 46 pairs

Answer: B) 23 pairs
Explanation: Humans have 23 pairs of chromosomes, making a total of 46 chromosomes in each cell.

4. Which of the following is a source of DNA suitable for forensic analysis?

A) Hair without root

B) Red blood cells

C) Saliva

D) Sweat

Answer: C) Saliva
Explanation: Saliva contains cells that are rich in DNA. In contrast, mature red blood cells do not contain DNA, and hair without a root has minimal DNA.

5. What is the significance of Short Tandem Repeats (STRs) in DNA profiling?

A) They determine eye color.

B) They are used to amplify DNA.

C) They are repetitive DNA sequences used in forensic profiling.

D) They encode for specific proteins.

Answer: C) They are repetitive DNA sequences used in forensic profiling.
Explanation: STRs are short sequences of DNA that repeat at specific loci. The number of repeats varies among individuals, making STRs useful for DNA profiling.

6. Which of the following is NOT a step in the DNA profiling process?

A) Amplification of selected genetic markers

B) Isolation of RNA

C) Quantitation of DNA

D) Statistical analysis of results

Answer: B) Isolation of RNA
Explanation: DNA profiling involves the isolation of DNA, not RNA. The process also includes amplification, quantitation, and statistical analysis.

7. What percentage of human DNA is identical across all individuals?

A) 50%

B) 75%

C) 90%

D) 99.9%

Answer: D) 99.9%
Explanation: Human DNA is 99.9% identical across individuals. The remaining 0.1% contains variations that are unique to each person and are used in forensic profiling.

8. Which of the following outcomes from a DNA profile comparison would suggest that the DNA samples came from the same source?

A) Match

B) Exclusion

C) Inconclusive

D) Amplification

Answer: A) Match
Explanation: A match in DNA profile comparison indicates that the DNA samples are indistinguishable and likely came from the same source.

9. What does the term “touch DNA” refer to?

A) DNA found in blood samples

B) DNA left behind on objects during physical contact

C) DNA extracted from bone marrow

D) DNA found in hair follicles

Answer: B) DNA left behind on objects during physical contact
Explanation: Touch DNA refers to DNA that is transferred to objects through physical contact. It contains very low amounts of DNA and is not ideal for profiling.

10. According to the Law Commission of India, how should DNA evidence be treated in legal contexts?

A) As conclusive proof of identity

B) As probabilistic evidence

C) As irrelevant to criminal cases

D) As the sole basis for conviction

Answer: B) As probabilistic evidence
Explanation: The Law Commission of India suggests that DNA evidence is based on probability and should be used cautiously. It cannot be the sole basis for conviction without corroborating evidence.