Molecular basis of Inheritance 09 - CUET Computer Science

Human Genome Project and DNA Fingerprinting

The Human Genome Project (HGP) and DNA fingerprinting are foundational topics in molecular genetics, focusing on the sequencing, analysis, and application of human and non-human DNA. These topics cover the methodologies, findings, and practical uses of genome sequencing and DNA profiling, especially in forensic science and genetic identification.

Bioinformatics and Model Organism Sequencing

Bioinformatics emerged as a crucial field for managing, analyzing, and interpreting the vast data generated by genome sequencing. The HGP also included sequencing of non-human model organisms to understand their natural capabilities and potential applications in healthcare, agriculture, energy…

Human Genome Project and DNA Fingerprinting

The Human Genome Project (HGP) and DNA fingerprinting are foundational topics in molecular genetics, focusing on the sequencing, analysis, and application of human and non-human DNA. These topics cover the methodologies, findings, and practical uses of genome sequencing and DNA profiling, especially in forensic science and genetic identification.

Bioinformatics and Model Organism Sequencing

Bioinformatics emerged as a crucial field for managing, analyzing, and interpreting the vast data generated by genome sequencing. The HGP also included sequencing of non-human model organisms to understand their natural capabilities and potential applications in healthcare, agriculture, energy production, and environmental remediation.

• Model organisms sequenced: Bacteria, yeast, free-living non-pathogenic nematodes, Drosophila, rice, and others.
• Purpose: To enable the use of these organisms in various applied fields and to facilitate vaccine and injection development by understanding their genetic sequences.

Approaches to Genome Sequencing

Two major approaches were used in the HGP: Expressed Sequence Tags (ESTs) and Sequence Annotation. These approaches differ in their focus on coding and non-coding DNA segments.

• EST (Expressed Sequence Tags): Focuses on sequencing only those genes that are expressed as RNA, i.e., coding sequences.
• Sequence Annotation: Involves sequencing the entire genome, including both coding and non-coding regions, and later assigning functions to different regions.
• The HGP adopted the sequence annotation approach, sequencing all DNA segments regardless of their coding potential.

Methodology of Genome Sequencing

Genome sequencing involves several key steps, from DNA isolation to sequence assembly. The process ensures that even large DNA molecules are efficiently analyzed and interpreted.

• DNA Isolation: DNA can be extracted from various sources such as hair roots, saliva, or blood.
• Fragmentation: Large DNA molecules (e.g., 2.2 meters in length) are digested using restriction endonucleases, producing fragments of varying lengths.
• Separation: DNA fragments are separated by gel electrophoresis, typically using agarose gel.
• Amplification: Multiple copies of DNA fragments are generated using cloning (with bacterial or yeast artificial chromosomes as vectors) or PCR (Polymerase Chain Reaction).
• Sequencing: Fragments are sequenced using automated DNA sequencers based on the Frederick Sanger method.
• Assembly: Overlapping regions among fragments are used to reconstruct the complete DNA sequence.
• Annotation: Computer-based programs assign sequences to specific chromosomes and identify gene functions.

Key Findings from the Human Genome Project

The HGP revealed several important features of the human genome, including the number of genes, the nature of coding and non-coding DNA, and the extent of genetic variation among individuals.

• Total nucleotide bases: 3,164,700,000 (3.16 billion) bases.
• Average gene length: About 3,000 bases, but can vary greatly (e.g., dystrophin gene: 2.4 million bases; TDF gene: 14 bases).
• Number of genes: Approximately 30,000, distributed across 23 pairs of chromosomes.
• Genetic similarity: 99.9% of nucleotide bases are identical among all humans; only 0.1% variation exists.
• Protein-coding DNA: Less than 2% of the genome codes for proteins.
• Repeated sequences: The majority of human DNA consists of repeated sequences, which do not code for proteins but are useful in DNA fingerprinting.
• Chromosome 1: Contains the highest number of genes (2,968); its sequencing was completed in 2006, causing a delay in the project.
• Single Nucleotide Polymorphisms (SNPs): 1.4 million locations identified where a single base differs among individuals.

DNA Fingerprinting: Principle and Basis

DNA fingerprinting, also called DNA profiling, is based on detecting differences in specific repetitive DNA regions among individuals. These differences are used for identification in forensic and paternity cases.

• Basis: The 0.1% genetic variation, especially in repetitive DNA (satellite DNA), forms the foundation of DNA fingerprinting.
• Repetitive DNA: Consists of stretches of DNA that repeat multiple times; these are non-coding and vary in number among individuals.
• Types of repetitive DNA: Classified based on length, base composition, and number of repeats.

Types of Satellite DNA Used in Fingerprinting

Satellite DNA is divided into two main types based on the size and number of repeat units. These are critical for generating unique DNA profiles.

• VNTR (Variable Number Tandem Repeats): Also called minisatellites; consist of longer repeat units (10–100 base pairs) and show high polymorphism. Used predominantly in early DNA fingerprinting.
• STR (Short Tandem Repeats) or Microsatellites: Consist of shorter repeat units (1–6 base pairs). Modern technology allows their use in DNA profiling.
• Inheritance: VNTRs and STRs are inheritable and show high variability among individuals and chromosomes.

Steps in DNA Fingerprinting

DNA fingerprinting involves a series of laboratory steps to isolate, fragment, separate, and visualize DNA bands unique to each individual.

1. Sample Collection: DNA is isolated from sources like blood, hair, saliva, or skin.
2. Digestion: DNA is cut into fragments using restriction endonucleases.
3. Separation: Fragments are separated by agarose gel electrophoresis; smaller fragments move faster.
4. Transfer: DNA bands are transferred from the gel to a nitrocellulose or nylon membrane (Southern blotting technique).
5. Hybridization: The membrane is exposed to radioactive single-stranded DNA probes complementary to the VNTR or STR regions.
6. Detection: The location of hybridized probes is visualized using autoradiography, revealing a pattern of bands (the DNA fingerprint).
7. Comparison: Band patterns from different samples are compared to establish identity or relationships.

Applications of DNA Fingerprinting

DNA fingerprinting is a powerful tool for individual identification, forensic investigations, and resolving biological relationships.

• Forensic science: Identifying criminals from biological evidence at crime scenes.
• Paternity disputes: Establishing biological parentage by comparing child and parent DNA fingerprints.
• Personal identification: Each individual (except monozygotic twins) has a unique DNA fingerprint.
• Population studies: Used for genetic mapping and evolutionary studies.

DNA Polymorphism and Its Importance

DNA polymorphism refers to the presence of multiple forms of DNA sequences among individuals, mainly due to mutations. This variation is essential for DNA fingerprinting and genetic mapping.

• Origin: Caused by mutations in non-coding regions, which are inherited through generations.
• Frequency: Only those polymorphisms with a population frequency greater than 0.1% are useful for fingerprinting.
• Inheritance: Polymorphisms in non-coding regions are more likely to be inherited as they do not affect protein function.
• Role in fingerprinting: High degree of polymorphism in satellite DNA ensures unique banding patterns for each individual.

Key Scientists and Historical Notes

The development and application of DNA fingerprinting involved significant contributions from specific scientists.

• Alec Jeffreys: Credited as the father of DNA fingerprinting (developed the technique in 1985).
• Lalji Singh and V.K. Kashyap: Recognized for establishing DNA fingerprinting technology in India and contributing to molecular human genetics research.

Summary Table: Types of Satellite DNA

• VNTRs and STRs are both non-coding, repetitive DNA sequences used for generating unique DNA profiles.
• The number of repeats varies among individuals and chromosomes, forming the basis for identification.