Molecular Genetics: Experiments, DNA Structure, and Biotechnology

1. Griffith Experiment (1928) — Transformation & the Discovery of Heritable Molecules

Purpose and Background

Frederick Griffith conducted experiments to understand how bacteria cause disease, leading to the discovery of the process of transformation. His work provided the first evidence that genetic information could be transferred between organisms.

• Smooth (S) strain: Encapsulated, protected from immune system — virulent.

• Rough (R) strain: Non-encapsulated, susceptible to immune system — non-virulent.

Experiments & Observations

• Live S strain → Mouse dies.

• Live R strain → Mouse lives.

• Heat-killed S strain → Mouse lives.

• Live R strain + Heat-killed S strain → Mouse dies. Live S cells recovered.

Conclusion — Transformation

• Some "transforming principle" from dead S cells turned live R cells into virulent S cells.

• This principle was later identified as DNA, showing that DNA carries heritable information.

Connection to DNA Technology

• Laid the foundation for understanding how genetic information could be transferred via DNA — a concept essential for modern genetic engineering and biotechnology.

2. Hershey-Chase Experiment (1952) — Proving DNA Is the Genetic Material

Purpose

To determine whether DNA or protein stores hereditary information in viruses.

Why Use Bacteriophages?

• Bacteriophages are viruses that infect bacteria, injecting their genetic material into host cells.

Radioactive Labeling Strategy

• DNA: Labeled with 32P (phosphorus isotope).

• Protein: Labeled with 35S (sulfur isotope).

Major Findings

• 32P (DNA) was found inside bacteria after infection.

• 35S (protein) remained outside the bacteria.

Conclusion

• DNA, not protein, is the hereditary molecule.

Video Integration — Connection to Modern Biotechnology

• Confirmed the role of DNA in carrying genetic information, foundational for genetic engineering, cloning, and molecular biology techniques.

3. Chargaff’s Rules — Base Pairing Logic

Key Findings

• DNA composition varies between species.

• In any species, the amount of adenine (A) equals thymine (T), and guanine (G) equals cytosine (C):

$[A] = [T]$ $[G] = [C]$

Why This Matters

• Provided evidence for complementary base pairing, crucial for understanding DNA replication and structure.

4. Discovery of DNA Structure — Franklin, Watson, and Crick

Rosalind Franklin’s Contribution

• Used X-ray diffraction (X-ray crystallography) to create high-resolution DNA images.

• Showed DNA is a helix.

• Identified repeating patterns and spacing of parts within the helix.

Watson and Crick Model

• Double helix structure, two antiparallel strands (5'→3' and 3'→5').

• Bases on the inside, sugar-phosphate backbone on the outside.

• Complementary base pairing: A with T, G with C.

Importance of Double Helix Structure

• Explains how DNA replicates — each strand serves as a template.

• Mutations — base changes alter genetic information.

5. Telomeres & Telomerase — Aging, Replication Limits, and Cancer

What Are Telomeres?

• Telomeres are repetitive, noncoding DNA at chromosome ends that protect genes from shortening during replication.

Role of Telomerase

• Telomerase extends telomeres by adding nucleotide repeats.

• Active in germ cells, stem cells, and most cancer cells.

• Most somatic cells lack telomerase, so telomeres shorten with each division, contributing to aging.

Biological Significance

• Cellular aging: Shorter telomeres = cellular senescence or apoptosis.

• Cancer: Telomerase reactivation allows unlimited cell division.

Connection to AP Bio Themes

• Links to cell cycle regulation, DNA replication mechanisms, cancer biology, and uncontrolled cell division.

6. Polymerase Chain Reaction (PCR) — Rapid DNA Amplification

Purpose

PCR is a laboratory technique used to make millions of copies of a specific DNA sequence without living cells.

Why It’s Powerful

• Used in medical diagnostics (COVID testing), forensics, DNA analysis, and genetic research.

PCR Steps (Repeated 20–40 cycles)

1. Denaturation (94–98°C): DNA strands separate.

2. Annealing (50–65°C): Primers bind to target sequences.

3. Extension (72°C): Taq polymerase synthesizes new DNA.

Each cycle doubles the DNA — exponential growth.

7. DNA Cloning — Using Bacteria to Copy Genes

Step 1 — Creating Recombinant DNA

• Restriction enzymes cut DNA at specific sequences, producing sticky ends.

• Target gene is inserted into a plasmid.

• DNA ligase seals the recombinant plasmid.

Step 2 — Transformation of Bacteria

• Bacteria take up recombinant plasmid.

• Plasmid replicates, copying the gene of interest.

Applications

• Production of vaccines and medicines.

• Creating genetically modified organisms (GMOs).

• Research and gene function studies.

8. Gel Electrophoresis — DNA Separation by Size

How It Works

• DNA is negatively charged and moves toward the positive electrode.

• Smaller fragments move faster and farther through the gel.

• Used to analyze DNA fragment size or fingerprint samples.

Applications

• Crime scene analysis

• Genetic testing

• Research (e.g., verifying PCR products)

9. Southern Blotting

Purpose

To detect specific DNA sequences in a mixture.

Process

• DNA fragments from electrophoresis are transferred to a membrane.

• Radioactive or fluorescent DNA probes bind to complementary sequences.

• Only sequences that match the probe are visible.

Importance

• Vital for identifying mutations, diagnosing genetic diseases, and measuring gene copy number.

10. DNA Fingerprinting & Genetic Markers

Key Concept — Polymorphisms

• DNA fingerprinting uses genetic variations such as SNPs (single nucleotide polymorphisms) and STRs (short tandem repeats).

Applications

• Forensic science

• Human identification

• Population genetics

• Diagnosing hereditary diseases