1. Understanding the Experiments Leading to Our Understanding of DNA as the Genetic Material

Frederick Griffith

Frederick Griffith's experiment was pivotal in demonstrating the phenomenon of transformation in bacteria. He showed that non-virulent bacteria could become virulent when exposed to heat-killed virulent bacteria, suggesting that some 'transforming principle' was responsible for transferring genetic information.

• Transformation: The process by which genetic material from one organism causes a heritable change in another organism.

• Key Evidence: Non-virulent bacteria acquired virulence after exposure to heat-killed virulent bacteria.

• Example: Streptococcus pneumoniae strains (smooth and rough) in Griffith's experiment.

Oswald Avery

Oswald Avery and colleagues expanded on Griffith's work by identifying DNA as the 'transforming principle.' Their experiments showed that only DNA, not proteins or other molecules, could induce transformation in bacteria.

• Key Point: DNA is the molecule responsible for heredity.

• Evidence: Enzymatic removal of DNA prevented transformation; removal of proteins or RNA did not.

• Additional info: Avery's work laid the foundation for molecular genetics.

Alfred Hershey and Martha Chase

Hershey and Chase used bacteriophages (viruses that infect bacteria) to demonstrate that DNA, not protein, is the genetic material. They labeled DNA with radioactive phosphorus and protein with radioactive sulfur, showing that only DNA entered the bacterial cells during infection.

• Key Point: DNA carries genetic information in viruses.

• Evidence: Radioactive phosphorus (DNA) was found inside bacteria; radioactive sulfur (protein) was not.

• Example: T2 bacteriophage experiments.

Erwin Chargaff

Erwin Chargaff discovered that the amount of adenine (A) in DNA equals the amount of thymine (T), and the amount of guanine (G) equals the amount of cytosine (C). These findings, known as Chargaff's rules, were crucial for understanding DNA structure.

• Chargaff's Rules: %A = %T and %G = %C in DNA.

• Significance: Provided evidence for base pairing in the double helix.

Watson, Crick, Franklin, and Wilkins

James Watson and Francis Crick, aided by Rosalind Franklin's X-ray diffraction images and Maurice Wilkins' research, proposed the double helix model of DNA. Their work explained how DNA's structure enables replication and genetic inheritance.

• Double Helix: DNA consists of two antiparallel strands forming a helical structure.

• Base Pairing: Adenine pairs with thymine; guanine pairs with cytosine.

• Example: Franklin's Photo 51 provided critical evidence for the helical structure.

• What does it mean that the strands of DNA are antiparallel? The two strands run in opposite directions (5' to 3' and 3' to 5').

2. DNA Replication and Repair

DNA Replication Models

DNA replication is the process by which a cell copies its DNA before cell division. The semiconservative model, supported by the Meselson-Stahl experiment, states that each new DNA molecule consists of one old strand and one new strand.

• Semiconservative Replication: Each daughter DNA molecule contains one parental and one newly synthesized strand.

• Meselson-Stahl Experiment: Used isotopes of nitrogen to distinguish old and new DNA strands, confirming the semiconservative model.

DNA Replication Details

DNA replication involves several key enzymes and steps to ensure accurate copying of genetic material.

• Helicase: Unwinds the DNA double helix.

• Topoisomerase: Relieves tension ahead of the replication fork.

• Primase: Synthesizes RNA primers to initiate DNA synthesis.

• DNA Polymerase: Adds nucleotides to the growing DNA strand; synthesizes in the 5' to 3' direction.

• Ligase: Joins Okazaki fragments on the lagging strand.

• Leading vs. Lagging Strand: Leading strand is synthesized continuously; lagging strand is synthesized in short fragments (Okazaki fragments).

• Equation:

DNA Repair

DNA repair mechanisms correct errors that occur during replication or due to environmental damage. These mechanisms are essential for maintaining genetic stability.

• Mismatch Repair: Corrects errors missed by DNA polymerase proofreading.

• Excision Repair: Removes damaged bases and replaces them with correct ones.

• Telomeres: Repetitive DNA sequences at chromosome ends; protect chromosomes from degradation.

• Telomerase: Enzyme that extends telomeres in germ cells and some cancer cells.

• Difference between 'end replication problem' and 'nucleotide excision repair': The end replication problem refers to the inability of DNA polymerase to fully replicate the ends of linear chromosomes, while nucleotide excision repair removes bulky DNA lesions.

3. Chromosomal Packaging

Chromatin

Chromatin is the complex of DNA and proteins that forms chromosomes within the nucleus of eukaryotic cells. Its organization is essential for DNA compaction and regulation of gene expression.

• Histones: Proteins that DNA wraps around to form nucleosomes.

• Nucleosome: The basic unit of chromatin, consisting of DNA wrapped around a histone octamer.

• Heterochromatin: Densely packed chromatin, generally transcriptionally inactive.

• Euchromatin: Loosely packed chromatin, generally transcriptionally active.