DNA Structure and Replication

Watson and Crick (1953): Introduced the double-helical model for DNA structure.
DNA: The molecule of inheritance, encoding hereditary information.
Role of DNA: Directs biochemical, anatomical, physiological, and some behavioral traits.

Early 20th Century: Identification of inheritance molecules was a major challenge.
Chromosomes and Genes: Research by T.H. Morgan demonstrated that genes are located on chromosomes, where DNA and proteins were candidates for genetic material.

Frederick Griffith's Work: Discovered transformation through experiments with pathogenic and harmless strains of a bacterium.
- Transformation Defined: Change in genotype and phenotype due to foreign DNA assimilation.

Transforming Substance Identified: The transforming factor was DNA, as evidenced by experiments showing that only DNA transformed harmless bacteria into pathogenic.
Skepticism: Some biologists doubted the results due to limited knowledge of DNA.

Viral Studies: Research on bacteriophages provided further evidence for DNA as the genetic material.
Hershey and Chase (1952): Their experiments showed that only DNA (not protein) entered E. coli cells during T2 phage infection, reinforcing the idea that DNA carries genetic information.

Nucleotide Composition: Erwin Chargaff found that the amount of adenine (A) = thymine (T) and the amount of guanine (G) = cytosine (C), adding credibility to DNA as the genetic material.

X-ray Crystallography: Techniques used by Maurice Wilkins and Rosalind Franklin.
- Franklin's images implied a helical structure for DNA.
- DNA model built by Watson and Crick identified antiparallel sugar-phosphate backbones with nitrogenous bases on the inside.

Watson-Crick Model: Specific pairing where A pairs with T and G pairs with C.
This specificity explains Chargaff’s observations: in any organism, A = T and G = C.

Base Pairing Mechanism: The complementary strands of DNA serve as templates in a replication process.
Semiconservative Replication: When DNA replicates, each daughter molecule consists of one parent strand and one new strand.

Origin of Replication: Sites where DNA strands are separated to form replication bubbles.
Key Enzymes in Replication:
- Helicases: Unwind the double helix.
- Single-strand binding proteins: Stabilize strands until they serve as templates.
- Topoisomerases: Relieve overwinding ahead of the replication fork.
- Primase: Synthesizes short RNA primers to initiate replication.
- DNA Polymerases: Enzymes that synthesize new DNA strands by adding nucleotides.

Leading Strand: Synthesized continuously towards the replication fork.
Lagging Strand: Synthesized as Okazaki fragments in a direction away from the fork.
DNA Ligase: Enzyme that joins Okazaki fragments on the lagging strand.

Error Correction: DNA polymerases proofread newly synthesized DNA, correcting mismatches.
Damage Repair: Enzymes repair DNA damaged by environmental factors through various repair mechanisms, including nucleotide excision repair.

Telomeres: Repetitive nucleotide sequences at the ends of eukaryotic chromosomes that protect genes during replication.
Role of Telomerase: Catalyzes the lengthening of telomeres in germ cells, potentially linked to aging and cancer cell proliferation.