Overview
This lecture explains the discovery of DNA’s double helix structure, the key experiments and scientists involved, and the molecular details of DNA's structure and function.
Components of DNA
- DNA is made of subunits called nucleotides, each containing a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases: adenine (A), thymine (T), guanine (G), or cytosine (C).
- Pyrimidines are single-ring bases (cytosine and thymine), while purines are double-ring bases (adenine and guanine).
- Nucleotides link together via covalent bonds forming a sugar-phosphate backbone.
Chargaff’s Rules
- Erwin Chargaff found that the amount of A equals T and G equals C in DNA, though overall base ratios differ among species.
- These relationships became crucial in identifying the base-pairing in DNA’s structure.
Key Contributors and Experiments
- Watson and Crick constructed the DNA double helix model mainly by analyzing existing research.
- Rosalind Franklin used X-ray crystallography to produce DNA diffraction images (notably "Photo 51") revealing its helical structure.
- Franklin’s data were shared with Watson and Crick without her consent, and her contributions were not fully acknowledged.
Structure of the DNA Double Helix
- DNA consists of two antiparallel strands (running in opposite 5' to 3' directions), forming a right-handed double helix.
- The sugar-phosphate backbone is on the outside, with paired nitrogenous bases inside.
- Base pairing is specific: A pairs with T (two hydrogen bonds), and G pairs with C (three hydrogen bonds), explaining Chargaff’s rules.
- The helix has major and minor grooves, which serve as binding sites for proteins.
Variations and Significance
- Most DNA is in the B-form (right-handed), but A-DNA is also right-handed, while Z-DNA is left-handed.
- The discovery of the structure enabled deeper understanding of DNA replication and gene function.
Key Terms & Definitions
- Nucleotide — DNA subunit composed of a sugar, a phosphate group, and a nitrogenous base.
- Purine — Double-ring nitrogenous base (adenine or guanine).
- Pyrimidine — Single-ring nitrogenous base (cytosine or thymine).
- Sugar-phosphate backbone — Repeating chain of sugars and phosphates forming the structure of DNA.
- Antiparallel — Strands running in opposite directions in the DNA double helix.
- X-ray crystallography — Technique to determine molecular structures by observing X-ray diffraction patterns.
- Major groove/Minor groove — Structural features of the DNA helix, important for protein binding.
- Complementary base pairs — Specific pairing of A with T and G with C through hydrogen bonds.
Action Items / Next Steps
- Review the function and structure of DNA nucleotides.
- Study Chargaff's rules and their implications for base pairing.
- Explore how DNA structure enables its function in replication and gene expression.