Overview
This lecture explains the key structural features of DNA and how these features contribute to its function in the cell.
DNA Structure Overview
- DNA stands for deoxyribonucleic acid and is typically found as a double-stranded helix in cells.
- The most common form of DNA is B-form DNA, a right-handed double helix.
- Each DNA strand is a polynucleotide made of repeating units called nucleotides.
Nucleotide Components
- A nucleotide consists of a five-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base.
- The nitrogenous base attaches to the 1' carbon of the sugar.
- Phosphate groups connect the 5' carbon of one sugar to the 3' carbon of the next via phosphodiester bonds, forming the backbone.
DNA Directionality & Backbone
- DNA strands have directionality: one runs 5' to 3' and the other 3' to 5', making them antiparallel.
- Sugars and phosphates form the DNA backbone, while bases extend inward.
Base Pairing & Structure
- The two strands are held together by hydrogen bonds between complementary bases (base pairs).
- Adenine (A) pairs with Thymine (T) via two hydrogen bonds; Guanine (G) pairs with Cytosine (C) via three.
- Thymine and cytosine are pyrimidines (single ring); adenine and guanine are purines (double rings).
- Only proper base pairs (AT/TA, GC/CG) fit the geometry of the double helix.
Helical Features & Stability
- The DNA helix is highly regular, with about 10 base pairs per turn.
- Base stacking (pi-pi interactions) between aromatic rings of bases further stabilizes the helix.
Major and Minor Grooves
- The helix has repeating major and minor grooves, which serve as binding sites for proteins.
- The major groove allows sequence-specific recognition; the minor groove is less sequence-specific.
Key Terms & Definitions
- Nucleotide — Basic unit of DNA, made of a sugar, phosphate, and nitrogenous base.
- Phosphodiester bond — Covalent bond linking nucleotides in a strand.
- Antiparallel — Orientation where DNA strands run in opposite 5' to 3' directions.
- Base pair — Two nitrogenous bases paired by hydrogen bonds, holding the DNA strands together.
- Purine — Double-ring nitrogenous base (adenine, guanine).
- Pyrimidine — Single-ring nitrogenous base (cytosine, thymine).
- Major groove — Large groove in the DNA helix, important for protein binding.
- Minor groove — Smaller groove in the DNA helix, less specific for protein binding.
Action Items / Next Steps
- Review the DNA structure diagrams to reinforce base pairing and backbone concepts.
- Practice identifying major and minor grooves and directionality in DNA models.