Overview
This lecture explains the structure, function, and replication of DNA, highlighting its significance, key scientists involved, and differences between DNA and RNA.
Structure and Role of DNA
- DNA (deoxyribonucleic acid) stores genetic instructions for cellular activities in all living organisms.
- DNA is a double helix: two polynucleotide chains held together by hydrogen bonds between nitrogenous base pairs.
- DNA molecules are tightly packed into chromosomes, with humans having 46 per cell.
- Each DNA molecule is a polymer of nucleotides, which consist of a deoxyribose sugar, a phosphate group, and one of four nitrogenous bases (A, T, C, G).
- The backbone of DNA is made of alternating sugar and phosphate groups, running anti-parallel (5' to 3' and 3' to 5').
DNA Base Pairing and Sequencing
- Adenine (A) pairs with Thymine (T) via two hydrogen bonds; Guanine (G) pairs with Cytosine (C) via three hydrogen bonds.
- The sequence of bases encodes genetic information; different sequences make different genetic instructions.
- Base sequences on one strand predict the sequence on the complementary strand (A with T, G with C).
DNA vs. RNA
- DNA is double-stranded; RNA (ribonucleic acid) is single-stranded.
- RNA's sugar is ribose (with one more oxygen than deoxyribose in DNA).
- RNA uses uracil (U) instead of thymine (T); uracil pairs with adenine.
History of DNA Discovery
- DNA was first discovered by Friedrich Miescher in 1869, initially called "nuclein."
- Rosalind Franklin used X-ray diffraction to reveal DNA's helical structure and phosphate-sugar backbone.
- Watson and Crick built on others' data, publishing the double helix model in 1953.
DNA Replication
- Replication creates an identical copy of DNA, using each original strand as a template.
- Helicase enzyme unwinds the double helix at the replication fork.
- Leading strand is synthesized continuously; lagging strand is synthesized in Okazaki fragments.
- RNA primase lays RNA primers; DNA polymerase extends new DNA from primers.
- DNA ligase joins Okazaki fragments on the lagging strand.
- DNA polymerase proofreads and corrects errors, maintaining genetic fidelity.
Key Terms & Definitions
- Nucleotide — building block of nucleic acids; consists of sugar, phosphate, and nitrogenous base.
- Polynucleotide — a long chain of nucleotides.
- Double helix — two intertwined DNA strands forming the DNA molecule’s structure.
- Base pair — two complementary nucleotides bonded by hydrogen bonds (A-T, G-C).
- Okazaki fragments — short DNA segments synthesized on the lagging strand during replication.
- Replication fork — the Y-shaped region where DNA is split for replication.
- RNA primer — short RNA sequence that initiates DNA synthesis.
Action Items / Next Steps
- Review the structure and replication steps of DNA.
- Prepare for next week's topic: how DNA creates individual traits.
- Ask any unclear questions in class or online discussion boards.