Overview
This lecture covers the structure, function, historical discovery, and replication process of DNA, with comparisons to RNA and key terminology defined.
DNA: Structure and Function
- DNA (deoxyribonucleic acid) stores genetic information for cell function and heredity.
- Each human somatic cell contains 46 chromosomes, each with a DNA molecule.
- DNA is a nucleic acid polymer made from repeating nucleotides.
- Each nucleotide contains a five-carbon sugar (deoxyribose), a phosphate group, and a nitrogenous base (A, T, C, G).
- DNA molecules exist as two antiparallel strands forming a double helix.
- Sugar-phosphate backbones run in opposite (5' to 3' and 3' to 5') directions.
- Nitrogenous bases pair specifically: adenine (A) with thymine (T), guanine (G) with cytosine (C).
- Base pairs are held together by hydrogen bonds (A-T: 2 bonds; G-C: 3 bonds).
- The sequence of nitrogenous bases encodes genetic information.
RNA vs DNA
- RNA (ribonucleic acid) is usually single-stranded, with ribose as its sugar.
- RNA uses uracil (U) instead of thymine (T), pairing with adenine (A).
- RNA plays a key role in protein production and DNA replication.
Historical Discovery of DNA
- DNA was first discovered by Friedrich Miescher in 1869 (called "nuclein").
- Rosalind Franklin used x-ray diffraction to help reveal the helical structure and sugar-phosphate backbone location.
- Watson and Crick elucidated the double helix model, building on Franklin's data; she was not awarded the Nobel due to her early death.
DNA Replication
- Replication allows each cell to copy its DNA before division.
- Helicase unwinds double helix, creating a replication fork.
- DNA polymerase synthesizes new strands using each original strand as a template.
- Leading strand is synthesized continuously after one RNA primer is laid down by RNA primase.
- Lagging strand is synthesized in short segments (Okazaki fragments), each starting with its own RNA primer.
- DNA ligase joins Okazaki fragments; DNA polymerases replace RNA primers and proofread to correct errors.
Key Terms & Definitions
- Nucleotide — building block of DNA or RNA, consisting of a sugar, phosphate, and nitrogenous base.
- Double Helix — the twisted ladder shape of DNA formed by two strands of nucleotides.
- Antiparallel — orientation of the two DNA strands running in opposite directions (5' to 3' and 3' to 5').
- Base Pair — two nitrogenous bases bonded across DNA strands (A-T, G-C).
- Replication Fork — the point where DNA is split for replication.
- Leading Strand — the DNA strand synthesized continuously during replication.
- Lagging Strand — the DNA strand synthesized discontinuously in Okazaki fragments.
- Okazaki Fragment — short DNA segments formed on the lagging strand.
- Primer — short RNA sequence that provides a starting point for DNA synthesis.
- DNA Ligase — enzyme that joins Okazaki fragments together.
Action Items / Next Steps
- Review notes on DNA structure and replication to prepare for next week's lesson on gene expression.
- Watch the next lecture to learn how DNA translates into traits.