Overview
This lecture explains the process of DNA replication, highlighting the roles of key enzymes, the directionality of synthesis, and the differences between the leading and lagging strands.
Purpose of DNA Replication
- DNA replication is required for a single cell to divide into two cells, each with a complete set of DNA.
- Both resulting cells must have identical copies of DNA after replication.
Key Enzymes and Directionality
- DNA polymerase is the main enzyme that reads the template strand and synthesizes new DNA.
- Replication proceeds in the 5' to 3' direction, always adding new nucleotides to the 3' end.
- Complementary base pairing is crucial: A pairs with T, and G pairs with C.
Steps in DNA Replication
- The process begins with initiation, where the enzyme helicase unwinds and unzips the DNA double helix.
- Elongation follows, where DNA polymerase synthesizes new DNA by matching complementary bases.
Leading vs. Lagging Strand
- The leading strand is synthesized continuously in the same direction as the replication fork.
- The lagging strand is synthesized discontinuously in small fragments called Okazaki fragments.
- Short RNA primers are required to start each Okazaki fragment on the lagging strand.
- DNA polymerase replaces RNA primers with DNA, and DNA ligase seals the gaps.
Semi-Conservative Replication
- DNA replication is semi-conservative: each new DNA molecule contains one old (template) strand and one newly synthesized strand.
- This method reduces errors compared to synthesizing DNA from scratch.
Key Terms & Definitions
- DNA replication — the process of copying DNA before cell division.
- DNA polymerase — enzyme that synthesizes new DNA strands.
- Helicase — enzyme that unwinds the DNA double helix.
- Leading strand — DNA strand synthesized continuously toward the replication fork.
- Lagging strand — DNA strand synthesized in short discontinuous Okazaki fragments away from the replication fork.
- RNA primer — short RNA segment providing a starting point for DNA synthesis.
- Okazaki fragment — short segment of DNA synthesized on the lagging strand.
- DNA ligase — enzyme that seals gaps between Okazaki fragments.
- Semi-conservative replication — each new DNA molecule has one old and one new strand.
Action Items / Next Steps
- Review the process and enzymes involved in DNA replication.
- Understand and be able to explain the differences between leading and lagging strand synthesis.