Overview
This lecture explains why DNA replication is necessary, details the stages and location of the process, and clarifies its importance in cell division.
Purpose and Timing of DNA Replication
- DNA replication ensures new cells receive identical genetic information.
- Replication occurs during interphase, specifically the S (synthesis) phase.
- Interphase is when the cell grows, develops, and specializes, spending most of its life cycle here.
- Accurate replication prevents mutations, which can be harmful.
- DNA replication enables genetic code inheritance and tissue repair.
The Cell Cycle Context
- Interphase includes growth phases (G1, G2) and DNA synthesis (S phase).
- DNA exists as chromatin (thread-like) during interphase, with the nucleus intact.
- Mitosis follows interphase, resulting in two identical daughter cells.
Steps of DNA Replication
- DNA is a double helix with two strands wound together, making the code inaccessible in this state.
- The double helix unwinds to expose the nucleotide bases (A, T, G, C).
- Hydrogen bonds between bases break, separating the two strands.
- An enzyme catalyzes the unwinding and separation of strands.
- Each original strand serves as a template for a new complementary strand.
- Free nucleotides in the nucleus pair up with their complementary bases: A pairs with T, G pairs with C.
- The sugar-phosphate backbone of new nucleotides connects to form a complete new strand.
- DNA polymerase is the main enzyme that reads, assembles, and checks the new DNA.
- Hydrogen bonds reform between the bases, resulting in two identical DNA molecules.
- Each DNA molecule consists of one old (parental) and one new (complementary) strand (semi-conservative replication).
- DNA rewinds into the double helix structure.
- The cell can now divide, ensuring each daughter cell gets one complete set of DNA.
Key Terms & Definitions
- Interphase — cell cycle phase where the cell grows and DNA replicates.
- Chromatin — thread-like form of DNA found during interphase.
- S phase — synthesis phase of interphase when DNA replication occurs.
- Double helix — two-stranded, spiral structure of DNA.
- Nucleotide — basic unit of DNA, consisting of a sugar, phosphate, and nitrogenous base.
- Hydrogen bond — weak bond holding complementary DNA bases together.
- DNA polymerase — enzyme that builds new DNA strands and checks for errors.
- Semi-conservative replication — each new DNA molecule contains one old and one new strand.
Action Items / Next Steps
- Review the stages of DNA replication and be able to describe them in detail.
- Use the exam guideline booklet to distinguish DNA replication from protein synthesis.
- Prepare to explain why replication accuracy is crucial for inheritance and cell division.