Overview
This lecture explains the structure of DNA and details the steps in DNA replication, highlighting key enzymes and the concept of semi-conservative replication.
DNA Structure
- DNA consists of two strands twisted into a double helix.
- Each DNA strand is made up of four nitrogenous bases: Adenine (A), Cytosine (C), Guanine (G), and Thymine (T).
- The two strands are complementary: A pairs with T, and C pairs with G.
- DNA strands have directionality, labeled 3' (three prime) and 5' (five prime).
- The two strands are antiparallel, running in opposite directions.
DNA Replication Process
- Replication begins with the enzyme helicase separating the two DNA strands, forming a replication fork.
- Each separated strand acts as a template for new DNA synthesis.
- Primase synthesizes a short RNA primer to start new strand creation.
- DNA polymerase binds to the primer and adds nucleotides in the 5' to 3' direction.
- The leading strand is synthesized continuously as DNA polymerase moves with the replication fork.
- The lagging strand is made in short segments called Okazaki fragments due to opposite fork movement.
- Each Okazaki fragment needs a new RNA primer from primase.
Final Steps in DNA Replication
- Exonuclease removes all RNA primers from the newly synthesized DNA.
- DNA polymerase fills the gaps left by removed RNA primers.
- DNA ligase joins the DNA fragments, ensuring a continuous double helix.
- The process results in semi-conservative replication: each new DNA molecule has one old and one new strand.
Key Terms & Definitions
- Double helix — the spiral shape formed by two DNA strands.
- Antiparallel — describes DNA strands running in opposite 5' to 3' directions.
- Replication fork — the Y-shaped structure formed during DNA strand separation.
- Primase — enzyme that creates RNA primers to start DNA synthesis.
- DNA polymerase — enzyme that adds DNA nucleotides in the 5' to 3' direction.
- Okazaki fragment — short DNA segment synthesized on the lagging strand.
- Exonuclease — enzyme that removes RNA primers from new DNA.
- DNA ligase — enzyme that joins DNA fragments together.
- Semi-conservative replication — each new DNA molecule contains one old strand and one new strand.
Action Items / Next Steps
- Review the steps of DNA replication and the role of each enzyme.
- Study the differences between leading and lagging strand synthesis.