Copy_of_2-DNA-Replication-March-2023

DNA Replication Overview

Definition: The process of copying one DNA molecule into two identical molecules, essential for cell reproduction.
Timing: Occurs during interphase, before mitosis begins.
Models of Replication:
- Conservative Model: Results in one new molecule and conserves the old one.
- Semi-Conservative Model: Produces two hybrid molecules, each containing one old and one new strand.
- Dispersive Model: Creates hybrid molecules where each strand is a mixture of old and new.

Purpose: To test the models of DNA replication by distinguishing between parent and daughter strands.
Method: Used isotopes of nitrogen, 14N (light) and 15N (heavy).
- Reason for Choosing Isotopes: Nitrogen is a DNA component and differing densities allow for separation.
- Density Difference: 15N DNA is more dense than 14N DNA.

Binding of Initiator Proteins: Initiator proteins attach to DNA, starting unwinding.
Role of Helicase: Multiple helicase enzymes cut hydrogen bonds between base pairs, unwinding the DNA.
Stabilization: Single-strand-binding proteins stabilize the unwound strands.
Topoisomerase II: Relieves strain in the double helix due to unwinding.
Replication Bubble Formation: Creates a Y-shaped replication fork.
Origins of Replication:
- Circular prokaryotic DNA has a single origin.
- Linear eukaryotic DNA may have thousands.

Template Usage: New DNA strands are assembled using parent DNA as templates.
Role of DNA Polymerase III: Catalyzes addition of new nucleotides, creating complementary strands to each parent strand.
Direction of Synthesis: Nucleotides added from 5' to 3' direction; the leading strand is synthesized continuously.
Lagging Strand Formation:
- Formed in short segments (Okazaki fragments) away from the replication fork in a discontinuous manner.
- Requires primase to synthesize RNA primers.
- DNA Polymerase I removes primers, fills spaces with DNA fragments.
- DNA ligase joins Okazaki fragments to complete the strand.

Completion of Synthesis: When new DNA strand synthesis is complete, the two DNA molecules separate.
Dismantling of Replication Machinery: The replication complex of proteins and DNA is dismantled.

Error Rate: Approximately one error per billion nucleotide pairs.
Causes of Errors:
- Mispairing of bases due to flexibility in DNA structure.
- Strand slippage causing additions or omissions in synthesized strands.

DNA Polymerase Proofreading: Enzymes I and II recognize and correct mismatch errors in new DNA strands, correcting about 99% of them.
Mismatch Repair: Groups of proteins recognize and repair deformities in synthesized DNA due to mispaired bases.
Evolution of Mutations: Errors that persist after proofreading or mismatch repair can lead to mutations during cell division.