DNA Replication Overview

Study Notes

Importance of Accuracy in DNA Replication

A human zygote has approximately 6.4 billion base pairs, necessitating high fidelity during replication.
Millions of cell divisions occur throughout human development, presenting significant opportunities for errors.
With a mutation rate of 1 error per million bases, roughly 6,400 mistakes could arise per cell division.
Speed is essential; E. coli replicates its 4.6 million base pairs at approximately 1000 bases per second, completing division in around 20 minutes.

Models for DNA Replication

Watson & Crick's double helix structure implies a template-based replication mechanism.
Conservative replication: Entire original double helix is copied into a new double helix without mixing.
Dispersive replication: New helices consist of hybrid DNA, mixing original and newly synthesized strands.
Semiconservative replication: Each original strand serves as a template for creating a complementary strand.

DNA Replication is Semi-Conservative

Meselson and Stahl's experiment in 1958 confirmed semi-conservative replication through density gradient centrifugation.
Their method utilized heavy (15N) and light (14N) nitrogen isotopes to trace DNA incorporation during replication.
Results indicated that DNA replication involves the separation of strands, with new strands being synthesized complementary to the templates.

Equilibrium Density Gradient Centrifugation

Involves spinning a centrifuge tube filled with a heavy salt solution and DNA, creating a density gradient.
Heavier DNA (with 15N) migrates to the bottom of the tube; lighter DNA (with 14N) remains closer to the top.

Meselson and Stahl's Experimental Design

Bacteria were cultured in heavy 15N, followed by transfer to light 14N media for subsequent replications.
After one round of replication, a single DNA band was observed, showing an intermediate density, eliminating the conservative model of DNA replication.

Implications of Dispersive Model

If dispersive replication were correct, second round analysis would show only hybrid density DNA that is somewhat lighter than after the first round.

Mechanisms of DNA Replication

Replication is driven by units called replicons, which include an origin of replication.
Theta replication: Circular DNA unwinds at the origin, creating replication forks; if two forks move bidirectionally, an intermediate structure resembling theta forms.
Rolling-circle replication: A break in one nucleotide strand initiates replication; the inner strand serves as the template while the outer strand is displaced, generating circular DNA molecules.

Linear Eukaryotic Replication

Replication occurs at a pace of 500-5000 base pairs per minute from a single replication fork, significantly slower than prokaryotic replication.
Eukaryotic chromosomes, often several hundred million bases long, require multiple origins of replication to ensure timely replication completion.

Description

Explore the fundamental concepts of DNA replication, including the importance of accuracy and various models such as semiconservative and conservative replication. Understand the implications of replication fidelity and the speed of DNA synthesis in both humans and E. coli.