Molecular and Cellular Basis of Life: DNA Replication

Dr. Alessandro Siani discusses DNA, its role as a substrate that encodes metabolism, and the fundamental aspects of DNA replication.

Three Fundamental Rules of Replication:
1. Replication is semi-conservative: Each new DNA molecule contains one old (parent) strand and one new (daughter) strand.
2. Replication begins at an origin and proceeds bidirectionally.
3. Synthesis of new DNA occurs in the 5’→3’ direction and is semidiscontinuous: New DNA strands are synthesized in the 5’ to 3’ direction, leading to a continuous leading strand and a discontinuous lagging strand.

Three Possible Models for DNA Replication:
1. Conservative Model: The original DNA molecule remains intact, making a separate copy.
2. Semiconservative Model: Each DNA molecule consists of one old strand and one new strand.
3. Dispersive Model: The original strands are dispersed throughout new DNA molecules, hybridizing sections of parents with new DNA.

Data and Methodology:
- All DNA is initially labeled with $^{15}N$ (heavy nitrogen).
- E. coli cells grown in $^{15}N$ medium (Generation 0) and later transferred to $^{14}N$ medium for growth over four generations.
- Samples were analyzed via ultracentrifugation in CsCl density gradients which showed the distribution of $^{14}N$ and $^{15}N$ in DNA.
Results:
- 1st Generation: 100% $^{15}N$ ;
- 2nd Generation: 50% $^{15}N$ , 50% hybrid (15N-14N);
- 3rd Generation: 25% $^{15}N$ , 75% hybrid;
- 4th Generation: 12.5% $^{15}N$ , 87.5% hybrid.
Findings:
- The dispersive model was rejected because the results showed distinct bands rather than a single band.
- The conservative model was rejected as there were not two distinct bands after round 1.
- The semiconservative model was supported as it showed one band after the first round and two bands after the second round.

The Meselson-Stahl experiment indicated that nitrogen from parental strands is distributed equally between daughter genomes, supporting the semiconservative mechanism.

Cairns’ Experiments: Demonstrated that replication is bidirectional by utilizing radiolabeled DNA within the cells and showing two replication forks.

Replication begins at a unique origin ( extbf{oriC}) in prokaryotes, specifically E. coli, which is about 245 bp long with conserved sequence elements.
Key sequences include:
- Five repeats of a 9-bp sequence (R sites) for the initiation protein DnaA.
- An A=T-rich DNA unwinding element (DUE).

There are at least five DNA polymerases in E. coli:
- DNA Polymerase I: High abundance but low processivity (not optimal for replication).
- DNA Polymerase III: The principal replication enzyme, exhibiting high processivity.
- DNA Polymerases II, IV, V: Primarily involved in DNA repair.

Composition: A complex with 10 types of subunits which increases processivity through interaction with a dimer of β subunits (the sliding clamp).

DNA Synthesis Direction: Always occurs in the 5’→3’ direction. This entails that new nucleotides are added to the 3’-OH end of a growing strand.

Primase (DnaG) synthesizes RNA primers, and DNA Polymerase III extends the DNA chains. The leading strand is synthesized continuously, whereas the lagging strand is synthesized discontinuously in Okazaki fragments.

The replication fork forms a DNA loop allowing replication of both strands simultaneously.
When DNA Polymerase III on the lagging strand reaches an existing Okazaki fragment, it releases the β clamp and the template strand, allowing DNA Polymerase I to fill in gaps created by the removal of RNA primers.

The replication forks meet at a designated site containing sequences from 20-bp (Ter) sites, bound by the protein Tus, which halts the progression of the DnaB helicase, effectively terminating replication.

These complexes are asymmetrical, allowing replication fork progression from one direction while blocking it from the opposite direction.

Understanding the molecular processes of DNA replication reveals the elegantly coordinated actions of multiple proteins, polymerases, and mechanisms ensuring genetic fidelity and cellular function.