DNA Replication Models and Polymerization

Original Models for Replication
- Three models proposed for DNA replication:
- Conservative Model: The original parental double-stranded DNA (dsDNA) is conserved and remains intact after division.
- Semi-Conservative Model: Each new dsDNA duplex consists of one parental strand and one daughter strand.
- Dispersive Model: New DNA is distributed equally among parental and daughter strands.
- The original models for replication were investigated in the 1958 Meselson and Stahl experiment.

Nucleic Acid Synthesis
- DNA polymerization is a condensation reaction that involves the joining of nucleotides.
- Hydrolysis of bonds between phosphates provides necessary energy, akin to how ATP hydrolysis can drive energetically unfavorable reactions.
Direction of Synthesis
- DNA synthesis occurs in a 5’ to 3’ direction.
Polymerase Activity
- Requires nucleoside triphosphates for the incorporation of nucleotides.
- A condensation reaction creates phosphodiester bonds between nucleotides.

Key Enzymes
- DNA Polymerases:
- DNA Polymerase III:
  - Synthesizes DNA in the 5’ to 3’ direction.
  - Requires a template to guide synthesis.
  - Needs a free 3' hydroxyl (3'-OH) end to initiate polymerization.
- DNA Polymerase I:
  - Replaces RNA primers with DNA in the replication process.
- RNA Polymerase (Primase):
- Synthesizes RNA primers in the 5’ to 3’ direction.
- Requires a template but does not need a free 3' end to start polymerization.
- DNA Ligase:
- Joins large DNA fragments to form a complete daughter strand.

Helicase:
- Unwinds and separates the DNA strands, creating two single-stranded DNA (ssDNA) templates.
Single-Stranded DNA Binding Proteins (SSBPs):
- Stabilize the separated ssDNA to prevent re-annealing.
RNA Primers:
- Synthesized by RNA Polymerases (primase) to create short RNA sequences that provide starting points for DNA polymerases.
- Each primer ends with a free 3’ end necessary for DNA polymerase to elongate the new DNA strand.

DNA Polymerase III:
- Extends the new DNA strand from the RNA primer, synthesizing DNA up to the primers and generating a series of DNA fragments, each having a free 3' end.
DNA Polymerase I:
- Replaces RNA primers with DNA by adding complementary nucleotides in the 5’ to 3’ direction.
DNA Ligase:
- Links the DNA fragments created by DNA polymerase III and I to complete the daughter strand.
- This involves sealing the remaining gaps (nicks) between the DNA fragments in the newly synthesized strand.

Origins of Replication:
- There is a single origin of replication (ori) present in the bacterial chromosome, which serves as the starting point for DNA synthesis.

Bidirectional Synthesis:
- DNA synthesis occurs in two directions from the origin of replication.
Semi-Discontinuous Synthesis:
- Synthesis of the leading strand occurs continuously as a single, long polymer, while the lagging strand is synthesized in smaller segments called Okazaki fragments, each less than 2000 base pairs in length.
Lagging Strand Requirements:
- Requires multiple RNA primers for the initiation of synthesis.
- DNA polymerase I replaces RNA primers on the lagging strand with DNA.
- DNA ligase connects Okazaki fragments and nicks.

Telomeres:
- In eukaryotic cells, telomeres are not fully replicated, which poses challenges for complete genome replication at chromosome ends.
Regulation of Replication:
- Replication is tightly regulated to ensure that each genome undergoes replication only once during a cell division cycle.
Efficiency of DNA Replication:
- High-speed replication: E.coli can complete its genome (4.6 million base pairs) in less than 1 hour, averaging about 500 base pairs per second.
- In humans, DNA replication proceeds at an approximate speed of 50 bases per second.
Fidelity of DNA Replication:
- DNA replication exhibits high fidelity, with errors occurring at a rate of approximately 1 in 10 billion base pairs of a completed DNA strand, maintaining the integrity of the genetic information.