In this article we will discuss about the six basic rules for DNA replication. The rules are: 1. Replication is a Semi-conservative Process 2. Replication has Direction 3. Replication Starts off a Unique Point on Bacterial and Viral Chromosomes 4. Replication of Both Strands Proceeds by the Addition of Nucleotide Monomers in the 5′-3′ Direction and Others.
Rule # 1. Replication is a Semi-Conservative Process:
Each strand of the parental DNA duplex is conserved and copied by base-pairing with matching nucleotides and the result is two duplexes identical to the parental one. Base pairing and chain extension create a growing point, or fork, at which the parental duplex is unwound.
Rule # 2. Replication has Direction:
Replication proceeds from a given point on the chromosome either in one or in both directions. Bi-directional movement involving two growing points or forks appears to be the most common. The movement of two forks away from each other creates a loop which appears as a “bubble” or “eye” in electron micrographs. It does not follow, however, that the speed on extent of movement of the two forks is the same in both directions.
Rule # 3. Replication Starts off a Unique Point on Bacterial and Viral Chromosomes:
By starting another copy at a unique point, called the origin, before the preceding one is completed, the chromosomes of a rapidly growing bacterium may have several forks and increase its rate of replication. By contrast, replication in eukaryotic chromosomes begins at many origins located at different positions along the chromosomal DNA.
Rule # 4. Replication of Both Strands Proceeds by the Addition of Nucleotide Monomers in the 5′-3′ Direction:
The addition of nucleotide monomers in the 5′-3′ direction proceeds in accordance with the requirements of DNA polymerases. Consequently, replication at the monomer level of nucleotides cannot occur simultaneously along both anti-parallel strands in the region of the fork; a transient single-stranded region is created on one side of the duplex while the other strand is being replicated.
Rule # 5. Replication Occurs in Short, Discontinuous Pulses:
The short fragments—about 100 nucleotides long in animal cells and 1,000 to 2,000 in prokaryotes—later join to the main body of the growing chains.
Rule # 6. Replication at the Level of Short Fragments is Initiated by the Production of a Short Segment of RNA to Serve as a Primer for DNA Polymerase:
The region of RNA is later excised, and the gap thus created is filled with DNA. The nascent pieces—or replication fragments of DNA—are joined to the growing chromosome. When two replicating forms moving in opposite directions on a circular chromosome approach each other at a point it will signal a division into the progeny chromosomes.