6 Basic Rules for DNA Replication | Genetics

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.