DNA replication models

DNA replication is the process by which transmission of genetic information stored in chromosomal DNA from one generation to successive generations is occurred. Proper development of succeeding generation-organisms can only be achieved by accurate replication of chromosomal DNA. Accurate replication conserves the genetic information present in chromosomes and that information is crucial to cell propagation. The replication process provides the two copies of the entire genome for faithful distribution into each daughter cell. However, theoretically, there are three modes of DNA replication possible. Finding the accurate mode of replication which can conserve the genetic information in the living cell is crucial for understanding. Theoretically, three DNA replication models are possible….

1) Conservative mode of DNA replication

2) Semiconservative mode of DNA replication

3) Dispersive mode of DNA replication

 

1) Conservative mode of DNA replication

In this mechanism of replication, the two parental DNA strands become a single daughter DNA molecule and second daughter DNA molecule will have 2 newly synthesized DNA strands.

In conservative mechanisms, no hybrid DNA containing the parental DNA strand and newly synthesized DNA strand will generate.

We can assume that conservative mechanism is similar to creating photocopy in which one copy always remain as original and another copy will be duplicated. This mechanism cannot lose the originality of the DNA molecule.

 

2) Semiconservative mode of DNA replication

In the semiconservative mechanism, after replication, each of the daughter DNA molecules contains a parental DNA strand and newly synthesized DNA strand.

By this mechanism, the produced each daughter DNA molecule always remain as a hybrid molecule having one strand from parental DNA another strand will be newly synthesized.

Since the parental DNA strand always joined with the newly synthesized strand, after the DNA replication immediately proofreading activity can make correction of newly synthesized strand based on nucleotides sequence present on a parental strand or template strand.

After the replication parental strand can be recognized based on the methylation of certain specific nucleotide, whereas the methylation process will not take place in the newly synthesized strand.

Hence, the immediate host proofreading replicative correction will take place only on unmethylated newly synthesized DNA strand.

 

3) Dispersive mode of DNA replication

In dispersive mechanisms, the parental DNA strands are fragmented and they combine with the newly synthesized segments to generate DNA duplexes, in which each strand is hybrid of the parental segment and newly formed segment.

In this mechanism whatever DNA molecules formed will be a hybrid DNA molecule.

The major considering limitation of the dispersive mechanism is the complete loss of sequential genetic information due to the fragmentation of overall DNA.

Note: Among all the three theoretically possible models, the semiconservative mechanism only was experimentally proved as the mechanism of DNA replication in all living organisms, in which methylation process distinguish the parental strand from newly synthesized strands that benefit the proofreading activity of replicative correction by without ambiguity and loss of conserved genetic information.

 

Meselson-Stahl experiment

Meselson and Stahl used E-coli as the experimental organism and used two different types nitrogen isotopes (14N- lighter and 15N- heavier) for incorporation into the nucleotides during their synthesis in the experimental organism.

In their experiment, as the source of nitrogen, they have used 14N ammonium chloride and 15N ammonium chloride in the bacterial growth medium.

This experimental technique was performed by isopycnic density gradient centrifugation using cesium chloride a gradient.

Experimental procedure

Initially, they grew the E. coli cells in 15N ammonium containing a medium for several newly grown bacterial.

After several generations of growth in 15N medium, the bacterial cells were transferred to a medium supplemented with only 14N ammonium chloride as a nitrogen source.

They extracted the DNA after each generation in 14N medium and subjected the obtained DNA to isopycnic density gradient centrifugation to find out the property of different forms of DNA with respective density.

The density difference in the isotope mass allows the separation of DNA molecules with different density in the density gradient centrifugation.

 

Results of this experiment

  1. First generation

After one generation in14N medium, they found only a single band by centrifugation.

The formation of the only single band corresponding to the hybrid DNA band strictly ruled out the possibility of the conservative mechanism. Because, if the DNA replication occurs by conservative mechanism then there should be two bands corresponding to one heavy DNA band and another one light DNA band after only one generation.

The formation of a single band corresponding to hybrid DNA after one generation in 14N medium can be due to semiconservative mechanisms or dispersive mechanism.

Note: Result of first-generation ruled out the possibility of the conservative mechanism but not ruled out the semiconservative mechanisms and dispersive mechanism

  1. Second generation

After two generations in14N medium they found two bands by centrifugation.

The formation of two bands corresponding to heavy DNA and light DNA band after two generations of growth ruled out the possibility of the dispersive mechanism. Because, if the DNA replication occurs by dispersive mechanism then there should be only one band corresponding to one hybrid DNA.

Note: Result of second-generation ruled out the possibility of dispersive replication mechanism. After completion of second and third generations in 14N medium, the formation of two bands corresponding to light DNA and hybrid DNA provides strong evidence that the replication of DNA in all living organisms is strictly by SEMICONSERVATIVE MECHANISM.

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