DNA Replication: 3 Important Concepts


Which model of DNA replication is accepte­d?

The models of DNA replication (DNA replication types) process are characterized in two different ways:

  1. Classification of DNA replication model based on persistence of parental DNA strands
  2. Classification of the DNA replication model based on the replication pattern

Based on the persistence of parental DNA strands

  1. Dispersive
  2. Conservative
  3. Semi-Conservative

Dispersive: This kind of replication brings about two strands having alternating sequences of both parental and recently incorporated strands.

Conservative: This sort of replication proposes that the two parental strands were by one way or another conserved altogether in a single offspring and the other one will have a newly synthesized strands.

Semi-Conservative: This kind of replication recommends that the DNA double helix in the offspring is composed of the parental strand also known as template strand and the other one will be the recently synthesized one.

dna replication types
Figure: DNA replication model based on persistence of parental DNA strands. Image Credit: Biology Libretexts

Based on the pattern of DNA replication

  1. Jacob and Brenner’s Model
  2. Carin’s Model
  3. Yoshikawa Model
  4. Rolling Circle Model

Jaboc and Brenner’s Model:

In a dividing eukaryotic cell, the sister chromosomes get arranged at equal distances in a cell. In eukaryotic cell, this segregation of sister chromosomes is accomplished by the mitotic spindle fibres.

In prokaryotes, this process is absent. Jacob and Brenner gave a model to clarify the replication and segregation of daughter chromosomes.

As indicated by this model, the chromosome gets connected to the mesosome at the site of replication origin.

At the end of replication, another membrane connection site appears nearby the former one. The strand which acted as a template remains in a circle, while new strand has been broken.

One of the free terminals of the recently synthesized chromosome connects to the new membrane binding site. During the replication process, the replication fork and the enzyme complex stays fixed while the DNA strand moves along and gets duplicated.

Parallelly, another layer is formed between and the two genomes get isolated. The recently synthesized DNA strand becomes circular afterwards.

Carin’s Model

At a particular point (replication origin) the DNA breaks and replication starts and continues in two reverse ways. As the replication continues, the DNA strand loosens up (Unwinding of DNA).

This one-sided unwinding (the other side can’t loosen up openly) makes a tension and it is moved to the un-replicated part bringing about super twisting in the DNA double helix. This causes cessation of DNA replication.

Launch a new replication event a ‘nick’ is created in one of the DNA strand through a turning protein. This initiates strand rotation and the break is closed consequently.

Yoshikawa Model

It is just a modification of the Cairn’s model. As mentioned by Yoshikawa, the newly synthesized DNA strands covalently join to the terminal of the template chromosome.

Rolling Circle Model:

This is the DNA replication model acknowledged for the replication of single strand DNA in Viruses (ɸ x 174). The circular single stranded DNA is converted into double stranded DNA by the formation of another strand starting at the replication origin.

One strand of these duplex DNA rings is instantly cut down at a site by an enzyme known by the name endonuclease. Accordingly, a linear DNA strand with 3′ and 5′ end is synthesized. The 3′ end acts as primer for the new strand synthesis.

The entire strand is utilized for this reason; with the parental strand itself going about as a template. The 5′ terminal of the strand connects to the membrane.

In the interim the original, the entire strand loosens up and segregates leaving the tail joined to the membrane. The tail cuts into a particular length that is known as the Progeny rod. The rod rolls and structures a circle, with the DNA ligase joining the two ends of the DNA.

The recently framed circular DNA double helix can thus become rolling circles. The hereditary information is protected in the single stranded DNA which stays circular consistently.

New DNA strand synthesis take place either from the single stranded DNA (original) or from the rolling circle’s strand. Proof for the moving circle model has been acquired from the replication of a few viruses (M13, P2 T4, X and so on).

rolling circle model
Figure: Rolling Circle model of DNA replication. Image Credit: Wikimedia

What is the theta model of DNA replication?

It is the replication process usually adapted by prokaryotic organisms since their DNA is circular.

As the name suggests, theta (θ) like structure is formed in the middle of the replication process hence it is known as theta model of DNA replication. Following are the key steps involved in the DNA replication through theta model:

Key steps in the Mechanism of Theta model of DNA Replication:

  • Start of DNA unwinding at origin of replication and the replication machinery starts to assemble at the site of replication
  • Since the construction matches the Greek letter theta (θ), it is named after it.
  • Then deoxyribonucleotides are added continuously, this process is known as elongation. The elongation process might be unidirectional or bidirectional
  • In the unidirectional elongation, only one replication fork performs the elongation gets back to its starting place and forms two daughter DNA strands. While in bidirectional elongation, two replication forks perform elongation and meet each other from opposite sides.
  • This is the most widely recognized method of DNA replication. It occurs specifically in gram negative protobacteria.
  • Commonly occurs plasmids such as RK2, ColE1, and F, as well as in bacteriophage like P1.
Figure: Theta model of DNA replication. Image Credit: Biology Stack Exchange

What is Semiconservative DNA replication?

Semiconservative DNA replication process is the replication process which provides the utmost genome integrity

Semiconservative replication portrays the system of DNA replication in almost every known cell. DNA replication happens on numerous replication origins along the template DNA strand. As the DNA helix is loosened up (unwinding) by helicase, replication happens independently on both the DNA strands in antiparallel manner.

This method of replication is known as semi-conservative DNA replication process since two replica of the original DNA strand are synthesized, each replica conserves the data from one portion of the original DNA double helix.

Each replica contains one original strand and one recently synthesized strand. (The two replica are ought to be identical, yet this isn’t totally guaranteed) The DNA structure indicate that every single strand of the double helix would serve as a template for synthesis of one more strand. It was not realized how recently synthesized strands join with the template strands to shape two double helical DNA strands.

Numerous studies were directed to decide how DNA replicates itself. The semiconservative model of DNA replication was first proposed by Nikolai Koltsov and is later illustrated by Meselson-Stahl experiment. This has confirmed that the DNA replicates in a semi-conservative way by utilizing two isotopes: nitrogen-14 (14N) and nitrogen-15 (15N). At the point when 14N was added to the heavier 15N-15N DNA, a DNA hybrid 15N-14N was found in first progeny.

After the one more division, the mixture remained, however light DNA (14N-14N) was viewed also. This demonstrated that DNA replicates semi-conservatively. This method of DNA replication considered every daughter strand to remain combined with its template strand

Figure: Meselson-Stahl experiment. Image Credit: Wikimedia

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