Main Difference – Prokaryotic vs Eukaryotic DNA Replication

Prokaryotic and eukaryotic DNA replications occur before the beginning of the cell division. DNA replication is a biological process by which the two genetically identical replicas of DNA are synthesized from a single, original DNA molecule. DNA replication ensures the receipt of the exact copy of the parent’s genetic material by each daughter cell. DNA replication is carried out by a class of enzymes called DNA polymerases. Both prokaryotic and eukaryotic DNA replications are semi-conservative DNA replications in which one old and one new strand can be found in the daughter cell. Though the process of DNA replication is nearly similar in both prokaryotes and eukaryotes, some differences may occur due to the size and the complexity of the genetic material. The main difference between prokaryotic and eukaryotic DNA replication is that prokaryotic DNA replication occurs through a single origin of replication whereas eukaryotic DNA replication occurs through multiple replication origins.  

Key Areas Covered

1. What is Prokaryotic DNA Replication
      – Definition, Features, Mechanism
2. What is Eukaryotic DNA Replication
      – Definition, Features, Mechanism
3. What are the Similarities Between Prokaryotic and Eukaryotic DNA Replication
      – Outline of Common Features
4. What is the Difference Between Prokaryotic and Eukaryotic DNA Replication
     – Comparison of Key Differences

Key Terms: DNA polymerase, Eukaryotic DNA Replication, Lagging Strand, Leading Strand, Origin of Replication, RNA Primer, Prokaryotic DNA Replication, Replication Bubble, Replication Fork

What is Prokaryotic DNA Replication

Prokaryotic DNA replication is the process by which prokaryotes such as bacteria and archaea duplicate their genome into a second copy, which can be transformed into a daughter cell. Prokaryotes consist of a double-stranded circular DNA molecule in their cytoplasm. Prokaryotic DNA comprises a single origin of replication. DNA helicase unwinds the DNA at the origin of replication by breaking the hydrogen bonds between the nitrogenous bases. The resultant Y-shaped structure is called the replication fork. Since prokaryotic DNA contains a single origin of replication, only two replication forks are formed during the replication process. These two replication forks process bi-directionally. The single-strand DNA-binding proteins (SSB) stabilizes the two unwound strands, which serve as the template strands for the replication. The enzyme, RNA primase synthesizes a five to ten base pairs long RNA primer, which is complementary to the template strand.

Figure 1: DNA replication in Prokaryotes

Three types of DNA polymerases are involved in the prokaryotic DNA replication; DNA polymerase I, II, and III. Both initiation and elongation of the prokaryotic DNA replication are carried out by DNA polymerase III. The DNA polymerase III adds nucleotides in 5’ to 3’ direction. Due to the antiparallel nature of the DNA double-helix, one strand runs from 5’ to 3’ direction (leading strand). The other strand runs from 3’ to 5’ direction (lagging strand). Since, the lagging strand requires RNA primers continuously in order to synthesize DNA in the 5’ to 3’ direction, new fragments of DNA called Okazaki fragments are continuously formed. The gap filling and DNA repair are carried out by DNA polymerase I and II. The RNA primer is removed by the DNA polymerase I. The process of DNA replication in prokaryotes is shown in figure 1. 

What is Eukaryotic DNA Replication

Eukaryotic DNA replication is the process by which the eukaryotic genome duplicates prior to cell division. Though the basic mechanism of the eukaryotic DNA replication is similar to prokaryotic DNA replication, there are some differences due to the size and the structure of eukaryotic DNA. Eukaryotic DNA is double-stranded linear molecules. The amount of the eukaryotic DNA is around 50 times more than the prokaryotic DNA. Moreover, eukaryotic DNA is tightly packed with histones inside the nucleus of the cell. Therefore, DNA replication occurs in three steps; initiation, elongation, and termination.

Initiation

The eukaryotic DNA replication occurs through multiple replication origins. The multiple replication origins form several replication bubbles per chromosome. DNA helicase and SSBs are involved in the unwinding and stabilization of the two templates at each origin of replication.    

Elongation

During elongation, DNA polymerases add new nucleotides to the existing 3’ ends. The three types of DNA polymerases which are involved in eukaryotic DNA replication are DNA polymerase α, δ, and ε. The DNA polymerase α initiates the DNA replication whereas the DNA polymerase δ and ε are involved in the elongation. DNA polymerase α also requires an RNA primer to synthesize the new DNA strand and the primer is removed by the DNA polymerase β. The leading and lagging strands are formed in the same manner as in prokaryotic DNA replication. Eukaryotic DNA replication elongation is shown in figure 2.

Figure 2: Elongation

Termination

Once the leading strand of a one replication bubble meets a lagging strand of a second replication bubble, the replication process is halted. Then, the RNA primer is removed, and the gap is filled by the freely-floating DNA polymerases. The nicks are joined by the DNA ligase. The multiple replication bubbles are shown in figure 3.

Figure 3: Multiple Replication Bubbles

Similarities Between Prokaryotic and Eukaryotic DNA Replication

• Both prokaryotic and eukaryotic DNA replications occur before entering the nuclear division.
• Both prokaryotic and eukaryotic DNA replication works upon double-stranded DNA.
• The unwinding of both prokaryotic and eukaryotic DNA is done by DNA helicase.
• The unwound DNA strands are stabilized by single-stranded DNA-binding proteins (SSB).
• Both prokaryotic and eukaryotic DNA replication are multistep processes, which are carried out by an enzyme complex called DNA polymerases.
• Each type of DNA polymerases works in the 5’ to 3’ direction.
• RNA primers are required for the initiation of both types of DNA replications.
• The synthesis of the RNA primer is done by the enzyme called primase.
• Both prokaryotic and eukaryotic DNA replications occur in a semi-conservative manner where one old strand of DNA and one new strand of DNA can be found in the daughter cell.
• Both prokaryotic and eukaryotic DNA replications are bi-directional since the replications progress in both ways.
• Leading and lagging strands are formed in both types of DNA replications.
• The lagging strand produces the small DNA fragments called Okazaki fragments, which are eventually joined together.
• The time taken for both types of replications are around one hour.

Difference Between Prokaryotic and Eukaryotic DNA Replication

Definition

Prokaryotic DNA Replication: Prokaryotic DNA replication is the process by which a prokaryotic organism duplicates its entire genome in order to pass the second copy to a daughter cell.

Eukaryotic DNA Replication: Eukaryotic DNA replication is the process by which the eukaryotic genome duplicates prior to cell division.

Occurrence

Prokaryotic DNA Replication: Prokaryotic DNA replication is a continuous process.

Eukaryotic DNA Replication: Eukaryotic DNA replication occurs during the S phase of the cell cycle.

Location

Prokaryotic DNA Replication: Prokaryotic DNA replication takes place in the cytoplasm.

Eukaryotic DNA Replication: Eukaryotic DNA replication takes place in the nucleus.

Type of DNA

Prokaryotic DNA Replication: Prokaryotic DNA is circular and double-stranded.

Eukaryotic DNA Replication: Eukaryotic DNA is linear and double-stranded with ends.

Amount of DNA

Prokaryotic DNA Replication: There is a small amount of Prokaryotic DNA.

Eukaryotic DNA Replication: The amount of eukaryotic DNA is 50 times more than the amount of prokaryotic DNA.

Packaging

Prokaryotic DNA Replication: Prokaryotic DNA forms loop-like structures by wrapping around histone-like protein molecules.

Eukaryotic DNA Replication: Eukaryotic DNA forms nucleosomes and shows higher order packaging.

Origin of Replication

Prokaryotic DNA Replication: Prokaryotic DNA consists of a single origin of replication.

Eukaryotic DNA Replication: Eukaryotic DNA consists of multiple origins of replication (over 1000).

DNA Polymerases

Prokaryotic DNA Replication: Prokaryotic DNA replication is carried out by DNA polymerase I and III.

Eukaryotic DNA Replication: Eukaryotic DNA replication is carried by DNA polymerase α, δ, and ε.

Size of the Okazaki Fragment

Prokaryotic DNA Replication: The Okazaki fragments are comparatively large, 1000-2000 nucleotides in length.

Eukaryotic DNA Replication: The Okazaki fragments are small, around 100-200 nucleotides in length.

DNA Gyrase

Prokaryotic DNA Replication: DNA gyrase is involved in the prokaryotic DNA replication.

Eukaryotic DNA Replication: DNA gyrase is not required for the eukaryotic DNA replication.

Rate of DNA replication

Prokaryotic DNA Replication: Prokaryotic DNA replication is a rapid process and around 2000 nucleotides are added per second.

Eukaryotic DNA Replication: Eukaryotic DNA replication is a slow process and around 100 nucleotides are added per second.

End Synthesis

Prokaryotic DNA Replication: Prokaryotic DNA does not contain ends.

Eukaryotic DNA Replication: Telomerase is involved in the end synthesis in Eukaryotic DNA during the replication.

Final Product of the Replication

Prokaryotic DNA Replication: The final product of the prokaryotic DNA replication is two circular chromosomes.

Eukaryotic DNA Replication: The final product of the eukaryotic DNA replication is two sister chromatids.

Conclusion

Prokaryotic and eukaryotic DNA replication are two processes that are involved in the duplication of genomes prior to cell division. The mechanism of both prokaryotic and eukaryotic DNA replication are similar. However, due to the size and the complexity of the eukaryotic genome, the eukaryotic DNA replication is a more complex process. Thus, eukaryotic DNA replication occurs through the formation of multiple replication origins. But, prokaryotic DNA replication occurs through a single replication origin. However, since prokaryotic DNA replication is a rapid process, both DNA replication process takes the same time. Therefore, the main difference between prokaryotic and eukaryotic DNA replication relies on the size and the complexity of each type of genomes.

Reference:

1.  “DNA Replication in Prokaryotes.” Boundless. N.p., 02 June 2016. Web. Available here. 08 Aug. 2017.
2. “DNA Replication in Eukaryotes.” Boundless. N.p., 26 May 2016. Web. Available here. 08 Aug. 2017. 

Image Courtesy:

1. “DNA replication en” By LadyofHats Mariana Ruiz – Own work. Image renamed from File:DNA replication.svg, (Public Domain) via Commons Wikimedia
2. “Replication complex” By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia
3. “DNA bubbles2” By Boumphreyfr – Own work (CC BY-SA 3.0) via Commons Wikimedia

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