Tuesday, 9 February 2021

Enzymes and Proteins Involved in the DNA Replication

Enzymes and proteins involved in DNA replication 

A number of enzymes and proteins are associated with the replication fork to help in the initiation and continuation of DNA synthesis, Most prominently, DNA polymerase synthesizes the new strands by adding nucleotides that complement each (template) strand.
   DNA replication occurs during the S-phase of interphase. At the replication fork, many replication enzymes assemble on the DNA into a complex molecular machine called the replisome. The following is a list of major DNA replication enzymes that participate in the replisome.


1) DNA Helicase :

Helicase enzyme opens up the DNA double helix by breaking hydrogen bond between two strands of DNA and provide single template strand.
DNA-B is a primary replicative Helicase it binds and move on lagging strand in 5' to 3' direction unwinding the duplex as it goes.
Helicase requires ATP as energy source

2) Single Stranded Binding Proteins (SSB proteins) :

SSB proteins binds to both seperated single stranded DNA and prevent the DNA double helix from re-annealing after helicase unwinds.
SSB proteins are maintaining the strand seperation and facilitating the synthesis of the nascent strand.

3). Topoisomerase :

  DNA Topoisomerase is a nuclease enzyme that break a phosphodiester bond in a DNA strand. The function of Topoisomerase is relaxes the DNA from its super coiled nature.

4).DNA Gyrase :

This enzyme is used to make sure the double stranded areas out side of the replication fork do not supercoil, DNA Gyrase is one type of topoisomerase.

5). Primase :

Primase provides a starting point of RNA (or DNA) for DNA polymerase to begin synthesise of the new DNA strand.
Because DNA polymerase requires free 3'-OH group for bind to DNA for starting replication.

6) DNA Polymerase :

DNA dependent DNA polymerase enzyme that can synthesise a new strand on a DNA tamplate.
DNA polymerase has different types in Prokaryotes and Eukaryotes.

a). Prokaryotic DNA polymerase -

Prokaryotes has 3 types of DNA polymerase, these are DNA pol-I, DNA pol-II and DNA pol-III.

DNA polymerase I -
It is made up of one subunits. It has 3' to 5' and 5' to 3' exonuclease activity.
Function - DNA repair, Gap filling and synthesis of new lagging strand.

DNA polymerase II -
It is made up of 7 subunits. It has only 3' to 5' exonuclease activity.
Function - DNA repair and DNA proof reading.

DNA polymerase III -
It is made up of at least 10 subunits. It has 3' to 5' exonuclease activity.
Function - This is the main replication enzyme in Prokaryotes.


b) Eukaryotic DNA polymerase -
Eukaryotes has 5 types of DNA polymerase which are DNA polymerase α, β, γ, δ and ε.

DNA polymerase α -
It has no any exonuclease activity.
Function - DNA replication in the  nucleous.

DNA polymerase β -
It has no any exonuclease activity.
Function - DNA replication and base excision repair.

DNA polymerase γ -
It has 3' to 5'  exonuclease activity.
Function - DNA replication in Mitochondria.

DNA polymerase δ -
It has 3' to 5'  exonuclease activity.
Function - Synthesis of lagging strand during DNA replication.

DNA polymerase ε -
It has 3' to 5'  exonuclease activity.
Function - Synthesis of leading strand during DNA replication.

7) Beta Clamp Proteins :

Beta clamps are the protein which prevents elongating DNA polymerase from dissociating from the DNA parent strand.
It helps hold the DNA polymerase in place on the DNA.

8) DNA Ligase :

DNA Ligase Catalyse the joining of ends of two DNA chains by forming phosphodiester bond between 3'-OH group at one end of DNA strand and  and 5'-Phosphate group at the end of other DNA strand.
DNA ligase joins the Okazaki fragments of two lagging strand.

9) Telomerase :

Lengthens the telomeric DNA by adsing repetitive nucleotide sequence to the ends of eukaryotic chromosomes. This allows germ cells and stem cells to avoid the Hayflick limit on cell division.

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