ENZYMES INVOLVED IN DNA REPLICATION

ENZYMES INVOLVED IN DNA REPLICATION
•Variety of enzymes help in DNA replication.
•They can be listed as below:
a) DNA Polymerase
b) Ligase
c) Primase
d) SSB
e) Helicase
f) Endonuclease
g) Pilot proteins

Let us first talk about DNA polymerase

1) DNA POLYMERASE : Also known as DNA replicase; it helps to synthesize a new DNA  strand on a template DNA strand. It helps to add deoxyribonucleotides to the 3'-end of a pre-existing polynucleotide and so replication takes place in 5'-3' direction.
They also help in correct base pairing.i.e. only correct bases are paired against the bases on the template DNA strand.

DNA POLYMERASES ARE DIFFERENT IN PROKARYOTES AND EUKARYOTES.

In the current post, we will discuss only about prokaryotic DNA polymerase.

IN PROKARYOTES:

There are 3 types of DNA polymerases.

• DNA polymerase I
• DNA polymerase II
• DNA polymerase III

DNA polymerase I was first isolated by KORNBERG from E.COLI.
It possesses the following three activities; but it is chiefly a DNA REPAIR enzyme.

1) 5'-3' polymerase; (replication of nicks)
2) 5'-3' exonuclease; (repair DNA damaged by radiations)
3) 3'-5' exonuclease; (repair incorrectly placed bases of DNA during replication)

3'-5' EXONUCLEASE - It is the chief repair enzyme. It helps in proof reading. It removes incorrect bases from the 3'- end. Now, correct base is added against the template strand by 5'-3' polymerase.

5'-3' EXONUCLEASE - It removes DNA segments damaged by UV light, irradiation and other agents. It excises up to 10 bases of nucleotides.
It also excises RNA primers during replication after an endonuclease produces a nick. 5'-3' polymerase then adds the correct DNA bases. This is called nick translation.

NOTE :so, as we see 3'-5' exonuclease excises a single base  as a result of proof reading; but 5'-3' exonuclease excises up to 10 bases in case of damage due to UV light and irradiation. and, in both cases 5'-3' POLYMERASE adds the missing bases. So, polymerase I helps in replication only in case of error, or damage or when RNA primers have been excised.

DNA polymerase I is encoded by the gene polA. The enzyme consists of two segments.
; one large and another small.
The larger segment is known as klenow fragment and exhibits the 5'-3' polymerase at the C-terminal  and 3'-5' exonuclease activities at the N-terminal.
Smaller segment encodes for the 5'-3' exonuclease activity.

DNA polymerase II-It only has a 5'-3' polymerase activity and 3'-5' exonuclease activity.
The exact in vivo function is not known. Although, it most likely function in DNA repair in the absence of DNA polymerase I and III.

DNA polymerase III-It has 5'-3' polymerase activity and 3'-5' exonuclease activity. It is the MAIN DNA REPLICATION ENZYME.
It is made up of several subunits such as α, β, γ, δ, ε, θ, ψ, χ, τ.

• α has the polymerase activity and ε has the proof reading activity. They generally function together.
• θ helps in assembly of enzymes and so bring α and ε together to form catalytic core.
• Though, the catalytic core has both polymerase and 3'-5' exonuclease activity but they dissociate after synthesizing only 11 bp long chain on only one strand.
• But, because there are two strands; so dimerization of this subunit is necessary in which τ helps.
• τ comes in contact with αθε to form αθετ complex which dimerizes to form α₂θ₂ε₂τ₂. this is called POL III*. 
• Now γ complex is added to POL III* to form POL III'. γ- complex is known as clamp loader and loads β dimer onto DNA strands.
• There are 2 β-dimers. Each dimer forms a ring shaped structure that surrounds the DNA duplex providing a sliding clamp. It allows the holoenzyme to slide along DNA.

now , we will deal with eukaryotic DNA polymerase along with other enzymes in next posts.