DNA REPLICATION

DNA replication

Content

• Introduction
• Types of replication
• Model of DNA replication
• Enzymes involved in DNA replication

Introduction

• It is the process of producing two identical copy of DNA from one original DNA molecule.
• DNA replication is one of the most essential or important properties exhibited by DNA.
• Evolution of all morphologically complex form of life is based on the replication.

Types of Replication

• There are possibly three modes of replication:
• Dispersive
• Conservative
• semi-conservative

Semi Conservative

• In this mode of replication, two  old parental strands serve as template for synthesis of new daughters strands and each new DNA contains one strand from the parent and one from newly formed progeny.
• The evidence of semi conservative replication of DNA was first presented by Meselson and Stahl in 1958.

Dispersive

• The old DNA molecule would break into several pieces, each fragment would replicate and the old and new segment would be combined to yield two progeny DNA molecules; each progeny molecule contains  both old and new strands along its length.

Conservative

• According to the conservative replication, the old parental strands remains together and newly formed daughter strands are also together.

Model of DNA Replication

• The brief discussion of DNA replication model is as follows:
• DNA applications initiate at certain unique and fixed point called origin.
• There is unwinding of complementary strands of DNA duplex with the help of two enzymes DNA gyrase and DNA helicase, this process is called Melting.
• Single strand binding protein are attached to two single stranded region so that they not join to form duplex.
• Due to melting in the origin region it produces two Forks(Y), in the DNA duplex and one fork is located at the end of the melted region. Generally, both the fox are involved in replication and become Replication Fork.
• After formation of replication fork ,Primase enzymes initiate transcription of the strand in the 3’-5’ direction. This results in the generation of 10-60 long primer RNA.
• The free 3’-OH of the  RNA primer provide initiation point for DNA polymerase for the sequential addition of deoxyribonucleotide.
• The replication of the second strand (5’-3’) DNA is discontinuous, Hence 3’-5’strand of DNA is termed as the Leading strand and 5’-3’ is termed as Lagging strand.
• Lagging strand generates small polynucleotides fragment called Okazaki fragments, during replication. This fragment is about 1000-2000 nucleotides long in E.coli.

Enzymes Involved in the DNA Replication.

• There are some important enzymes which are involved in the replication of DNA :
• DNA polymerase
• Primase
• Polynucleotide ligase
• Endonuclease
• Helicase
• Single stand binding protein (SSB).

DNA polymerase

• This enzyme synthesizes new strand on a template DNA strand.
• It is also known as DNA replicase.
•  Its activity was first explained by Kornberg in 1956.
• In prokaryotes, there are five types of DNA polymerase:

DNA polymerase	Gene	Functions
1	polA	Major repair enzyme
2	polB	Minor repair enzyme
3	polC	DNA replication
4	dinB	SOS repair
5	umuD2’C	SOS repair

Primase

• It involves in the synthesis of RNA primer, which are required for initiation of DNA replication. It is RNA polymerase that are used only to synthesize primer during replication.

Ligases

• This involves in the formation of phophodiester linkage and joining of two newly formed DNA strands.

Endonucleases

• It produces an internal cut in a DNA molecule, while Restriction Endonucleases are those that cuts at only specific site or sequences.

Single strand binding protein

• It prevents from forming duplex by binding to single strand DNA.