DNA REPLICATION:-

        The process of copying a parent DNA molecule to form two daughter

DNA molecule is called DNA Replication.

      Any organism can grow through the division of its own calls. before

a Cell can divide, it must replicate all of its DNA. The process oo replication

occurs during the S phase of the cell cycle. During  replication the cellular

DNA machinery  duplicates each DNA molecule. Each resulting  DNA molecule

is called a chromatid.

      Due to replication, the each daughter cell thus receives an identical  set

Of chromosomes when the parental cell divides.DNA replication is controlled

by DNA itself.

     The self complementary nature of DNA allows each of the two strands

of a parental  DNA molecule to serve as template for the formation  of a daughter 

Stands. This process is Called DNA replication. However, the replication is complex 

multi-step process, involving several enzymes.

    Many proteins are involved in replication. these proteins included DNA hwlicase,                    single stranded DNA binding proteins, topoisomerases, primase, DNA polymerase,                   sliding clamps, RNAse H and DNA ligase.

       DNA helicases are a class of enzymes that couple ATP hydrolases to

the separation of DNA strands. The DNA helicases involved in replication are

typically hexameric proteins in the shape of a ring. The junction between  

the newly separated template strands and the unreplicated double

DNA is called the replication fork.

      The replicated fork can be looked at as moving continuously towards

the unreplicated double stranded DNA.

       Single stranded DNA binding protein or SSBS, bind to the single

DNA -to stabilize the separated strands.

       As the DNA unwinds the twist number decreases. Therefore the unwinds

number must increase, causing the DNA to become positively super coilrd.

These super coils are removed .by topoisomerases.

       DNA replication requires an RNA primer to begin. Primase is a specialized 

RNA polymerase which makes short RNA primers using small ssDNA as a template.               Primase is activated by association with other DNA replication proteins,

such as DNA helicase.

     The synthesis of DNA is catalyses by an enzyme called DNA polymerase.

DNA polymerase is only able to add dNTPs to the 3' end of a polynucleotide 

Due to the anti parallel nature of DNA one strand is synthesized continuously

toward the replication fork. While the other strand is synthesized discontinuously

away from the replication fork.

     The continuously replicated strand is called leading strand, and the

discontinuously replicated strand is called lagging strand.

    It takes approximately one second for a DNA polymerase to bind to

DNA. However once replication has begun, DNA polymerase are capable of

adding up to thousand nucleotides to the growing daughter strand every second.

The ability of an enzyme to catalyze many reaction before releasing' its substrate

is called processivity.

    To increase the degree of processivity of DNA polyomerases, a sliding

DNA clamp binds to the DNA and DNA polymerase  them together.

     DNA polymerase can only add small dNTP to the 3’ end of a

polynucleotide of lagging strand because both DNA strands must be paired

In the same overall direction through the replication machinery the lagging 

strand is synthesized discontinuously in small fragmen

      The short fragments of DNA found on the lagging Strands are called

okazaki fragments.

   To complete let DNA replication, the RNA primers must be removed.

The RNAse H specifically degrades RNA that is base pair with DNA.

      Removal of the RNA primers leaves gaps in the double stranded DNA.

These gaps are filled in by a DNA polymerase.

       Finally, the nick between The 3' hydroxyle of the repaired section and 5'                               phosphoryl of the replicated section are repaired by the DNA ligase.