Question

how DNA replicates?the enzymes which are nesseary and there function?

Answer 1

DNA replicates in the following way:
1.Activation of Deoxyribonucleotides: They occur freely in the nucleoplasm.They are of four types-deAMP,deGMP,deCMP and deTMP.
2.Exposure of DNA strands:Enzyme helicase acts and opens the two strands of DNA . The whole strands however does not opens at one stretch  but the point of separation proceeds slowly towards the direction hence giving it a Y-shaped structure called replication fork.
3.RNA Primer: It is essential for the formation of new DNA chains. 
4.DNA Polymerases: Prokaryotes have three major types of DNA synthesizing enzymes  called DNA Polymerases III, II and I. All of them add nucleotides in 5'-->3' direction on 3-->5 strand. 
5.Base Pairing: The two separated DNA strands in the replication fork function as templates. Deoxyriboribonucleoside triphosphates come to lie opposite the nitrogen bases of exposed DNA templates. 
6.Chain Formation: It requires DNA Polymerase III in prokaryotes and Polymeraseδ/ε in eukaryotes.
7.Proof Reading and DNA repair: A wrong base is sometimes introduced during replication. The frequency is one in ten thousand. DNA Polymerase III is able to sense the same. It goes back, removes the wrong base, allows addition to proper base and proceeds forward.

Answer 2

DNA
 Deoxyribonuclicacid is present in nucleus in chromosomes
.