in PCR reaction occursin5'-3'.why not in 3'-5' direction?

Answer 1

Most simple explanation is the properties of DNA polymerases. Polymerases possess a 5´to 3´ polymerase and some possess 3´→5´ exonuclease activity.

Answer 2

You wont be asking this question if you knew that elongation of DNA during replication occurs continuosly in leading strand that is in the 5'-3' direction. Because addition of next nuclotiede reqiures fromation of some bond with the pre-existing attached nucleotide.Phosho-di-ester bond is created between the 5' P group and 3' OH group and this way nuclotides are added and for this OH group availability is a must.The same reaction occurs in vitro in PCR carried out by Taq polemerase so same direction.

Answer 3

All DNA synthesis occurs in the 5' to 3' direction (including PCR) because DNA polymerases must attach the next nucleotide in the chain to a 3' Oxygen.

Answer 4

No DNA replication occurs in a 3'-5' direction because of the nature of nucleotides. PCR simply exploits what happens in nature.

The way it currently works... Think of a bunch of energized beads floating around in solution (A,G,C,T) the beads can each add on to an existing string of already joined beads. If something snags the energy from a bead another one is ready to take its place. The energy is used when a bead snaps on to the existing string.

If the end of the string was energized and this energy is what is needed for a new bead to attach, once the energy was used (whether on purpose or accident) by something other than another energy bead attaching, the string is done.

In nature there is always something in the cell willing to snag energy and use it. I doubt DNA replication would ever get very far (let alone an entire chromosome) if it had to start over every time something ran into it. Keep in mind that ATP (a precursor to the A that gets incorporated into DNA) is the major energy currency of the cell.