Why does dna replication take place at many places on the molecule simultaneously?

DNA is the molecule that caries genetic replication in living organisms. DNA replication is the process that involves copying of DNA during cell division (meiosis and mitosis). DNA replication takes place on the DNA helix which first unwounds itself into two strands the leading strand and the lagging strand. DNA replication takes place at multiple location along the DNA simultaneously simply because human DNA is very long with many millions of base pairs thus it unzips at multiple places so that replication can occur simultaneously at hundreds places along the length of the strand.

Mitochondria produce atp, a high energy compound. atp is used for all sorts of enzyme-mediated reactions, such as making proteins/nucleotides/etc, certain compounds cross the cell membrane, muscles move, etc.  without this source of energy, a cell will die. it will not be able to repair itself, defend against pathogens, etc.