1) DNA copying or replication is the process by which two copies of a DNA molecule is produced from one.
Since the two strands of a DNA molecule have complementary base pairs, the nucleotide sequence of each strand automatically supplies the information needed to produce its partner. If the two strands of a DNA molecule are separated, each can be used as a template to produce a complementary strand. Each template and its new complement together then form a new DNA double helix, identical to the original.
Before replication can occur, the length of the DNA double helix about to be copied must be unwound. In addition, the two strands must be separated by breaking the weak hydrogen bonds that link the paired bases. Once the DNA strands have been unwound, they must be held apart to expose the bases so that new nucleotide partners can hydrogen-bond to them. The enzyme DNA polymerase then moves along the exposed DNA strand, joining newly arrived nucleotides into a new DNA strand that is complementary to the template. Each cell contains a family of more than thirty enzymes to ensure the accurate replication of DNA.
2) This problem is taken care of by the process of meiosis occuring during formation of gametes. Meiosis is a reduction division, wherein the number of chromosomes and thereby the DNA is reduced tro half during cell division. So the daughter cells i.e. the gametes have half the number of chromosomes than the parent cells. When two such haploid gametes combine, it results in the formation of a diploid cell (zygote). In this zygote, the number of chromosomes become equal to the somatic cells of the body. The zygote gives rise to all body cells by mitosis division, so that all body cells have the same chromosome number specific to the species.