why is the genetic code identical in humans, bacteria, and other organisms.?

Answer 1

Good question. The reason the code has diverged slightly in mitochondria but not in nuclear DNA has been of great interest. It is possibly though horizontal or lateral DNA transfer.
When we compare genomes of ancient lineages of living creatures, we find evidence of numerous transfers of genetic information from one lineage to another between viruses, bacteria, archea, and eukaryotes. In early times, horizontal gene transfer, the sharing of genes between unrelated organisms, was prevalent. It becomes more prevalent the further back you go in time. Evolution appears a communal affair, the whole community advancing in metabolic and reproductive efficiency as the genes of the most efficient cells were shared by mobile genetic elements. Chance had not created a way to prevent this movement. Once the means of blocking genetic movement by DNA restriction enzymes or the RNA interference pathway appeared a new selective pressure appeared but by then the DNA code was established as the universal system between all organisms.
Slight divergences in start or stop codons have appeared and continued but the basic code remains shared. In organisms more sheltered from the general horizontal transfer there is more divergence but all still share three base codons, tRNA & reading direction.
How the code arose is another question with several theories.
origin of the genetic code
http://www.ncbi.nlm.nih.gov/sites/entrez?db=pubmed&uid=9299300&cmd=showdetailview&indexed=google

Answer 2

The presumption is that all life today is descended from a common ancestor which used this system more than three and a half billion years ago.

Answer 3

1] It isn't, not completely. Mitochondria have a different code.
2] Maybe something to do with EVOLUTION ?

Answer 4

Because all have DNA! No cell can exist with out DNA. DNA is passed down from generation to generation of cells.

Answer 5

its not identical....
its 99.9% identical......