The bacterial DNA polymerases that have been characterized to date have certain patterns of similarities and differences which has led some to divide these enzymes into two groups: those whose genes contain introns--intervening non-coding nucleotide sequences--(Class B DNA polymerases), and those whose DNA polymerase genes are roughly similar to that of E. coli DNA polymerase I and do not contain introns (Class A DNA polymerases).By "non-coding" is meant that the nucleotides comprising both nucleic acid strands in such sequences do not contain 3-nucleotide codons that encode and correspond to amino acid residues of the mature protein. Introns are most often found in the genes of eukaryotic higher organisms but have also been found in lower organisms such as archaebacteria.
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