Polymerase (Dna and Rna)
Polymerases are present in prokaryotes and eukaryotes, where they are essential to the manufacture of genetic material.
DNA polymerase functions to duplicate DNA (deoxyribonucleic acid). This process is called replication. Each time a cell divides, DNA polymerase duplicates its entire DNA, and the cell passes one copy to each daughter cell. This process ensures that genetic material is faithfully passed from generation to generation. DNA polymerase is extremely accurate in its task. Its error rate is less than one base in each billion duplicated.
The task of DNA replication requires a number of polymerases. Three distinct DNA polymerases (I, II, and III) have been isolated from Escherichia coli. Arthur Kornberg discovered the first DNA polymerase (I) in E. coli in 1956. Studies of mutant strains of E. coli led to the discovery of the other DNA polymerases II and III in the 1970s. Such studies also showed that the polymerase I is primarily involved in the repair of DNA damage, although it is also required for replication, and that polymerase III is the major replicative enzyme. The function of DNA polymerase II is still unknown.
Eukaryotes contain five DNA polymerases. Three are found in the mitochondria and are responsible for replication of mitochondrial DNA. Four (including two of the mitochondrial species) are active in the cell nucleus, where they function in replication.
In DNA replication, whether mitochondrial or nuclear, the involved polymerase functions to synthesize a new strand of DNA from each one of the old DNA strands, once unwinding of the old strands has occurred prior to replication. The polymerase begins synthesis at the site of RNA primers, which were previously synthesized on the two unwound DNA strands. Once synthesis of new DNA is complete, another DNA polymerase acts to remove the RNA primers and replace them with DNA, completing the synthetic process.
RNA polymerase functions to synthesize RNA using a DNA template and molecules called ribonucleotide triphosphates as the substrates. This process is called transcription. The DNA may code for a messenger RNA, which will be used for protein synthesis, for a ribosomal RNA, or for a transfer RNA. The latter two RNA species are used in the manufacture of protein. In prokaryotes, one RNA polymerase synthesizes all three of the RNAs. In eukaryotes, this function is performed by three separate RNA polymerases.
In a cell, DNA replication and DNA transcription will be occurring simultaneously, so both DNA and RNA polymerases will be in action at the same time. A mechanism must exist to coordinate the action of the polymerases in order to prevent a collision of the polymerase complexes. The best interpretation of available evidence suggests that the movement of the DNA polymerase through the RNA transcription complex accomplish this. The exact means by which this occurs remain unclear.
This is the complete article, containing 459 words
(approx. 2 pages at 300 words per page).
