RNA Polymerase II (RNA Pol II) is the enzyme that adds nucleotides to a new DNA chain produced during transcription. It is recruited to the transcription start site of a gene by a cluster of transcription factors that bind the TATA box, which is a sequence of nucleotides near the starting line of the gene. These transcription factors are called the TFII family (for transcription factor for polymerase II) of proteins. These transcription factors help RNA Polymerase II to start traveling along the unwound DNA. As it moves along, it links nucleotides into a new chain by matching the free floating nucleotides with their corresponding base-pairs on the template strand of DNA.
Bacterial RNA Polymerase II is a multi-subunit protein complex. Instead of being recruited to the transcription start site by TFII family proteins -- as happens with the eukaryotic version -- bacterial RNA Pol II has a subunit called the sigma factor. The sigma factor brings the whole RNA Pol II complex to the starting line of the gene. The sigma factor helps pry open the DNA double helix, allowing the bacterial RNA Pol II complex to slide along one strand of DNA and start adding new nucleotides.
DNA replication is generally similar between eukaryotes and prokaryotes. Replication is different than transcription in that both strands of DNA are copied at the same time -- both strands of DNA serve as templates. In DNA replication, one strand of new DNA is produced as a continuous chain (called the leading strand), while the other strand of new DNA is made in short discontinuous pieces (called the lagging strand). DNA Polymerase III is the enzyme that adds nucleotides to make the continuous leading strand. Another polymerase, DNA Polymerase I, adds nucleotides to make the discontinuous fragments (called Okazaki fragments) on the lagging strand.
More Than One Polymerase
There are five DNA polymerases in bacteria, and 15 in humans. They generally belong to three different classes: A, B and X. DNA Pol III, which makes the leading strand during DNA replication, is a class A type and makes very long strands (30,000 nucleotides) before falling off the DNA. DNA Pol I, which makes the short discontinuous Okazaki fragments on the lagging strand, belongs to class B -- it makes fragments that are around 600 nucleotides long. Lastly, class X contains polymerases that are involved in repairing damaged DNA. They also add nucleotides, but in the form of short chains.