<P> For a cell to divide, it must first replicate its DNA . This process is initiated at particular points in the DNA, known as "origins", which are targeted by initiator proteins . In E. coli this protein is DnaA; in yeast, this is the origin recognition complex . Sequences used by initiator proteins tend to be "AT - rich" (rich in adenine and thymine bases), because A-T base pairs have two hydrogen bonds (rather than the three formed in a C-G pair) and thus are easier to strand - separate . Once the origin has been located, these initiators recruit other proteins and form the pre-replication complex, which unwinds the double - stranded DNA . </P> <P> DNA polymerase has 5' - 3' activity . All known DNA replication systems require a free 3' hydroxyl group before synthesis can be initiated (note: the DNA template is read in 3' to 5' direction whereas a new strand is synthesized in the 5' to 3' direction--this is often confused). Four distinct mechanisms for DNA synthesis are recognized: </P> <Ol> <Li> All cellular life forms and many DNA viruses, phages and plasmids use a primase to synthesize a short RNA primer with a free 3' OH group which is subsequently elongated by a DNA polymerase . </Li> <Li> The retroelements (including retroviruses) employ a transfer RNA that primes DNA replication by providing a free 3 ′ OH that is used for elongation by the reverse transcriptase . </Li> <Li> In the adenoviruses and the φ29 family of bacteriophages, the 3' OH group is provided by the side chain of an amino acid of the genome attached protein (the terminal protein) to which nucleotides are added by the DNA polymerase to form a new strand . </Li> <Li> In the single stranded DNA viruses--a group that includes the circoviruses, the geminiviruses, the parvoviruses and others--and also the many phages and plasmids that use the rolling circle replication (RCR) mechanism, the RCR endonuclease creates a nick in the genome strand (single stranded viruses) or one of the DNA strands (plasmids). The 5 ′ end of the nicked strand is transferred to a tyrosine residue on the nuclease and the free 3 ′ OH group is then used by the DNA polymerase to synthesize the new strand . </Li> </Ol> <Li> All cellular life forms and many DNA viruses, phages and plasmids use a primase to synthesize a short RNA primer with a free 3' OH group which is subsequently elongated by a DNA polymerase . </Li>

What is the role of nucleotides in dna replication