Prokaryotes, daughter strand

Postreplicational mismatch repair has been found to correct errors in base substitution occurring during DNA replication in prokaryotes.48 This lowers the error rate for the polymerase from 1 in 106 to 107 to the observed range of values of 1 in 108 to 1010 in E. coli. How does the repair system know in this case which base in a mispair is the incorrect one The answer appears to be that the parent strand is tagged by methylation. A small proportion, some 0.2%, of the cytosine residues are methylated at the 5 position, and a similar proportion of the adenine residues are methylated at the 6 position. As methylation is a postreplicative event, the daughter strand is temporarily undermethylated after replication. 

Identification of daughter strand in prokaryotes Since DNA methylation lags behind DNA synthesis, the daughter strand is under methylated in comparison to the parental strand. The mismatch repair system seeks to act on the undermethylated DNA, which may elude the editing function of Pol during DNA replication. 

In-vitro studies with DNA polymerases from both prokaryote and eukaryote sources have shown these enzymes are capable of synthesizing DNA only by the incorporation of deoxyribonucleotides in a 5 to 3 direction. Autoradiographic studies indicate that at any one growing point, both daughter strands extend in one direction on the parental duplex... 

Bacteria are prokaryotes and their cellular organization is much simpler than that of eukaryotic cells. They are spherical or rod-shaped and often possess a tough protective coat or cell wall. The only membrane that bacterial cells possess is the outer cell membrane or plasma membrane they have no proper nucleus or other membrane-bound organelles, such as mitochondria. Instead of a nucleus they possess a single chromosome consisting of a circular molecule of double-stranded DNA. Bacteria normally reproduce by an asexual process in which the daughter cells each receive a genome which is identical with that of the parent cell. Their ribosomes are appreciably smaller than those of eukaryotes. 

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