The relationship between the shape of the bacterial DNA and the blocking of replication machinery
This has been covered elsewhere (I highly recommend this page ) but it basically has to do with the priming of the strands and the fact that all polymerases work from the 5' prime to the the 3'. I made a quick schematic to illustrate (the explanation is below):



The priming is done with RNA during DNA replication in the cell, unlike PCR where you use DNA oligo's to prime. This RNA-DNA is called the Ozakazi fragments. After replication RNAse H will remove these primers leaving only DNA's and gaps. In the middle of the sequence the gaps can be filled in by the normal repair machinery but at the ends there is no 3' terminus to fill in the gaps. That is where the telomerase comes in and fill these gaps with a fixed sequence (also based on an RNA template but that is going into details). 
If you have a circular chromosome there is always a 3' to complete the gap.