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chap 12 and 14 notes

chap 12 and 14 notes - Chapter 12 Extrachromosomal...

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Chapter 12 Extrachromosomal Replicons Plasmids: self-replicating circular molecules that are maintained in the cell in a stable and characteristic number of copies Lysogeny: phages that are found as part of the bacterial chromosome Episomes: plasmids that also have the ability to integrate into the chromosome -they boh maintain a selfish possession of their bacterium and often make it impossible for another element of the same type to become established:immunity 12.2 The ends of linear DNA are a problem for replication No replicons so far had a linear end: either circular or they were part of longer segregation units Linear replicons do occur: as a single extra chromosomal unit and at ends of eukaryotic chromosomes -To synthesize a complement at eh end of the upper strand, it must start right at the last base, or else the strand would become shorter each cycle of replication Types of solution to accommodate the need to copy a terminus: -covert linear replicon into a circular or multimeric molecule -DNA may form an unusual structure: ex by creating a hairpin -the end may be variable -a mechanism to add or remove units makes it unnecessary to replicate right up to the very end -protein may intervene to make initiation possible at the actual terminus 12.3 Terminal proteins enable initiation at the ends of viral DNAs When the replication fork reaches the other end of the molecule, the displaced strand is released as a free single strand. Then its replicated independently by the formation of a duplex origin by base pairing between some short complementary sequences at the ends of the molecule A terminal protein is linked to the mature viral DNA via a phosphodiester bond to serine -it carries a cytidine nucleotide that provides the primer and associated with DNA polymerase -The free 3’ OH end of the C nucleotide is used to prime the elongation reaction by the DNA polymerase -this generates a new strand whose 5’ end is linked to the initiating C nucleotide 12.4 Rolling Circles Produce Mulitmers of a Replicon A nick opens one strand, then a free 3-OH end generated by the nick is extended by the DNA polymerase. It replaces the original parental strand -It is a rolling circle: generates single-stranded multimers of the original sequence Cleavage of a unit length tail generates a copy of the original circular replicon in linear form.
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