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Acta Medica Medianae
Vol. 46, No 4, December, 2007
UDK 61
YU ISSN 0365-4478




Contakt: Biljana Miljkovic-Selimovic
Public Health Institute, Clinical Center
Dr Zoran Djindjic 50 Blvd.
18000 Nis, Srbija
Phone:+381 18 226448
E-mail: biljams@eunet.yu


Biljana Miljkovic-Selimovic, Tatjana Babic, Branislava Kocic, Predrag Stojanovic, Ljiljana Ristic and Marina Dinic

Public Health Institute Nis, Clinical Center Nis


Plasmids, extrachromosomal DNA, were identified in bacteria pertaining to family of Enterobacteriacae for the very first time. After that, they were discovered in almost every single observed strain. The structure of plasmids is made of circular double chain DNA molecules which are replicated autonomously in a host cell. Their length may vary from few up to several hundred kilobase (kb). Among the bacteria, plasmids are mostly transferred horizontally by conjugation process. Plasmid replication process can be divided into three stages: initiation, elongation, and termination. The process involves DNA helicase I, DNA gyrase, DNA polymerase III, endonuclease, and ligase. Plasmids contain genes essential for plasmid function and their preservation in a host cell (the beginning and the control of replication). Some of them possess genes which control plasmid stability. There is a common opinion that plasmids are unnecessary for a growth of bacterial population and their vital functions; thus, in many cases they can be taken up or kicked out with no lethal effects to a plasmid host cell. However, there are numerous biological functions of bacteria related to plasmids. Plasmids identification and classification are based upon their genetic features which are presented permanently in all of them, and these are: abilities to preserve themselves in a host cell and to control a replication process. In this way, plasmids classification among incompatibility groups is performed. The method of replicon typing, which is based on genotype and not on phenotype characteristics, has the same results as incompatibility grouping. Acta Medica Medianae 2007;46(4):61-65.

Key words: bacteria, plasmid