Topology of DNA Replication

A chromosome contains a single DNA molecule, which is generally very large e.g., some bacterial chromosomes are composed of as many as 4 x 106 base pairs. Furthermore, in many cases, the DNA is a closed or circular structure. Some bacterial chromosomes are linear. We will concentrate first on the topology of replication of the circular bacterial ( . coli) chromosome. Little is known about linear bacterial chromosomes in this regard. 

Question What is the form taken by a replicating bacterial chromosome (circular DNA molecule)  

The replicating bacterial chromosome remains in a closed form, with a portion of its length duplicated and joined to the rest of the DNA at replication forks (Fig. 16-2). 

Question The replicating bacterial chromosome shown in Fig. 16-2 contains two forks. Are both of these replication forks  

both are actively involved in DNA replication and move at approximately equal rates in opposite directions around the circular molecule. This is known as bidirectional replication. The replicated portion of the molecule is referred to as a replication bubble or eye form (because of its appearance in diagrams). The size of the bubble varies from being extremely small up to nearly twice the size of the nonreplicating chromosome. Obviously, the site on the circular molecule of a very small bubble represents the region within which replication was initiated. 

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