Replicating chromosome, diagram

Synthesis at the ends of a eukaryotic chromosome. One end of the linear DNA of a eukaryotic chromosome is diagrammed. A flush-ended DNA duplex presents a problem for completing synthesis at the 5 end (a). This is because of the RNA primer requirement for DNA synthesis. When the primer at the 5 end is removed there is no conventional way to fill the gap. A solution to this problem is shown in (b). The ends of eukaryotic chromosomal DNAs consist of highly repetitious tandem repeats (telomeres). These repeats on the 3 end serve as both primer and template for extending the 3 end. The extended 3 end can accommodate a primer RNA, so after chromosomal DNA replication no loss occurs from the 5 end of the DNA. Another process is needed to remove the extension from the 3 end. New synthesis is indicated in red. The zigzag represents primer. 

Yes, 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. 

Fig. 1. Comparative chromosomal organization of prokaryotes and eukaryotes. The question mark indicates that we do not know whether archaebacteria have a single origin or multiple replication origins. The diagrams correspond to the average situation a few eubacteria have linear chromosomes but without telomeres whereas some eubacteria and Haloferax species have several circular chromosomes and/or giant plasmids. The cell size is also only indicative a few eubacteria are bigger...

Figure 5.16 Diagram of a yeast artificial chromosome (YAC). These vectors include features necessary for replication and stability in yeast ceils.

FIGURE 2.12. Diagram of a replicated and condensed eukaryotic chromosome (1) chromatid, (2) centromere, (3) short arm and (4) long arm (Wikimedia Commons M. Manske). 

FIGURE 18.2 Schematic diagram of two models of DNA replication in a circular chromosome, (a) Initial organization, (b) Unidirectional model, (c)... 

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