Unidirectional and bidirectional DNA replication
John Cairns, from his autoradiography experiments which are described earlier in this section, had concluded that DNA synthesis starts at a fixed point on the chromosome and proceeds in one direction. It was subsequently realized that Cairns' results could be interpreted in terms of bidirectional replication also. Additional evidences supporting bidirectional replication were obtained from (i) autoradiography, (ii) electron microscopy and (iii) genetic studies. Evidence of bidirectional replication is also available in eukaryotes like yeast, fruitfly and mammals, where DNA synthesis starts at multiple sites, which start as loops and can be seen as expanding bubbles or eyes in electron micrographs (Fig. 26.8). Number of eyes within an initiation zone (5000-55000 bp) indicates the number of initiation sites for DNA replication (each about 100-500 bases; see later for details).

In view of the above evidences, it is now known that DNA replication can be unidirectional or bidirectional, depending upon whether the replication from the point of origin proceeds only in one direction or proceeds in both the directions, (Fig. 26.9, 26.10). A replication eye may appear in both the situations, unless the replication starts from one of the two ends of a linear DNA molecule. However, in unidirectional replication, one of the two ends of the replication eye will be stationary and the other end will move with replication (Fig. 26.9). On the other hand, in bidirectional replication, none of the two ends will be stationary and both will be moving (Fig. 26.10). If radioactively labelled nucleotides are used during DNA synthesis, distinction between unidirectional and bidirectional replication can be made by the study of autoradiographs prepared after DNA synthesis has already proceeded for some time with the use of labelled nucleotides. Labelling on both the replication forks will indicate bidirectional replication and labelling only at one fork will suggest unidirectional replication (Fig. 26.11). An example of unidirectional replication is the replication of niitochondrial DNA (mtDNA) by D-loops in vertebrates (see Maternal Effects and Cytoplasmic Inheritance).