Unidirectional replication

Schematic diagrams of two different modes of DNA synthesis at the growth fork(s). In unidirectional replication (a) one growth fork occurs in bidirectional replication (b) two occur. Red indicates regions containing newly synthesized DNA.

Bidirectional replication. Replication in both directions away from the origin, as opposed to replication in one direction only (unidirectional replication). 

Not exactly, in that there is a defined origin of replication, yes. However, in some cases, e.g., some plasmids, replication proceeds in a single unique direction around the DNA molecule. This is called unidirectional replication. In other situations, e.g., mitochondrial DNA (Chap. 1), there is a more complex sequence of events which involves the unidirectional copying of only one strand over most of the DNA length, in a different type of eye form, referred to as a D loop. 

Replication of the bacterial chromosome commences at a unique site, the origin, and replication forks diverge from this site at approximately equal rates. Consequently, the origin is duplicated completely and each daughter origin remains at the central portion of the expanding bubble. This is in contrast to unidirectional replication, which would leave the origin only partially replicated and located at the stationary fork as the replication fork moved away. 

Mitochondria - Figure 24.39 illustrates the unusual DNA replication initiation scheme employed by animal cell mitochondria. This involves two unidirectional replication processes. 

The F factor not only codes for genes that are essential to regulate its own replication but also controls its copy number. The regulatory genes include oriS, repE, par A, and parB. The oriS and repE genes mediate the unidirectional replication of the F factor, whereas parA and parB maintain a copy number at a level of one or... 

DNA replication proceeds by the synthesis of one new strand on each of the parental strands. This mode of replication is called semiconservative, and it appears to be universal. DNA synthesis initiates from a primer at a unique point on a prokaryotic template such as the E. coli chromosome. From the initiation point, DNA synthesis proceeds bidirectionally on the circular bacterial chromosome. The bidirectional mode of synthesis is not followed by all chromosomes. For some chromosomes, usually small in size, replication is unidirectional. 

In the second mechanism replication of both strands is initiated at one origin. The site at which the two strands are replicated is called the replication fork. Since the fork moves in one direction from the origin this type of replication is called unidirectional. Some types of bacteria use this type of mechanism. 

Replication of bacterial DNA with an integrated F factor. There are two origins of DNA replication, ori - and ori . Replication is bidirectional from ori - and unidirectional from orl . Somehow the cell solves this topological dilemma when cell division occurs. 

If replication were unidirectional, tracks "with a low-grain density at one end and a high-grain density at the other end would be seen. On the other hand, if replication were bidirectional, the middle of a track would have a low density, as shown in the margin. For E. coli, the grain tracks are denser on both ends than in the middle, indicating that replication is bidirectional. 

Because natural tissues have anisotropic mechanical properties, in order to better replicate the properties of soft tissue, specifically cardiovascular tissues such as heart valve leaflet and vascular conduits, these anisotropic mechanical properties should be incorporated into the design of PVA-C. PVA-C prepared by the standard freeze-thaw cycling process is isotropic, with mechanical properties independent of sample orientation. MiUon et al. [59] produced anisotropic PVA-C by subjecting the hydrogel to an initial controlled unidirectional strain after the PVA solution... 

Fig. 3. Unidirectional and bidirectional replication of prokaryotic and viral DNA. The bidirectional mechanism is found in E. coli and other bacteria and many DNA viruses. The unidirectional mechanism has been observed in certain plasmids. Autoradiography indicates that the ring structure is retained during the entire process. Some viral DNAs are replicated by a unidirectional rolling circle mechnaism (Fig.4).

Unidirectionally reinforced [0°4]AS4/3501-6 graphite/epoxy coupons were immersed in simulated sea water at room temperature within pressure chambers at 13.8 and 20.7 MPa for 2 months and weight gain data recorded periodically up to saturation (Gao and Weitsman 1998). A third set of specimens was immersed at atmospheric pressure 0.1 MPa. Each circumstance involved at least four replicate samples. All weight data could be matched with linear Fickian predictions. Results are summarized in Table 5.1. 

The single-stranded DNA genomes of certain small E. coli viruses (such as M13 and ( X174) are replicated in the form of rolling circles in which unidirectional synthesis of one (virus genome) strand occurs by continuous displacement from the template (complementary strand Fig. 4A). The initial duplex DNA (called the repUcative... 

---