Bidirectional Replication

Bidirectional replication is a method of DNA replication found in organism from each of the main kingdoms. Bidirectional replication involves replicating DNA in two directions at the same time resulting in a leading strand (were replication occurs more rapidly) and a lagging strand (with slower replication). The properties of each of these strands is caused by DNA polymerase and its ability to only replicate in the 5' to 3' direction. In the leading strand, a single DNA polymerase can replicate large portions of the strand (approximately X1000-5000 bases before it falls off the DNA due to its high processivity) before dissociating. However, in the lagging strand, the DNA is replicate in chunks which are called Okasaki fragments. Each of these fragments is later fused together by DNA ligase to produce the full, unfragmented strand. 

The chromosome also has two replication forks which are the regions where nucleotides are actively added to growing strands. Prokaryotes have a circular chromosome with a single origin of replication (OriC) and a single termination site. However the linear chromosomes, like those in eukaryotes, have several origins of replication and two replication forks for each of these, replication therefore occurs much more quickly^[1]. At all replication origins, replication takes place in a bidirectional format which results in the formation of ‘replication bubbles’. These bubbles grow in size as replication continues. Eventually, two replication forks (at each end of a bubble) meet, at which point they fuse together producing a larger bubble. Ultimately, all the replication bubbles along the chromosome merge into one large bubble joint only at the telomeres; these split to give two identical strands of DNA. This process continues to produce a many strands of DNA which are then passed on to daughter cells ^[2]

References

1. ↑ J.M Berg, J.L Tymoczko, L Stryer, N.D Clarke. (2002). Chapter 27, Section 4: DNA Replication of Both Strands Proceeds Rapidly from Specific Start Sites. Biochemistry. W.H. Freeman and Company.
2. ↑ Ruvolo, D. L. (2012). Eighth Edition Genetics Analysis of Genes and Genomes. Burlington: Jones & Bartlett Learning.