What is Eukaryotic DNA Replication?

The replication events at the replication fork arc much the same in eukaryotes as in prokaryotes except that the enzymes and protein factors are different. The main polymerizing enzyme is polymerase a, (3, y, 8 & e.

This polymerase enzyme is much slower in comparison to that of prokaiyote. DNA pol III adds about 1000 nucleotides per second where as DXA pol a ads about 50 nucleotides per second. The SSB protein is known as Replication factor A in eukaryote and the topoisomerase is Type I topoisomerase.

Another big difference is the sheer amount of DXA and the larger size of DXA. Eukaryotes have more than one chromosomes and cach chromosome has DXA larger than the genome of a bacterium. For example, the total length of human DXA of one cell is about 2 meter where as that of E.coli is only l mm. So for larger DXA to replicate in quick time eukaryotic DXA have multiple origin and cach eukaryotic DXA is a multiple replicon.

The yeast cell chromosomes have about 400 origin and cach human DXA with about 1000 origins. Imagine a situation of human genome with 4xio⁹base pairs to rcplicate as a single replieon: it will take several weeks. But, the cell cyclc is completed in 24 hours and for that cycle to operate in time, the DXA replication in human is completed in 6-8 hours of S-phase. This is achieved due to the presence of multiple origins.

During the S-phase of the cell cycle, the DXA replicates only once and then the cell divides. Hence the amount of total DXA is first doubled in S-phase and then equally divided between the two daughter cells during the cell division. Thus the DXA level (and the chromosome number) is kept constant after successive cell divisions.

The accurate replication of DXA and their equal distribution among the daughter cells form the bases of transmission of hereditary characters. Any error in DXA replication is taken care of by DXA repair mechanism available in the cell. But, imagine a situation where DXA divides not once but many times before a cell division. In such a situation the total DXA will increase two times, four times or many more times and the subsequent cell division will produce polyploid cells (cells with more than the normal number of chromosomes).

This does not happen as the cells have a replication licensing system. During the cell division, in the anaphase stage the replication origins are licensed by a nondiffusible Replication Licensing Factors or RLF. After the anaphase, no further licensing can occur due to the presence of nuclear membrane.

The RLF allows DNA to replicate once in the S-phase and the RLF get destroyed during replication. Further round of replication will require further licensing. Unless the cell undertakes division cycle, it cannot come to anaphase and licensing of origins cannot occur. This mechanism ensures that a cell must divide after a single round of DNA replication.