In this article we will discuss about the replication of eukaryotic chromosome.
Taylor in 1958 used radioactive thymidine (3H-thymidine) to label Viciafaba root tip chromosomes. He showed that the chromosomes replicated in a semiconservative manner, and proposed that each chromatid was made up of a single DNA molecule.
Thus each sister chromatid is composed of one of the two progeny DNA molecules produced by the replication of the parent DNA molecule, and each progeny DNA molecules has one old and one new strand.
Replication of the eukaryotic chromosome differs for the rates of replication per unit time, the total time taken for replication of the whole genome, and for the number of replicons per chromosome. It has been estimated that E. coli chromosome replicates at a rate of 50 kb/minute, whereas for eukaryotic chromosomes this rate is very low (Table 3.6).
The average rate is about 2 kb/minute; thus the replication of E. coli chromosome is about 25 times as fast as that of the eukaryotes for the reason briefly described earlier.
Each bacterial chromosome has only one origin so that the whole chromosome functions as a single replicon. It takes about 30-40 minutes for the E. coli chromosome to replicate completely. But each eukaryotic chromosome has several to thousands origins and as many replicons; e.g., the largest chromosome of Drosophila has 7000 replicons.
Therefore, the length of a single replicon may vary from few µm to several hundred µm with an average of 15-60 which corresponds roughly to 40 to 180 kb.
In each replicon, DNA replication begins at the origin and proceeds bi-directionally. Thus, in each replicon, two replication forks are produced which keep on moving in the opposite directions till they meet with the advancing replication forks of the adjacent replicons.
When the replication of the adjacent replicons is completed, a nick remains in the newly synthesized strands at their junction; this nick is sealed by the enzyme DNA ligase (Fig. 3.17).
Thus the DNA segments synthesized in the multiple replication units are joined together to form the complete DNA strand. Various enzymes involved in the replication are given in Table 3.7. All the replicons of a single chromosome do not begin to replicate simultaneously since the initiation of the replication occurs at different times.
Certain chromosomes and parts of a single chromosome replicate early while the others may replicate later during the S-phase. In general, heterochromatic regions replicate later than the euchromatic ones. It has been shown that the early replicating chromosome regions are rich in guanine and cytosine, while those replicating late are rich in adenine and thymine.
The substance responsible for replication initiation is some protein. Treatment of late G1 phase cells with inhibitors of protein synthesis prevents the initiation of DNA replication in them. Inhibitors of protein synthesis are also known to depress the eukaryotic DNA synthesis that is already in progress.
The nucleosome histones remain as intact particles when they separate from DNA during the replication process; they rapidly re-associate with DNA to form nucleosomes as soon as the replication of a sufficient length of DNA double helix is completed. As a result, the segment of chromosomal DNA is free from the nucleosome organisation only so long as it is actually undergoing replication.
The progeny DNA double helices reassemble immediately into nucleosomes just as they are being produced through replication. The arrangement or distribution of old and newly synthesized nucleosomes on the chromosome is not clearly understood. Experiments have indicated that histones are reassembled afresh and consist of both old and new histone molecules in the histone octamer.