Origin of replication in eukaryotes

The process of eukaryotic DNA replication closely follows that of prokaryotic DNA synthesis. Some differences, such as the multiple origins of replication in eukaryotic cells versus single origins of replication in prokaryotes, have already been discussed. Eukaryotic single-stranded DNA-binding proteins and ATP-dependent DNA helicases have been identified, whose functions are analogous to those of the prokaryotic enzymes previously discussed. In contrast, RNA primers are removed by RNase H. 

Question What are the structural features of origins of replication in eukaryotes and how is initiation... 

Eukaryotes have more DNA polymerases, which tend to be larger molecules. Eukaryotic DNA polymerases tend not to have exonuclease activity. There are more origins of replication in eukaryotes and shorter Okazaki fragments. See Table 10.5. 

DNA replication is initiated at a single site within the E. coli genome. A set of specific proteins recognize this origin of replication and assemble the enzymes needed for DNA synthesis, including a he-licase that promotes strand separation. The initiation of replication in eukaryotes is more complex. DNA synthesis is initiated at thousands... 

The multiple sites that serve as origins for DNA replication in eukaryotes are poorly defined except in a few animal viruses and in yeast. However, it is clear that initiation is regulated both spatially and temporaUy, since clusters of adjacent sites initiate rephcation synchronously. There are suggestions that functional domains of chromatin replicate as intact units, implying that the origins of rephcation are specificaUy located with respect to transcription units. 

Each eukaryotic chromosome contains one linear molecule of DNA having multiple origins of replication. Bidirectional replication occurs by means of a pair of replication forks produced at each origin. Completion of the process results in the production of two identical linear molecules of DNA. DNA replication occurs in the nucleus during the S phase of the eukaryotic cell cycle. The two identical sister chromatids are separated om each other when the ceU divides during mitosis. 

Research work with large genomes and the associated need for high-capacity cloning vectors led to the development of yeast artificial chromosomes (YACS Fig. 9-8). YAC vectors contain all the elements needed to maintain a eukaryotic chromosome in the yeast nucleus a yeast origin of replication, two selectable markers, and specialized sequences (derived from the telomeres and centromere, regions of the chromosome discussed in Chapter 24) needed for stability and... 

The size of the genomic DNA in eukaryotic cells (such as the cells of yeast, plants, or mammals) is much larger (up to 10+11 base pairs) than in E. coli (ca. 10+6 base pairs). The rate of the eukaryotic replication fork movement is about fifty nucleotides per second, which is about ten times slower than in E. coli. To complete replication in the relatively short time periods observed, multiple origins of replication are used. In yeast cells, these multiple origins of replication are called autonomous replication sequences (ARSs). As with prokaryotic cells, eukaryotic cells have multiple DNA polymerases. DNA polymerase S, complexed with a protein called proliferating... 

Although DNA replication is basically similar in prokaryotes and eukaryotes, the size and organization of the eukaryotic DNA introduce additional complexities into an already complex process. There is need for multiple origins of replication operating simultaneously for the DNA doubling to take place in a reasonable length of time (Figure 11.6). Nucleosomes must be disassembled in front of the replication and the newly formed DNA reassembled into nucleosomes with the newly formed histone octamers (Chapter 10). There are also some differences in the enzymes involved. 

Figure 11.6 Multiple origin of replication forks in eukaryotic DNA replication. Termination of replication occurs where two growth forks come together.

Fig. 16-5 Tandemly arranged replicons in eukaryotic DNA generate a chain of replication bubbles. O denotes an origin of replication.

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