Replication fork, eukaryotic

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... 

Kunkel TA, Burgers PM. (2008) Dividiitg the workload at a eukaryotic replication fork. Trends Cell Biol 18(11), 521-7. Also Juhani Syvoaja, personal communication. 

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. 

In bacteria, some 1000 nucleotides are replicated per second. In eukaryotes, replication takes place more slowly (about 50 nucleotides s ) and the genome is larger. Thousands of replication forks are therefore active simultaneously in eukaryotes. 

The rate of replication fork movement in eukaryotes (-50 nucleotides/s) is only one-twentieth that observed in E. coli. At this rate, replication of an average human chromosome proceeding from a single origin... 

Another difference between bacterial and eukaryotic replication is the presence of nucleosomes in eukaryotes. Some evidence suggests that nucleosomes may open and close to allow replication forks to pass through.509 Studies of SV40 minichromosomes indicate that passage of the replication machinery does destabilize nucleosomes, which must be partially reconstructed about 260 nucleotides past the elongation point.510 Another factor is the variable extent and location of modifications to histones, in particular to... 

DNA in eukaryotic chromosomes is complexed with histone proteins in complexes called nucleosomes. These DNA-protein complexes are disassembled directly in front of the replication fork. The nucleosome disassembly may be rate-limiting for the migration of the replication forks, as the rate of migration is slower in eukaryotes than prokaryotes. The length of Okazaki fragments is also similar to the size of the DNA between nucleosomes (about 200 bp). One model that would allow the synthesis of new eukaryotic DNA and nucleosome formation would be the disassembly of the histones in front of the replication fork and then the reassembly of the histones on the two duplex strands. Histone synthesis is closely coupled to DNA replication. 

The basic DNA replication mechanism is identical for prokaryotes and eukaryotes in that an asymmetric replication fork and a RNA primer are in-... 

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

Many of the proteins and enzymes involved in initiation at replication origins and DNA chain growth at replication forks have the same biochemical activities as their counterparts in bacteria. However, the situation regarding terminators and terminator proteins is less clear. Whether they exist to delineate to any extent individual replicons or clusters of replicons is not known. In the case of eukaryotic chromosomes, however, there is a special mechanism to replicate their ends which are known as telomeres. 

Question What is known about the process of DNA chain growth at replication forks in eukaryotes ... 

The structure of replication forks in eukaryotes is essentially the same as in bacteria. Chain growth is continuous on the leading strand and discontinuous on the lagging strand. There are equivalents of the polymerases, helicase, primase, SSB, etc., but there are clearly some differences. For example, two different polymerases, DNA polymerase 8 and DNA polymerase a, function on the leading and lagging strand, respectively. Also, the mitochondrion has its own DNA polymerase. 

S. Waga and B. Stillman. 1998. The DNA replication fork in eukaryotic cells Anna. Rev. Biochem. 67 721-751. (PubMed)... 

In eubacteria and eukaryotes, several types of DNA polymerases have been characterized three in eubacteria (DNA polymerases I, II and III), and five in eukaryotes (DNA polymerases a, 3, 6, e and )). Some of these enzymes, named DNA replicases , are specifically involved in DNA-chain elongation at the replication fork. They have a multi-subunit structure and can prime and perform DNA replication in a processive way when they are associated with the other replicative proteins. In eubacteria, only one DNA replicase has been isolated (DNA polymerase III), whereas several DNA replicases co-exist in eukaryotes DNA polymerases a, 6 and e, which are essential for the replication of nuclear DNA, and DNA polymerase y, which is responsible for the replication of the mitochondrial genome. The other eubacterial and eukaryotic DNA polymerases are monomeric and are preferentially involved in mechanisms which require replication of short DNA fragments, in the course of either DNA repair (DNA polymerases I and II from E. coli, eukaryotic DNA polymerase 3), or DNA replication (maturation of Okasaki fragments by E. coli DNA polymerase I). 

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