Eukaryotes chromosome replication

Figure 11.12 A schematic representation of eukaryotic chromosome replication. The new...

F g,2. Repicatlon of a eukaryotic chromosome. Replication occurs simultaneously in different subregions, which eventually join with one another, producing two separate daughter duplexes. 

Edenberg, H. J., and Huberman, J. A., 1975, Eukaryotic chromosome replication, Annu. 

Abstract. In eukaryotic cells, replicated DNA molecules remain physically connected from their synthesis in S phase until they are separated during anaphase. This phenomenon, called sister chromatid cohesion, is essential for the temporal separation of DNA replication and mitosis and for the equal separation of the duplicated genome. Recent work has identified a number of chromosomal proteins required for cohesion. In this review we discuss how these proteins may connect sister chromatids and how they are removed from chromosomes to allow sister chromatid separation at the onset of anaphase. 

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. 

Telomeres are r etitive sequences at the ends of linear DNA molecules in eukaryotic chromosomes. With each round of replication in most normal cells, the telomeres are shortened because DNA polymerase cannot complete synthesis of the 5 end of each strand. This contributes to the aging of cells, because eventually the telomeres become so short that the chromosomes cannot function properly and the cells die. 

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... 

FIGURE 24-5 Eukaryotic chromosomes, (a) A pair of linked and condensed sister chromatids from a human chromosome. Eukaryotic chromosomes are in this state after replication and at metaphase during mitosis, (b) A complete set of chromosomes from a leukocyte from one of the authors. There are 46 chromosomes in every normal human somatic cell. 

Artificial chromosomes (Chapter 9) have been constructed as a means of better understanding the functional significance of many structural features of eukaryotic chromosomes. A reasonably stable artificial linear chromosome requires only three components a centromere, telomeres at each end, and sequences that allow the initiation of DNA replication. Yeast artificial chromosomes (YACs see Fig. 9-8) have been developed as a research tool in biotechnology. Similarly, human artificial chromosomes (HACs) are being developed for the treatment of genetic diseases by somatic gene therapy. 

The termination of replication on linear eukaryotic chromosomes involves the synthesis of special structures called telomeres at the ends of each chromosome, as discussed in the next chapter. 

Replication of the Escherichia coli Chromosome Initiation and Termination of Escherichia coli Chromosomal Replication DNA Replication in Eukaryotic Cells Eukaryotic Chromosomal DNA SV40 Is Similar to Its Host in Its Mode of Replication... 

Bacterial Reverse Transcriptase Catalyzes Synthesis of a DNA-RNA Molecule Telomerase Facilitates Replication at the Ends of Eukaryotic Chromosomes Other Enzymes That Act on DNA... 

Multiple-origin model for eukaryotic chromosomal DNA replication, (a) Autoradiograph of short-term labeling of a eukaryotic chromosome during replication and its interpretation. (b) Overall replication scheme for a eukaryotic chromosome. Only a short region of the chromosome is shown. It is believed that replication origins are relatively free of proteins. 

Telomerase Facilitates Replication at the Ends of Eukaryotic Chromosomes... 

Since eukaryotic chromosomes are linear, the ends of these chromosomes require a special solution to ensure complete replication. This can be seen in figure 26.26. At the very end of a linear duplex a primer is necessary to initiate DNA replication. After RNA primer removal there is bound to be a gap at the 5 end of the newly synthesized DNA chains. Since DNA synthesis always requires a primer the usual way of filling this gap is not going to solve the problem. This dilemma is overcome by a special structure at the ends (telomeres) of eukaryotic chromosomes and a special type of reverse transcriptase (telomerase) that synthesizes telomeric DNA. In many eukaryotes the telomeres contain short sequences (frequently hexamers) that are tan-demly repeated many times. Telomerase contains an RNA that binds to the 3 ends and also serves as a template for the extension of these ends. Prior to replication, the 3 ends of the chromosome are extended with additional tandemly repeated hexamers. The 3 ends are extended sufficiently so that there is room to accommodate an RNA primer. In this way there is no net loss of DNA from the 5 ends as a result of replication. After replication the 3 end is somewhat... 

Synthesis at the ends of a eukaryotic chromosome. One end of the linear DNA of a eukaryotic chromosome is diagrammed. A flush-ended DNA duplex presents a problem for completing synthesis at the 5 end (a). This is because of the RNA primer requirement for DNA synthesis. When the primer at the 5 end is removed there is no conventional way to fill the gap. A solution to this problem is shown in (b). The ends of eukaryotic chromosomal DNAs consist of highly repetitious tandem repeats (telomeres). These repeats on the 3 end serve as both primer and template for extending the 3 end. The extended 3 end can accommodate a primer RNA, so after chromosomal DNA replication no loss occurs from the 5 end of the DNA. Another process is needed to remove the extension from the 3 end. New synthesis is indicated in red. The zigzag represents primer. 

Enzymes that catalyze the synthesis of DNA using an RNA template are known as reverse transcriptases. The first reverse transcriptase discovered was encoded by an RNA retrovirus. This enzyme is needed in the virus replication cycle. Some animal viruses pass through an RNA intermediate and also require a reverse transcriptase to replicate the viral DNA. Similarly, a number of transposable elements found in cellular chromosomes replicate through RNA intermediates they usually encode a reverse transcriptase. A unique reverse transcriptase called telomerase is used to synthesize the DNA at the ends of linear eukaryotic chromosomes. 

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. 

Fig. 2. Replication of eukaryotic chromosomal DNA. Replication begins at many origins and proceeds bi-directionally at each location. Eventually the replication eyes merge together to produce two daughter DNA molecules, each of which consists of one parental DNA strand (thin line) and one newly synthesized DNA strand (thick line).

The replication of a linear DNA molecule in a eukaryotic chromosome creates a problem that does not exist for the replication of bacterial circular DNA molecules. The normal mechanism of DNA synthesis (see above) means that the 3 end of the lagging strand is not replicated. This creates a gap at the end of the chromosome and therefore a shortening of the double-stranded replicated portion. The effect is that the chromosomal DNA would become shorter and shorter after each replication. Various mechanisms have evolved to solve this problem. In many organisms the solution is to use an enzyme called telom-erase to replicate the chromosome ends (telomeres). 

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 special about the ends of eukaryotic chromosomes and how are they replicated ... 

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