Chromosome replication ends

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. 

Many proteins are required for DNA synthesis and chromosomal replication. These include polymerases helicases, which unwind the parental duplex enzymes that fill in the gaps and join the ends in the case of lagging-strand synthesis enzymes that synthesize RNA primers at various points along the DNA tem-... 

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. 

Since DNA in chromosomes is a linear molecule, problems arise when replication comes to the ends of the DNA. Synthesis of the lagging strand at each end of the DNA requires a primer so that replication can proceed in a 5 to 3 direction. This becomes impossible at the ends of the DNA and 50-100 bp is lost each time a chromosome replicates. Thus, at each mitosis of a somatic cell, the DNA in chromosomes becomes shorter and shorter. Ultimately, after a limited number of divisions, a cell enters a nondividing state, called replicative senescence, which may play an important role in biological aging. 

Replication ends when the replication forks meet on the other side of the circular chromosome at the termination site, the ter (t) region. The ter region is composed of a pair of 20-bp inverted repeat ter sequences separated by a 20-bp segment. Each ter sequence prevents further progression of one of the replication forks when a 36-kD ter binding protein (TBP) is bound. How the two daughter DNA molecules separate is not understood, although a type II topoisomerase is believed to be involved. 

Fig, 49. Model of chromosome replication (Deepesh, 1964). Histone surrounding the DNA segments is not shown for greater simplicity of the model. Arrows on the DNA chains indicate free ends. Broken lines show newly synthesized components. Each daughter chromo-neme is a hybrid of DNA and protein components. 

ITowever, most normal somatic cells lack telomerase. Consequently, upon every cycle of cell division when the cell replicates its DNA, about 50-nucleotide portions are lost from the end of each telomere. Thus, over time, the telomeres of somatic cells in animals become shorter and shorter, eventually leading to chromosome instability and cell death. This phenomenon has led some scientists to espouse a telomere theory of aging that implicates telomere shortening as the principal factor in cell, tissue, and even organism aging. Interestingly, cancer cells appear immortal because they continue to reproduce indefinitely. A survey of 20 different tumor types by Geron Corporation of Menlo Park, California, revealed that all contained telomerase activity. 

S phase (DNA synthesis) is the period of time during which DNA replication occurs. At the end of S phase, each chromosome has doubled its DNA content and is composed of two identical sister chromatids linked at the centromere. 

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. 

Telomeres are seqnences of six-nucleotide repeats found at the ends of the chromosomal DNA strands. Many thon-sands of repeat nnits (TTAGGG) may be present at the end of the 3 strand and (AATCCC) at the end of the 5 strand. These are present at the ends of the strands to overcome a problem posed by the semi-conservative mechanism of DNA replication, known as the end replication problem . Replication of the ends of the chromosomes presents par-ticnlar difficnlties, since DNA polymerase can only elon-... 

An enzyme that uses an RNA template to add DNA to the ends of chromosomes. Telomerase is normally active only in stem cells and those cells giving rise to sperm and egg, but telomerase also undergoes activation when cells become cancerous. In the latter case, telomerase action allows transformed cells to replicate without a limit, a process termed immortalization . 

Cancer cells avoid replicative senescence by maintaining integrity of their chromosome ends through increased activity of which of the following enzymes ... 

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. 

The ends of a linear chromosome are not readily replicated by cellular DNA polymerases. DNA replication requires a template and primer, and beyond the end of a linear DNA molecule no template is available for the pairing of an RNA primer. Without a special mechanism for replicating the ends, chromosomes would be shortened somewhat in each cell generation. The enzyme telomerase solves this problem by adding telomeres to chromosome ends. 

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