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7. Beskrive hvordan DNA syntesen termineres.
Devlin, s.181, fig 4.17
Stryer, s.765

The termination of the replication is different in eukaryotes and prokaryotes.

In prokaryotes (that have a circular genome), the termination generally occurs 180 degrees away from the OriC. In some small viruses, the termination occurs wherever the two replication forks meet.

On the other hand, in E.Coli, there are special termination sequences that constrain termination within a region, by preventing forks from proceeding past the region.

In eukaryotes (having a linear genome) the free ends of the linear DNA molecules introduce several problems.

The continuous, leading strand can be theoretically synthesized all the way to the end of its template, by the 5´-3´ polymerase activity of  DNA polymerase III.

The discontinuous, lagging strand can not be synthesized all the way to the end of the template strand, because there is no place to synthesize a primer to which the nucleotides opposite the end of the template can be added. If the last nucleotides should be added, then the primer should be placed just outside the template strand, and that is not possible.

Even if a primer is created complementary to the very end of the template strand, after the removal of the primer, it will leave a gap and an incomplete 5´end of the lagging strand is created, since no polymerase adds nucleotides to the 5´-end of the strand.

This may not be a problem in a single generation, but after many cycles of replication, chromosome ends would be shortened to that degree that essential genes become lost and the cell dies.

In order to avoid this problem, the ends of the eukaryotic linear chromosomes are special structures called telomeres, which contain many repeats of a six-nucleotide, G-rich repeat sequence.

Human telomeres contain hundreds of tandem repeats of the sequence TTAGGG. The 3´-end of one strand of the chromosome extends about 18 nucleotides beyond the complementary strand with 5´-end, leaving three repeats as an overhang.

The overhanging 3´-end fold back on itself and makes G-G hydrogen bonds. It also binds extra proteins that define the chromosomes length and protect it from recombination. (since the end of the DNA molecule tend to trigger recombination)  

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