Editing DNA replication (section)

=== Termination ===
Eukaryotes initiate DNA replication at multiple points in the chromosome, so replication forks meet and terminate at many points in the chromosome.  Because eukaryotes have linear chromosomes, DNA replication is unable to reach the very end of the chromosomes.  Due to this problem, DNA is lost in each replication cycle from the end of the chromosome.  [[Telomeres]] are regions of repetitive DNA close to the ends and help prevent loss of genes due to this shortening. Shortening of the telomeres is a normal process in [[somatic cell]]s.  This shortens the telomeres of the daughter DNA chromosome.  As a result, cells can only divide a certain number of times before the DNA loss prevents further division. (This is known as the [[Hayflick limit]].)  Within the [[germ cell]] line, which passes DNA to the next generation, [[telomerase]] extends the repetitive sequences of the telomere region to prevent degradation. Telomerase can become mistakenly active in somatic cells, sometimes leading to [[cancer]] formation. Increased telomerase activity is one of the hallmarks of cancer.<ref>{{Cite journal |last=Shay |first=Jerry W. |date=2016-06-01 |title=Role of Telomeres and Telomerase in Aging and Cancer |journal=Cancer Discovery |language=en |volume=6 |issue=6 |pages=584–593 |doi=10.1158/2159-8290.CD-16-0062 |pmid=27029895 |pmc=4893918 |issn=2159-8274}}</ref>

Termination requires that the progress of the DNA replication fork must stop or be blocked. Termination at a specific locus, when it occurs, involves the interaction between two components: (1) a termination site sequence in the DNA, and (2) a protein which binds to this sequence to physically stop DNA replication. In various bacterial species, this is named the DNA replication terminus site-binding protein, or [[Ter protein]].<ref>{{Cite journal |last1=Neylon |first1=Cameron |last2=Kralicek |first2=Andrew V. |last3=Hill |first3=Thomas M. |last4=Dixon |first4=Nicholas E. |date=September 2005 |title=Replication Termination in Escherichia coli : Structure and Antihelicase Activity of the Tus- Ter Complex |journal=Microbiology and Molecular Biology Reviews |language=en |volume=69 |issue=3 |pages=501–526 |doi=10.1128/MMBR.69.3.501-526.2005 |pmid=16148308 |pmc=1197808 |issn=1092-2172}}</ref>

Because bacteria have circular chromosomes, termination of replication occurs when the two replication forks meet each other on the opposite end of the parental chromosome. ''E. coli'' regulates this process through the use of termination sequences that, when bound by the [[Tus protein]], enable only one direction of replication fork to pass through. As a result, the replication forks are constrained to always meet within the termination region of the chromosome.<ref>{{Cite book |title=Genomes |vauthors=Brown TA |publisher=[[BIOS Scientific Publishers]] |year=2002 |isbn=1-85996-228-9 |chapter=Chapter 13.2.3. Termination of replication |chapter-url=https://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=genomes.section.8121#8156}}</ref>