Changes

[[Prokaryotic DNA]] is circular (a closed loop).  Whereas, [[eukaryotic DNA|eukaryotic chromosomes]] are linear (with ends), with the notable exception of [[Mitochondrial DNA]] and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus.<ref>Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the [[endosymbiotic hypothesis]]</ref>  The ends of eukaryotic chromosomes are protected by [[telomere]]s, which are always tightly condensed except during S phase of [[mitosis]].<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref> Telomeres are partially lost during cell division, and must be regrown later. Most of the cells in animal body are not capable of restoring they telomeres so can divide only limited number of times (Hayflick limit <ref>{{cite journal |author=Watson JD |title=Origin of concatemeric T7 DNA |journal=Nature New Biol. |volume=239 |pages=197–201 |year=1972 |pmid=4507727 |issue=94 |doi=10.1038/newbio239197a0}}</ref>).

[[Prokaryotic DNA]] is circular (a closed loop).  Whereas, [[eukaryotic DNA|eukaryotic chromosomes]] are linear (with ends), with the notable exception of [[Mitochondrial DNA]] and plastid (chloroplast) DNA, which is separate from the DNA in the nucleus.<ref>Mitochondria and chloroplasts are thought to have originated as prokaryotic cells living symbiotically inside of primitive eukaryotic cells, this is called the [[endosymbiotic hypothesis]]</ref>  The ends of eukaryotic chromosomes are protected by [[telomere]]s, which are always tightly condensed except during S phase of [[mitosis]].<ref>Campbell, Neil A, et. al. ''Biology''. 6th ed. San Francisco: Benjamin Cummings, 2002. 299, 530-31.</ref> Telomeres are partially lost during cell division, and must be regrown later. Most of the cells in animal body are not capable of restoring they telomeres so can divide only limited number of times (Hayflick limit <ref>{{cite journal |author=Watson JD |title=Origin of concatemeric T7 DNA |journal=Nature New Biol. |volume=239 |pages=197–201 |year=1972 |pmid=4507727 |issue=94 |doi=10.1038/newbio239197a0}}</ref>).

== Mutations ==

== Mutations ==