Editing Chromosome (section)

=== Metaphase chromatin and division ===
{{See also|chromosome condensation|mitosis|meiosis}}
[[File:HumanChromosomesChromomycinA3.jpg|thumb|left|upright=0.9|Human chromosomes during [[metaphase]]]]
[[File:Stages of early mitosis in a vertebrate cell with micrographs of chromatids.svg|thumb|right|Stages of early mitosis in a vertebrate cell with micrographs of chromatids]]

In the early stages of [[mitosis]] or [[meiosis]] (cell division), the chromatin double helix becomes more and more condensed. They cease to function as accessible genetic material ([[Transcription (genetics)|transcription]] stops) and become a compact transportable form. The loops of thirty-nanometer chromatin fibers are thought to fold upon themselves further to form the compact metaphase chromosomes of mitotic cells. The DNA is thus condensed about ten-thousand-fold.<ref name="Cooper-2019">{{Cite book|last1=Cooper|first1=G.M.|title=The Cell|publisher=[[Oxford University Press]]|year=2019|isbn=978-1605357072|edition=8}}</ref>

The [[chromosome scaffold]], which is made of proteins such as [[condensin]], [[TOP2A]] and [[KIF4A|KIF4]],<ref>{{Cite journal|last1=Poonperm|first1=Rawin|last2=Takata|first2=Hideaki|last3=Hamano|first3=Tohru|last4=Matsuda|first4=Atsushi|last5=Uchiyama|first5=Susumu|last6=Hiraoka|first6=Yasushi|last7=Fukui|first7=Kiichi|date=1 July 2015|title=Chromosome Scaffold is a Double-Stranded Assembly of Scaffold Proteins|journal=Scientific Reports|volume=5|issue=1|pages=11916|doi=10.1038/srep11916|pmid=26132639|pmc=4487240|bibcode=2015NatSR...511916P}}</ref> plays an important role in holding the chromatin into compact chromosomes. Loops of thirty-nanometer structure further condense with scaffold into higher order structures.<ref>{{Cite book|last1=Lodish|first1=U.H.|title=Molecular Cell Biology|last2=Lodish|first2=H.|last3=Berk|first3=A.|last4=Kaiser|first4=C.A.|last5=Kaiser|first5=C.|last6=Kaiser|first6=U.C.A.|last7=Krieger|first7=M.|last8=Scott|first8=M.P.|last9=Bretscher|first9=A.|year=2008|publisher=W. H. Freeman|isbn=978-0-7167-7601-7|last10=Ploegh|first10=H.|last11=others}}</ref>

This highly compact form makes the individual chromosomes visible, and they form the classic four-arm structure, a pair of sister [[chromatid]]s attached to each other at the [[centromere]]. The shorter arms are called ''[[p arm]]s'' (from the French ''petit'', small) and the longer arms are called ''[[q arm]]s'' (''q'' follows ''p'' in the Latin alphabet; q-g "grande"; alternatively it is sometimes said q is short for ''queue'' meaning tail in French<ref>"[http://www.nature.com/scitable/topicpage/chromosome-mapping-idiograms-302 Chromosome Mapping: Idiograms]" ''Nature Education'' – 13 August 2013</ref>). This is the only natural context in which individual chromosomes are visible with an optical [[microscope]].

Mitotic metaphase chromosomes are best described by a linearly organized longitudinally compressed array of consecutive chromatin loops.<ref name="Naumova-2013">{{cite journal | vauthors = Naumova N, Imakaev M, Fudenberg G, Zhan Y, Lajoie BR, Mirny LA, Dekker J | title = Organization of the mitotic chromosome | journal = Science | volume = 342 | issue = 6161 | pages = 948–53 | date = November 2013 | pmid = 24200812 | pmc = 4040465 | doi = 10.1126/science.1236083 | bibcode = 2013Sci...342..948N }}</ref>

During mitosis, [[microtubule]]s grow from centrosomes located at opposite ends of the cell and also attach to the centromere at specialized structures called [[kinetochore]]s, one of which is present on each sister [[chromatid]]. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region. The microtubules then pull the chromatids apart toward the centrosomes, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and DNA can again be transcribed. In spite of their appearance, chromosomes are structurally highly condensed, which enables these giant DNA structures to be contained within a cell nucleus.