Editing Chromosome (section)

== Eukaryotes ==
{{Main|Chromatin}}
{{See also|DNA condensation|Chromosome condensation|Nucleosome|Histone|Protamine}}
{{See also|Eukaryotic chromosome fine structure}}
[[File:Eukaryote DNA-en.svg|thumb|upright=1.15|Organization of DNA in a eukaryotic cell]]

Each eukaryotic chromosome consists of a long linear [[DNA|DNA molecule]] associated with [[protein]]s, forming a compact complex of proteins and DNA called ''[[chromatin]].'' Chromatin contains the vast majority of the DNA in an organism, but a [[Mitochondrial DNA|small amount]] inherited maternally can be found in the [[mitochondria]]. It is present in most [[cell (biology)|cells]], with a few exceptions, for example, [[red blood cell]]s.

[[Histone]]s are responsible for the first and most basic unit of chromosome organization, the [[nucleosome]].

[[Eukaryote]]s ([[cell (biology)|cells]] with nuclei such as those found in plants, fungi, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one [[centromere]], with one or two arms projecting from the centromere, although, under most circumstances, these arms are not visible as such. In addition, most eukaryotes have a small circular [[mitochondrial genome]], and some eukaryotes may have additional small circular or linear [[cytoplasm]]ic chromosomes.

[[File:Chromatin Structures.png|thumb|center|upright=3.9|The major structures in DNA compaction: [[DNA]], the [[nucleosome]], the 10&nbsp;nm "beads-on-a-string" fibre, the 30&nbsp;nm fibre and the [[metaphase]] chromosome]]
In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around [[histone]]s (structural proteins), forming a composite material called chromatin.

=== Interphase chromatin ===
The packaging of DNA into nucleosomes causes a 10 nanometer fibre which may further condense up to 30&nbsp;nm fibres.<ref name="Cooper-2019" /> Most of the euchromatin in interphase nuclei appears to be in the form of 30-nm fibers.<ref name="Cooper-2019" /> Chromatin structure is the more decondensed state, i.e. the 10-nm conformation allows transcription.<ref name="Cooper-2019" />

[[File:Heterochromatin vs. euchromatin.svg|thumb|Heterochromatin vs. euchromatin|upright=1.8]]

During [[interphase]] (the period of the [[cell cycle]] where the cell is not dividing), two types of chromatin can be distinguished:
* [[Euchromatin]], which consists of DNA that is active, e.g., being expressed as protein.
* [[Heterochromatin]], which consists of mostly inactive DNA. It seems to serve structural purposes during the chromosomal stages. Heterochromatin can be further distinguished into two types:
** ''Constitutive heterochromatin'', which is never expressed. It is located around the centromere and usually contains [[repeated sequence (DNA)|repetitive sequences]].
** ''Facultative heterochromatin'', which is sometimes expressed.

=== 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.

=== Human chromosomes ===
Chromosomes in humans can be divided into two types: [[autosome]]s (body chromosome(s)) and allosome ([[sex chromosome]](s)). Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the [[mitochondrial genome]]. [[DNA sequencing|Sequencing]] of the [[human genome]] has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the [[Sanger Institute]]'s human genome information in the [[Vertebrate and Genome Annotation Project|Vertebrate Genome Annotation (VEGA) database]].<ref>[http://vega.sanger.ac.uk/Homo_sapiens/index.html Vega.sanger.ad.uk], all data in this table was derived from this database, 11 November 2008.</ref> Number of genes is an estimate, as it is in part based on [[gene prediction]]s. Total chromosome length is an estimate as well, based on the estimated size of unsequenced [[heterochromatin]] regions.

{| class="wikitable sortable" style="text-align:right"
|+
|-
! Chromosome !! [[Gene]]s<ref>{{Cite web|url=http://apr2013.archive.ensembl.org/Homo_sapiens/Location/Chromosome?r=1:1-1000000|title=Ensembl genome browser 71: Homo sapiens – Chromosome summary – Chromosome 1: 1–1,000,000|website=apr2013.archive.ensembl.org|access-date=11 April 2016}}</ref> !! Total [[nucleobase|base pairs]] !! % of bases 
|-
| [[Chromosome 1|1]] ||2000|| 247,199,719 ||8.0
|-
| [[Chromosome 2|2]] ||1300|| 242,751,149 ||7.9
|-
| [[Chromosome 3|3]] ||1000|| 199,446,827 ||6.5
|-
| [[Chromosome 4|4]] ||1000|| 191,263,063 ||6.2
|-
| [[Chromosome 5|5]] ||900|| 180,837,866 ||5.9
|-
| [[Chromosome 6|6]] ||1000|| 170,896,993 ||5.5
|-
| [[Chromosome 7|7]] ||900|| 158,821,424 ||5.2
|-
| [[Chromosome 8|8]] ||700|| 146,274,826 ||4.7
|-
| [[Chromosome 9|9]] ||800|| 140,442,298 ||4.6
|-
| [[Chromosome 10|10]] ||700|| 135,374,737 ||4.4
|-
| [[Chromosome 11|11]] ||1300|| 134,452,384 ||4.4
|-
| [[Chromosome 12|12]] ||1100|| 132,289,534 ||4.3
|-
| [[Chromosome 13|13]] ||300|| 114,127,980 ||3.7
|-
| [[Chromosome 14|14]] ||800|| 106,360,585 ||3.5
|-
| [[Chromosome 15|15]] ||600|| 100,338,915 ||3.3
|-
| [[Chromosome 16|16]] ||800|| 88,822,254 ||2.9
|-
| [[Chromosome 17|17]] ||1200|| 78,654,742 ||2.6
|-
| [[Chromosome 18|18]] ||200|| 76,117,153 ||2.5
|-
| [[Chromosome 19|19]] ||1500|| 63,806,651 ||2.1
|-
| [[Chromosome 20|20]] ||500|| 62,435,965 ||2.0
|-
| [[Chromosome 21|21]] ||200|| 46,944,323 ||1.5
|-
| [[Chromosome 22|22]] ||500|| 49,528,953 ||1.6
|-
| [[X chromosome|X (sex chromosome)]] ||800|| 154,913,754 ||5.0
|-
| [[Y chromosome|Y (sex chromosome)]] ||200<ref name="NCBI-1998">{{Cite book| title = Genes and Disease| chapter = Chromosome Map| publisher = National Center for Biotechnology Information| location = Bethesda, Maryland|url = https://www.ncbi.nlm.nih.gov/books/NBK22266/#A296| year = 1998}}</ref>|| 57,741,652 ||1.9
|- class="sortbottom"
! Total ||style="text-align:right"| 21,000 ||style="text-align:right"| 3,079,843,747 ||style="text-align:right"| 100.0 
|}

Based on the micrographic characteristics of size, position of the [[centromere]] and sometimes the presence of a [[chromosomal satellite]], the human chromosomes are classified into the following groups:<ref>The colors of each row match those of the karyogram (see Karyotype section)</ref><ref>{{cite journal|author1=Erwinsyah, R.|author2=Riandi|author3=Nurjhani, M.|year=2017|title=Relevance of human chromosome analysis activities against mutation concept in genetics course. IOP Conference Series.|journal=Materials Science and Engineering|doi=10.1088/1757-899x/180/1/012285|s2cid=90739754 |doi-access=free}}</ref>
{|class=wikitable
! Group
! Chromosomes
! Features
|- style="background:lavenderblush"
| '''A'''
| 1–3
| Large, metacentric or submetacentric
|- style="background:honeydew"
| '''B'''
| 4–5
| Large, submetacentric
|- style="background:lightyellow"
| '''C'''
| 6–12, X
| Medium-sized, submetacentric
|- style="background:linen"
| '''D'''
| 13–15
| Medium-sized, acrocentric, with [[Satellite chromosome|satellite]]
|- style="background:lightcyan"
| '''E'''
| 16–18
| Small, metacentric or submetacentric
|- style="background:lavender"
| '''F'''
| 19–20
| Very small, metacentric
|- style="background:lavenderblush"
| '''G'''
| 21–22, Y
| Very small, acrocentric (and 21, 22 with [[Satellite chromosome|satellite]])
|}