Chromosome

A **chromosome** (frae auncient Greek: χρωμόσωμα, *chromosoma, chroma* means colour, *soma* means bouk) is a DNA molecule wi pairt or aw o the genetic material (genome) o an organism. Maist eukaryotic chromosomes include packagin proteins that, aidit bi chaperone proteins, bind to an condense the DNA molecule tae prevent it frae acomin an unmanageable tangle.

Chromosomes are normally veesible unner a licht microscope anly whan the cell is unnergaein the metaphase o cell diveesion. Afore this happens, ivery chromosome is copied ance (S phase), an the copy is jyned tae the oreeginal bi a centromere, resultin aither in an X-shapit structur (picturt tae the richt) if the centromere is locatit in the middle o the chromosome or a twa-airm structur if the centromere is locatit near ane o the ends. The oreeginal chromosome an the copy are nou cried sister chromatids. In metaphase the X-shape structur is cried a metaphase chromosome. In this heichly condensed form chromosomes are easiest tae distinguish an study. In ainimal cells, chromosomes reach thair heichest compaction level in anaphase in segregation.

Chromosomal recombination in meiosis an subsequent sexual reproduction play a seegneeficant role in genetic diversity. If thir structurs are manipulatit incorrectly, throu processes kent as chromosomal instability and translocation, the cell mey unnergae mitotic catastrophe an dee, or it mey unexpectitly evade apoptosis, leadin to the progression o cancer.

Some uise the term chromosome in a wider sense, tae refer tae the individualised portions o chromatin in cells, aither veesible or nae unner licht microscopy. Houiver, ithers uise the concept in a narraer sense, tae refer tae the individualised portions o chromatin in cell diveesion, veesible unner licht microscopy due tae heich condensation.

## Etymology

The wird *chromosome* comes frae the Greek  (*chroma*, "colour") an  (*soma*, "bouk"), descrivin thair strang stainin bi pairteecular dyes. The term wis crautit bi von Waldeyer-Hartz, referrin tae the term chromatin, that wis introduced bi Walther Flemming.

## History o diskivery

The German scientists Schleiden, Virchow an Bütschli war amang the first scientists that recognised the structurs nou fameeliar as chromosomes.

Wilhelm Roux suggestit that ilk chromosome cairies a different genetic laid. Boveri wis able tae test an confirm this hypothesis. Aidit bi the rediskivery at the stairt o the 1900s o Gregor Mendel's earlier wark, Boveri wis able tae pynt oot the connection atween the rules o inheritance an the behaviour o the chromosomes. Boveri influenced twa generations o American cytologists: Edmund Beecher Wilson, Nettie Stevens, Walter Sutton an Theophilus Painter war aw influenced bi Boveri (Wilson, Stevens, an Painter actually wirkit wi him).

In his famous textbeuk *The Cell in Development and Heredity*, Wilson airtit thegither the independent wark o Boveri an Sutton (baith aroond 1902) bi namin the chromosome theory o inheritance the Boveri–Sutton chromosome theory (the names are whiles reversed). Ernst Mayr remerks that the theory wis hetly contestit bi some famous geneticists: William Bateson, Wilhelm Johannsen, Richard Goldschmidt an T.H. Morgan, aw o a raither dogmatic turn o mynd. Eventually, complete pruif cam frae chromosome maps in Morgan's awn lab.

The nummer o human chromosomes wis published in 1923 by Theophilus Painter. Bi inspection throu the microscope, he coontit 24 pairs, that wad mean 48 chromosomes. His error wis copied bi ithers an it wis nae till 1956 that the true nummer, 46, wis determined bi Indonesie-born cytogeneticist Joe Hin Tjio.

## Prokaryotes
The prokaryotes&nbsp;– bacteria an archaea&nbsp;– teepically hae a single circular chromosome, but mony variations exeest. The chromosomes o maist bacteria, that some authors prefer tae cry genophores, can range in size frae anerly 130,000 base pairs in the endosymbiotic bacteria *[[Candidatus Hodgkinia cicadicola]]* an *[[Candidatus Tremblaya princeps]]*, tae mair nor 14,000,000 base pairs in the sile-dwallin bacterium *[[Sorangium cellulosum]]*. Spirochaetes o the genus *Borrelia* are a notable exception tae this arrangement, wi bacteria sic as *[[Borrelia burgdorferi]]*, the cause o Lyme disease, conteenin a single *linear* chromosome.

### Structur in sequences
Prokaryotic chromosomes hae less sequence-based structur nor eukaryotes. Bacteria teepically hae a ane-pynt (the oreegin o replication) frae that replication starts, whauras some archaea conteen multiple replication oreegins. The genes in prokaryotes are eften organised in operons, an dae nae uisually conteen introns, unlik eukaryotes.

### DNA packagin
Prokaryotes dae nae possess nuclei. Insteid, thair DNA is organised intae a structur cried the nucleoid. The nucleoid is a distinct structur an occupies a defined region o the bacterial cell. This structur is, houiver, dynamic an is mainteened an remodeled bi the actions o a range o histone-lik proteins, that associate wi the bacterial chromosome. In archaea, the DNA in chromosomes is even mair organised, wi the DNA packaged within structurs seemilar tae eukaryotic nucleosomes.

Certaint bacteria an aw conteen plasmids or ither extrachromosomal DNA. Thir are circular structurs in the cytoplasm that conteen cellular DNA an play a role in horizontal gene transfer. In prokaryotes an viruses, the DNA is eften densely packed an organised; in the case o archaea, bi homology tae eukaryotic histones, an in the case o bacteria, bi histone-lik proteins.

Bacterial chromosomes tend tae be tethered tae the plasma membrane o the bacteria. In molecular biology application, this allous for its isolation frae plasmid DNA bi centrifugation o lysed bacteria an pelletin o the membranes (an the attached DNA).

Prokaryotic chromosomes an plasmids are, lik eukaryotic DNA, generally supercoiled. The DNA maun first be released intae its relaxed state for access for transcription, regulation, an replication.

## Eukaryotes

Chromosomes in eukaryotes are componed o chromatin feebre. Chromatin feebre is made o nucleosomes (histone octamers wi pairt o a DNA straund attached tae an wrappit aroond it). Chromatin feebres are packaged bi proteins intae a condensed structur cried chromatin. Chromatin conteens the vast majority o DNA and a smaw amoont inheritit maternally, can be foond in the mitochondria. Chromatin is present in maist cells, wi a few exceptions, for ensaumple, reid bluid cells.

Chromatin allous the verra lang DNA molecules tae fit intae the cell nucleus. In cell diveesion chromatin condenses forder tae form microscopically veesible chromosomes. The structur o chromosomes varies throu the cell cycle. In cellular division chromosomes are replicatit, dividit, an passed successfully tae thair dauchter cells sae as tae ensur the genetic diversity an survival o their progeny. Chromosomes mey exeest as aither duplicatit or unduplicatit. Unduplicatit chromosomes are single dooble helixes, whauras duplicatit chromosomes conteen twa identical copies (called chromatids or sister chromatids) jynt bi a centromere.

Eukaryotes (cells wi nuclei sic as thae foond in plants, fungi, an ainimals) possess multiple lairge linear chromosomes conteened in the cell's nucleus. Ilk chromosome haes ane centromere, wi ane or twa airms projectin frae the centromere, awtho, unner mist circumstances, thir airms are nae veesible as sic. In addeetion, maist eukaryotes hae a smaw circular mitochondrial genome, an some eukaryotes mey hae additional smaw circular or linear cytoplasmic chromosomes.

In the nuclear chromosomes o eukaryotes, the uncondensed DNA exists in a semi-ordered structur, whaur it is wrapped aroond histones (structural proteins), formin a composite material cried chromatin.

### Human chromosomes
Chromosomes in humans can be dividit intae twa teeps: autosomes (bouk chromosome(s)) an allosome (sex chromosome(s)). Certaint genetic traits are airtit tae a person's sex an are passed on throu the sex chromosomes. The autosomes conteen the rest o the genetic hereditary information. Aw act in the same wey in cell diveesion. Human cells hae 23 pairs o chromosomes (22 pairs o autosomes an ane pair o sex chromosomes), giein a tot o 46 per cell. In addeetion tae thir, human cells hae mony hunders o copies o the mitochondrial genome. Sequencin o the human genome haes providit a great deal o information aboot ilk o the chromosomes. Ablo is a table compilin stateestics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database. Nummer o genes is an estimate, as it is in pairt based on gene predeections. Tot chromosome lenth is an estimate as weel, based on the estimatit size o unsequenced heterochromatin regions.

## Karyotype

In general, the **karyotype** is the chairactereestic chromosome complement o a eukaryote species. The preparation an study o karyotypes is pairt o cytogenetics.

## Aberrations
Chromosomal aberrations are disruptions in the normal chromosomal content o a cell an are a major cause o genetic conditions in humans, sic as Down syndrome, awtho maist aberrations hae little tae na effect. Some chromosome abnormalities dae nae cause disease in cairiers, sic as translocations, or chromosomal inversions, awtho thay mey lead tae a heicher chance o beirin a bairn wi a chromosome disorder. Abnormal numbers o chromosomes or chromosome sets, cried aneuploidy, mey be lethal or mey gie rise tae genetic disorders. Genetic coonselin is offered for faimilies that mey cairy a chromosome rearrangement.

The gain or loss o DNA frae chromosomes can lead tae a variety o genetic disorders. Human ensaumples include:
* Down syndrome, the maist common trisomy, usually caused bi an extra copy o chromosome 21 (trisomy 21). Chairactereestics include decreased muscle tone, stockier big, asymmetrical skull, slantin een an mild tae moderate developmental disability.
* Edwards syndrome, or trisomy-18, the seicont maist common trisomy. Symptoms include motor retardation, developmental disability an numerous congenital anomalies causin serious heal problems. Ninety percent o thae affectit die in infancy. Thay hae chairactereestic clenched haunds an owerlaippin fingers.
* Klinefelter syndrome (XXY). Men wi Klinefelter syndrome are uisually sterile an tend tae be tawer an hae langer airms an legs nor thair peers. Boys wi the syndrome are eften shy an quiet an hae a heicher incidence o speech delay an dyslexia. Withoot testosterone treatment, some mey develop gynecomastia in puberty.

## References