Centromere - wikipedia Centromere This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. Please help to improve this article by introducing more precise citations. ( May 2011 ) In this diagram of a duplicated chromosome, ( 2 ) identifies the centromere -- the region that joins the two sister chromatids, or each half of the chromosome. In prophase of mitosis, specialized regions on centromeres called kinetochores attach chromosomes to spindle fibers. The centromere is the specialized DNA sequence of a chromosome that links a pair of sister chromatids ( a dyad ). During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first thought to be genetic loci that direct the behavior of chromosomes. The physical role of the centromere is to act as the site of assembly of the kinetochores -- a highly complex multiprotein structure that is responsible for the actual events of chromosome segregation -- i.e. binding microtubules and signalling to the cell cycle machinery when all chromosomes have adopted correct attachments to the spindle, so that it is safe for cell division to proceed to completion and for cells to enter anaphase. There are, broadly speaking, two types of centromeres. `` Point centromeres '' bind to specific proteins that recognize particular DNA sequences with high efficiency. Any piece of DNA with the point centromere DNA sequence on it will typically form a centromere if present in the appropriate species. The best characterised point centromeres are those of the budding yeast, Saccharomyces cerevisiae. `` Regional centromeres '' is the term coined to describe most centromeres, which typically form on regions of preferred DNA sequence, but which can form on other DNA sequences as well. The signal for formation of a regional centromere appears to be epigenetic. Most organisms, ranging from the fission yeast Schizosaccharomyces pombe to humans, have regional centromeres. Regarding mitotic chromosome structure, centromeres represent a constricted region of the chromosome ( often referred to as the primary constriction ) where two identical sister chromatids are most closely in contact. When cells enter mitosis, the sister chromatids ( the two copies of each chromosomal DNA molecule resulting from DNA replication in chromatin form ) are linked along their length by the action of the cohesin complex. It is now believed that this complex is mostly released from chromosome arms during prophase, so that by the time the chromosomes line up at the mid-plane of the mitotic spindle ( also known as the metaphase plate ), the last place where they are linked with one another is in the chromatin in and around the centromere. Contents 1 Positions 1.1 Metacentric 1.2 Submetacentric 1.3 Acrocentric 1.4 Telocentric 1.5 Subtelocentric 1.6 Holocentric 1.7 Human chromosomes 2 Sequence 3 Inheritance 4 Structure 5 Centromeric aberrations 6 Dysfunction and disease 7 Etymology and pronunciation 8 See also 9 References 9.1 Further reading Positions Classifications of Chromosomes I : Telocentric - centromere placement very close to the top, p arms barely visible if visible at all II : Acrocentric - q arms are still much longer than the p arms, but the p arms are longer than those in telocentric III : Submetacentric - p and q arms are very close in length but not equal IV : Metacentric - pq arms are equal in length A : Short arm ( p arm ) B : Centromere C : Long arm ( q arm ) D: Sister Chromatid Each chromosome has two arms, labeled p ( the shorter of the two ) and q ( the longer ). Many remember that the short arm ' p ' is named for the French word `` petit '' meaning ' small ', although this explanation was shown to be apocryphal. They can be connected in either metacentric, submetacentric, acrocentric or telocentric manner. Categorization of chromosomes according to the relative arms length Centromere position Arms length ratio Sign Description Medial sensu stricto 1.0 -- 1.6 Metacentric Medial region 1.7 m Metacentric Submedial 3.0 sm Submetacentric Subterminal 3.1 -- 6.9 st Subtelocentric Terminal region 7.0 Acrocentric Terminal sensu stricto ∞ Telocentric Notes -- Metacentric : M+m Atelocentric : M+m+sm+st+t Metacentric These are X-shaped chromosomes, with the centromere in the middle so that the two arms of the chromosomes are almost equal. A chromosome is metacentric if its two arms are roughly equal in length. In a normal human karyotype, five chromosomes are considered metacentric : chromosomes 1, 3, 16, 19, and 20. In some cases, a metacentric chromosome is formed by balanced translocation : the fusion of two acrocentric chromosomes to form one metacentric chromosome. Submetacentric If arms ' lengths are unequal, the chromosome is said to be submetacentric. Their shape is L shape. Acrocentric If the p ( short ) arm is so short that it is hard to observe, but still present, then the chromosome is acrocentric ( the `` acro - '' in acrocentric refers to the Greek word for `` peak '' ). The human genome includes six acrocentric chromosomes : 13, 14, 15, 21, 22 and the Y chromosome. In an acrocentric chromosome the p arm contains genetic material including repeated sequences such as nucleolar organizing regions, and can be translocated without significant harm, as in a balanced Robertsonian translocation. The domestic horse genome includes one metacentric chromosome that is homologous to two acrocentric chromosomes in the conspecific but undomesticated Przewalski 's horse. This may reflect either fixation of a balanced Robertsonian translocation in domestic horses or, conversely, fixation of the fission of one metacentric chromosome into two acrocentric chromosomes in Przewalski 's horses. A similar situation exists between the human and great ape genomes ; in this case, because more species are extant, it is apparent that the evolutionary sequence is a reduction of two acrocentric chromosomes in the great apes to one metacentric chromosome in humans ( see Karyotype # Aneuploidy ). Telocentric A telocentric chromosome 's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome, their shape is similar to letter `` i '' during anaphase. For example, the standard house mouse karyotype has only telocentric chromosomes. Humans do not possess telocentric chromosomes. Subtelocentric If the chromosome 's centromere is located closer to its end than to its center, it may be described as subtelocentric. Holocentric With holocentric chromosomes, the entire length of the chromosome acts as the centromere. Examples of this type of centromere can be found scattered throughout the plant and animal kingdoms, with the most well - known example being the nematode Caenorhabditis elegans. Human chromosomes Chromosome Centromere position ( Mbp ) Chromosome Size ( Mbp ) Centromere size ( Mbp ) 125.0 metacentric 247.2 7.4 93.3 submetacentric 242.8 6.3 91.0 metacentric 199.4 6.0 50.4 submetacentric 191.3 5 48.4 submetacentric 180.8 6 61.0 submetacentric 170.9 7 59.9 submetacentric 158.8 8 45.6 submetacentric 146.3 9 49.0 submetacentric 140.4 10 40.2 submetacentric 135.4 11 53.7 submetacentric 134.5 12 35.8 submetacentric 132.3 13 17.9 acrocentric 114.1 14 17.6 acrocentric 106.3 15 19.0 acrocentric 100.3 16 36.6 metacentric 88.8 17 24.0 submetacentric 78.7 18 17.2 submetacentric 76.1 19 26.5 metacentric 63.8 20 27.5 metacentric 62.4 21 13.2 acrocentric 46.9 22 14.7 acrocentric 49.5 X 60.6 submetacentric 154.9 Y 12.5 acrocentric 57.7 Sequence There are two types of centromeres. In regional centromeres, DNA sequences contribute to but do not define function. Regional centromeres contain large amounts of DNA and are often packaged into heterochromatin. In most eukaryotes, the centromere 's DNA sequence consists of large arrays of repetitive DNA ( e.g. satellite DNA ) where the sequence within individual repeat elements is similar but not identical. In humans, the primary centromeric repeat unit is called α - satellite ( or alphoid ), although a number of other sequence types are found in this region. Point centromeres are smaller and more compact. DNA sequences are both necessary and sufficient to specify centromere identity and function in organisms with point centromeres. In budding yeasts, the centromere region is relatively small ( about 125 bp DNA ) and contains two highly conserved DNA sequences that serve as binding sites for essential kinetochore proteins. Inheritance Since centromeric DNA sequence is not the key determinant of centromeric identity in metazoans, it is thought that epigenetic inheritance plays a major role in specifying the centromere. The daughter chromosomes will assemble centromeres in the same place as the parent chromosome, independent of sequence. It has been proposed that histone H3 variant CENP - A ( Centromere Protein A ) is the epigenetic mark of the centromere. The question arises whether there must be still some original way in which the centromere is specified, even if it is subsequently propagated epigenetically. If the centromere is inherited epigenetically from one generation to the next, the problem is pushed back to the origin of the first metazoans. Structure The centromeric DNA is normally in a heterochromatin state, which is essential for the recruitment of the cohesin complex that mediates sister chromatid cohesion after DNA replication as well as coordinating sister chromatid separation during anaphase. In this chromatin, the normal histone H3 is replaced with a centromere - specific variant, CENP - A in humans. The presence of CENP - A is believed to be important for the assembly of the kinetochore on the centromere. CENP - C has been shown to localise almost exclusively to these regions of CENP - A associated chromatin. In human cells, the histones are found to be most enriched for H4K20me3 and H3K9me3 which are known heterochromatic modifications. In the yeast Schizosaccharomyces pombe ( and probably in other eukaryotes ), the formation of centromeric heterochromatin is connected to RNAi. In nematodes such as Caenorhabditis elegans, some plants, and the insect orders Lepidoptera and Hemiptera, chromosomes are `` holocentric '', indicating that there is not a primary site of microtubule attachments or a primary constriction, and a `` diffuse '' kinetochore assembles along the entire length of the chromosome. Centromeric aberrations In rare cases in humans, neocentromeres can form at new sites on the chromosome. There are currently over 90 known human neocentromeres identified on 20 different chromosomes. The formation of a neocentromere must be coupled with the inactivation of the previous centromere, since chromosomes with two functional centromeres ( Dicentric chromosome ) will result in chromosome breakage during mitosis. In some unusual cases human neocentromeres have been observed to form spontaneously on fragmented chromosomes. Some of these new positions were originally euchromatic and lack alpha satellite DNA altogether. Centromere proteins are also the autoantigenic target for some anti-nuclear antibodies, such as anti-centromere antibodies. Dysfunction and disease It has been known that centromere misregulation contributes to mis - segregation of chromosomes, which is strongly related to cancer and abortion. Notably, overexpression of many centromere genes have been linked to cancer malignant phenotypes. Overexpression of these centromere genes is thought to increase genomic instability in cancers. Elevated genomic instability on one hand relates to malignant phenotypes ; on the other hand, it makes the tumor cells more vulnerable to specific adjuvant therapies such as certain chemotherapies and radiotherapy. Instability of centromere repetitive DNA was recently shown in cancer and aging. Etymology and pronunciation The word centromere ( / ˈsɛntrəˌmɪər / ) uses combining forms of centro - and - mere, yielding `` central part '', describing the centromere 's location at the center of the chromosome. See also Cell biology Chromatid Diploid Genetics Monopolin References Jump up ^ Alberts, Bruce ; Bray, Dennis ; Hopkin, Karen ; Johnson, Alexander ; Lewis, Julian ; Raff, Martin ; Roberts, Keith ; Walter, Peter ( 2014 ). Essential Cell Biology ( 4 ed. ). New York, NY : Garland Science. p. 183. ISBN 978 - 0 - 8153 - 4454 - 4. access - date = requires url = ( help ) Jump up ^ Pollard, T.D. 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( 2002 ). `` Initial sequencing and comparative analysis of the mouse genome ''. Nature. 420 ( 6915 ) : 520 -- 62. doi : 10.1038 / nature01262. PMID 12466850. Jump up ^ `` subtelocentric chromosome definition ''. groups.molbiosci.northwestern.edu. Retrieved 2017 - 10 - 29. Jump up ^ Margulis, Lynn ; Matthews, Clifford ; Haselton, Aaron ( 2000 - 01 - 01 ). Environmental Evolution : Effects of the Origin and Evolution of Life on Planet Earth. MIT Press. ISBN 9780262631976. Jump up ^ Dernburg, A.F. ( 2001 ). `` Here, There, and Everywhere : Kinetochore Function on Holocentric Chromosomes ''. The Journal of Cell Biology. 153 ( 6 ) : F33 -- 8. doi : 10.1083 / jcb. 153.6. F33. PMC 2192025. PMID 11402076. Jump up ^ Pluta, A.F. ; MacKay, A.M. ; Ainsztein, A.M. ; Goldberg, I.G. ; Earnshaw, W.C. ( 1995 ). `` The Centromere : Hub of Chromosomal Activities ''. Science. 270 ( 5242 ) : 1591 -- 4. doi : 10.1126 / science. 270.5242. 1591. PMID 7502067. ^ Jump up to : Mehta, G.D. ; Agarwal, M. ; Ghosh, S.K. ( 2010 ). `` Centromere Identity : a challenge to be faced ''. Mol. Genet. Genomics. 284 ( 2 ) : 75 -- 94. doi : 10.1007 / s00438 - 010 - 0553 - 4. PMID 20585957. Jump up ^ Dalal, Yamini ( 2009 ). `` Epigenetic specification of centromeres ''. Biochemistry and Cell Biology. 87 ( 1 ) : 273 -- 82. doi : 10.1139 / O08 - 135. PMID 19234541. Jump up ^ Bernad, Rafael ; Sánchez, Patricia ; Losada, Ana ( 2009 ). `` Epigenetic specification of centromeres by CENP - A ''. Experimental Cell Research. 315 ( 19 ) : 3233 -- 41. doi : 10.1016 / j. yexcr. 2009.07. 023. PMID 19660450. Jump up ^ Chueh, A.C. ; Wong, LH ; Wong, N ; Choo, KH ( 2004 ). `` Variable and hierarchical size distribution of L1 - retroelement - enriched CENP - A clusters within a functional human neocentromere ''. Human Molecular Genetics. 14 ( 1 ) : 85 -- 93. doi : 10.1093 / hmg / ddi008. PMID 15537667. Jump up ^ Rosenfeld, Jeffrey A ; Wang, Zhibin ; Schones, Dustin E ; Zhao, Keji ; Desalle, Rob ; Zhang, Michael Q ( 2009 ). `` Determination of enriched histone modifications in non-genic portions of the human genome ''. BMC Genomics. 10 : 143. doi : 10.1186 / 1471 - 2164 - 10 - 143. PMC 2667539. PMID 19335899. Jump up ^ Volpe, T.A. ; Kidner, C ; Hall, IM ; Teng, G ; Grewal, SI ; Martienssen, RA ( 2002 ). `` Regulation of Heterochromatic Silencing and Histone H3 Lysine - 9 Methylation by RNAi ''. Science. 297 ( 5588 ) : 1833 -- 7. doi : 10.1126 / science. 1074973. PMID 12193640. Jump up ^ Marshall, Owen J. ; Chueh, Anderly C. ; Wong, Lee H. ; Choo, K.H. Andy ( 2008 ). `` Neocentromeres : New Insights into Centromere Structure, Disease Development, and Karyotype Evolution ''. The American Journal of Human Genetics. 82 ( 2 ) : 261 -- 82. doi : 10.1016 / j. ajhg. 2007.11. 009. PMC 2427194. PMID 18252209. Jump up ^ Warburton, Peter E. ( 2004 ). `` Chromosomal dynamics of human neocentromere formation ''. Chromosome Research. 12 ( 6 ) : 617 -- 26. doi : 10.1023 / B : CHRO. 0000036585.44138. 4b. PMID 15289667. Jump up ^ Zhang, W. ; Mao, J-H. ; Zhu, W. ; Jain, A.K. ; Liu, L. ; Brown, J.B. ; Karpen, G.H. ( 2016 ). `` Centromere and kinetochore gene misexpression predicts cancer patient survival and response to radiotherapy and chemotherapy ''. Nature Communications. 7 : 12619. doi : 10.1038 / ncomms12619. PMC 5013662. PMID 27577169. Jump up ^ Giunta, S ; Funabiki, H ( 21 February 2017 ). `` Integrity of the human centromere DNA repeats is protected by CENP - A, CENP - C, and CENP - T ''. Proceedings of the National Academy of Sciences of the United States of America. 114 ( 8 ) : 1928 -- 1933. doi : 10.1073 / pnas. 1615133114. PMC 5338446. PMID 28167779. Jump up ^ `` Centromere ''. Merriam - Webster Dictionary. Jump up ^ `` Centromere ''. Dictionary.com Unabridged. Random House. Further reading Mehta, G.D. ; Agarwal, M. ; Ghosh, S.K. ( 2010 ). `` Centromere Identity : a challenge to be faced ''. Mol. Genet. Genomics. 284 ( 2 ) : 75 -- 94. doi : 10.1007 / s00438 - 010 - 0553 - 4. PMID 20585957. Lodish, Harvey ; Berk, Arnold ; Kaiser, Chris A. ; Krieger, Monty ; Scott, Matthew P. ; Bretscher, Anthony ; Ploegh, Hiddle ; Matsudaira, Paul ( 2008 ). Molecular Cell Biology ( 6th ed. ). New York : W.H. Freeman. ISBN 978 - 0 - 7167 - 7601 - 7. Nagaki, Kiyotaka ; Cheng, Zhukuan ; Ouyang, Shu ; Talbert, Paul B ; Kim, Mary ; Jones, Kristine M ; Henikoff, Steven ; Buell, C Robin ; Jiang, Jiming ( 2004 ). `` Sequencing of a rice centromere uncovers active genes ''. Nature Genetics. 36 ( 2 ) : 138 -- 45. doi : 10.1038 / ng1289. PMID 14716315. Lay summary -- Science Daily ( January 13, 2004 ). Wikimedia Commons has media related to Centromere. Cytogenetics : chromosomes Basic concepts Karyotype Ploidy Genetic material / Genome Chromatin Euchromatin Heterochromatin Chromosome Chromatid Nucleosome Nuclear organization Types Autosome / Sex chromosome ( or allosome or heterosome ) Macrochromosome / Microchromosome Circular chromosome / Linear chromosome Extra chromosome ( or accessory chromosome ) Supernumerary chromosome A chromosome / B chromosome Lampbrush chromosome Polytene chromosome Dinoflagellate chromosomes Homologous chromosome Isochromosome Satellite chromosome Centromere position Metacentric Submetacentric Telocentric Acrocentric Holocentric Centromere number Acentric Monocentric Dicentric Polycentric Processes and evolution Mitosis Meiosis Structural alterations Chromosomal inversion Chromosomal translocation Numerical alterations Aneuploidy Euploidy Polyploidy Paleopolyploidy Polyploidization Structures Telomere : Telomere - binding protein ( TINF2 ) Protamine Histone H1 H2A H2B H3 H4 Centromere C1 C2 Q See also Extrachromosomal DNA Plasmid List of organisms by chromosome count List of chromosome lengths for various organisms List of sequenced genomes International System for Human Cytogenetic Nomenclature Antigens : Autoantigens Dehydrogenase Branched - chain alpha - keto acid dehydrogenase complex Oxoglutarate dehydrogenase Pyruvate dehydrogenase Transglutaminase Epidermal transglutaminase Tissue transglutaminase Nucleoporins NUP35 NUP37 NUP43 NUP50 NUP54 NUP62 NUP85 NUP88 NUP93 NUP98 NUP107 NUP133 NUP153 NUP155 NUP160 NUP188 NUP210 NUP205 NUP214 Other Acetylcholine receptor Actin Apolipoprotein H Cardiolipin Centromere Filaggrin ( Citrullinate ) Gangliosides Sp100 nuclear antigen Thrombin Topoisomerase Retrieved from `` https://en.wikipedia.org/w/index.php?title=Centromere&oldid=828793041 '' Categories : Chromosomes Hidden categories : Pages using citations with accessdate and no URL Articles lacking in - text citations from May 2011 All articles lacking in - text citations Talk Contents About Wikipedia Afrikaans Беларуская Català Čeština Dansk Deutsch Español Euskara فارسی Français Gaeilge Galego 客家 語 / Hak - kâ - ngî Hrvatski Bahasa Indonesia Italiano עברית ქართული Қазақша Kreyòl ayisyen Kurdî Кыргызча Magyar Македонски Nederlands 日本 語 Norsk Norsk nynorsk Occitan Polski Português Русский Scots Simple English Slovenčina Српски / srpski Srpskohrvatski / српскохрватски Suomi Svenska Tagalog Türkçe Українська 中文 35 more Edit links This page was last edited on 4 March 2018, at 20 : 07. 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Question: what is the major constituent of a chromosome
Answer: centromere