Centromere
Chromosomal components:
(1) Chromatid
(2) Centromere
(3) Short arm
(4) Long arm

A centromere is a region of DNA typically found near the middle of a chromosome where two identical sister chromatids come closest in contact. It is involved in cell division as the point of mitotic spindle attachment. The sister chromatids are attached all along their length, but they are closest at the centromere.[1]

Contents

Centromere positions

Each chromosome has two arms, labeled p (the shorter of the two) and q (the longer). The p arm is named for "petit" meaning 'small'; the q arm is named q simply because it follows p in the alphabet. (According to the NCBI, "q" refers to the French word "queue" meaning 'tail'.) They can be connected in either metacentric, submetacentric, acrocentric or telocentric manner.[citation needed]

Metacentric

A chromosome is metacentric if its two arms are roughly equal in length. In some cases, a metacentric chromosome is formed by balanced translocation: the fusion of two acrocentric chromosomes to form one metacentric chromosome.[citation needed]

Submetacentric

If arms' lengths are unequal, the chromosome is said to be submetacentric.[citation needed]

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 five acrocentric chromosomes: 13, 14, 15, 21 and 22.[citation needed]

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.[2] 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#Historical note).[citation needed]

Telocentric

A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. For example, the standard house mouse karyotype has only acrocentric chromosomes.[3][4] Humans do not possess telocentric chromosomes. Some authors[who?] denote extreme acrocentric chromosomes as telocentric- 21, 22, Y.[citation needed]

Subtelocentric

If the chromosome's centromere is located closer to its end than to its center, it may be described as subtelocentric.[citation needed]

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,[5] with the most well known example being the nematode Caenorhabditis elegans.

The centromeric sequence

There are two types of centromeres.[6] 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 has no defined DNA sequence. It typically 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.[7] 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.[8] 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.[9] 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[10] which are known heterochromatic modifications.

In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi.[11] 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.[12][13] The formation of a neocentromere must be coupled with or followed or proceeded by the inactivation of the 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.

Related links

References

  1. ^ "Sister chromatid cohesion". Genetics Home Reference. United States National Library of Medicine. May 15, 2011. http://ghr.nlm.nih.gov/glossary=sisterchromatidcohesion. 
  2. ^ Myka, J.L.; Lear, T.L.; Houck, M.L.; Ryder, O.A.; Bailey, E. (2003). "FISH analysis comparing genome organization in the domestic horse (Equus caballus) to that of the Mongolian wild horse (E. przewalskii)". Cytogenetic and Genome Research 102 (1–4): 222–5. doi:10.1159/000075753. PMID 14970707. 
  3. ^ Silver, Lee M. (1995). "Karyotypes, Chromosomes, and Translocations". Mouse Genetics: Concepts and Applications. Oxford: Oxford University Press. pp. 83–92. ISBN 978-0-19-507554-0. http://www.informatics.jax.org/silver/chapters/5-2.shtml. 
  4. ^ Chinwalla, Asif T.; Cook, Lisa L.; Delehaunty, Kimberly D.; Fewell, Ginger A.; Fulton, Lucinda A.; Fulton, Robert S.; Graves, Tina A.; Hillier, Ladeana W. et al. (2002). "Initial sequencing and comparative analysis of the mouse genome". Nature 420 (6915): 520–62. doi:10.1038/nature01262. PMID 12466850. 
  5. ^ 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2192025. 
  6. ^ 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. 
  7. ^ Dalal, Yamini (2009). "Epigenetic specification of centromeres". Biochemistry and Cell Biology 87 (1): 273–82. doi:10.1139/O08-135. PMID 19234541. 
  8. ^ 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. 
  9. ^ 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. 
  10. ^ 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. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=2667539. 
  11. ^ 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. 
  12. ^ 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. 
  13. ^ 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. 

Further reading

  • 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 et al. (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). 

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Look at other dictionaries:

  • Centromere — Centromère Chromosome. (1) Chromatide. Chacune formée d un brin d ADN parental et d un brin d ADN néoformé obtenu après réplication durant la phase S ; (2) Centromère. Le point de contact des deux chromatides, et le point de séparation lors… …   Wikipédia en Français

  • centromère — [ sɑ̃trɔmɛr ] n. m. • 1973; de centro et mère ♦ Biol. Petit granule ne fixant pas les colorants, situé en général au centre du chromosome, auquel le fuseau se fixe lors de la division cellulaire. ● centromère nom masculin Constriction présente… …   Encyclopédie Universelle

  • centromere — n. (Biol.) a specialized condensed region of a chromosomes that appears during mitosis where the chromatids are held together to form an X shape. [WordNet 1.5] …   The Collaborative International Dictionary of English

  • centromere — centromere. См. центромера. (Источник: «Англо русский толковый словарь генетических терминов». Арефьев В.А., Лисовенко Л.А., Москва: Изд во ВНИРО, 1995 г.) …   Молекулярная биология и генетика. Толковый словарь.

  • centromere — [sen′trō mir΄] n. [ CENTRO + MERE] a small, nonstaining structure, usually near the center of a chromosome to which the spindle fiber attaches during mitosis centromeric [sen′trōmer′ik, sen′trōmir′ik] adj …   English World dictionary

  • Centromère — Chromosome. (1) Chromatide. Chacune formée d un brin d ADN parental et d un brin d ADN néoformé obtenu après réplication durant la phase S ; (2) Centromère. Le point de contact des deux chromatides, et le point de séparation lors de la… …   Wikipédia en Français

  • Centromere — The cinched waist of the chromosome essential for the division and the retention of the chromosome in the cell. The centromere is a uniquely specialized region of the chromosome to which spindle fibers attach during cell division. The human… …   Medical dictionary

  • centromere — The region in eukaryote chromosomes where daughter chromatids are joined together. The kinetochore, to which the spindle chromosomes are attached, lies adjacent to the centromere. The centromeric DNA codes for the kinetochore …   Dictionary of molecular biology

  • centromere — centromeric /sen treuh mer ik, mear /, adj. /sen treuh mear /, n. Cell Biol. a specialized structure on the chromosome, appearing during cell division as the constricted central region where the two chromatids are held together and form an X… …   Universalium

  • centromere — centromera statusas T sritis augalininkystė apibrėžtis Pirminė chromosomos persmauga, prie kurios prisitvirtina achromatinės verpstės siūlai. atitikmenys: angl. centromere; kinetochore rus. кинетохор; центромера …   Žemės ūkio augalų selekcijos ir sėklininkystės terminų žodynas

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