What is Telocentric The centromere is the special DNA sequence of a chromosome that links a pair of sister chromatids (a group of two). [1] During mitosis , spindle fibers attach to the centromere via the kinetochore . [2] The centromere was previously thought to be the genetic locus that dictates the behavior of chromosomes .
The physiological role of the centromere is to act as the site of assembly of the centromeres . Connected to the signaling spindle , so that it is safe for cells to proceed and enter anaphase for the completion of cell division .
Broadly there are two types of centromere. “Point centromeres” bind to specific proteins that recognize specific DNA sequences with high efficiency . [4] Any piece of DNA that has a point centromere DNA sequence usually forms a centromere if present in the appropriate species. The best characterized point centromeres are those of the emerging yeast, Saccharomyces cerevisiae . The term “regional centromere” has been coined to describe most of the centromere, which usually form on regions of the preferred DNA sequence, but which can also form on other DNA sequences. [4] Epigenetic signaling of the formation of a regional centromere appears to be . Most organisms from the fission yeast Schizosaccharomyces pombe to humans have regional centromeres.
With respect to mitotic chromosome structure, the centromere represents a narrower region of the chromosome (often called the primary constriction) where two identical sister chromatids are most closely in contact. When cells enter mitosis, sibling chromatids ( two copies of each chromosomal DNA molecule as a result of 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 the chromosome arms during prophase, so that by the time the chromosomes are lined up at the mid-plane of the mitotic spindle (also known as the metaphase plate), the final location Where they are attached occurs with each other in the chromatin and around the centromere.
Post
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. [6] They can be combined in a metacentric, submetacentric, acrocentric or telocentric manner.
classification of chromosomes
| I | telocentric | The centromere placement is very close to the top, the p arms are barely visible if visible at all. |
| Second | Anterior | The q arms are still much longer than the p arms, but the p arms are longer than the telocentric ones. |
| third | submetacentric | The sides p and q are very close in length but not equal. |
| IV | metacentric | The lengths of the sides p and q are equal. |
A : Short arm (P arm)
B : Centromere
C : Long arm (Q arm)
D : Sister chromatids
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 combined in a metacentric, submetacentric, acrocentric or telocentric manner.
| Classification of chromosomes according to relative arm length | |||
| centromere position | Arms Length Ratio | clue | description |
| medial sensu stricto | 1.0 – 1.6 | I | metacentric |
| central zone | १.७ | I | metacentric |
| submedial | 3.0 | SM | submetacentric |
| subterminal | 3.1 – 6.9 | scheduled tribe | subtelocentric |
| terminal area | 7.0 | Tea | Anterior |
| terminal sensu stricto | ∞ | Tea | 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 chromosome are approximately equal.
A chromosome is metacentric if its two arms are approximately equal in length. In a normal human karyotype , five chromosomes are considered metacentric: 1 , 3 , 16 , 19 , 20 . In some cases, a metacentric chromosome is formed by balanced translocation: the fusion of two acrocentric chromosomes to form a metacentric chromosome.
Submetacentric
If the arm lengths are unequal, then the chromosome is said to be centromeric. They are L shaped. [11 1]
Anterior
If the p (short) arm is so short that it is hard to see, but still present, then the chromosome is acrocentric (“acro-” in acrocentric refers to the Greek word for “peak”). The human genome: contains five acrosome chromosomes 13, 14, 15, 21, 22. [12] The Y chromosome is also centromeric. [12]
The P arm in an acrocentric chromosome contains genetic material that includes repeated sequences such as nuclear organizing regions, and can be translated without loss, as in a balanced Robertsonian translocation. The domestic horse genome includes a metacentric chromosome that is homologous to the two procentric chromosomes in the conspecific but undomesticated Przewalski horse. [13]This may reflect either the determination of a balanced Robertsonian translocation in domestic horses or, conversely, the determination of one metacentric chromosome to two acrocentric chromosomes in Przewalski’s horses. A similar situation exists between the human and great ape genomes, with great apes lacking two acrocentric chromosomes and one metacentric chromosome in humans (see aneuploidy and human chromosome 2). [11 1]
Strikingly, deleterious translocations in the context of disease, particularly unbalanced translocations in blood cancers, involve acrocentric chromosomes more frequently than non-acrocentric chromosomes. [14] Although the cause is not known, it is probably related to the physical location of the acrocentric chromosomes within the nucleus. Acrocentric chromosomes are usually located in and around the nucleolus, therefore in the center of the nucleus, where chromosomes are less dense than chromosomes at the nuclear periphery. [15] Consistent, low-density chromosomal regions are also more prone to chromosomal translocations in cancer.
Telocentric
The centromere of a telocentric chromosome is located at the end of the chromosome. Therefore a telocentric chromosome has only one arm. Telomeres can extend from both ends of the chromosome, their shape is similar to the letter “i” during anaphase. For example, the standard house mouse karyotype has only telocentric chromosomes. [16] [17] Humans do not have telocentric chromosomes.
Subtelocentric
If the centromere of a chromosome is located closer to its end than to its centre, it can be described as subtelocentric. [18] [19]
Centromere number
Acentric
If a chromosome lacks a centromere, it is called acentric. The ciliates of the macronucleus for example contain hundreds of acentric chromosomes. [20] Chromosome-breaking events can also generate acentric chromosomes or acentric fragments.
Dicentric
A bicentral chromosome is an abnormal chromosome with two centromeres. It is formed through the fusion of two chromosome segments, each with a centromere, resulting in the loss of acentric fragments (lacking a centromere) and the formation of dicentric fragments. [21] The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation [12] and paracentric inversion. [22] Binucleated chromosomes play an important role in mitotic stability of chromosomes and in the formation of pseudocentric chromosomes. [23]
Monocentric
Monocentric chromosome A chromosome has only one centromere in a chromosome and forms a narrow constriction.
The monocentric centromere is the most common structure on highly repetitive DNA in plants and animals. [24]
Holocentric
In holocentric chromosomes, unlike monocentric chromosomes, the entire length of the chromosomes serves as the centromere. Holocentric chromosomes do not have a primary constriction, but the centromere contains several CENH3 loci that are spread throughout the chromosome. [25] Examples of this type of centromere can be found scattered throughout the plant and animal kingdoms, [26] the most famous example being the nematode Caenorhabditis elegans .
Polycentric
Human chromosome
| chromosome | centromere position (MBP) | square | Chromosome size (MBP) | Centromere Size (Mbp) |
|---|---|---|---|---|
| 1 | 125.0 | metacentric | 247.2 | 7.4 |
| 2 | 93.3 | submetacentric | 242.8 | 6.3 |
| 3 | 91.0 | metacentric | १९९.४ | 6.0 |
| 4 | 50.4 | submetacentric | 191.3 | – |
| 5 | 48.4 | submetacentric | १८०.८ | – |
| 6 | 61.0 | submetacentric | १७०.९ | – |
| 7 | 59.9 | submetacentric | १५८.८ | – |
| 8 | 45.6 | submetacentric | 146.3 | – |
| 9 | 49.0 | submetacentric | १४०.४ | – |
| 10 | 40.2 | submetacentric | १३५.४ | – |
| 1 1 | 53.7 | submetacentric | १३४.५ | – |
| 12 | 35.8 | submetacentric | १३२.३ | – |
| १३ | 17.9 | Anterior | 114.1 | – |
| 14 | 17.6 | Anterior | 106.3 | – |
| 15 | 19.0 | Anterior | 100.3 | – |
| 16 | 36.6 | metacentric | 88.8 | – |
| 17 | 24.0 | submetacentric | 78.7 | – |
| १८ | 17.2 | submetacentric | 76.1 | – |
| 19 | 26.5 | metacentric | 63.8 | – |
| 20 | २७.५ | metacentric | 62.4 | – |
| 21 | 13.2 | Anterior | 46.9 | – |
| 22 | 14.7 | Anterior | 49.5 | – |
| x | ६०.६ | submetacentric | 154.9 | – |
| You | 12.5 | Anterior | 57.7 | – |
Sequence
There are two types of centromere. [27] In the regional centromere, DNA sequences contribute but do not define function. The regional centromere contains large amounts of DNA and is often packaged in heterochromatin. In most eukaryotes, the DNA sequence of the centromere consists of large arrays of repetitive DNA (such as satellite DNA), where the sequence within individual repeat elements is similar but not identical. In humans, the primary centromeric repeat unit is called the α-satellite (or alphoid), although many other sequence types are found in this region. [28]
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 short (about 125 bp of DNA) and contains two highly conserved DNA sequences that serve as binding sites for essential kinetochore proteins. [28]
Heritage
Since the centromeric DNA sequence is not the major determinant of centromeric identity in metazoans, it is thought that epigenetic inheritance plays a major role in specifying the centromere. [29] The offspring will assemble the centromere at the same location as the parent chromosome, independent of chromosome order. It has been proposed that the histone H3 variant CENP-A (centromere protein A) is the epigenetic mark of the centromere. [30] The question arises whether there must still be some original way in which the centromere is specified, even though it may have been later propagated indigenously. If the centromere is genetically inherited from one generation to the next, the problem is pushed back to the origin of the first metazoans.
Structure
Centromeric DNA is normally in a heterochromatin state, which is required for the recruitment of the cohesin complex that mediates sister chromatid cohesion following DNA replication as well as coordinating sister chromatid separation during anaphase. In this chromatin, the normal histone H3 is replaced by a centromere-specific variant, CENP-A in humans. [31] The presence of CENP-A is believe to be important for the assembly of the kinetochore at the centromere. CENP-C has been shown to localize almost exclusively in these regions of CENP-A associated chromatin. In human cells, H4K20me 3 and H3K9me3 [32]Histones are found to be most enriched for what are known as heterochromatic modifications, in Drosophila, islands of retroelements are major components of the centromere. [33]
In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is associate with RNAi, in nematodes such as Caenorhabditis elegans , some plants, and insect orders Lepidoptera and Hemiptera, chromosomes are “holocentric”, indicating that there is no primary site of microtubule attachment or primary constriction, and a “diffuse” kinetochore assemblage. along the entire length of the chromosome.
Centromeric aberration
In rare cases, neocentromeres can form at new sites on the chromosome as a result of centromere repositioning. This phenomenon is best known from human clinical studies and there are currently over 90 known human neocentromers identified on 20 different chromosomes. [35] [36]The formation of a neocentromere must be combined with inactivation of the previous centromere, as chromosomes containing two functional centromeres (dicentric chromosomes) will result in chromosome breakage during mitosis. In some unusual cases, the human neocentromere has been observed to form spontaneously on fragmented chromosomes. Some of these new positions were euchromatic in origin and lacked alpha satellite DNA altogether. Neocentromeres lack the repetitive structure seen in normal centromeres suggesting that centromere formation is predominantly epigenetically controlled. [37] [38]Over time a neocentromere can accumulate repetitive elements and form what is known as an evolutionary new centromere. There are several well-known examples in primate chromosomes where the position of the centromere differs from that of the human centromere of the same chromosome and is considered an evolutionary new centromere. [37] Centromere repositioning and the formation of evolutionary new centromeres have been suggested to be a mechanism of speciation. [39]
Centromere proteins are also autoantigenic targets for some anti-nuclear antibodies, such as anti-centromere antibodies.
Dysfunction and disease
It is known that centromere misregulation contributes to the incorrect segregation of chromosomes, which is strongly related to cancer and miscarriage. Notably, overexpression of several centromere genes has been linked to a cancer malignant phenotype. Overexpression of these centromere genes may lead to increased genomic instability in cancer. On the one hand the elevated genomic instability is related to the lethal phenotype; On the other hand, it makes tumor cells more sensitive to specific adjuvant therapies, such as certain chemotherapies and radiotherapy. [40] Instability of DNA containing centromere repeats was recently shown in cancer and aging.
Etymology and pronunciation
word centromere uses the combination of forms centro- and -mere , describing the location of the centromere in the center of the chromosome yielding the “middle part”.
