Centromere is a specialized DNA sequence in a chromosome that holds together the two daughter chromatids. Centromeres represent a constricted region of the chromosome where two identical sister chromatids are most closely in contact. The centromere is the point of attachment of the kinetochore, a highly complex multiprotein structure to which the microtubules of the mitotic spindle become anchored. Kinetochore is responsible for the actual events of chromosome segregation when all chromosomes have adopted correct attachments to the spindle. Errors in centromere or kinetochore function are catastrophic for cells. Such errors can lead to aberrant division and chromosomal instability, both of which are often observed in cancerous cells.
Each chromosome has two arms, labeled p (the shorter of the two) and q (the longer). According to the positions of centromere, chromosome can be connected in telocentric, acrocentric, submetacentric, and metacentric manner. The centromere of a telocentric chromosome is located at the terminal end of the chromosome, and p arms is barely visible. The shape of telocentric chromosome is similar to the letter "i" during anaphase. The p arms of acrocentric chromosome is very short and hard to observe, but still present. The human chromosomes 13, 14, 15, 21, and 22 are acrocentric. The shape of submetacentric chromosome is "L", of which the length of p arms and q arms are unequal. The metacentric chromosomes are X-shaped, with the centromere in the middle so that the two arms of the chromosomes are almost equal. Human chromosomes 1, 3, 16, 19, and 20 are metacentric.
Figure 1. Classifications of Chromosomes.
Centromere is the major constriction of mitotic chromosomes. It is a densely packed, heterogeneous domain capped by the trilaminar kinetochore that binds to microtubules in the spindle. The centromere is a chromosomal locus in which DNA assembles into a unique multicomponent nucleoprotein complex that integrates microtubule binding and spindle mechanics with the chromatin fiber.
The peri-centromere is the physical region responsible for the geometry of bi-oriented sister kinetochores in metaphase. In budding yeast the 125 bp point centromere is sufficient to specify kinetochore assembly. The flanking region is enriched (3X) in cohesin and condensin relative to the remaining chromosome arms. The enrichment spans about 30–50 kb around each centromere. The flanking chromatin is referred as the peri-centromere in yeast. In mammals, a 5–10 Mb region dictates where the kinetochore is built. The kinetochore interacts with a very small fraction of DNA on the surface of the centromeric region. The remainder of the centromere lies between the sister kinetochores. This is typically called centromere chromatin. The chromatin sites that directly interface to microtubules cannot be identified due to the repeated sequence within the mammalian centromere. However in both yeast and mammals, the total amount of DNA between the sites of microtubule attachment in metaphase is highly conserved.
Centromere proteins (CENPs) are highly conserved across organisms. CENPA and CENPC are the most extensively studied and shown to be required to form centromeres and associated kinetochores. CENP-A is a centromere-specific form of the core nucleosome protein histone H3 and is thought to be the mammalian homolog of yeast Cse4p. Biochemical analyses reveal that CENP-A is found in a nucleosome-like particle. The centromere is differentiated from the chromosome arms at the most fundamental level of chromosome organization. CENP-A can target to centromeres in non-human cells, demonstrating that it recognizes a conserved feature of mammalian centromeres. Although CENP-A has a unique N-terminal domain unrelated to that of histone H3, this does not specify its distinctive assembly properties, rather; DNA-binding and self-assembly structures within the histone H3 homology domain are necessary for centromeric localization of CENP-A.
CENPC is a DNA-binding protein that associates with the inner-kinetochore plate, and has been shown to be essential for proper progression through mitosis and chromosome segregation. CENPC homologues have been identified in many model organisms, including yeasts, flies, plants and mammals. Loss of CENPC at the centromere has been shown in human cells to result in small or absent kinetochores. Like CENP-A, CENP-C can assemble at centromeres in a variety of vertebrate species and thus is recognizing some common feature related to kinetochore structure. CENP-C has a general DNA-binding activity and the sequences necessary for targeting CENP-C to centromeres are found adjacent to this DNA binding element in the central region of the molecule. CENPC binds two different groups of proteins that serve distinct functions: to the Mis12 complex, which is part of the KMN network of the outer kinetochore that is also needed for recruitment of checkpoint proteins; and to other CCAN components, such as CENPH, CENPI, CENPK, and CENPT43.
CENP-B was the first centromere protein cloned from any species and functions as a sequence-specific alpha satellite DNA-binding protein. A highly conserved single copy gene in mammalian cells, CENP-B has a distinctive N-terminal DNA binding domain that is functional as a monomer, but also associates to form homodimer complexes capable of binding two DNA molecules in vitro. CENP-B has been proposed to function as a cross-linker for higher order assembly of centromeric DNA because the 17bp CENP-B box sequence, the DNA-binding site for CENP-B, is iterated with high frequency in alphoid DNA and CENP-B is co-localized with alpha satellite throughout the inner heterochromatic domain of the centromere.
The CENPT–CENPW subcomplex is recruited to H3-containing centromeric chromatin and has been proposed to be an alternative to CENPC for the connection between the centromere and the kinetochore. The CENPT–CENPW subcomplex functions upstream of the CENPH subcomplex, which comprises CENPH, CENPI and CENPK. These proteins are recruited to the centromere by CENPC43. They are essential for kinetochore function in vertebrates and have been found to play a part in CENPA loading and the recruitment of other, more distal centromere complexes. The CENPO subcomplex and the similar CENPU protein are needed to prevent premature separation upon spindle damage. The CENPS subcomplex is required for proper and stable formation of the outer kinetochore, and localization of these proteins to the centromere requires CENPT or CENPK47.
Centromere plays an essential role in proper chromosome segregation during mitosis and meiosis in eukaryotic cells. Centromere function includes sister chromatid adhesion and separation, microtubule attachment, chromosome movement, establishment of heterochromatin and mitotic checkpoint control. A remarkable discovery of the past few years has been that centromeres function as sophisticated signal processing centers, regulating cell cycle progression through checkpoint controls that sense the disposition of the chromosomes and their proper assembly on the spindle. The noteworthy feature of these systems is that they appear, at least in part, to be mechanosensory pathways that convert tension exerted across the centromere into biological signals that affect global features of mitotic progression as well as local features of individual kinetochores and spindle fibers. Through their regulatory elements, centromeres act as guidance systems that coordinate and report the activities of the chromosomes throughout the critical phase of mitosis leading to the onset of anaphase. At anaphase they switch roles from signal generators to receptors as APC-mediated proteolysis severs the link between sister chromatids, unleashing them for transport to the spindle poles and the next generation.
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