- Concepts
- Centromere characteristics
- Position
- Telocentric chromosomes
- Acrocentric chromosomes
- Metacentric chromosomes
- Function
- References
The centromeres are a fundamental chromosomal structures that hold together the sister chromatids during cell division. In addition, these are the place where the microtubules of the spindle join to separate the chromosomes at the end of cell division.
Centromeres were first described in 1882 by the physician and researcher Walther Flemming (1843-1905), when he carried out a detailed characterization of cell division.
Basic structure of a chromosome, with the centromere in the center of it. Source: lifeder.com
Centromeres are also known as "adhesion regions" or "kinetochores." However, it is now known that these are the binding sites of the complex of DNA and proteins that form the kinetochore.
Concepts
The function of the centromere in all living beings is the same, but each species shows unique characteristics, and there may be interspecific differences in terms of structure, size and complexity.
Graphic representation of a human centromere (Source: Silvia3 via Wikimedia Commons) The DNA that is part of the centromeres undergoes constant modifications (evolves), which implies that significant differences are found between species, even when these are evolutionarily very close.
For scientists, the study of the centromere is not an easy task, since in plants and animals these “structure” or “regions” are contained in portions of the satellite-like genome (highly repetitive), making it difficult to map using techniques of conventional sequencing.
Mutations in the centromeric region have serious physiological implications in humans. Abnormalities in its structure and functions are lethal or associated with congenital and acquired diseases, with cancer, infertility, and birth disorders.
Centromere characteristics
Centromeres are portions of chromosomes that contain highly repetitive regions of DNA in the form of heterochromatin. These regions are specialized for the attachment and segregation of sister chromatids during cell division.
Generally, centromeres contain the oldest DNA sequences, ordered in succession and close to the border between heterochromatin and euchromatin, that is, centromeres are highly heterochromatic regions.
Centromeric sequences are regularly classified into two types: satellite DNA and transposable elements. Both types of sequence represent most of the DNA contained in the centromeres.
Organization of DNA in the centromeric regions of different species (Source: Gouttegd via Wikimedia Commons) At present, centromeres are considered as complex structures composed of genomic DNA, which is subjected to different epigenetic processes.
Since the centromeres are a chromatin portion of the chromosomes, they are formed by a complex of DNA and histone proteins that favor their "packaging".
The nucleosomes of the centromeric regions, however, do not possess the histone H3 protein; instead they have a variant that those skilled in the art have identified as, which is centromere specific.
This histone-like protein varies considerably between different species. In mammals this is known as CENP-A, in arthropods it is called CID and in fungi and yeasts it has been called Cse4.
Thanks to the specific differences of the CENH3 protein in the centromeres, its characteristics and properties are used for the identification of species, especially of the centromeric region in the chromosomes.
Position
The location of the centromere on the chromosomes is visualized in karyotypes as a "narrowing", which is generally called a "primary constriction."
In some organisms the centromeres are not found in one region, but are rather "diffuse", so that the fibers of the spindle can join along the entire chromosome. These chromosomes are known as diffuse centromere.
Diagram of a holocentric or diffuse centromere chromosome and another metacentric chromosome (also known as “monocentric”, since they have only one centromere) (Source: Mandrioli & Manicardi via Wikimedia Commons) The position of the centromere indicates the shape that the chromosome will take during the division of the nucleus. If the centromere is at the midpoint of a chromosome, it will take the shape of a “V” as it is segregated toward opposite poles of the dividing cell.
On the contrary, if the centromere is near one of the ends of a chromosome, this, when separating from its sister chromatid, will have a "J" shape during segregation. Likewise, if the centromere is located at the ends of a chromosome, the separation will give it a “rigid rod” appearance.
It is important to mention that the position of the centromere on a chromosome indicates the relationship between the lengths of its two arms (the short or "p" and the long or "q"). This relationship is quite specific for each type of chromosome.
According to the position of the centromere, three types of chromosomes are recognized:
Chromosome types and location of the centromere. A: short arm (p). B: centromere. C. Long arm (q). D: sister chromatid. I-Telocentric: the centromere is near the top. Arms p somewhat visible. II-Acrocentric: q arms are longer than p arms, but these are longer than in telocentrics. III-Submetacentric: arms p and q are similar in length but not equal. IV-Metacentric: q and p arms equal in length. Fockey003
Telocentric chromosomes
These chromosomes have the centromere at the end of one of the two chromatin “arms”. They are those that move in the form of rigid rods during segregation towards the poles in cell division.
Acrocentric chromosomes
In this type of chromosomes, the centromere is shown to be shifted more towards one of the ends than towards the other. When the cell divides and the chromosomes separate, the acrocentric chromosomes are the ones that acquire a "J" shape.
Metacentric chromosomes
Metacentric chromosomes have centromeres located throughout the center of the chromosome, separating two arms of equal length. Due to the location of their centromere, metacentric chromosomes secrete in a V shape during anaphase of cell division.
Function
Centromeres are the universal means for the effective secretion of chromosomes in all eukaryotic organisms. They are the attachment sites for microtubules to exert the precise mechanical force to separate chromosomes or chromatids during meiosis or mitosis.
The specific functions of the centromere are the adhesion and separation of sister chromatids, the fixation of microtubules, the movement of chromosomes during segregation towards daughter cells, the establishment of heterochromatin and, in addition, represent a checkpoint of mitosis.
In mammals, CENP-like proteins are found in the centromere heterochromatin. These can be of three types CENP-A, CENP-B and CENP-C, all of them participating in the assembly of the kinetochore.
The absence of the CENP-C protein can cause serious errors in the segregation of chromosomes, since this is a protein that has DNA-binding and “self-association” properties and is directly related to the segregation of chromosomes and kinetochore malfunction.
It is currently known that some regions of the centromeres are transcriptionally active. These encode for small interference RNAs, which participate in the transcriptional silencing of some regions of the genome.
These small double-band RNA transcripts from the pericentromeric regions are essential for the assembly of heterochromatin and are the transcriptional regions to regulate the steps prior to cell division.
References
- Choo, KA (1997). The centromere (Vol. 320). Oxford: Oxford University Press.
- Fincham, JRS (2001). Centromere.
- Fukagawa, T., & Earnshaw, WC (2014). The centromere: chromatin foundation for the kinetochore machinery. Developmental cell, 30 (5), 496-508.
- Henikoff, S., Ahmad, K., & Malik, HS (2001). The centromere paradox: stable inheritance with rapidly evolving DNA. Science, 293 (5532), 1098-1102.
- Plohl, M., Meštrović, N., & Mravinac, B. (2014). Centromere identity from the DNA point of view. Chromosome, 123 (4), 313-325.
- Westhorpe, FG, & Straight, AF (2015). The centromere: epigenetic control of chromosome segregation during mitosis. Cold Spring Harbor perspectives in biology, 7 (1), a015818.