||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (May 2011) (Learn how and when to remove this template message)|
The centromere is the part of a chromosome that links sister chromatids or a dyad. During mitosis, spindle fibers attach to the centromere via the kinetochore. Centromeres were first defined as genetic loci that direct the behavior of chromosomes.
The physical role of the centromere is to act as the site of assembly of the kinetochore - 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 recognise 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 (Deoxyribonucleic acid ) 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.
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.
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.
If arms' lengths are unequal, the chromosome is said to be submetacentric.
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).
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 telocentric chromosomes. Humans do not possess telocentric chromosomes.
If the chromosome's centromere is located closer to its end than to its center, it may be described as subtelocentric.
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.
|Chromosome||Centromere position (Mbp)||Chromosome Size (Mbp)||Centromere size (Mbp)|
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.
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.
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.
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.
|Wikimedia Commons has media related to Centromere.|