User:Jordanmackenzienelson/Centromere

Article Draft
The centromere links a pair of sister chromatids together during cell division. This constricted region of chromosome connects the sister chromatids, creating a short arm (p) and a long arm (q) on the chromatids. During mitosis, spindle fibers attach to the centromere via the kinetochore.

The physical role of the centromere is to act as the site of assembly of the kinetochores – a highly complex multiprotein structure that is responsible for the actual events of chromosome segregation – i.e. binding microtubules and signaling 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 recognize 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 characterized 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 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.

Metacentric
Metacentric means that the centromere is positioned midway between the chromosome ends, resulting in the arms being relatively equal in length. When the centromeres are metacentric, the chromosomes appear to be "x-shaped."

Submetacentric
Submetacentric means that the centromere is positioned near the middle, but one chromosome arm is much shorter than the other often resulting in an L shape.

Acrocentric
An acrocentric chromosome's centromere is situated so that one of the chromosomal arms is significantly shorter than the other. The "acro-" in acrocentric refers to the Greek word for "peak." The human genome includes five acrocentric chromosomes: 13, 14, 15, 21, 22. The Y chromosome is also acrocentric.

Acrocentric arms contain genetic material and can be translocated without significant harm, as in a balanced Robertsonian translocation. However, a proportion of acrocentric p-arms in cell lines and from normal human donors do not contain detectable rDNA. 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, with a reduction of two acrocentric chromosomes in the great apes to one metacentric chromosome in humans (see aneuploidy and the human chromosome 2).

Many diseases from the result of unbalanced translocations more frequently involve acrocentric chromosomes than other non-acrocentric chromosomes. Acrocentric chromosomes are usually located in and around the nucleolus. As a result these chromosomes tend to be less densely packed than chromosomes in the nuclear periphery. Consistently, chromosomal regions that are less densely packed are also more prone to chromosomal translocations in cancers.

Telocentric
Telocentric chromosomes' centromeres are located at one end of the chromosome. Telocentric centromeres often result in the p arms being barely or not visible at all.

If the telocentric chromosome's centromere is located at the terminal end of the chromosome, then the chromosome only has one arm. Naturally occurring telocentric chromosomes with a terminal centromere are rare, but do exist. Telocentric chromosomes are not present in healthy humans. Misdivision of centromeres in normal chromosomes lead to the development of telosomes. The structure of the telosomes kinetochores determines their cytological stability.

The standard house mouse karyotype has only telocentric chromosomes.

Subtelocentric
Subtelocentric chromosomes' centromeres are located between the middle and the end of the chromosomes, but reside closer to the end of the chromosomes.

Acentric
An acentric chromosome is fragment of a chromosome that lacks a centromere. Since centromeres are the attachment point for spindle fibers in cell division, acentric fragments are not evenly distributed to daughter cells during cell division. As a result, a daughter cell will lack the acentric fragment and deleterious consequences could occur.

Chromosome-breaking events can also generate acentric chromosomes or acentric fragments.

Dicentric
A dicentric 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. The formation of dicentric chromosomes has been attributed to genetic processes, such as Robertsonian translocation and paracentric inversion. Dicentric chromosomes have important roles in the mitotic stability of chromosomes and the formation of pseudodicentric chromosomes.

Monocentric
Most chromosomes are monocentric chromosomes, meaning they only possess one centromere.

Monocentric centromeres are the most common structure on highly repetitive DNA in plants and animals.

Holocentric
Main article: Holocentric chromosome

Unlike monocentric chromosomes, in holocentric chromosomes the entire length of the chromosome acts as the centromere. In holocentric chromosomes there is not one primary constriction but the centromere has many CenH3 loci spread over the whole chromosome. 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.