(1) Chromatid. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division One of the two identical parts of the chromosome after S phase.
(2) Centromere. The point where the two chromatids touch.
(3) Short arm.
(4) Long arm.
Because the arms are unequal, this chromosome is called "submetacentric. "

The centromere is a region, often found in the middle of the chromosome, involved in cell division and the control of gene expression. A chromosome is an organized structure of DNA and Protein that is found in cells. Gene expression is the process by which inheritable information from a Gene, such as the DNA sequence, is made into a functional Gene product, such

Function

The centromere is, together with telomeres and origin of replications, one of the essential parts of any eukaryotic chromosomes. A telomere is a region of repetitive DNA at the end of Chromosomes which protects the end of the chromosome from destruction The centromere is usually defined by specific DNA sequences which are in higher eukaryotes typical tandem repetitive sequences, often called "satellite DNA". These sequences bind specific proteins called "cen"-Proteins. During mitosis the centromeres can be identified in particular during the metaphase stage as a constriction at the chromosome. At this centromeric constriction the two mostly identical halves of the chromosome, the sister chromatids, are held together until late metaphase. A chromatid is one of two identical copies of DNA making up a replicated Chromosome, which are joined at their Centromeres for the process of Cell division During mitotic division, a transient structure called kinetochore is formed on top of the centromeres. The kinetochore (pronounced kin et' o core is the protein structure on Chromosomes where the Spindle fibers attach during division to pull the chromosomes apart The kinetochores are the sites where the spindle fibers attach. In Cell biology, the spindle apparatus (also called spindle fibers) is the structure that separate the Chromosomes into the daughter cells during Kinetochores and the spindle apparatus are responsible for the movement of the two sister chromatids to opposite poles of dividing cell nucleus during anaphase. Usually the mitosis is immediately followed by a cell division cytokinesis. Cytokinesis is the process whereby the Cytoplasm of a single Eukaryotic cell is divided to form two daughter cells However, mitosis and cytokinesis are separate processes and can be uncoupled.

A centromere functions in sister chromatid adhesion, kinetochore formation, and pairing of homologous chromosomes. The kinetochore (pronounced kin et' o core is the protein structure on Chromosomes where the Spindle fibers attach during division to pull the chromosomes apart Homologous chromosomes are Chromosomes in a Biological cell that pair ( synapse) during Meiosis, or alternatively non-identical chromosomes that

A centromere is the region where sister chromatids join in the double chromosomal structure during mitosis, prophase and metaphase. Mitosis is the process in which a Eukaryotic cell separates the Chromosomes in its Cell nucleus, into two identical sets in two daughter nuclei Prophase is a stage of Mitosis in which the Chromatin condenses into a highly ordered structure called a Chromosome Metaphase from the Ancient Greek μετά (after and φάσις (stage is a stage of Mitosis in the eukaryotic Cell cycle in which The centromere is also where kinetochore formation takes place: proteins bind on the centromeres that form an anchor point for the spindle formation required for the pull of chromosomes toward the centrioles during the anaphase and telophase of mitosis. A Centriole is a barrel shaped Organelle found in most animal Eukaryotic cells though absent in Higher plants and Fungi. Anaphase, from the Ancient Greek ἀνά (up and φάσις (stage is the stage of Mitosis when Chromosomes Telophase (sometimes spelled telephase) from the Ancient Greek "τελος" (end and "φασις" (stage is a stage in either Meiosis

When the centromere doesn't function properly, the chromosomes don't align and separate properly, resulting in the wrong number of chromosomes in the daughter cells (aneuploidy), and conditions such as Down syndrome, if the cells survive at all. Aneuploidy is defined as an abnormal number of Chromosomes Syndromes caused by an extra or missing chromosome are among the most widely recognized Genetic disorders Down syndrome, Down's syndrome, or trisomy 21 is a Chromosomal disorder caused by the presence of all or part of an extra 21st chromosome. ^[1]

Centromere Positions

Each chromosome has two arms, labeled p (the shorter of the two) and q (the longer). They can be connected in either metacentric, submetacentric, acrocentric or telocentric manner. (While the p arm is named for petite meaning small, the q arm is named thus simply because it follows p in the alphabet. )

Metacentric

If both arms are equal in length, the chromosome is said to be metacentric. Robertsonian -- fusion of 2 p arms by centric fusion to form metacentric chromosome.

Submetacentric

If arms' lengths are unequal, the chromosome is said to be submetacentric

Acrocentric

If the p (short) arm is so short that is hard to observe, but still present, then the chromosome is acrocentric (The "acro-" in acrocentric refers to the Greek word for "peak. ").

There are five acrocentric chromosomes in the human genome: 13, 14, 15, 21 and 22. Chromosome 13 is one of the 23 pairs of Chromosomes in Humans People normally have two copies of this chromosome Chromosome 14 is one of the 23 pairs of Chromosomes in Humans People normally have two copies of this chromosome Chromosome 15 is one of the 23 pairs of Chromosomes in Humans People normally have two copies of this chromosome Chromosome 21 is one of the 23 pairs of Chromosomes in Humans People normally have two copies of this chromosome Chromosome 22 is one of the 23 pairs of Chromosomes in Humans People normally have two copies of Chromosome 22 These five chromosomes are the site of genes encoding rRNAs.

Telocentric

A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. A telomere is a region of repetitive DNA at the end of Chromosomes which protects the end of the chromosome from destruction All mouse chromosomes are telocentric ^[2]; in humans, the Y sex chromosome is telocentric.

The centromeric sequence

In most eukaryotes, the centromere has no defined DNA sequence. Animals Plants fungi, and Protists are eukaryotes (juːˈkærɪɒt or -oʊt Organisms whose cells are organized into complex A DNA sequence or genetic sequence is a succession of letters representing the Primary structure of a real or hypothetical DNA Molecule It typically 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. However, 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.

Inheritance

Epigenetic inheritance plays a major role in specifying the centromere in most organisms. In Biology, the term epigenetics refers to changes in Gene expression caused by mechanisms other than changes in the underlying DNA sequence The daughter chromosomes will assemble centromeres in the same place as the parent chromosome, independent of sequence. However, there must still be some original way in which the centromere is specified, even if it is subsequently propagated epigenetically.

Structure

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. Heterochromatin is a tightly packed form of DNA Its major characteristic is that transcription is limited Cohesin is the Protein complex responsible for binding the sister Chromatids during Mitosis through the G2 phase and into mitosis phase In this chromatin, the normal histone H3 is replaced with a centromere-specific variant, CENP-A in humans (Lodish et al. In Biology, histones are the chief Protein components of Chromatin. 2004). 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 the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi. Schizosaccharomyces pombe, also called "fission yeast" is a Species of Yeast. RNA interference ( RNAi) is a mechanism that inhibits Gene expression at the stage of translation or by hindering the transcription of specific ^[3] 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. Caenorhabditis elegans (ˌsiːnoʊræbˈdaɪtɪs ˈɛlɪgænz is a free-living Nematode (roundworm about 1  mm in length which

Centromeric aberrations

In rare cases in humans, neocentromeres can form at new sites on the chromosome. This 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. Dicentric chromosome is an aberrant Chromosome having two Centromeres Dicentric chromosomes form when two chromosome segments (from different chromosomes or from 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.

Centromere proteins are also the autoantigenic target for some anti-nuclear antibodies such as anti-centromere antibodies

Related links

• Monopolin
• Genetics
• Cell biology
• Chromatid
• Diploid

References

Further reading

• Lodish et al. ; Molecular Cell Biology; fifth edition; 2004; ISBN 0716743663

External links

• Report about Nature Genetics article that centromere of rice genome carries active genes

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