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Encyclopedia > Centromere
Chromosome.
(1) Chromatid. One of the two identical parts of the chromosome after S phase.
(2) Centromere. The point where the two chromatids touch, and where the microtubules attach.
(3) Short arm
(4) Long arm.

The centromere is a region in the middle of the chromosome involved in cell division and the control of gene expression. From Nupedia File links The following pages link to this file: Chromosome Categories: GFDL images ... From Nupedia File links The following pages link to this file: Chromosome Categories: GFDL images ... A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. ... A scheme of a condensed (metaphase) chromosome. ... Gene expression, or simply expression, is the process by which the inheritable information which comprises a gene, such as the DNA sequence, is made manifest as a physical and biologically functional gene product, such as protein or RNA. Several steps in the gene expression process may be modulated, including the...

Contents

Function

The centromere is the strongest and thinnest region of the chromosome. It holds together the two similar halves of the chromosome, the sister chromatids. During cell division, the spindle fibers pull the chromatids apart. Each chromatid has its own centromere, and the spindle fibers attach to the centromeres. The centromere looks like a constriction along the length of the chromosome. A chromatid forms one part of a chromosome after it has coalesced for the process of mitosis or meiosis. ... Micrograph showing condensed chromosomes in blue and the mitotic spindle in green during prometaphase of mitosis During cell division, the spindle apparatus pulls apart the chromosomes into the two daughter cells. ...


A centromere functions in sister chromatid adhesion, kinetochore formation, pairing of homologous chromosomes and is involved in the control of puberty. The kinetochore is the protein structure in eukaryotes which assembles on the centromere and links the chromosome to microtubule polymers from the mitotic spindle during mitosis. ... Homologous chromosomes are chromosomes in a biological cell that pair (synapse) during meiosis, or alternatively, non-identical chromosomes that contain information for the same biological features and contain the same genes at the same loci but possibly different genetic information, called alleles, at those genes. ...


A centromere is the region where sister chromatids join in the double chromosomal structure during mitosis, prophase and metaphase. 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. Mitosis divides genetic information during cell division. ... Newt lung cell in Prophase, with the mitotic spindles stained green and the cell nucleus and chromatin stained blue. ... An image of a newt lung cell stained with fluorescent dyes during metaphase. ... A centriole in biology is a hollow cylindrical organelle found in most animal cells, and cells of fungi and algae though not frequently in plants. ... Newt lung cell during late anaphase. ... A cell during telophase that has almost completed cytokinesis. ...


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 a change in the number of chromosomes that can lead to a chromosomal disorder. ...


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 short, 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.


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. These five chromosomes are the site of genes encoding rRNAs. Chromosome 13 is one of the 23 pairs of chromosomes in humans. ... Chromosome 14 is one of the 23 pairs of chromosomes in humans. ... Chromosome 15 is one of the 23 pairs of chromosomes in humans. ... Chromosome 21 is one of the 23 pairs of chromosomes in humans. ... Chromosome 22 is one of the 23 pairs of chromosomes in humans. ...


Telocentric

A telocentric chromosome's centromere is located at the terminal end of the chromosome. Telomeres may extend from both ends of the chromosome. All mouse chromosomes are telocentric[1]; humans do not possess any telocentric chromosomes. A telomere is a region of highly repetitive DNA at the end of a chromosome, which functions as an aglet. ...


The centromeric sequence

In most eukaryotes, the centromere has no defined DNA sequence. 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. Kingdoms Eukaryotes are organisms with complex cells, in which the genetic material is organized into membrane-bound nuclei. ... part of a DNA sequence A DNA sequence (sometimes genetic sequence) is a succession of letters representing the primary structure of a real or hypothetical DNA molecule or strand, The possible letters are A, C, G, and T, representing the four nucleotide subunits of a DNA strand (adenine, cytosine, guanine...


Inheritance

Epigenetic inheritance plays a major role in specifying the centromere in most organisms. 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. It has been suggested that this article or section be merged with Epigenetics. ...


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. In this chromatin, the normal histone H3 is replaced with a centromere-specific variant, CENP-A in humans (Lodish et al. 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. For differently-colored eyes, see Heterochromia. ... Cohesin is the protein responsible for binding the sister chromatids during mitosis after S phase. ... Schematic representation of the assembly of the core histones into the nucleosome. ...


In the yeast Schizosaccharomyces pombe (and probably in other eukaryotes), the formation of centromeric heterochromatin is connected to RNAi.[2] 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. Binomial name Schizosaccharomyces pombe Schizosaccharomyces pombe, also called fission yeast, is a species of yeast. ... In molecular biology, RNA interference (RNAi) is a mechanism in which the presence of small fragments of double-stranded RNA (dsRNA) whose sequence matches a given gene interferes with the expression of that gene. ... Binomial name Maupas, 1900 Caenorhabditis elegans (IPA: ) is a free-living nematode (roundworm), about 1 mm in length, which lives in temperate soil environments. ...


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. 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. Dicentric chromosome is an aberrant chromosome having two centromeres. ...


Centromere proteins are also the autoantigenic target for some anti-nuclear antibodies such as anti-centromere antibodies Anti-nuclear antibodies (ANAs, also known as anti-nuclear factor or ANF) are antibodies present in higher than normal numbers in autoimmune disease. ... schematic representation of antibody. ...


Related links

Monopolin is a protein complex required for the segregation of homologous centromeres to opposite poles of a dividing cell during meiosis I of cell division. ... This article is about the general scientific term. ... Cell biology (also called cellular biology or formerly cytology, from the Greek kytos, container) is an academic discipline that studies cells. ...

References

  1. ^ Mouse Genome Sequencing and Comparative Analysis of the Mouse Genome, Nature, 2002
  2. ^ Volpe TA, Kidner C, Hall IM, Teng G, Grewal SI, Martienssen RA (2002-09-13). "Regulation of heterochromatic silencing and histone H3 lysine-9 methylation by RNAi". Science: 297(5588):1818–9. 

Also see: 2002 (number). ... is the 256th day of the year (257th in leap years) in the Gregorian calendar. ... Science is the journal of the American Association for the Advancement of Science (AAAS). ...

Further reading

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

External links


  Results from FactBites:
 
Centromere - Wikipedia, the free encyclopedia (520 words)
A centromere functions in sister chromatid adhesion, kinetochore formation, pairing of homologous chromosomes and is involved in the control of gene expression.
A centromere is the region where sister chromatids join in the double chromosomal structure during mitosis, prophase and metaphase.
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.
  More results at FactBites »

 
 

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