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Any of the organized components of each cell which carry the individual's hereditary material, deoxyribonucleic acid (DNA). Chromosomes are found in all organisms with a cell nucleus (eukaryotes) and are located within the nucleus. Each chromosome contains a single extremely long DNA molecule that is packaged by various proteins into a compact domain. A full set, or complement, of chromosomes is carried by each sperm or ovum in animals and each pollen grain or ovule in plants. This constitutes the haploid (n) genome of that organism and contains a complete set of the genes characteristic of that organism. Sexually reproducing organisms in both the plant and animal kingdoms begin their development by the fusion of two haploid germ cells and are thus diploid (2n), with two sets of chromosomes in each body cell. These two sets of chromosomes carry virtually all the thousands of genes of each cell, with the exception of the tiny number in the mitochrondria (in animal), and a few plant chloroplasts. See also
Chromosomes can change their conformation and degree of compaction throughout the cell cycle. During interphase, the major portion of the cycle, chromosomes are not visible under the light microscope because, although they are very long, they are extremely thin. However, during cell division (mitosis or meiosis), the chromosomes become compacted into shorter and thicker structures that can be seen under the microscope. At this time they appear as paired rods with defined ends, called telomeres, and they remain joined at a constricted region, the centromere, until the beginning of anaphase of cell division. See also
Chromosomes are distinguished from one another by length and position of the centromere. They are metacentric (centromere in the middle of the chromosome), acrocentric (centromere close to one end), or telocentric (centromere at the end, or telomere). The centromere thus usually lies between two chromosome arms, which contain the genes and their regulatory regions, as well as other DNA sequences that have no known function. In many species, regional differences in base composition and in the time at which the DNA is replicated serve as the basis for special staining techniques that make visible a series of distinctive bands on each arm, and these can be used to identify the chromosome.
Compaction
Each nucleus in the cell of a human or other mammal contains some 6 billion base pairs of DNA which, if stretched out, would form a very thin thread about 6 ft (2 m) long. This DNA has to be packaged into the chromosome within a nucleus that is much smaller than a printed dot. Each chromosome contains a single length of DNA comprising a specific portion of the genetic material of the organism. Tiny stretches of DNA, about 140 base pairs long and containing acidic phosphate groups, are individually wrapped around an octamer consisting of two molecules of each of the four basic histone proteins H2a, H2b, H3, and H4. This arrangement produces small structures called nucleosomes and results in a sevenfold compaction of the DNA strand. Further compaction is achieved by binding the histone protein H1 and several nonhistone proteins, resulting in a supercoiled structure in which the chromosome is shortened by about 1600-fold in the interphase nucleus and by about 8000-fold during metaphase and anaphase, where the genetic material must be fully compacted for transport to the two daughter cells. At the point of maximum compaction, human chromosomes range in size from about 2 to 10 micrometers in length, that is, less than 0.0004 in. See also Nucleosome.
Number and size
Each diploid (2n) organism has a characteristic number of chromosomes in each body (somatic) cell, which can vary from two in a nematode worm and one species of ant, to hundreds in some butterflies, crustaceans, and plants. The diploid number of chromosomes includes a haploid (n) set from each parent. Many one-celled organisms are haploid throughout most of their life cycle. The human diploid number is 46.
There is some relationship between the number of chromosomes and their size. Some of the chromosomes in certain classes of organisms with large numbers of chromosomes are very tiny, and have been called microchromosomes. In birds and some reptiles, there are about 30–40 pairs of microchromosomes in addition to 5–7 or so pairs of regular-sized macrochromosomes. The number of microchromosomes is constant in any species carrying them, and only their size distinguishes them from the widespread macrochromosomes. At least seven microchromosomes in birds have been shown to contain genes, and all are thought to.
In some species of insects, plants, flatworms, snails, and rarely vertebrates (such as the fox), the number of chromosomes can vary because of the presence of a variable number of accessory chromosomes, called B chromosomes. It is not clear what role, if any, B chromosomes play, but they appear to be made primarily of DNA that neither contains functional genes nor has much effect on the animal or plant even when present in multiple copies.
Structure
A telomere caps each end of every chromosome and binds specific proteins that protect it from being digested by enzymes (exonucleases) present in the same cell. Most important, the telomere permits DNA replication to continue to the very end of the chromosome, thus assuring its stability. The telomere is also involved in attachment of the chromosome ends to the nuclear membrane and in pairing of homologous chromosomes during meiosis. The structure of telomeric DNA is very similar in virtually all eukaryotic organisms except the fruit fly (Drosophila). One strand of the DNA is rich in guanine and is oriented toward the end of the chromosome, and the other strand is rich in cytosine and is oriented toward the centromere. In most organisms, the telomere consists of multiple copies of a very short DNA repeat.
The centromere is responsible for proper segregation of each chromosome pair during cell division. The chromatids in mitosis and each pair of homologous chromosomes in meiosis are held together at the centromere until anaphase, when they separate and move to the spindle poles, thus being distributed to the two daughter cells. The kinetochore, which is the attachment site for the microtubules that guide the movement of the chromosomes to the poles, is organized around the centromere. The molecular structures of centromeres in most species are still unclear. The repetitive DNA making up and surrounding the centromere is called heterochromatin because it remains condensed throughout the cell cycle and hence stains intensely.
One or more pairs of chromosomes in each species have a region called a secondary constriction which does not stain well. This region contains multiple copies of the genes that transcribe, within the nucleolus, the ribosomal RNA (rRNA). The number of active rRNA genes may be regulated, and an organism that has too few copies of the rRNA genes may develop abnormally or not survive. See also Ribosomes.
Staining
Staining with quinacrine mustard produces consistent, bright and less bright fluorescence bands (Q bands) along the chromosome arms because of differences in the relative amounts of CG (cytosine-guanine) or AT (adenine-thymine) base pairs. The distinctive Q-band pattern of each chromosome makes it possible to identify every chromosome in the human genome. Quinacrine fluorescence can also reveal a difference in the amount or type of heterochromatin on the two members of a homologous pair of chromosomes, called heteromorphism or polymorphism. Such differences can be used to identify the parental origin of a specific chromosome, such as the extra chromosome in individuals who have trisomy 21. Two other methods involve treating chromosomes in various ways before staining with Giemsa. Giemsa or G-band patterns are essentially identical to Q-band patterns; reverse Giemsa or R-band patterns are the reverse, or reciprocal, of those seen with Q or G banding. In humans, most other mammals, and birds (macrochromosomes only), the Q-, G-, and R-banding patterns are so distinctive that each chromosome pair can be individually identified, making it possible to construct a karyotype, or organized array of the chromosome pairs from a single cell (Fig. 1). The chromosomes are identified on the basis of the banding patterns, and the pairs are arranged and numbered in some order, often based on length. In the human karyotype, the autosomes are numbered 1 through 22, and the sex chromosomes are called X and Y. The short arm of a chromosome is called the p arm, and the long arm is called the q arm; a number is assigned to each band on the arm. Thus, band 1q23 refers to band 23 on the long arm of human chromosome 1.
G-banded metaphase karyotype of a human male cell. Every chromosome pair can be identified by its banding pattern. Chromosome 1 is about 12 μm long.
Imprinting
A chromosome carries the same complement of genes whether it is transmitted from the father or the mother, and most of these genes appear to be functionally the same. However, a small number of mammalian genes are functionally different depending on whether they were transmitted by the egg or by the sperm. This phenomenon is known as imprinting. It appears to be caused by the inactivation of certain genes in sperm or ova, probably by methylation of cytosine residues within the regulatory (promotor) region of the imprinted gene. As a result of imprinting, normal development of the mammalian embryo requires the presence of both a maternal and a paternal set of chromosomes. Parthenogenesis, the formation of a normal individual from two sets of maternal chromosomes, is therefore not possible in mammals.
Sex chromosomes
In most mammals, the sex of an individual is determined by whether or not a Y chromosome is present because the Y chromosome carries the male-determining SRY gene. Thus XX and the rare XO individuals are female, while XY and the uncommon XXY individuals are male. In contrast, sex in the fruit fly depends on the balance of autosomes (non-sex chromosomes) and X chromosomes. Thus, in diploids, XX and the rare XXY flies are female, while XY and the rare XO flies are male. In both mammals and fruit flies, males are the heterogametic sex, producing gametes that contain either an X or a Y chromosome; and females are the homogametic sex, producing only gametes containing an X. In birds and butterflies, however, females are the heterogametic sex and males the homogametic sex. Other sex-determining systems are used by some classes of organisms, while sex in some species is determined by a single gene or even by environmental factors such as temperature (some turtles and alligators) or the presence of a nearby female (Bonellia, a marine worm) rather than by a chromosome-mediated mechanism.
More than 900 gene loci have been mapped to the human X chromosome. If the genes on both X chromosomes were fully expressed in female mammalian cells, then male cells, which have only one X, would exhibit only half as much gene product as female cells. However, dosage compensation is achieved, because genes on only one X chromosome are expressed, and genes on any additional X chromosomes are inactivated. This X inactivation randomly occurs during an early stage in embryonic development, and is transmitted unchanged to each of the daughter cells. Mammalian females are therefore mosaics of two types of cells, those with an active maternally derived X and those with an active paternally derived X. Species other than mammals do not show this type of dosage compensation mechanism for sex-linked genes, and some show none at all.
The Y chromosome is one of the smallest chromosomes in the genome in most mammalian species. Usually the mammalian Y chromosome has a very high proportion of heterochromatin, as does the large Y chromosome in Drosophila. Very few genes are located on the Y chromosome in mammals or in Drosophila, and most of these genes are concerned with either sex determination or the production of sperm. In some species of insects and other invertebrates, no Y chromosome is present, and sex in these species is determined by the X:autosome balance (XX female, XO male). See also Cell nucleus; Genetics; Human genetics; Sex determination;
One of a number of small, dark-staining, and more or less rod-shaped bodies situated in the nucleus of a cell. At the time of cell division, chromosomes divide and distribute equally to the daughter cells. They contain genes arranged along their length. The number of chromosomes in the somatic cells of an individual is constant (the diploid number), whereas just half this number (the haploid number) appears in germ cells.
For more information on chromosome, visit Britannica.com.
The threadlike bodies in the nucleus of a cell that contain the strands of DNA which carry genes. In sexual reproduction, chromosomes pair up and divide in ways which randomly allocate genes to the resulting gametes, ensuring inherited diversity in the resulting progeny.
Coiled structure in the nucleus of cells, consisting of DNA and proteins.
The small bodies in the nucleus of a cell that carry the chemical “instructions” for reproduction of the cell. They consist of strands of
Emanating from or pertaining to chromosome.
One change in even one chromosome can make a big difference in an organism.
A chromosome is a single large macromolecule of DNA, and constitutes a physically organized form of DNA in a cell. It is a very long, continuous piece of DNA (a single DNA molecule), which contains many genes, regulatory elements and other intervening nucleotide sequences. A broader definition of "chromosome" also includes the DNA-bound proteins which serve to package and manage the DNA. The word chromosome comes from the Greek χρῶμα (chroma, color) and σῶμα (soma, body) due to its capacity to be stained very strongly with vital and supravital dyes.
Chromosomes vary extensively between different organisms. The DNA molecule may be circular or linear, and can contain anything from tens of kilobase pairs to hundreds of megabase pairs. Typically eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without nuclei) smaller circular chromosomes, although there are many exceptions to this rule. Furthermore, cells may contain more than one type of chromosome; for example mitochondria in most eukaryotes and chloroplasts in plants have their own small chromosome in addition to the nuclear chromosomes.
In eukaryotes nuclear chromosomes are packaged by proteins (particularly histones) into chromatin to fit the massive molecules into the nucleus. The structure of chromatin varies through the cell cycle, and is responsible for the compaction of DNA into the classic four-arm structure during mitosis and meiosis. Prokaryotes do not form chromatin, because the cells lack proteins required and the circular configuration of the molecule prevents this.
"Chromosome" is a rather loosely defined term. In prokaryotes, a small circular DNA molecule may be called either a plasmid or a small chromosome. In viruses, mitochondria, and chloroplasts their DNA molecules are commonly referred to as chromosomes, despite being naked molecules, as they constitute the complete genome of the organism or organelle.
Chromosomes were first observed in plant cells by a Swiss botanist named Karl Wilhelm von Nägeli in 1842, and independently in Ascaris worms by Belgian scientist Edouard Van Beneden (1846-1910). The use of basophilic aniline dyes was a fundamentally new technique for effectively staining the chromatin material in the nucleus. Their behavior in animal (salamander) cells was later described in detail by German cytologist and professor of anatomy Walther Flemming, the discoverer of mitosis, in 1882. The name was invented later by another German anatomist, Heinrich von Waldeyer in 1888.
Eukaryotes (cells with nuclei such as plants, yeast, and animals) possess multiple large linear chromosomes contained in the cell's nucleus. Each chromosome has one centromere, with one or two arms projecting from the centromere, although under most circumstances these arms are not visible as such. In addition most eukaryotes have a small circular mitochondrial genome, and some eukaryotes may have additional small circular or linear cytoplasmic chromosomes.
In the nuclear chromosomes of eukaryotes, the uncondensed DNA exists in a semi-ordered structure, where it is wrapped around histones (structural proteins), forming a composite material called chromatin.
Chromatin is the complex of DNA and protein found in the eukaryotic nucleus which packages chromosomes. The structure of chromatin varies significantly between different stages of the cell cycle, according to the requirements of the DNA.
During interphase (the period of the cell cycle where the cell is not dividing) two types of chromatin can be distinguished:
Individual chromosomes cannot be distinguished at this stage - they appear in the nucleus as a homogeneous tangled mix of DNA and protein.
In the early stages of mitosis or meiosis (cell division), the chromatin strands become more and more condensed. They cease to function as accessible genetic material (transcription stops) and become a compact transportable form. This compact form makes the individual chromosomes visible, and they form the classic four arm structure, a pair of sister chromatids attached to each other at the centromere. The shorter arms are called p arms (from the French petit, small) and the longer arms are called q arms (q follows p in the Latin alphabet). This is the only natural context in which individual chromosomes are visible with an optical microscope.
During divisions long microtubules attach to the centromere and the two opposite ends of the cell. The microtubules then pull the chromatids apart, so that each daughter cell inherits one set of chromatids. Once the cells have divided, the chromatids are uncoiled and can function again as chromatin. In spite of their appearance, chromosomes are structurally highly condensed which enables these giant DNA structures to be contained within a cell nucleus (Fig. 2).
The self assembled microtubules form the spindle, which attaches to chromosomes at specialized structures called kinetochores, one of which is present on each sister chromatid. A special DNA base sequence in the region of the kinetochores provides, along with special proteins, longer-lasting attachment in this region.
Prokaryotes (eg. Bacteria) typically have a single circular chromosome, but many variations do exist. Bacterial DNA also exists as plasmids, essentially miniature chromosomes, which are small circular pieces of DNA that are readily transmitted between bacteria. The distinction between plasmids and chromosomes is poorly defined, though size and necessity are generally taken into account.
Prokaryotes chromosomes have less sequence-based structure than eukaryotes. They do, however, typically have a single point, the origin of replication, from which replication starts.
The genes in prokaryotes are often organised in operons, and do not contain introns, unlike eukaryotes.
Bacterial chromosomes tend to be tethered to the plasma membrane of the bacteria. In molecular biology application, this allows for its isolation from plasmid DNA by centrifugation of lysed bacteria and pelleting of the membranes (and the attached DNA).
Prokaryotes do not possess histones or nuclei, and so do not possess chromatin like eukaryotes. There is, however, thought to be some structural organisation to help condense the large molecule into the small prokaryotic cell.
Prokaryotic chromosomes and plasmids are, like eukaryotic DNA, generally supercoiled. The DNA must first be released into its relaxed state for access for transcription, regulation, and replication.
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 Normal members of a particular eukaryotic species all have the same number of nuclear chromosomes (see the table). Other eukaryotic chromosomes, i.e., mitochondrial and plasmid-like small chromosomes, are much more variable in number, and there may be thousands of copies per cell. Asexually reproducing species have one set of chromosomes, which is the same in all body cells. Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes, one from the mother and one from the father. Gametes, reproductive cells, are haploid [n]: they have one set of chromosomes. Gametes are produced by meiosis of a diploid germ line cell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed. Some animal and plant species are polyploid [Xn]: they have more than two sets of homologous chromosomes. Agriculturally important plants such as tobacco or wheat are often polyploid compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more common pasta and bread wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes compared to the 14 (diploid) chromosomes in the wild wheat.[10] Historical note: In 1921, Theophilus Painter claimed, based on his observations, that human sex cells had 24 chromosomes each, giving humans 48 chromosomes total. It wasn't until 1955 that the number of chromosomes was clearly shown to be 23. ProkaryotesProkaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies. Plasmids and plasmid-like small chromosomes are, like in eukaryotes, very variable in copy number. The number of plasmids in the cell is almost entirely determined by the rate of division of the plasmid - fast division causes high copy number, and vice versa. Karyotype Karyotyping is a technique used to determine the (diploid) number of nuclear chromosomes of a eukaryotic organism, and may be used for determining sex and spotting chromosomal abnormalities. Cells can be locked part way through division (in metaphase) in vitro (in a reaction vial) with colchicine. These cells are then stained, photographed and arranged into a karyotype (an ordered set of chromosomes, Fig. 3), also called karyogram. Like many sexually reproducing species, humans have special gonosomes (sex chromosomes, in contrast to autosomes). These are XX in females and XY in males, and can be seen in the karyotype, Fig. 3. Chromosomal aberrations
Chromosomal aberrations are disruptions in the normal chromosomal content of a cell, and are a major cause of genetic conditions in humans, such as Down syndrome. Some chromosome abnormalities do not cause disease in carriers, such as translocations, or chromosomal inversions, although they may lead to a higher chance of having a child with a chromosome disorder. Abnormal numbers of chromosomes or chromosome sets, aneuploidy, may be lethal or give rise to genetic disorders. Genetic counseling is offered for families that may carry a chromosome rearrangement. The gain or loss of chromosome material can lead to a variety of genetic disorders. Human examples include:
Chromosomal mutations produce changes in whole chromosomes (more than one gene) or in the number of chromosomes present.
Most mutations are neutral- have little or no effect A detailed graphical display of all human chromosomes and the diseases annotated at the correct spot may be found at [2]. Human chromosomesHuman cells have 23 pairs of large linear nuclear chromosomes, giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome. Sequencing of the human genome has provided a great deal of information about each of the chromosomes. Below is a table compiling statistics for the chromosomes, based on the Sanger Institute's human genome information in the Vertebrate Genome Annotation (VEGA) database.[11] Number of genes is an estimate as it is in part based on gene predictions. Total chromosome length is an estimate as well, based on the estimated size of unsequenced heterochromatin regions.
See also
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