Chromosome

chromosome

 [kro´mo-sōm]

in animal cells, a structure in the nucleus, containing a linear thread of deoxyribonucleic acid (DNA), which transmits genetic information and is associated with ribonucleic acid and histones. In bacterial genetics, a closed circle of double-stranded DNA that contains the genetic material of the cell and is attached to the cell membrane; the bulk of this material forms a compact bacterial nucleus. adj., adj chromoso´mal.

During cell division the material composing the chromosome is compactly coiled, making it visible with appropriate staining and permitting its movement in the cell with minimal entanglement. Each organism of a species is normally characterized by the same number of chromosomes in its somatic cells, 46 being the number normally present in humans, including 22 pairs of autosomes and the two sex chromosomes (XX or XY), which determine the sex of the organism. (See also heredity.)

Chromosome Analysis. This can be done on fetal cells obtained by amniocentesis or chorionic villus sampling, on lymphocytes from a blood sample, on skin cells from a biopsy, or on cells from products of conception such as an aborted fetus. The cells are then cultured in the laboratory until they divide. Cell division is arrested in mid-metaphase by the drug Colcemid. The chromosomes can be stained by one of several techniques that produce a distinct pattern of light and dark bands along the chromosomes, and each chromosome can be recognized by its size and banding pattern. The chromosomal characteristics of an individual are referred to as the karyotype. It is also possible to make a photomicrograph of a cell nucleus, cut it apart, and rearrange it so that the individual chromosomes are in order and labeled. The autosomes are numbered 1–22, roughly in order of decreasing length. The sex chromosomes are labeled X and Y. Karyotyping is useful in determining the presence of chromosome defects.

Before the chromosomes could be precisely identified they were placed in seven groups: A (chromosomes 1–3), B (4–5), C (6–12 and X), D (13–15), E (16–18), F (19–20), and G (21–22 and Y).

Chromosomal Abnormalities. The prevalence of chromosomal disorders cannot be fully and accurately determined because many of these disorders do not permit full embryonic and fetal development and therefore end in spontaneous abortion. About one in every 100 newborn infants do, however, have a gross demonstrable chromosomal abnormality. A large majority of cytogenetic abnormalities can be identified by cytogenetic analysis either before birth, by means of chorionic villus sampling or amniocentesis, or after birth.

Cytogenetic disorders with visible chromosomal abnormalities are evidenced by either an abnormal number of chromosomes or some alteration in the structure of one or more chromosomes. In the language of the geneticist, trisomy refers to the presence of an additional chromosome that is homologous with one of the existing pairs so that that particular chromosome is present in triplicate. An example of this type of disorder is a form of down syndrome (trisomy 21). Another example is patau's syndrome (trisomy 13), which produces severe anatomical malformations and profound mental retardation.

The term monosomy refers to the absence of one of a pair of homologous chromosomes. Monosomy involving an autosome usually results in the loss of too much genetic information to permit sufficient fetal development for a live birth. Either trisomy or monosomy involving the sex chromosomes yields relatively mild abnormalities.

A condition known as mosaicism results from an error in the distribution of chromosomes between daughter cells during an early embryonic cell division, producing two and sometimes three populations of cells with different chromosome numbers in the same individual. Mosaicism involving the sex chromosomes is not uncommon.

Other abnormal structural changes in the chromosome are consequences of some kind of chromosomal breakage, with either the loss or rearrangement of genetic material. translocation involves the transfer of a segment of one chromosome to another. inversion refers to a change in the sequence of genes along the chromosome, which occurs when there are two breaks in a chromosome and the segment between the breaks is reversed and reattached to the wrong ends. deletion occurs when a portion of a chromosome is lost. An example of this type of chromosomal abnormality is cri du chat syndrome, a deletion in the short arm of chromosome 5, marked by mental retardation and sometimes congenital heart defects. When deletion occurs at both ends of the chromosome, the two damaged ends can unite to form a circle and the rearrangement produces a ring chromosome. isochromosomes form when the centromere divides along the transverse plane rather than the normal long axis of the chromosome so that both arms are identical. All of the previously described structural abnormalities can affect both autosomal and sex chromosomes.

The causes of chromosomal errors are not completely understood. In some conditions such as Down syndrome, late maternal age seems to be a factor. Other factors may include the predisposition of chromosomes to nondisjunction (failure to separate during meiosis), exposure to radiation, and viruses.

homologous c's the chromosomes of a matching pair in the diploid complement that contain alleles of specific genes.

chromosome painting fluorescent in situ hybridization.

Ph¹ chromosome (Philadelphia chromosome) an abnormality of chromosome 22, characterized by the translocation of genetic material from its long arm to chromosome 9, seen in the marrow cells of most patients with chronic myelogenous leukemia.

ring chromosome a chromosome in which both ends have been lost (deletion) and the two broken ends have reunited to form a ring-shaped figure.

sex c's the chromosomes responsible for determination of the sex of the individual that develops from a zygote; in mammals they are an unequal pair, the X and Y chromosomes.

somatic chromosome autosome.

X chromosome the female sex chromosome, being carried by half the male gametes and all female gametes; female diploid cells have two X chromosomes.

Y chromosome the male sex chromosome, being carried by half the male gametes and none of the female gametes; male diploid cells have an X and a Y chromosome.

Miller-Keane Encyclopedia and Dictionary of Medicine, Nursing, and Allied Health, Seventh Edition. © 2003 by Saunders, an imprint of Elsevier, Inc. All rights reserved.

chro·mo·some

(krō'mō-sōm),

One of the bodies (normally 46 in somatic cells in humans) in the cell nucleus that is the bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons. In bacteria and other prokaryotes, the chromosome is not enclosed within a nuclear membrane and not subject to a mitotic mechanism. Prokaryotes may have more than one chromosome.

[chromo- + G. sōma, body]

Farlex Partner Medical Dictionary © Farlex 2012

chromosome

(krō′mə-sōm′)

n.

1. A linear strand of DNA and associated proteins in the nucleus of eukaryotic cells that carries the genes and functions in the transmission of hereditary information.

2. A circular strand of DNA in bacteria and archaea that contains the hereditary information necessary for cell life.

---

chro′mo·so′mal (-sō′məl), chro′mo·so′mic (-sō′mĭk) adj.

chro′mo·so′mal·ly adv.

The American Heritage® Medical Dictionary Copyright © 2007, 2004 by Houghton Mifflin Company. Published by Houghton Mifflin Company. All rights reserved.

chromosome

adjective Referring to a chromosome. (Etymologically incorrect, but widely preferred to chromosomal.)

noun Any of a number of paired units of the self-replicating genetic material in the eukaryotic nucleus belonging to the organism’s genome, which allows the palette of phenotypic expression of individual organisms. Human chromosomes consist of 23 long (100–-300-million bp, each) paired DNA molecules that are associated with RNA and histone proteins, and most readily recognised during mitosis as they align themselves on the metaphase plate. Chromosomes are divided into structurally similar groups based on length from the centromere.

Human chromosome groups
A—Chromosomes 1–3.
B—Chromosomes 4, 5.
C—Chromosomes 6–12; X chromosome.
D—Chromosomes 13–15.
E—Chromosomes 16–18.
F—Chromosomes 19, 20.
G—Chromosomes 21, 22; Y chromosome.

Segen's Medical Dictionary. © 2012 Farlex, Inc. All rights reserved.

chromosome

Genetics adjective Etymologically incorrect, but widely preferred noun Any of a number of paired units of the self-replicating genetic material in the eukaryotic nucleus, the 'master genetic database' containing the complete information present in a cell or virus which results in the palette of phenotypic expression of the individual; human chromosomes consist of 23 long–100-300 million bp, each–paired DNA or, in some organisms, RNA molecules that in humans are associated with RNA and histone proteins, and most readily recognized during mitosis as they align themselves on the metaphase plate; chromosomes are divided into structurally similar groups based on length from the centromere: group A–chromosomes 1-3; B–chromosomes 4, 5; C–chromosomes 6–12, X chromosome; D–chromosomes 13–15; E–chromosomes 16–18; F–chromosomes 19, 20; G–chromosomes 21, 22, Y chromosome. See Acentric chromosome, Accessory chromosome, Autosomal chromosome, B chromosome, Bacterial artificial chromosome, C banding, Christchurch chromosome, Eukaryote, Flow cytometry, G banding, Gene, Harlequin chromosome, Honorary chromosome, Homologous chromosome, Human genome project, Isochromosome, Lampbrush chromosome, Marker chromosome, Minichromosome chromosome, Nucleotide, Philadelphia chromosome, Ploidy analysis, Polytene chromosome, Protein, Q Banding, Ring chromosome, Sex chromosome, Translation, Transcription. ,Unbanded chromosome, X chromosome, Y chromosome.

McGraw-Hill Concise Dictionary of Modern Medicine. © 2002 by The McGraw-Hill Companies, Inc.

chro·mo·some

(krō'mŏ-sōm)

A body in the cell nucleus (of which there are normally 46 in humans) that is a bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons.

Medical Dictionary for the Health Professions and Nursing © Farlex 2012

chromosome

One of the discrete coiled DNA and protein structures, present in all animal and plant cells, which carry the genetic code for the construction of the body of the organism. Chromosomes are DNA bound to the protein histone in an enormously condensed manner. The packing ratio (DNA length divided by the chromosome length) may be as great as 7000. The code is represented by the GENES, of which there are about 100,000 in humans, strung along the chromosomes. Each normal human body cell contains 46 chromosomes, arranged in 23 pairs. A deficiency of chromosomes is incompatible with life but it is not uncommon for a person to have an extra copy of one of the chromosomes, invariably with undesirable effect. Chromosomes carry most, but not all, of the cell's DNA. Some of it is carried by the MITOCHONDRIA. The term arose when these ‘coloured bodies’ were first distinguished under the microscope by means of specific stains. So the term ‘chromosome’ actually refers to a characteristic not present in life.

Collins Dictionary of Medicine © Robert M. Youngson 2004, 2005

chromosome

a coiled structure found in the nucleus of EUKARYOTE cells which contains DNA (the genetic material making up the genes), basic proteins called HISTONES, and nonhistone acidic proteins which may regulate the activity of DNA (see NUCLEOSOME). Each species of organism has a typical number of chromosomes (e.g. 46 in man, 20 in maize) which come in identical HOMOLOGOUS pairs in DIPLOID (1) types, although lower types such as some fungi have only one chromosome of each type (see HAPLOID, sense 1). Chromosomes are not visible during the interphase parts of the CELL CYCLE, but during MITOSIS and MEIOSIS they shorten and thicken and, after suitable preparation, may be observed under the microscope. Individual chromosomes can be recognized by their overall length and the position of the CENTROMERES. In PROKARYOTES, the chromosome consists of an intact DNA molecule lacking a centromere and is often circular. In viruses, the chromosomal material can be DNA or RNA. See NUCLEOID.

Collins Dictionary of Biology, 3rd ed. © W. G. Hale, V. A. Saunders, J. P. Margham 2005

Chromosome

A structure composed of deoxyribonucleic acid (DNA) contained within a cell's nucleus (center) where genetic information is stored. Human have 23 pairs of chromosomes, each of which has recognizable characteristics (such as length and staining patterns) that allow individual chromosomes to be identified. Identification is assigned by number (1-22) or letter (X or Y).

Mentioned in: Acoustic Neuroma, Amniocentesis, Birth Defects, Cerebral Amyloid Angiopathy, Cri Du Chat Syndrome, Down Syndrome, Edwards' Syndrome, Fragile X Syndrome, Gene Therapy, Genetic Testing, Hereditary Hemorrhagic Telangiectasia, Huntington Disease, Klinefelter Syndrome, Neurofibromatosis, Patau Syndrome, Retinoblastoma, Turner Syndrome, Von Willebrand Disease, Wilson Disease

Gale Encyclopedia of Medicine. Copyright 2008 The Gale Group, Inc. All rights reserved.

chromosome 

One of the thread-like structures located within the cell nucleus composed of an extremely long, double-stranded DNA (deoxyribonucleic acid) helix tightly folded around proteins called histones. Each chromosome carries genes that contain the hereditary material that controls the growth and characteristics of the body. There are 46 chromosomes in each human somatic cell organized in 23 pairs, of which 22 pairs are similar in appearance but differ at the molecular level. They are called autosomal chromosomes or autosomes and are designated by a number (with chromosome 1 being the longest, followed by chromosome 2, etc.). The other pair, the sex chromosomes determines the sex of the individual. In mammals the two sex chromosomes of females are alike (homologous) and are referred to as X chromosomes. Males carry one X chromosome along with a much shorter chromosome, the Y chromosome. Each chromosome has a centromere that divides it into two arms, the short arm 'p' and the long arm 'q'. Disorders of chromosome number in which the number of chromosomes is above or below the normal (46) are called aneuploidy. Common forms of aneuploidy are trisomy in which there is one extra chromosome and monosomy in which there is one less, than the normal 46. They rarely cause specific eye diseases but affected individuals present ocular manifestations. Examples: Down's syndrome (trisomy of chromosome 21), Edwards' syndrome (trisomy 18), Turner's syndrome (monosomy 45 XO). There are other chromosome abnormalities such as translocation (one segment of a chromosome is transferred to another chromosome) as may occur in congenital anterior polar cataract, deletion (a loss of a piece of chromosome) as in aniridia, choroideremia, retinoblastoma, etc. Other cases involve damage of a chromosome (e.g. fragile X syndrome). See defective colour vision; gene; mitosis; mutation.

Millodot: Dictionary of Optometry and Visual Science, 7th edition. © 2009 Butterworth-Heinemann

chro·mo·some

(krō'mŏ-sōm)

One of the bodies (normally 46 in somatic cells in humans) in the cell nucleus that is the bearer of genes, has the form of a delicate chromatin filament during interphase, contracts to form a compact cylinder segmented into two arms by the centromere during metaphase and anaphase stages of cell divison, and is capable of reproducing its physical and chemical structure through successive cell divisons.

Medical Dictionary for the Dental Professions © Farlex 2012