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Definition of «Chromosome»

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Chromosome: A visible carrier of the genetic information.

The 3 billion bp (base pairs) in the human genome are organized into 24 distinct, physically separate microscopic units called chromosomes. All genes are arranged linearly along the chromosomes. The nucleus of most human cells contains two sets of chromosomes, one set given by each parent. Each set has 23 single chromosomes--22 autosomes and an X or Y sex chromosome. (A normal female will have a pair of X chromosomes; a male will have an X and Y pair.) Chromosomes contain roughly equal parts of protein and DNA; chromosomal DNA contains an average of 150 million bases. DNA molecules are among the largest molecules now known.

Chromosomes can be seen under a light microscope and, when stained with certain dyes, reveal a pattern of light and dark bands reflecting regional variations in the amounts of A and T vs G and C. Differences in size and banding pattern allow the 24 chromosomes to be distinguished from each other, an analysis called a karyotype. A few types of major chromosomal abnormalities, including missing or extra copies or gross breaks and rejoinings (translocations), can be detected by microscopic examination; Down's syndrome, in which an individual's cells contain a third copy of chromosome 21, is diagnosed by karyotype analysis.

Most changes in DNA, however, are too subtle to be detected by this technique and require molecular analysis. These subtle DNA abnormalities (mutations) are responsible for many inherited diseases such as cystic fibrosis and sickle cell anemia or may predispose an individual to cancer, major psychiatric illnesses, and other complex diseases.

The foregoing definition and discussion pertain to the chromosomes in the nucleus of the cell. The chromosome of the mitochondrion, which is in the cytoplasm of the cell, is a somewhat different story.

For More Information «Chromosome»

  • chromosome: Definition from Answers.com

    Library > Literature & Language > Dictionary ( krō ' mə-sōm ' ) n. A threadlike linear strand of DNA and associated proteins in the nucleus of eukaryotic cells ...

  • Chromosome | Define Chromosome at Dictionary.com

    noun Genetics . any of several threadlike bodies, consisting of chromatin, that carry the genes in a linear order: the human species has 23 pairs, designated 1 to 22 ...

  • Chromosomes

    The molecule of DNA in a single human chromosome ranges in size from 50 x 10 6 nucleotide pairs in the smallest chromosome (stretched full-length this molecule would extend ...

  • chromosome - definition of chromosome in the Medical ...

    chromosome /chro·mo·some/ (kro´mah-sōm) in animal cells, a structure in the nucleus containing a linear thread of DNA which transmits genetic information and is ...

  • What is a chromosome? - Genetics Home Reference

    In the nucleus of each cell, the DNA molecule is packaged into thread-like structures called chromosomes. Each chromosome is made up of DNA tightly coiled many ...

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