chromosome (krō`məsōm'), structural carrier of hereditary characteristics, found in the nucleus of every cell and so named for its readiness to absorb dyes. The term chromosome is usually reserved for the structure when it is condensed and readily visible during cell division (see mitosis interphase the chromosomes are dispersed in the nucleus and appear as a network of long, thin threads or filaments, called the chromatin. At some point before prophase begins, the chromosomes replicate themselves to form pairs of identical sister chromosomes, or chromatids; the
..... Click the link for more information. ). At other times the chromosome appears as a fibrous structure, called the chromonema, consisting of accumulations (called chromomeres) of chromatin, the dye-absorbing material. During nuclear division, when each chromosome splits, each of the duplicate chromosomes is called a chromatid. A certain number of chromosomes is characteristic of each species of plant and animal; e.g., the human has 46 chromosomes, the potato has 48, and the fruit fly Drosophila has 8. Each of these chromosome numbers is the so-called diploid number, i.e., the number found in the somatic (body) cells and in the germ cells that give rise to the gametes, or reproductive cells. When the germ cells divide in the two-step process of meiosis meiosis (mīŏ`sĭs)
..... Click the link for more information. , the chromosomes are separated in such a way that each daughter cell receives a haploid (half the diploid) number of chromosomes. Fusion of the male and female gametes in fertilization restores the diploid number in the fertilized egg, or zygote, which thus contains two sets of homologous chromosomes, one from each parent. The principal constituents of the chromosomes are nucleoproteins containing deoxyribonucleic acid, or DNA (see nucleic acid nucleic acid, any of a group of organic substances found in the chromosomes of living cells and viruses that play a central role in the storage and replication of hereditary information and in the expression of this information through protein synthesis.
..... Click the link for more information. ). Chromosomes appear microscopically as a linear arrangement of genes, the factors that determine the inherited characteristics of all living organisms. The very large chromosomes in the salivary gland cells of Drosophila and other insects have furnished valuable material for the study of genetics genome, or characteristic set of genes, that contains the total genetic information for an individual organism. In many familiar organisms two genes for each trait are present in each individual, and these paired genes, both governing the same trait, are called

alleles.
..... Click the link for more information. .

chromosome

During the first stages of cell division, the recognizable double-stranded chromosome is formed by …

Microscopic, threadlike part of a cell that carries hereditary information in the form of genes. The structure and location of chromosomes differentiate prokaryotic cells from eukaryotic cells (see prokaryote, eukaryote). Every species has a characteristic number of chromosomes; humans have 23 pairs (22 pairs of autosomal, or nonsex, chromosomes and one pair of sex chromosomes). Human chromosomes consist primarily of DNA. During cell division (see meiosis, mitosis), chromosomes are distributed evenly among daughter cells. In sexually reproducing organisms, the number of chromosomes in somatic (nonsex) cells is diploid, while gametes or sex cells (egg and sperm) produced by meiosis are haploid (see ploidy). Fertilization restores the diploid set of chromosomes in the zygote.

chromosome (redirected from X chromosome)

chromosome

chromosome
(redirected from X chromosome)