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chromosome / chromosomes; eukaryotic chromosome; eucariotic chromosome; procariotic chromosome

A chromosome is a single, long molecule of DNA. These highly organized structures store genetic information in living organisms. Small sections of the chromosome, called genes, code for the RNA and protein molecules required by an organism. In some organisms, like humans, chromosomes are linear, but in other organisms, like bacteria, chromosomes are typically circular. In prokaryotes, the circular chromosome is contained in the cytoplasm in an area called the nucleoid. In contrast, in eukaryotes, all of the cell's chromosomes are stored inside a structure called the nucleus. Each eukaryotic chromosome is composed of DNA coiled and condensed around nuclear proteins called histones. Humans inherit one set of chromosomes from their mother and a second set from their father. In total, most human cells contain 46 chromosomes with 22 pairs of autosomes, or non-sex chromosomes, and two sex-determining chromosomes. The sex chromosomes in humans are called X and Y. Females carry two X chromosomes, while males carry one X and one Y chromosome. Cells of the body that contain two sets of chromosomes are called diploid. Meanwhile, germ line cells, which go on to produce egg or sperm cells, are called haploid because they contain half the chromosomes of diploid cells. Chromosomes are often observed and depicted as X-shaped structures. DNA takes this form following DNA replication during the process of cell division when the two replicated chromosomes, called chromatids, are highly condensed and still attached to one another at a point called the centromere. Human chromosomes can be differentiated from one another under a microscope by their lengths and by the position of the centromere.
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