Biology X
What is X chromosome ?
The nuclei of human cells contain 22 autosomes (any chromosome that is not a sex chromosome) and two sex chromosomes. In females, the sex chromosomes are the two X chromosomes, and in males there is one X chromosome and one Y chromosome.
What is xenobiotic ?
A xenobiotic (Greek xenos “foreign” bios “life”) is a compound that is foreign to a living organism. Principal xenobiotics include drugs, carcinogens, and various compounds that have been introduced into the environment by artificial means.
What is XO ?
Sex is determined by genes, called sex-linked genes. An XO individual has only one chromosome, and the Y or X is missing. An XO human is viable and develops ovaries but fails to produce steroid hormones, and so the individual has no puberty and is infertile. Those with Turner’s syndrome are missing or have a damaged X chromosome, a condition that affects about one in every 2,500 females. Certain organisms, such as caddis, butterflies and moths, birds, and some fish, have XO sex determination. A male is produced when two X chromosomes are present, and a female is produced when only one X chromosome is present. Thus XY produces a female and XX produces a male in Lepidoptera, the heterogametic sex being the female in this select group of organisms.
What is xylem ?
A tissue found in a plant’s vascular bundle (above the phloem) and in the center of the plant, where secondary growth occurs. Its purpose is to bring water and minerals (nutrients) from the roots upward throughout the plant.
There is a primary and secondary xylem. Xylem is made up of tracheids (long, dead cells that function as conductors), vessel elements (individual short, wide cells that make up the vessel, also responsible for water transport), fibers, and parenchyma cells (act as storage cells and help in assimilation or wound healing). Both tracheids and vessels have stiff lignin-containing secondary cell walls that give them rigidity.
Primary xylem comes from the plants procambium and contains protoxylem (annular and spiral vessels) and metaxylem (scalariform, reticulate, and pitted vessels). The secondary xylem comes from the vascular cambium in plants exhibiting secondary growth, such as wood.
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