Allele - any of the variations of a GENE that may occupy the same position (locus) on a CHROMOSOME. The gene controlling a particular trait or function always occupies the same locus on the same chromosome. Genes occur as pairs, with one gene coming from each parent. The pairing determines how the gene’s traits are expressed in the individual. For example, the gene for BLOOD TYPE occurs at region 34 on the long arm of chromosome 9, indicated as
Apoptosis - the natural mechanism through which a cell engages in actions that lead to its death, often called programmed cell death or cell suicide. Apoptosis appears linked to SENESCENCE, an inherent limitation on the number of times a cell can divide. Both apoptosis and senescence play significant roles in the aging process. Once the cell initiates apoptosis there is no reversal; the process proceeds until the cell dies. Apoptosis begins when the
Autosomal Trisomy is a chromosomal disorder in which there are three instead of the normal two copies of an AUTOSOME (nonsex chromosome). An autosomal trisomy may be complete (affect all cells) or mosaic (affect only some cells). The most commonly occurring complete autosomal trisomies that are survivable are those involving chromosomes 21, 18, and 13, which result in the chromosomal disorders DOWN SYNDROME (trisomy 21), EDWARDS SYNDROME (trisomy 18), and
Autosome is a CHROMOSOME that appears as a pair in which both chromosomes are the same in either sex, also called a nonsex chromosome. In contrast, the sex chromosomes appear as a pair that is different in males and females. The human GENOME contains 22 autosomes and one pair of sex chromosomes for a total complement of 46 chromosomes as 23 pairs.
The cell is the basic structural and functional unit of all living organisms. About 100 trillion cells make up one of the most complex of such organisms, the human. Types of Cells in the Human Body There are three basic types of cells in the body: stem cells, germ cells, and somatic cells. The foundation of life: stem cells Stem cells are the primal, undifferentiated cells that give rise to all other cells. They are primarily abundant and functional
Centromere - the position on a CHROMOSOME where the chromosome separates during cell division. The centromere is a structure of noncoding DNA (DNA that does not convey genetic information). When the cell divides the strands of the chromatids migrate in opposite directions (pull apart) at the centromere. In a photomicrograph, the centromere appears as an indented, waistlike area on the chromosome. Geneticists use the centromere’s position, along with
Chromatid is a replica of a CHROMOSOME that develops in preparation for cell division. Chromatids are “sister” pairs of each chromosome that contain identical genetic material. They remain attached to each other at the CENTROMERE until cell division. When the mother cell divides, the sister chromatids separate at the centromere and migrate into the new daughter cells, forming the chromosome pairs for the new cells. Though minor variations are
Chromosomal Disorders - abnormalities affecting the chromosomes that result in syndromes (constellations of symptoms) having characteristic physical or functional anomalies. Most chromosomal disorders occur because of alterations in the number of chromosomes or the structure of chromosomes. Though an individual may inherit a chromosomal disorder, more commonly chromosomal disorders represent random occurrences. Typically all the cells in the body reflect