Zygote - the fertilized ovum (egg) before it begins to divide. The spermatozoon (SPERM cell) and the ovum are each haploid cells (gametes); they contain half the complement of chromosomes necessary to create an organism. When two gametes join they form a single diploid cell that contains the full complement of chromosomes. The zygote then divides as a haploid cell, becoming a blastocyst and eventually forming an EMBRYO.
For further discussion of zygotes within the context of the structures and functions of genetics, please see the overview section “Genetics and Molecular Medicine.”
Resource: Facts On File Encyclopedia Of Health And Medicine
RNA - the abbreviation for ribonucleic acid. RNA is a single-strand molecule consisting of ribose; a sugar; and nucleotides made up of the nitrogen bases adenine, uracil, guanine, and cytosine. RNA exists in a number of forms, all of which serve as biochemical messengers that carry the instructions of DNA to the ribosomes, structures in the cell’s
DNA - the abbreviation for deoxyribonucleic acid. DNA is the molecule of heredity; its sequences form the body’s GENETIC CODE. Each cell in the body contains DNA within the chromosomes in its nucleus (except erythrocytes, which do not have nuclei). DNA has a characteristic double-helix structure that resembles a gently twisting ladder. The supporting
Gene is a segment of coding DNA (DNA that instructs the structure and function of cells throughout the body) composed of a specific sequence of nucleotides. The gene is the basic unit of inheritance that directs every facet of the body’s appearance and functions. Genes align along chromosomes in pairs. Each CHROMOSOME (AUTOSOME) contains thousands of
Stem Cell - an undifferentiated, primal cell that has the capability to endlessly divide and develop into numerous types of cells. Totipotent stem cells exist primarily in the early EMBRYO (blastocyst) and can differentiate into (become) virtually any type of cell in the body. As the body becomes more complex and develops beyond the blastocyst stage, stem
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
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
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,
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.
Chromosome is a coiled DNA molecule within the cell’s nucleus that carries an individual’s GENETIC CODE. Most of the time the chromosome’s structure is loose and indistinguishable. Only in the stage of cell division immediately before the cell divides (the metaphase) does the chromosome draw itself into a compact, rodlike structure the
Cloning - the creation of exact copies of a GENE, cell, or entire organism. Such exact copies occur naturally when a ZYGOTE divides to become identical multiples such as twins or, less commonly, triplets. Manipulated cloning is primarily a research method at present, though scientists use cloning for therapeutic applications in creating RECOMBINANT DNA