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| A nonmitotic, or nondivision, form of cell death that occurs when cells die without attempting division during the interphase portion of the cell life cycle is termed apoptosis, or programmed cell death. This was formerly called interphase death. Apoptosis occurs spontaneously in both normal tissue and in tumors. It can occur in human beings and other vertebrate animals and amphibians, both in the embryo and in the adult. An example of this process is the sequence of events during embryonic development whereby tadpoles lose their tails. |
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| A cell survival curve is a curve used in radiobiology. It depicts the relationship between the fraction of cells retaining their reproductive integrity and the absorbed dose of radiation. Conventionally, the surviving fraction is depicted on a logarithmic scale, and is plotted on the y-axis against dose on the x-axis |
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| syndromes are a group of genetic disorders that are typically transmitted in an autosomal recessive mode of inheritance. ... The disorders are characterized by a defect in DNA repair mechanisms or genomic stability, and patients with these disorders show increased predisposition to cancer |
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Definition
When ionizing particles interact directly with vital biologic macromolecules such as:
• DNA
• Ribonucleic acid (RNA)
• Proteins
• Enzymes
damage to these molecules occurs from the absorption of energy through photoelectric and Compton interactions. The ionization or even the excitation of the atoms of the biologic macromolecules can often result in breakage of the macromolecules' intricate chemical bonds causing them to become abnormal structures. This change could in turn lead to inappropriate cellular chemical reactions. |
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Definition
| an uncharged molecule (typically highly reactive and short-lived) having an unpaired valence electron. |
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| Indirect action, instead, is always a multistage process that first involves the production of free radicals that are created by the interaction of the radiation with water (H2O) molecules. These unstable agents, then, may proceed to interact with cellular molecules. They are so highly reactive that they have the capability to substantially disrupt master molecules, with resulting cell death. |
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Term
| law of Bergonié and Tribondeau |
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Definition
| It states that the radiosensitivity of cells is directly proportional to their reproductive activity and inversely proportional to their degree of differentiation. Thus the most pronounced radiation effects occur in cells with the least maturity and specialization or differentiation, the greatest reproductive activity, and the longest mitotic phases.6 Although the law was originally applied only to germ cells, it is actually true for all types of cells in the human body. |
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Term
| linear energy transfer (LET) |
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Definition
| when passing through a medium, ionizing radiation may interact with it during its passage and as a result lose energy along its path (called a track). The average energy deposited per unit length of track is called linear energy transfer (LET) (Fig. 7.1). The energy average is calculated by dividing the total energy deposited in the medium by the total length of the track. LET is generally described in units of kiloelectron volts (keV) per micron (1 micron [µm] = 10−6 m). The rate of transfer of energy from ionizing radiation used for diagnostic purposes (x-rays) to soft biologic tissue is estimated to be 3 keV/µm. This is considered to be relatively low-LET radiation compared with other types of radiation, which can have much higher keV/µm values. |
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| teractions of ionizing radiation with DNA molecules may cause the loss of or change in a nitrogenous base on the DNA chainBecause the genetic information to be passed on to future generations is contained in the strict sequence of these bases, the loss or change of a base in the DNA chain represents a mutation. It may not be reversible and may generate acute consequences for the cell, but, more important, if the cell remains viable, incorrect genetic information will be transferred to one of the two daughter cells when the cell divides. |
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| If ionizing radiation interacts with a DNA macromolecule, the energy transferred could rupture one of its chemical bonds and possibly sever one of the sugar–phosphate chain side rails, or strands, of the ladder-like molecular structure (single-strand break). This type of injury to DNA is called a point lesion. Such a single alteration along the sequence of nitrogenous bases can result in a gene abnormality. Point lesions commonly occur with low-LET radiation. Repair enzymes, however, are often capable of reversing this damage |
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Term
| radiation weighting factor (WR) |
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Definition
| The radiation weighting factor (WR) is a dimensionless constant that accounts for the relative biological effectiveness (RBE) of various types of ionizing radiation |
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Term
| relative biologic effectiveness (RBE) |
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Definition
| In radiobiology, the relative biological effectiveness is the ratio of biological effectiveness of one type of ionizing radiation relative to another, given the same amount of absorbed energy |
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Definition
| The biologic effects of exposure to radiation stem primarily from the ionizations occurring at sensitive cellular points secondary to energy transfers from that radiation. These affected locations in a cell or, more specifically, on a vital molecule within the cell are known as targets. Whether or not such locations are struck by radiation is a random process. From all existing evidence, it appears that producing a serious effect usually requires more than one radiation “hit” on a specific target. The damage from a single hit normally is not conclusive because of repair mechanisms. This concept of radiation damage resulting from discrete and random events is known as target theory. |
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Term
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Definition
| Wave–particle duality is the concept in quantum mechanics that every particle or quantum entity may be described as either a particle or a wave. It expresses the inability of the classical concepts "particle" or "wave" to fully describe the behaviour of quantum-scale objects |
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Definition
| The oxygen enhancement ratio (OER) is given by the dose in hypoxia divided by the dose in air to achieve the same survival level, and at high dose levels the value of OER for most cell types is in the range of about 2.5–3.0 |
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Term
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Definition
| Anaphase begins with the breakdown of a protein called securin that inhibits the action of a protein called separase whose function is to break down the protein cohesin. The latter is ultimately responsible for maintaining the integrity of the centromeres attached to the microtubules making up the mitotic spindle. With the breakdown of cohesin, the centromeres are severed and the sister chromatids move apart and are subsequently pulled toward opposite poles of the spindle. |
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Definition
| As metaphase begins, the fibers collectively referred to as the mitotic spindle form between the centrioles. Each chromosome, which now consists of two chromatids, lines up in the center, or equator, of the cell attached by its centromere to the mitotic spindle. This configuration forms the equatorial plate. During metaphase, cell division can be stopped, and visible chromosomes can be examined under a microscope. Chromosome damage caused by radiation can then be evaluated. |
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Term
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Definition
| During telophase, the chromatids undergo changes in appearance by uncoiling and becoming long, loosely spiraled threads. Simultaneously, the nuclear membrane forms anew, and two nuclei (one for each new daughter cell) appear. The cytoplasm also divides (cytokinesis) near the equator of the cell to surround each new nucleus. After this cell division completes, each daughter cell has a complete cell membrane and contains exactly the same amount of genetic material (46 chromosomes) as the parent cell. |
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Term
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Definition
| During prophase, the first phase of cell division, the nucleus enlarges, the DNA complex (the chromatid network of threads) coils up more tightly, and the chromatids become more visible on stained microscopic slides. Chromosomes enlarge, and the DNA begins to take structural form. The nuclear membrane disappears, and the centrioles (small hollow, cylindrical structures) migrate to opposite sides of the cell and begin to regulate the formation of the mitotic spindle, the delicate fibers that are attached to the centrioles and extend from one side of the cell to the other across the equator of the cell. |
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Term
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Definition
| Meiosis is a special type of cell division that reduces the number of chromosomes in each daughter cell to half the number of chromosomes in the parent cell provides terms associated with the female reproductive cell. Male and female germ cells, or sperm and ova, of sexually mature individuals each begin meiosis with 46 chromosomes. However, before the male and female germ cells unite to produce a new organism, the number of chromosomes in each must be reduced by one half to ensure that the daughter cells (called zygotes) formed when they unite will contain only the normal number of 46 chromosomes. Hence meiosis is really a process of reduction division |
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