cellular metabolic activities. Ribosomes are made up of protein and ribo-
nucleic acid (RNA) and are responsible for protein synthesis in living
matter. Endoplasmic reticula are tubular structures mostly responsible for
protein synthesis. Mitochondria are ellipsoidal structures with a central
cavity and contain specific enzymes to oxidize carbohydrate and lipid to
produce energy. Lysosomes are small organelles in the cytoplasm that
contain enzymes capable of lysing many nutrients and cells.
The entire cytoplasm is enclosed within a cell membrane made of lipids
and proteins. Its primary function is to selectively prohibit or permit the
passage of substances into and out of the cell.
The growth of living matter is caused by proliferation of cells by cell divi-
sion—a process in which a cell divides into two cells. The cell division of
somatic cells is called mitosisand that of germ cells is called meiosis. Both
mitosis and meiosis, designated as M, consist of four phases:prophase,
metaphase,anaphase, and telophase. Each of these phases involves the
rearrangement of the number of chromosomes and represents the pro-
gression of cell division (Fig. 15.3) and is described below.
In prophase, the chromosome thickens in the shape of dumbbell with a
constriction at the center, called centromere. The nuclear membrane breaks
open, leading to the mixture of cytoplasm and nuclear material, and spin-
dles made of fibers are formed extending from one end (pole) of the cell
to the other. Next in the metaphase, the chromosomes move to and line up
at the central (or equatorial) plane of the cell, and the centromeres divide
into two, each attaching to the spindle. Anaphase then follows and two chro-
matids move to the two poles of the cell. The last step of cell division
involves the deconvolution of the chromosomes leading to the regenera-
tion of the nuclear membrane and nucleoli around both poles. Division of
cytoplasm (cytokinesis) sets in, and ultimately two daughter cells are
formed.
Before cell division, each cell undergoes a long period, termed interphase,
in which DNA molecules are synthesized. In DNA synthesis, two new DNA
molecules are produced from each DNA molecule, which are exact repli-
cas of the original DNA molecule. This period of DNA synthesis is desig-
nated the “S” phase, which takes place around the middle of the interphase.
The period between the telophase and the S phase is termed G 1 , and the
period between the S phase and the prophase is termed G 2 (Fig. 15.4).
During the G 1 and G 2 periods, no functional activity related to cell division
occurs. The period of the entire cell cycle including the M and S phases
varies with the types of cells. The S phase normally is the longest and G 1 is
the most variable phase in the cell cycle. The duplicate DNA molecules lead
to two identical chromosomes during mitosis, which are termed sister
chromatids.
One important difference between mitosis and meiosis is that in meiosis,
for a given series of cell division, every alternate cell division skips DNA
synthesis, thus keeping the number of chromosomes the same in germ
cells.
228 15. Radiation Biology