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All living systems are ordinarily capable of reproducing themselves. The replication of DNA, however, lies at the heart of all other forms of reproduction. Though cell division might seem the supreme act of replication, enzymes and other proteins are continually replicated at the ribosomes. So, too, are membranes and mitochondria. Hence, replication is going on within a cell whether the cell itself is dividing or not.

When cell division does occur, the parent cell splits into two daughter cells, each of which has the same parts the parent had. In this replication process, the two strands of each DNA molecule separate, and each daughter cell receives a strand. Afterward, the DNA strands that the daughter cells receive act as the templates on which their complementary strands are built. As a consequence, the total genetic package received in part from the parent is reestablished in each daughter cell.

In one-celled organisms cell division is the means of reproduction. In many-celled organisms it is the means whereby the organisms' tissues grow and are maintained. Cell division in higher organisms begins when the cell's nuclear membrane breaks down. Then DNA's paired structures, called chromosomes, line up in the middle of the cell and separate through a series of complex maneuvers. Finally, a cleavage furrow forms, and the cell splits in half, providing each daughter cell with its critical structures. Amid this spectacle, membranes are replicated. Many of the details of replication have yet to be determined.

Living things and their components have distinct shapes because the architecture of their molecules is tailored for their specific tasks. For example, each of the thousand or more protein molecules has a special job. It might be involved in catalysis, in electron transfer, or in membrane construction, to name a few. A protein molecule has a shape uniquely suited for its assignment. The hemoglobin molecule, for example, has a pocket for carrying oxygen or carbon dioxide during respiration. The rod-shaped collagen molecule stiffens tissues and organs. The same notion of fit-to-function applies to most other cell molecules. DNA is designed for the storage of genetic information, phospholipids for use in membranes, and ATP for the storage of usable energy.

DNA. Deoxyribonucleic acid, a molecule that holds the genetic information needed for heredity.

electromechanochemical energy. The interconversion of electrical, mechanical, and chemical energy by the cell's energy-gathering systems to unleash, gather, and store the power locked in ATP.

enzyme. A protein that catalyzes, or speeds up, biochemical reactions without itself undergoing a lasting change.

phospholipid. A bimodal molecule composed of two contradictory elements a phosphate group attractive to water and a lipid, which repels water.

protein. A large molecule composed of amino acids strung together in a unique arrangement.

RNA. Ribonucleic acid, the complement to DNA, which transcribes DNA's genetic instructions for the manufacture of proteins.

BIOCHEMISTRY. Scientists in the field of biochemistry study the chemical basis of life's activities. They have shown that all living things amoebas and elephants alike share many similarities at the level of atoms and molecules. Without exception, all animals and plants operate on the basis of a few unvarying biological principles. These principles are: all forms of life consist of basic units called cells; every living thing has a heredity; all vital activities require energy; all cells undergo certain key chemical reactions; and all living groups reproduce. What goes on in the life of a cell stems from an interplay of these few important principles.

To understand cell activities one must know about membranes and their functions. A cell is surrounded by a continuous membrane. It walls the cell's interior from the outer environment. The life processes go on inside the cytoplasm, or cell interior. The cell interior contains tiny organelles with membranes. These organelles include the mitochondrion (plural, mitochondria), the chloroplast (in plants only), the endoplasmic reticulum, and the nucleus.

All the membranes of a cell are so thin that their width can be seen only under the extremely high magnification of the electron microscope (see Microscope). A membrane is constructed from two types of molecules proteins and phospholipids. They nest together to form the membrane. Both types of molecules have two surfaces. One surface, the hydrophilic one, water. The other surface, the hydrophobic one, water but likes oil. Membrane proteins and phospholipids are arranged in paired tiers, with protein tiers alternating with phospholipid tiers. Since water is a major component of the cytoplasm and also of the outside environment, the fashion in which protein and phospholipid surfaces react to water forms the unique basis of membranes. Arranged in paired tiers, the membrane molecules expose their water-loving surfaces to the water both inside and outside the cell. By contrast, their water-hating, oil-loving surfaces avoid the water by lining up opposite each other at the middle of the membrane. This tightly organized molecular arrangement is so stable that it tenaciously resists disruption. Even when disrupted by strong forces, it tries to reseal any momentary holes to keep a continuous surface. Only membrane proteins, however, are designed for membrane service. Ordinary proteins having only water-loving surfaces cannot be used in membranes.

Each of the cell's organelles has its own distinctive membrane containing specific types of proteins and phospholipids. The specificity of membranes is possible because they can contain an endless variety of water-loving and oil-loving components as long as their bimodal character is kept.

What does a membrane do? One of its functions is to serve as a container. Another is to act as a barrier for preventing molecules from moving into and out of a cell at random. A membrane does this by providing molecular that regulate which molecules can enter and which cannot. Still another function is fulfilled by a membrane: it houses some of the cell's enzymes as well as its energy-converting .The membrane enzymes, which are special proteins themselves, carry out respiration needed for energy production, active transport of materials across membranes, metabolic cycles essential for life, and many molecule-building activities (see Biophysics; Enzymes).

Enzymes can be easily assembled on membranes. This feature pays enormous dividends to a cell because its vital biochemical reactions are facilitated by these important proteins. The manner in which protein molecules pair together in the double-tier arrangement of membranes is akin to the way in which complementary strands of deoxyribonucleic acid (DNA) pair. Since DNA, the important molecule of heredity, directs the assembly of enzymes and other proteins by complementarily matching certain chemical groups, it is clear why complementarity plays such a major role in cellular activities.

Every living system has a blueprint for replication, or making copies of itself. This blueprint is commonly called heredity. The key structure of the hereditary process is the long, spiral DNA molecule. DNA consists of two complementary strands coiled around each other to form a twisting ladder called a double helix (see Genetics). The strands are made up of varying sequences of chemical groups which are called nucleotides. A nucleotide consists of a sugar and a phosphate group plus either of two purine bases adenine (A) and guanine (G) or either of two pyrimidine bases thymine (T) and cytosine (C).

DNA contains the genetic code for making proteins from smaller molecules called amino acids. [3]