Virtually everyone knows that the genes, located in the nuclei of all cells of the body, control heredity from parents to children, but most people do not realize that these same genes also control day-today function of all the body's cells. The genes control cell function by determining which substances are synthesized within the cell—which structures, which enzymes, which chemicals. Figure 3-1 shows the general schema of genetic control. Each gene, which is a nucleic acid called deoxyribonucleic acid (DNA), automatically controls the formation of another nucleic acid, ribonucleic acid (RNA); this RNA then spreads throughout the cell to control the formation of a specific protein. Because there are more than 30,000 different genes in each cell, it is theoretically possible to form a very large number of different cellular proteins.
Some of the cellular proteins are structural proteins, which, in association with various lipids and carbohydrates, form the structures of the various intracellu-lar organelles discussed in Chapter 2. However, by far the majority of the proteins are enzymes that catalyze the different chemical reactions in the cells. For instance, enzymes promote all the oxidative reactions that supply energy to the cell, and they promote synthesis of all the cell chemicals, such as lipids, glyco-gen, and adenosine triphosphate (ATP).
In the cell nucleus, large numbers of genes are attached end on end in extremely long double-stranded helical molecules of DNA having molecular weights measured in the billions. A very short segment of such a molecule is shown in Figure 3-2. This molecule is composed of several simple chemical compounds bound together in a regular pattern, details of which are explained in the next few paragraphs.
Basic Building Blocks of DNA. Figure 3-3 shows the basic chemical compounds involved in the formation of DNA. These include (1) phosphoric acid, (2) a sugar called deoxyribose, and (3) four nitrogenous bases (two purines, adenine and guanine, and two pyrimidines, thymine and cytosine). The phosphoric acid and deoxyribose form the two helical strands that are the backbone of the DNA molecule, and the nitrogenous bases lie between the two strands and connect them, as illustrated in Figure 3-6.
Nucleotides. The first stage in the formation of DNA is to combine one molecule of phosphoric acid, one molecule of deoxyribose, and one of the four bases to form an acidic nucleotide. Four separate nucleotides are thus formed, one for each of the four bases: deoxyadenylic, deoxythymidylic, deoxyguanylic, and deoxycytidylic acids. Figure 3-4 shows the chemical structure of deoxyadenylic acid, and Figure 3-5 shows simple symbols for the four nucleotides that form DNA.
Organization of the Nucleotides to Form Two Strands of DNA Loosely Bound to Each Other.
Figure 3-6 shows the manner in which multiple numbers of nucleotides are
RNA formation \
Cell structure Cell enzymes
Cell structure Cell enzymes
General schema by which the genes control cell function.
The helical, double-stranded structure of the gene. The outside strands are composed of phosphoric acid and the sugar deoxyri-bose. The internal molecules connecting the two strands of the helix are purine and pyrimidine bases; these determine the "code" of the gene.
Phosphoric acid O
Was this article helpful?
This ebook provides an introductory explanation of the workings of the human body, with an effort to draw connections between the body systems and explain their interdependencies. A framework for the book is homeostasis and how the body maintains balance within each system. This is intended as a first introduction to physiology for a college-level course.