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Dogma, DNA, and Enzymes

BIO. "What is Biotechnology?" Washington, D.C.: Biotechnology Industry Organization, 1990.

The Central Dogma

Though it comes as no surprise that the composition of DNA between different organisms is different, it is not immediately obvious why the muscle cells, blood cells, and brain cells of any one particular vertebrate are so different in their structure and composition when the DNA of every one of their cells is identical. This is the key to one of the most exciting areas of modern cell biology. In different cell types, different sets of the total number of genes (genome) are expressed. In other words, different regions of the DNA are "active" in the muscle cells, blood cells, and brain cells.

To understand how this difference in DNA activity can lead to differences in cell structure and composition, it is necessary to consider what is often known as the central dogma of molecular biology: "DNA makes RNA makes protein." In molecular terms, a gene is that portion of DNA that encodes for a single protein. The dictum "one gene makes one protein" has required some modification with the discovery that some proteins are composed of several different polypeptide chains, but the "one gene makes one polypeptide" rule does hold.

DNA Contains the Blueprint for all Cell Proteins

Messenger RNA is a precise copy (transcript) of the coded sequence of nucleic acid bases in DNA, and this message is translated into a unique protein molecule on specialist organelles (ribosomes) present in the cytoplasm of all cells. Proteins, which are largely made up of carbon (C), hydrogen (H), oxygen (0), and nitrogen (N), are constructed from 20 different, common amino acids. The versatility of proteins, the workhorse molecules of the cell, stems from the immense variety of molecular shapes that can be created by linking amino acids together in different sequences. The smaller proteins consist of only a few dozen amino acids, whereas the larger ones may contain in excess of 200 amino acids, all linked together in a linear chain by peptide bonds.

As the proteins are released from the ribosome, they fold into unique shapes, under the influence of chemical forces that depend on the particular sequence of amino acids. So the protein primary sequence, encoded in the gene and faithfully transcribed and translated into an amino acid chain, determines the three-dimensional structure of the emerging molecule. The human body possesses some 30,000 different kinds of proteins and several million copies of many of these. Each plays a specific role - for example, hemoglobin carries oxygen in the blood, actin and myosin interact to generate muscle movement, and acetylcholine receptor molecules mediate chemical transmission between nerve and muscle cells.

Enzymes - Protein Biocatalysts

An essential group of proteins - the enzymes - act as biological catalysts and regulate all aspects of cell metabolism. They enable breakdown of high-energy food molecules (carbohydrates) to provide energy for biological reactions, and they control the synthetic pathways that result in the generation of lipids (e.g., fats, cholesterol, and other vital membrane components), carbohydrates (sugars, starch, and cellulose - the key components of plant cell walls), and many vital small biomolecules essential for cell function.

Though grouped together for their capacity to speed up chemical reactions that would proceed only very slowly at room temperature, different classes of enzymes vary greatly in their structure and function. Most cells contain about a thousand different enzymes, each capable of catalyzing a unique chemical reaction.

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