Chapter 6: Cell Signaling. Chapter 7: Metabolism. Chapter 8: Cellular Respiration. Chapter 9: Photosynthesis. Chapter Meiosis. Chapter Classical and Modern Genetics. Chapter Gene Expression. Chapter Biotechnology. Chapter Viruses. Chapter Nutrition and Digestion.
Chapter Nervous System. Chapter Sensory Systems. Chapter Musculoskeletal System. Chapter Endocrine System. Chapter Circulatory and Pulmonary Systems. Chapter Osmoregulation and Excretion. Chapter Immune System. Chapter Reproduction and Development. Chapter Behavior. Chapter Ecosystems. Chapter Population and Community Ecology. Chapter Biodiversity and Conservation. Chapter Speciation and Diversity. Chapter Natural Selection. Chapter Population Genetics.
Chapter Evolutionary History. Chapter Plant Structure, Growth, and Nutrition. Chapter Plant Reproduction. Chapter Plant Responses to the Environment. Full Table of Contents. This is a sample clip. Sign in or start your free trial. DNA sequences outside this 1 percent are involved in regulating when, how and how much of a protein is made.
DNA's instructions are used to make proteins in a two-step process. First, enzymes read the information in a DNA molecule and transcribe it into an intermediary molecule called messenger ribonucleic acid, or mRNA. Next, the information contained in the mRNA molecule is translated into the "language" of amino acids, which are the building blocks of proteins.
This language tells the cell's protein-making machinery the precise order in which to link the amino acids to produce a specific protein. This is a major task because there are 20 types of amino acids, which can be placed in many different orders to form a wide variety of proteins. But nearly a century passed from that discovery until researchers unraveled the structure of the DNA molecule and realized its central importance to biology.
For many years, scientists debated which molecule carried life's biological instructions. Most thought that DNA was too simple a molecule to play such a critical role. Instead, they argued that proteins were more likely to carry out this vital function because of their greater complexity and wider variety of forms. By studying X-ray diffraction patterns and building models, the scientists figured out the double helix structure of DNA - a structure that enables it to carry biological information from one generation to the next.
Despite his scientific achievements, Dr. Scientist use the term "double helix" to describe DNA's winding, two-stranded chemical structure. This shape - which looks much like a twisted ladder - gives DNA the power to pass along biological instructions with great precision. To understand DNA's double helix from a chemical standpoint, picture the sides of the ladder as strands of alternating sugar and phosphate groups - strands that run in opposite directions.
Each "rung" of the ladder is made up of two nitrogen bases, paired together by hydrogen bonds. Because of the highly specific nature of this type of chemical pairing, base A always pairs with base T, and likewise C with G. So, if you know the sequence of the bases on one strand of a DNA double helix, it is a simple matter to figure out the sequence of bases on the other strand.
DNA is made up of four different building blocks, called nucleotides, which are each made up of one of four nitrogenous bases demonstrated in Figure 1. These are the purines: guanine G and adenine A , and the pyrimidines: thymine T and cytosine C. These nucleotides are coupled to a deoxyribose sugar and are able to bind to other deoxyribose sugars via phosphate linkages to form long chains, some of which can be well over ,, molecules long.
Since each deoxyribose in a DNA chain is coupled to one of the four nitrogenous bases G, A, T, or C , these long chains can carry information. Codons are used to call for specific amino acids to be bonded together to form proteins. For instance the codon adenosine-adenosine-guanosine AAG calls for the amino acid lysine lys to be incorporated into a protein molecule. The codon AGG calls for the amino acid arginine arg. There are also codons that, under the right circumstances, call for a protein to begin to be formed start codons , or for a protein chain to be finished stop codons.
As you can see from this simple example, DNA can carry a massive amount of information. Figure 1: Adenine binds to thymine; guanine binds to cytosine. A gene is a set of codons that specify a specific protein chain, along with the associated start and stop codons. In nature, the process for information to be passed on from DNA can occur through either replication or gene expression.
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