The Blueprint of Life from DNA to Protein


t n 1866, the Czech monk Gregor Mendel showed that traits rn are inherited by means of physical units, which we now call * genes. It was not until 1941, however, that the precise function of genes was revealed when George Beadle, a geneticist, and Edward Tatum, a chemist, published a scientific paper reporting that genes determine the structure of enzymes. Biochemists had already shown that enzymes catalyze the conversion of one compound into another in a biochemical pathway.

Beadle and Tatum studied Neurospora crassa, a common bread mold that grows on a very simple medium containing sugar and simple inorganic salts. Beadle and Tatum created N. crassa strains with altered properties, mutants, by treating cells with X rays, which were known to alter genes. Some of these mutants could no longer grow on the glucose-salts medium unless growth factors such as vitamins were added to the medium. To isolate these Beadle and Tatum had to laboriously screen thousands of progeny to find the relatively few that required the growth factors. Each mutant presumably contained a defective gene.

The next task for Beadle and Tatum was to identify the specific biochemical defect of each mutant. To do this, they added different growth factors, one at a time, to each mutant culture. The one that allowed a particular mutant to grow had presumably bypassed the function of a defective enzyme. In this manner, they were able to pinpoint in each mutant the specific step in the biochemical pathway that was defective. Then, using these same mutants, Beadle and Tatum showed that the requirement for each growth factor was inherited as a single gene, ultimately leading to their conclusion that a single gene determines the production of one enzyme. Their conclusion has been modified somewhat, because we now know that some enzymes are made up of more than one protein. A single gene determines the production of one protein. In 1958, Beadle and Tatum shared the Nobel Prize in Medicine, largely for these pioneering studies that ushered in the era of modern biology.

As so often occurs in science, the answer to one question raised many more questions. How do genes specify the synthesis of enzymes? What are genes made of? How do genes replicate? Numerous other investigators won more Nobel Prizes for answering these questions, many of which are covered in this chapter.

—A Glimpse of History

CONSIDER FOR A MOMENT THE VAST DIVERSITY OF cellular life forms that exist. Our world contains a remarkable variety of microorganisms and specialized cells that make up

DNA double helix plants and animals. Every characteristic of each of these cells, from its shape to its function, is dictated by information contained in its deoxyribonucleic acid (DNA). DNA encodes the master plan, the blueprint, for all cell structures and processes. Yet for all the complexity this would seem to require, DNA is a string composed of only four different nucleotides, each containing a particular nitrogenous base: adenine (A), thymine (T), cytosine (C), or guanine (G). ■ nucleotides, p. 31

While it might seem improbable that the vast array of life forms can be encoded by a molecule consisting of only four different units, think about how much information can be transmitted by binary code, the language of all computers, which has a base of only two. A simple series of ones and zeros can code for each letter of the alphabet. String enough of these series together in the right sequence and the letters become words, and the words can become complete sentences, chapters, books, or even whole libraries.

The four nucleotides of a DNA molecule create information in a similar fashion. A set of three nucleotides encodes a specific amino acid. In turn, a string of amino acids makes up a protein, the function of which is dictated by the order of the amino acid subunits. Some proteins serve as structural components of a cell. Others, such as enzymes, mediate cellular activities including biosynthesis and energy conversion.

168 Chapter 7 The Blueprint of Life, from DNA to Protein

Together, proteins synthesized by a cell are responsible for every aspect of that cell. Thus, the sequential order of nucleotide bases in a cell's DNA ultimately dictates the characteristics of that cell. ■ amino acids, p. 25 ■ protein structure, p. 27 ■ flagella, p. 63 ■ enzymes, pp. 131,138

This chapter will focus on the cellular process of converting the information encoded within DNA into proteins, concentrating primarily on the mechanisms used by prokaryotic cells. The eukaryotic processes have many similarities, but they are considerably more complicated and will only be discussed briefly.

7.1 Overview

The complete set of genetic information for a cell is referred to as its genome. Technically, this includes plasmids as well as the chromosome; however, the term genome is often used interchangeably with chromosome. The genome of all cells is composed of DNA, but some viruses have an RNA genome. The functional unit of the genome is a gene. A gene encodes a product, the gene product, most commonly a protein. The study of the function and transfer of genes is called genetics, whereas the study and analysis of the nucleotide sequence of DNA is called genomics. ■ chromosome, p. 66 ■ plasmid, p. 66

All living cells must accomplish two general tasks in order to multiply. The double-stranded DNA must be duplicated before cell division so that its encoded information can be passed on to future generations. This is the process of DNA replication. In addition, the information encoded by the DNA must be deciphered, or expressed, so that the cell can synthesize the necessary gene products at the appropriate time. Gene expression involves two interrelated processes, transcription and translation. Transcription copies the information encoded in DNA into a slightly different molecule, RNA. The RNA serves as a transitional, temporary form of the genetic information and is the one that is actually deciphered. Translation interprets information carried by RNA to synthesize the encoded protein. The chemistry and structure of DNA and RNA ensure that each of these processes can occur with great accuracy.

The flow of information from DNA to RNA to protein is often referred to as the central dogma of molecular biology (figure 7.1). It was once believed that information flow proceeded only in this direction. Although this direction is by far the most common, certain viruses, such as the one that causes AIDS, have an RNA genome but copy that information into the form of DNA.

Characteristics of DNA

A single strand of DNA is composed of a series of deoxyri-bonucleotide subunits, more commonly called nucleotides. These are joined in a chain by a covalent bond between the 5'PO4 (5 prime phosphate) group of one nucleotide and the 3'OH (3 prime hydroxyl) group of the next. Note that the designations 5' and 3' refer to the numbered carbon atoms of the pentose sugar of the nucleotide (see figure 2.22). Joining of the nucleotides in this manner creates a series of alternating sugar and phosphate moieties, called the sugar-phosphate backbone. Connected to

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