Molecular Biology

I

Introduction

Molecular Biology, branch of biology that seeks to understand the molecular basis of life. In particular, it relates the structure of specific molecules of biological importance—such as proteins, enzymes, and the nucleic acids DNA and RNA—to their functional roles in cells and organisms.

II

The Structure of DNA

The field of molecular biology effectively began with the discovery of the structure of DNA in 1953. Francis Crick and James Watson published the first description of the structure using research performed by Rosalind Franklin and Maurice Wilkins. This discovery was of importance not only because DNA is the molecule that transmits hereditary information from generation to generation, but also because its structure immediately provided an insight into how this transmission is achieved.

In cells DNA is a double-stranded helical molecule in which the two single-stranded chains are joined together by bonds between the bases adenine (A), guanine (G), cytosine (C), and thymine (T). In this structure, an A in one strand always pairs with a T in the other strand, and a G always pairs with a C.

When DNA replicates, the two single strands separate and the information is precisely reproduced. Each single strand becomes double-stranded by an A being inserted in the new strand to pair with a T in the old strand, a G being inserted to pair with a C, and so on. To assure accuracy, there is a proofreading capacity in cells, resulting in identical copies of DNA with each replication. In this way the hereditary information, which controls the properties of the cell and of the organism, is transmitted to daughter cells when a cell divides, and to the offspring when an organism reproduces.

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III

DNA Makes RNA, and RNA Makes Protein

After DNA was modeled, scientists began to investigate how hereditary information actually influences the activities of the cell. It was discovered that the DNA is copied, in a process called transcription, into a single-stranded molecule of the related RNA. As in the replication of DNA, the information in the bases of DNA is precisely copied by base pairing to produce the RNA by a family of enzymes called DNA-dependent RNA polymerases. RNA polymerases substitute U for every T found in DNA; that is, whenever an A is found in the DNA strand being copied into RNA, a U is inserted.

After further processing, the so-called messenger RNA (mRNA) moves to subcellular particles called ribosomes, where it is translated into protein. This translation is governed by the genetic code in which each combination of three bases, or triplet, directs the addition of a particular amino acid onto the protein chain: ACC directs addition of threonine, CCC of proline, and so on. Hence the genetic information contained in the linear array of bases in the DNA directs the production of a linear array of amino acids within a protein.

Consequently, genetic changes in the bases in the DNA result in specific changes in the protein produced. For example, an A-to-C change in an ACC triplet would lead to the addition of a proline instead of a threonine. As specific proteins have particular biological effects, changes affecting the function of the protein will lead to an alteration in the appearance or function of an organism. In this way differences in the information in the DNA are observed as inherited differences between individuals, such as eye color, or a genetic disorder such as hemophilia. The conclusion that DNA makes RNA makes protein has been referred to as the “central dogma of molecular biology.”

IV

Gene Cloning and Hybridization

Although the major advances described above were made in the 1950s and 1960s, the explosion in molecular biology began in the 1970s with the development of techniques for gene cloning. These techniques allowed the isolation of large amounts of a pure DNA fragment, free from all the other DNA sequences that together constitute the organism’s genome (all the genes in the chromosomes). This process enabled a DNA fragment—perhaps representing a particular gene—to be generated and characterized.

Gene cloning was coupled with the development of hybridization procedures in which a cloned DNA molecule is radioactively labeled and then made single-stranded. Some techniques use nonradioactive labels. The resulting molecule will bind by base pairing to any DNA or RNA that contains the same linear order of the four bases. As a result, it can be used as a probe to locate a particular DNA sequence within a DNA sample. The procedure used to accomplish this is known as Southern blotting after its inventor, Ed Southern.

In the related technique of Northern blotting, DNA from a gene is hybridized to the RNA prepared from different tissues, which allows the RNA corresponding to the gene to be detected and quantified in different tissues. These techniques have revealed much information on gene structure and expression.