Genetic Implications of DNA Structure

Watson and Crick’s great contribution was their elucidation of the genotype’s chemical structure, making it possible for geneticists to begin to examine genes directly, instead of looking only at the phenotypic consequences of gene action. The determination of the structure of DNA led to the birth of molecular genetics: the study of the chemical and molecular nature of genetic information.

Watson and Crick’s structure did more than create the potential for molecular genetic studies, however; it was an immediate source of insight into key genetic processes. At the beginning of this chapter, four fundamental properties of genetic material were identified. First, it must be capable of carrying large amounts of information. Watson and Crick’s model suggested that genetic instructions are encoded in the base sequence, the only variable part of the DNA molecule.

A second necessary property of genetic material is its ability to replicate faithfully. The complementary polynucleotide strands of DNA make this replication possible. Watson and Crick proposed that in replication, the two polynucleotide strands unzip, breaking the weak hydrogen bonds between them, and each strand serves as a template on which a new strand is synthesized. The specificity of the base pairing means that only one possible sequence of bases—the complementary sequence—can be synthesized from each template strand. Newly replicated double-stranded DNA molecules are therefore identical with the original double-stranded DNA molecule (see Chapter 9 on DNA replication).

A third essential property of genetic material is the ability to express its instructions as a phenotype. DNA expresses its genetic instructions by first transferring its information to an RNA molecule in a process termed transcription (see Chapter 10). The term transcription is appropriate because, although the information is transferred from DNA to RNA, the information remains in the language of nucleic acids. In some cases, the RNA molecule then transfers the genetic information to a protein by specifying its amino acid sequence. This process is termed translation (see Chapter 11) because the information must be translated from the language of nucleotides into the language of amino acids. A fourth property of DNA is that it must be capable of varying. This variation, as we have seen, consists of differences in the sequence of bases found among different individuals.

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Figure 8.14: Pathways of information transfer within the cell.

We can now identify three major pathways of information flow in the cell (Figure 8.14a): in replication, information passes from one DNA molecule to other DNA molecules; in transcription, information passes from DNA to RNA; and in translation, information passes from RNA to protein. This concept of information flow was formalized by Francis Crick in a concept that he called the central dogma of molecular biology. The central dogma states that genetic information passes from DNA to protein in a one-way information pathway. We now realize, however, that the central dogma is an oversimplification. In addition to the three general information pathways of replication, transcription, and translation, other transfers may take place in certain organisms or under special circumstances. Retroviruses (see Chapter 7) and some transposable elements (see Chapter 13) transfer information from RNA to DNA (in reverse transcription), and some RNA viruses transfer information from RNA to RNA (in RNA replication; Figure 8.14b).