Principles of Biochemistry, Volume 3 - A. Lehninger 1985

Molecular mechanisms of genetic information transfer
DNA: structure of chromosomes and genes
The nucleotide sequence of DNA serves as a template

DNA molecules are long polymer chains featuring specific sequences of the four primary bases A, T, G, and C, which act as symbols designed to encode Genetic information. Consequently, we say that The nucleotide sequence in DNA serves as a template for METABOLISM/36.html">DNA Replication. However, it is essential to understand why templates are required for the accurate replication, Transcription, and Translation of genetic information.

During The Biosynthesis of the non-informational macromolecule Glycogen, which consists of repeating units of a single type—D-glucose—the identity and purity of the final product are ensured by the Active Site of glycogen synthase (Section 20.13). This enzyme is characterized by substrate Specificity; that is, its active site is capable of binding only a UDP-glucose molecule and the non-reducing end of the glycogen chain that is to be elongated. In principle, the active site of this enzyme (like that of all Other Enzymes) can be viewed as a template (meaning a mold or form), since a complementary "fit" is established between the substrate molecule(s) and the active site.

In the case of DNA, RNA, and Polypeptides, a single enzyme active site is insufficient to ensure the specific sequence of coding units. Enzyme active sites are relatively small and can bind simultaneously only one or a few building-block molecules, positioning them to ensure their precise assembly in the correct sequence. Nucleic Acids, containing thousands or millions of nucleotide units, are so large that enzyme active sites are simply too small to directly dictate the complete sequence in which the nucleotide units must be assembled. Therefore, a single DNA strand must serve as the template for forming the nucleotide sequence of its complementary partner.

Another aspect of the Watson-Crick hypothesis is that The Structure of the DNA double helix suggests a mechanism by which the genetic information contained within DNA can be precisely reproduced (Fig. 27-13). Because the two strands of the DNA double helix are structurally complementary, their nucleotide sequences carry complementary information. Watson and Crick postulated that DNA replication during Cell Division begins with the Separation of the two strands, with each strand acting as a template that dictates the nucleotide sequence of a new complementary strand synthesized by replication enzymes. It was suggested that The fidelity of replication for each DNA strand is ensured by the precise pairing and Stability of the complementary Base Pairs A=T and G=C in the two daughter duplexes, each containing one parental DNA strand and a newly synthesized strand complementary to it. It was also postulated that each newly formed daughter double helix is passed on to a daughter cell unchanged. In Chapter 28, we will see how this hypothesis was experimentally confirmed.

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Fig. 27-13. The Watson-Crick model of DNA replication. The complementary strands of the parental DNA separate, and each serves as a template for the biosynthesis of a complementary daughter strand (daughter strands are shown in red).