Nucleic Acid Structure and DNA Replication

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CHAPTER 33 The Structure of Informational Macromolecules: DNA and RNA
CHAPTER 34 DNA Replication
CHAPTER 35 DNA Repair and Recombination

Thus far in our study of biochemistry, we have focused on the extraction of energy from fuels and the synthesis of fuel stores for later use. We have examined the metabolism of many small molecules, including carbohydrates, amino acids, and nucleotides. In this section and the following ones, we will examine how energy and small molecules are used to construct the macromolecules of biochemistry. Nucleic acids—DNA and RNA—function as the informational molecules in the cell. DNA is the stable genetic information that is passed from one generation to the next, whereas RNA, a biologically accessible copy of DNA, is a more transient copy of this information. Information carried by nucleic acids is represented by a sequence of only four letters: A, G, C, and T for DNA; and A, G, C, and U for RNA. The arrangement of these four letters into sequences of various lengths provides the blueprint for every living organism on Earth.

To preserve the information encoded in DNA through many cell divisions and prevent the introduction of mutations, the copying of the genetic information must be extremely faithful: an error rate of less than 1 base pair (bp) per 3 × 109 bp is required. Such remarkable accuracy is achieved through a multilayered system of accurate DNA synthesis, proofreading during DNA synthesis, and repair after replication.

Even after DNA has been replicated, the genome is still not safe. A variety of environmental elements can damage DNA, introducing changes in the DNA sequence (mutations), or cause lesions that can block DNA replication. All organisms contain DNA-repair systems that detect DNA damage and act to preserve the original sequence. Mutations in genes that encode components of DNA-repair systems are key factors in the development of cancer.

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We begin our study of nucleic acids by focusing on structure. Especially noteworthy is the double-helical structure of DNA and how this structure facilitates the transfer of genetic information from one generation to the next. After this structural examination, we will undertake a mechanistic examination of the process of DNA replication and repair.

✓ By the end of this section, you should be able to: