Division base pairs

The double helix model provides a simple explanation for cell division and reproduction. In the reproduction process, the two DNA chains unwind from each other. As this happens, a new matching chain of DNA is synthesized on each of the original ones, creating two double helices. Since the base pairs in each new double helix must match in the same way as in the original, the two new double helices must be identical to the original. Exact replication of genetic data is thereby accomplished, however complex that data may be. 

Rephcation errors, even with a very efficient repair system, lead to the accumulation of mutations. A human has 10 nucleated cells each with 3 X 10 base pairs of DNA. If about 10 cell divisions occur in a lifetime and 10 mutations per base pair per cell generation escape repair, there may evenmaUy be as many as one mutation per 10 bp in the genome. Formnately, most of these will probably occur in DNA that does not encode proteins or will not affect the function of encoded proteins and so are of no consequence. In addition, spontaneous and chemically induced damage to DNA must be repaired. 

During cell division, the DNA molecule is replicated so that each daughter cell will carry its own DNA molecule. During the process, the two strands of DNA unwind, and each strand then acts as the template for synthesis of a new strand in each case, the new strand is complementary to the original because of the base-pairing restrictions (Figure 14.3). 

During cell division (replication), part of the double helix unwinds, temporarily separating the base pairs. The individual strands then act as templates for producing new, complementary strands. After replication, each double helix contains one old and one new strand, thereby ensuring that all genes are intact. DNA is responsible also for the synthesis of KNA. 

DNA replication must be very rapid, given the sizes of the genomes and the rates of cell division. The E. coli genome contains 4.8 million base pairs and is copied in less than 40 minutes. Thus, 2000 bases are incorporated per second. We shall examine some of the properties of the macromolecular machines that replicate DNA with such high accuracy and... 

The gully complementary nature of the base pairs in DNA means that each strand can act as a blueprint for the other. Assisted by the enzyme DNA polymerase, the DNA molecule can be unzipped and replicate itself exactly by the assemblage of new nucleotides from the cell s pool. This process is known as semiconservative replication because the two identical DNA molecules produced consist of one original and one new strand of DNA. DNA is able to replicate itself, which is essential to the process of cell division. As a result, an exact copy of the cell s genetic instructions can be handed on to the next generation of cells. 

Section 27.27 Nucleic acids derived from 2-deoxy-D-ribose (DNA) are responsible for storing and transmitting genetic information. DNA exists as a double-stranded pair of helices in which hydrogen bonds are responsible for complementary base pairing between adenine (A) and thymine (T), and between guanine (G) and cytosine (C). During cell division the two strands of DNA unwind and are duplicated. Each strand acts as a template on which its complement is constructed. 

RNA is formed by the transcription of DNA. On cell division, the two chains of the helix unwind, and each strand is used as a template for the construction of an RNA molecule. The complementary bases pair up, and the completed RNA (which corresponds to only a section of the DNA) then unwinds from DNA and travels to the nucleus. Unlike DNA, RNA remains a single strand of nucleotides. 

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