Chap 6

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CHAPTER 6

  • The ability of a cell to maintain order in a chaotic environment depends on the accurate duplication on the accurate duplication of the vast quantity of genetic information carried in its DNA
  • Each of the two strands can act as a template for the synthesis of the other strand. A DNA double helix thus carries the same information in each of its strands.
  • A DNA molecule is duplicated (replicated) by the polymerization of new complementary strands using each of the old strands of the DNA double helix as a template. Two identical DNA molecules are formed, enabling the genetic information to be copied and passed on from cell to daughter cell and from parent to offspring.
  • As a DNA molecule replicates, its two strands are pulled apart to form one or more Y-shaped replication forks. DNA polymerase enzymes, situated at the fork, produce a new complementary DNA strand on each parental strand.
  • DNA polymerase replicates a DNA template with remarkable fidelity, making less than one error in every 107 bases read. This accuracy is made possible, in part, by a proofreading process in which the enzyme removes its own polymerization errors as it moves along the DNA.
  • Because DNA polymerase synthesizes new DNA in only one direction, only the leading strand at the replication fork can be synthesized in a discontinuous backstitching process, producing short fragments of DNA that are later joined by the enzyme DNA ligase to complete that DNA strand.
  • DNA polymerase is incapable of starting a new DNA chain from scratch. Instead, DNA synthesis is primed by an RNA polymerase called primase, which makes short lengths of RNA (primers) that are elongated by DNA polymerase. The polymers are subsequently erased and replaced with DNA.
  • DNA replication requires the cooperation of many proteins; these form a multi-enzyme replication machine that catalyzes DNA synthesis.
  • In eukaryotes, a special enzyme called telomerase replicates the DNA at the ends of the chromosomes.
  • The rare copying mistakes that slip through the DNA replication machinery are dealt with by the mismatch repair proteins. The overall accuracy of the DNA replication, including mismatch repair, is one mistake per 109 nucleotides copied.
  • DNA damage caused by unavoidable chemical reactions is repaired by a variety of enzymes that recognize damaged DNA that excise is a short stretch of the DNA strand that contains it. The missing DNA is resynthesized by a repair of DNA polymerase that uses the undamaged strand as a template.
  • Nonhomologous end-joining allows the rapid repair of double-strand DNA breaks; the process often alters the DNA sequence at the site of the repair.
  • Homologous recombination can faithfully repair double-strand DNA breaks using a homologous chromosome sequence as a guide. During meiosis, a related homologous recombination process causes a shuffling of genetic information that creates DNA molecules with novel sequences.
  • Mobile genetic elements, or transposons, move from place in the genomes of their hosts, providing a sourse of genetic variation.
  • Nearly half of the human genome consists of mobile genetic elements. Two classes of these elements have multiplied to especially high copy numbers.
  • Viruses are little more than genes packaged in protective protein coats. They require host cells to reproduce themselves.
  • Some viruses have RNA instead of DNA as their genomes. One group of RNA viruses — the retroviruses — must copy their RNA genomes with DNA to replicate.

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