Chapter Summary

• Griffith’s experiments in the 1920s demonstrated that some substance in cells can cause heritable changes in other cells. Review Figure 13.1

• The location and quantity of DNA in the cell suggested that DNA might be the genetic material. Avery and his colleagues isolated the transforming principle from bacteria and identified it as DNA. Review Figure 13.2

• The Hershey–Chase experiments established conclusively that DNA (and not protein) is the genetic material, by tracing the DNA of radioactively labeled viruses, which were used to infect bacterial cells. Review Figure 13.4, Animation 13.1

• Genetic transformation of eukaryotic cells is often called transfection. Transformation and transfection can be studied with the aid of a genetic marker gene that confers a known and observable phenotype.

• Chargaff’s rule states that the amount of adenine in DNA is equal to the amount of thymine, and that the amount of guanine is equal to the amount of cytosine; thus the total abundance of purines (A + G) equals the total abundance of pyrimidines (T + C).

• X-ray diffraction showed that the DNA molecule is a double helix. Watson and Crick proposed that the two strands in DNA are antiparallel. Review Figure 13.6

• Complementary base pairing between A and T and between G and C accounts for Chargaff’s rule. The bases are held together by hydrogen bonding.

• Reactive groups are exposed in the paired bases, allowing for recognition by other molecules such as proteins. Review Figure 13.7

• Meselson and Stahl showed that DNA undergoes semiconservative replication. Each parent strand acts as a template for the synthesis of a new strand; thus the two replicated DNA molecules each contain one parent strand and one newly synthesized strand. Review Investigating Life: The Meselson–Stahl Experiment, Animation 13.3

• In DNA replication, the enzyme DNA polymerase catalyzes the addition of nucleotides to the 3′ end of each strand. Which nucleotides are added is determined by complementary base pairing with the template strand. Review Figure 13.9

• The pre-replication complex is a huge protein complex that attaches to the chromosome at the origin of replication (ori).

• Replication proceeds from the origin of replication on both strands in the 5′-to-3′ direction, forming a replication fork. Review Figure 13.10

• Primase catalyzes the synthesis of a short RNA primer to which nucleotides are added by DNA polymerase. Review Figure 13.11

• Many proteins assist in DNA replication. DNA helicase separates the strands, and single-strand binding proteins keep the strands from reassociating. Review Figure 13.13, Activity 13.1

• The leading strand is synthesized continuously and the lagging strand in pieces called Okazaki fragments. The fragments are joined together by DNA ligase. Review Focus: Key Figure 13.13, Figure 13.14, Animation 13.4

• The speed with which DNA polymerization proceeds is attributed to the processive nature of DNA polymerases, which can catalyze many polymerizations at a time. A sliding DNA clamp helps ensure the stability of this process. Review Figure 13.16

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• At the ends of eukaryotic chromosomes are regions of repetitive DNA sequence called telomeres. Unless the enzyme telomerase is present, a short segment at the end of each telomere is lost each time the DNA is replicated. After multiple cell cycles, the telomeres shorten enough to cause chromosome instability and cell death. Review Figure 13.17

• DNA polymerases make about one error in 100,000 bases replicated. DNA is also subject to natural alterations and chemical damage. DNA can be repaired by at least three different mechanisms: proofreading, mismatch repair, and excision repair. Review Figure 13.18

• The polymerase chain reaction technique uses DNA polymerase to make multiple copies of DNA in the laboratory. Review Figure 13.19, Activity 13.2