Summary

Concept 9.1: DNA Structure Reflects Its Role as the Genetic Material

• Circumstantial evidence for DNA as the genetic material includes its presence in the nucleus, its doubling during S phase of the mitotic cell cycle, and its injection into host cells by viruses. Review Figures 9.1 and 9.2
• Experimental evidence for DNA as the genetic material is provided by the transformation of one genotype into another by adding DNA. Review Figure 9.3
• In DNA, the amount of A equals the amount of T and the amount of G equals the amount of C. This observation, along with X-ray crystallography data, helped Watson and Crick unravel the helical structure of DNA. Review Figures 9.4 and 9.5

Concept 9.2: DNA Replicates Semiconservatively

• DNA exhibits 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 (the template) and one newly synthesized strand. Review ANIMATED TUTORIALS 9.1 and 9.2
• In DNA replication, the enzyme DNA polymerase catalyzes the addition of nucleotides to the 3′ end of each new strand. Review Figure 9.7
• Replication proceeds in both directions from the origin of replication. The parent DNA molecule unwinds to form a replication fork. Review Figure 9.8 and ANIMATED TUTORIAL 9.3
• Primase catalyzes the synthesis of a short RNA primer to which nucleotides are added by DNA polymerase. Review Figure 9.9 and ACTIVITY 9.1
• The leading strand is synthesized continuously. The lagging strand is synthesized in pieces called Okazaki fragments. The fragments are joined together by DNA ligase. Review Figures 9.11 and 9.12 and ANIMATED TUTORIAL 9.4
• Eukaryotic chromosomes have repetitive sequences at each end called telomeres. DNA replication leaves a short, unreplicated sequence at the 5′ end of each new DNA strand. Unless the enzyme telomerase is present, the sequence is removed. After multiple cell cycles the telomeres shorten, leading to chromosome instability and cell death. Review Figure 9.13
• DNA polymerases make errors, which can be repaired by proofreading and mismatch repair. Review Figure 9.14
• The polymerase chain reaction, or PCR technique uses DNA polymerase to make multiple copies of DNA in the laboratory. Review Figure 9.15 and ANIMATED TUTORIAL 9.5

Concept 9.3: Mutations Are Heritable Changes in DNA

• Somatic mutations occur in the body cells of an individual and are passed on to daughter cells during mitosis. Only germline mutations (mutations in the cells that give rise to gametes) can be passed on to sexually produced offspring.
• Point mutations are alterations in single base pairs of DNA. Silent mutations can occur in genes or nontranscribed regions and do not affect the amino acid sequences of proteins. A mutation in a protein-coding region can lead to an alteration in the amino acid sequence of the protein. Review Figure 9.16
• Chromosomal mutations (deletions, duplications, inversions, and translocations) involve large regions of chromosomes. Review Figure 9.17
• Spontaneous mutations occur because of instabilities in DNA or chromosomes. Induced mutations occur when a mutagen damages DNA. Review Figure 9.18