|DNA: The Genetic Material|
14.1 What is the genetic material?
The Hammerling Experiment: Cells Store Hereditary Information in the Nucleus
• Hammerling conducted a series of experiments and discovered that hereditary information in Acetabularia resided in the foot, which is also the location of the nucleus. (p. 280)
Transplantation Experiments: Each Cell Contains a Full Set of Genetic Instructions
• Later experiments in the mid-1950s showed that the nucleus of eukaryotic cells includes a full set of genetic information. (p. 281)
The Griffith Experiment: Hereditary Information Can Pass Between Organisms
• Griffith found that transformation occurs when genetic material is transferred from one cell to another, and that live cells can be transformed by dead cells. (p. 282)
The Avery and Hershey-Chase Experiments: The Active Principle Is DNA
• Avery provided conclusive evidence that DNA is the heredity material for the bacterial specimens under investigation. (p. 283)
• Hershey and Chase provided further evidence that heredity material in bacteriophages was found in DNA, not in proteins. (p. 283)
14.2 What is the structure of DNA?
The Chemical Nature of Nucleic Acids
• Both DNA and RNA are formed of nucleotides joined together in series. Each nucleotide is composed of a five-carbon sugar, a phosphate group, and a nitrogen-containing base. (p. 284)
• Chargaff's Rule states that in reference to the nitrogen-containing bases, adenine always pairs with thymine, and guanine always pairs with cytosine. Thus, there are always equal proportions of purines and pyrimidines. (p. 285)
The Three-Dimensional Structure of DNA
• Franklin was able to obtain the first glimpse of DNA using X-ray diffraction in 1953, while Watson and Crick theorized that DNA exists in a double-helical, antiparallel configuration. (pp. 286-287)
• Using a spiral staircase analogy, the handrails of the staircase represent the sugar-phosphate backbone of the DNA double helix, and the steps represent the hydrogen-bonded base pairs. (p. 287)
14.3 How does DNA replicate?
The Meselson-Stahl Experiment: DNA Replication Is Semiconservative
• Meselson and Stahl demonstrated that DNA replication is semiconservative because each strand of the original duplex becomes one of the two strands in each new duplex. (p. 288)
The Replication Process
• Replication of E. coli begins at a specific origin, proceeds bidirectionally, and ends at a specific terminus. (p. 290)
• Many enzymes function in DNA replication, including DNA primase, which creates a short RNA primer complementary to a DNA template; DNA helicase, which unwinds the helix in front of DNA polymerase, which then synthesizes new DNA by adding nucleotides to the growing strands; and DNA ligase, which creates phosphodiester bonds between adjacent Okazaki fragments. (pp. 292-293)
• Replication can be divided into three stages: initiation, elongation, and termination. (p. 294)
Eukaryotic DNA Replication
• The major difference between prokaryotic and eukaryotic replication is that eukaryotic chromosomes have multiple replication origins, whereas prokaryotic chromosomes have a single point of origin. (p. 295)
14.4 What is a gene?
The One-Gene/One-Polypeptide Hypothesis
• Beadle and Tatum concluded that genes produce their effects by specifying the structure of enzymes, and that each gene encodes the structure of one enzyme. Today, this is commonly referred to as the one-gene/one-polypeptide relationship. (p. 297)
How DNA Encodes Protein Structure
• Over 50 years of research has yielded clear evidence that DNA is the molecule responsible for the inheritance of traits from one generation to the next, and that DNA is divided into functional subunits, or genes, located on chromosomes. (p. 298)