DNA Structure and Function Chapter 13
Impacts, Issues Here Kitty, Kitty, Kitty, Kitty, Kitty Clones made from adult cells have problems; the cell s DNA must be reprogrammed to function like the DNA of an egg
13.1 The Hunt for DNA Investigations that led to our understanding that DNA is the molecule of inheritance reveal how science advances
Early and Puzzling Clues 1800s: Miescher found DNA (deoxyribonucleic acid) in nuclei Early 1900s: Griffith transferred hereditary material from dead cells to live cells Mice injected with live R cells lived Mice injected with live S cells died Mike injected with killed S cells lived Mice injected with killed S cells and live R cells died; live S cells were found in their blood
Griffith s Experiments
R S R A Mice injected with live cells of harmless strain R do not die. Live R cells are in their blood. B Mice injected with live cells of killer strain S die. Live S cells are in their blood. C Mice injected with heat-killed S cells do not die. No live S cells are in their blood. D Mice injected with live R cells plus heat-killed S cells die. Live S cells are in their blood. Fig. 13-2, p. 204
R S R A Mice injected with live cells of harmless strain R do not die. Live R cells are in their blood. B Mice injected with live cells of killer strain S die. Live S cells are in their blood. C Mice injected with heat-killed S cells do not die. No live S cells are in their blood. D Mice injected with live R cells plus heat-killed S cells die. Live S cells are in their blood. Stepped Art Fig. 13-2, p. 204
Animation: Griffith s experiment
Avery and McCarty Find the Transforming Principle 1940: Avery and McCarty separated deadly S cells (from Griffith s experiments) into lipid, protein, and nucleic acid components When lipids, proteins, and RNA were destroyed, the remaining substance, DNA, still transformed R cells to S cells Conclusion: DNA is the transforming principle
Confirmation of DNA s Function 1950s: Hershey and Chase experimented with bacteriophages (viruses that infect bacteria) Protein parts of viruses, labeled with 35 S, stayed outside the bacteria DNA of viruses, labeled with 32 P, entered the bacteria Conclusion: DNA, not protein, is the material that stores hereditary information
The Hershey-Chase Experiments
Fig. 13-3a, p. 205
Fig. 13-3b, p. 205
Virus particle coat proteins labeled with 35 S 35 S remains outside cells DNA being injected into bacterium B In one experiment, bacteria were infected with virus particles labeled with a radioisotope of sulfur ( 35 S). The sulfur had labeled only viral proteins. The viruses were dislodged from the bacteria by whirling the mixture in a kitchen blender. Most of the radioactive sulfur was detected in the viruses, not in the bacterial cells. The viruses had not injected protein into the bacteria. Fig. 13-3b, p. 205
Fig. 13-3c, p. 205
Virus DNA labeled with 32 P Labeled DNA being injected into bacterium 32 P remains inside cells C In another experiment, bacteria were infected with virus particles labeled with a radioisotope of phosphorus ( 32 P). The phosphorus had labeled only viral DNA. When the viruses were dislodged from the bacteria, the radioactive phosphorus was detected mainly inside the bacterial cells. The viruses had injected DNA into the cells evidence that DNA is the genetic material of this virus. Fig. 13-3c, p. 205
Virus particle coat proteins labeled with 35 S 35 S remains outside cells DNA being injected into bacterium Virus DNA labeled with 32 P Labeled DNA being injected into bacterium 32 P remains inside cells Stepped Art Fig. 13-3, p. 205
Animation: Hershey-Chase experiments
13.1 Key Concepts Discovery of DNA s Function The work of many scientists over more than a century led to the discovery that DNA is the molecule that stores hereditary information about traits
13.2 The Discovery of DNA s Structure Watson and Crick s discovery of DNA s structure was based on almost fifty years of research by other scientists
DNA s Building Blocks Nucleotide A nucleic acid monomer consisting of a fivecarbon sugar (deoxyribose), three phosphate groups, and one of four nitrogen-containing bases DNA consists of four nucleotide building blocks Two pyrimidines: thymine and cytosine Two purines: adenine and guanine
Four Kinds of Nucleotides in DNA
adenine (A) deoxyadenosine triphosphate, a purine Fig. 13-4a, p. 206
guanine (G) deoxyguanosine triphosphate, a purine Fig. 13-4b, p. 206
thymine (T) deoxythymidine triphosphate, a pyrimidine Fig. 13-4c, p. 206
cytosine (C) deoxycytidine triphosphate, a pyrimidine Fig. 13-4d, p. 206
Chargaff s Rules The amounts of thymine and adenine in DNA are the same, and the amounts of cytosine and guanine are the same: A = T and G = C The proportion of adenine and guanine differs among species
Franklin, Watson and Crick Rosalind Franklin s research in x-ray crystallography revealed the dimensions and shape of the DNA molecule: an alpha helix This was the final piece of information Watson and Crick needed to build their model of DNA
Watson and Crick s DNA Model A DNA molecule consists of two nucleotide chains (strands), running in opposite directions and coiled into a double helix Base pairs form on the inside of the helix, held together by hydrogen bonds (A-T and G-C)
Patterns of Base Pairing Bases in DNA strands can pair in only one way A always pairs with T; G always pairs with C The sequence of bases is the genetic code Variation in base sequences gives life diversity
Structure of DNA
Fig. 13-5a, p. 207
2-nanometer diameter 0.34 nanometer between each base pair 3.4-nanometer length of each full twist of the double helix The numbers indicate the carbon of the ribose sugars (compare Figure 13.4). The 3 carbon of each sugar is joined by the phosphate group to the 5 carbon of the next sugar. These links form each strand s sugar phosphate backbone. The two sugar phosphate backbones run in parallel but opposite directions (green arrows). Think of one strand as upside down compared with the other. Fig. 13-5b, p. 207
Animation: DNA close up
13.2 Key Concepts Discovery of DNA s Structure A DNA molecule consists of two long chains of nucleotides coiled into a double helix Four kinds of nucleotides make up the chains, which are held together along their length by hydrogen bonds
13.3 DNA Replication and Repair A cell copies its DNA before mitosis or meiosis I DNA repair mechanisms and proofreading correct most replication errors
Semiconservative DNA Replication Each strand of a DNA double helix is a template for synthesis of a complementary strand of DNA One template builds DNA continuously; the other builds DNA discontinuously, in segments Each new DNA molecule consist of one old strand and one new strand
Enzymes of DNA Replication DNA helicase Breaks hydrogen bonds between DNA strands DNA polymerase Joins free nucleotides into a new strand of DNA DNA ligase Joins DNA segments on discontinuous strand
DNA Replication
adenine (A) deoxyadenosine triphosphate, a purine Fig. 13-4a, p. 206
guanine (G) deoxyguanosine triphosphate, a purine Fig. 13-4b, p. 206
thymine (T) deoxythymidine triphosphate, a pyrimidine Fig. 13-4c, p. 206
cytosine (C) deoxycytidine triphosphate, a pyrimidine Fig. 13-4d, p. 206
A A DNA molecule is double-stranded. The two strands of DNA stay zippered up together because they are complementary: their nucleotides match up according to base-pairing rules (G to C, T to A). B As replication starts, the two strands of DNA are unwound. In cells, the unwinding occurs simultaneously at many sites along the length of each double helix. C Each of the two parent strands serves as a template for assembly of a new DNA strand from free nucleotides, according to base-pairing rules (G to C, T to A). Thus, the two new DNA strands are complementary in sequence to the parental strands. D DNA ligase seals any gaps that remain between bases of the new DNA, so a continuous strand forms. The base sequence of each half-old, half-new DNA molecule is identical to that of the parent DNA molecule. Stepped Art Fig. 13-6, p. 208
Animation: DNA replication details
Semiconservative Replication of DNA
Discontinuous Synthesis of DNA
A Each DNA strand has two ends: one with a 5 carbon, and one with a 3 carbon. DNA polymerase can add nucleotides only at the 3 carbon. In other words, DNA synthesis proceeds only in the 5 to 3 direction. Fig. 13-8a, p. 209
Only one new DNA strand is assembled continuously. 3 5 3 The parent DNA double helix unwinds in this direction. The other new DNA strand is assembled in many pieces. Gaps are sealed by 3 5 DNA ligase. 3 5 B Because DNA synthesis proceeds only in the 5 to 3 direction, only one of the two new DNA strands can be assembled in a single piece. The other new DNA strand forms in short segments, which are called Okazaki fragments after the two scientists who discovered them. DNA ligase joins the fragments into a continuous strand of DNA. Fig. 13-8b, p. 209
Checking for Mistakes DNA repair mechanisms DNA polymerases proofread DNA sequences during DNA replication and repair damaged DNA When proofreading and repair mechanisms fail, an error becomes a mutation a permanent change in the DNA sequence
13.3 Key Concepts How Cells Duplicate Their DNA Before a cell begins mitosis or meiosis, enzymes and other proteins replicate its chromosome(s) Newly forming DNA strands are monitored for errors Uncorrected errors may become mutations
13.4 Using DNA to Duplicate Existing Mammals Reproductive cloning is a reproductive intervention that results in an exact genetic copy of an adult individual
Cloning Clones Exact copies of a molecule, cell, or individual Occur in nature by asexual reproduction or embryo splitting (identical twins) Reproductive cloning technologies produce an exact copy (clone) of an individual
Reproductive Cloning Technologies Somatic cell nuclear transfer (SCNT) Nuclear DNA of an adult is transferred to an enucleated egg Egg cytoplasm reprograms differentiated (adult) DNA to act like undifferentiated (egg) DNA The hybrid cell develops into an embryo that is genetically identical to the donor individual
Somatic Cell Nuclear Transfer (SCNT)
Fig. 13-9a, p. 210
A A cow egg is held in place by suction through a hollow glass tube called a micropipette. The polar body (Section 10.5) and chromosomes are identified by a purple stain. Fig. 13-9a, p. 210
Fig. 13-9b, p. 210
B A micropipette punctures the egg and sucks out the polar body and all of the chromosomes. All that remains inside the egg s plasma membrane is cytoplasm. Fig. 13-9b, p. 210
Fig. 13-9c, p. 210
C A new micropipette prepares to enter the egg at the puncture site. The pipette contains a cell grown from the skin of a donor animal. skin cell Fig. 13-9c, p. 210
Fig. 13-9d, p. 210
D The micropipette enters the egg and delivers the skin cell to a region between the cytoplasm and the plasma membrane. Fig. 13-9d, p. 210
Fig. 13-9e, p. 210
E After the pipette is withdrawn, the donor s skin cell is visible next to the cytoplasm of the egg. The transfer is complete. Fig. 13-9e, p. 210
Fig. 13-9f, p. 210
F The egg is exposed to an electric current. This treatment causes the foreign cell to fuse with and empty its nucleus into the cytoplasm of the egg. The egg begins to divide, and an embryo forms. After a few days, the embryo may be transplanted into a surrogate mother. Fig. 13-9f, p. 210
A Clone Produced by SCNT
Fig. 13-10, p. 211
Animation: How Dolly was created
Therapeutic Cloning Therapeutic cloning uses SCNT to produce human embryos for research purposes Researchers harvest undifferentiated (stem) cells from the cloned human embryos
13.4 Key Concepts Cloning Animals Knowledge about the structure and function of DNA is the basis of several methods of making clones, which are identical copies of organisms
13.5 Fame and Glory In science, as in other professions, public recognition does not always include everyone who contributed to a discovery Rosalind Franklin was first to discover the molecular structure of DNA, but did not share in the Nobel prize which was given to Watson, Crick, and Wilkins
Rosalind Franklin s X-Ray Diffraction Image Franklin died of cancer at age 37, possibly related to extensive exposure to x-rays
13.5 Key Concepts The Franklin Footnote Science proceeds as a joint effort; many scientists contributed to the discovery of DNA s structure
Animation: DNA replication
Animation: Semidiscontinuous DNA replication
Animation: Structure of DNA
Animation: Subunits of DNA
ABC video: DNA ark promise hope for the future
Video: Goodbye, dolly