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DNA Structure and Function

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Presentation on theme: "DNA Structure and Function"— Presentation transcript:

1 DNA Structure and Function
Chapter 13

2 DNA Deoxyribonucleic Acid



5 Impacts, Issues Cloning DNA can lead to problems for the cloned offspring

6 Fig. 13-1a, p.206

7 Dolly lived for 6 ½ years - Fig. 13-9, p.214



10 Nucleus being injected into a donor cell whose nucleus was removed

11 This Yellow lab Died of Cancer Owners Were so Sad…

12 So they cloned him

13 It cost them $155,000

14 13.1 The Hunt for DNA Investigations that led to our understanding that DNA is the molecule of inheritance reveal how science advances

15 The proof that DNA is the carrier of Genetic information was made by:
Watson and Crick Wilkins Rosalind Franklin Avery-MacLeod-McCarty Hershey-Chase

16 How was DNA linked to heredity?
The role of DNA in heredity was discovered by studying bacteria and the viruses that infect them. Viruses that infect bacteria are called bacteriophages

17 These cells were “transformed” by some type of
Substance – what was that substance? 1 Mice injected with live cells of harmless strain R 2 Mice injected with live cells of killer strain S 3 Mice injected with heat-killed S cells 4 Mice injected with live R cells plus heat-killed S cells Mice don’t die. No live R cells in their blood Mice die. Live S cells in their blood Mice don’t die. No live S cells in their blood Mice die. Live S cells in their blood Fig. 13-3, p.208

18 Oswald Avery Experiment
Identified the transformed substance He looked at: proteins, RNA and DNA to see which one contained the genetic information. He heat killed one at a time until he figured it was DNA Avery and partners McCarty and MacLeod announced: transforming substance is DNA

19 Avery, McCarty and MacLeod
people were skeptical about their findings because: DNA too simple to be genetic material Proteins way more complicated so were thought to be genetic material Didn’t know a lot about DNA at the time

20 Hershey and Chase Experiments
Used bacteriophages Viruses very simple: protein and DNA (or RNA) Looked at a bacteriophage that infects E. coli. So which viral component: protein or DNA? They radiolabeled protein with sulfur They radiolabeled DNA with phosphorus

21 T2 virus – Bacteriophage That infects E. Coli Fig. 13-4c1, p.209

22 Fig. 13-4c2, p.209

23 35S remains outside cells virus particle labeled with 35S DNA (blue)
being injected into bacterium virus particle labeled with 32P 35P remains inside cells DNA (blue) being injected into bacterium Fig. 13-4ab, p.209

24 Discoveries Avery, McCarty and MacCloed figured out that DNA was the transformation factor in pathogens injected in mice Hershey and Chase – looked at bacteriophages and found that DNA is the genetic component Now that the genetic component (DNA) has been identified, what about its structure?

25 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

26 DNA Structure Wilkins and Rosalind Franklin – London
Rosalind Franklin developed an Xray diffraction technique in Wilkin’s lab, where Crick also studied Watson visited Wilkin’s lab and saw Franklin’s work and the images she came up with He figured out from these images that DNA is a double helix

27 DNA structure So, Watson and Crick used Rosalind Franklin’s x-ray crystalography diffraction image to determine the shape of DNA They read one of Franklin’s unpublished studies where she figured out that the sugar phosphate is part of the DNA backbone, with the hydrophobic nitrogenous bases in the center 1953 – published a 1 page paper on the structure of DNA

28 Fig. 13-2, p.207

29 DNA’s Building Blocks Nucleotide
A nucleic acid monomer consisting of a five-carbon sugar (deoxyribose), phosphate group, and one of four nitrogen-containing bases DNA consists of four nucleotide building blocks Two pyrimidines: thymine and cytosine Two purines: adenine and guanine

30 thymine (T) base with a single-ring structure adenine A base with a double-ring structure sugar (deoxyribose) guanine (G) base with a double-ring structure cytosine (C) base with a single-ring structure Fig. 13-5, p.210

31 p.211

32 2-nanometer diameter overall
0.34-nanometer distance between each pair of bases- 10 bases in each “twist” Fig. 13-6, p.211

33 Fig. 13-7, p.212


35 Enzymes Involved DNA polymerase Ligase RNA polymerase Helicase

36 DNA Replication in short:
Step 1: DNA is unwound at a replication fork by helicases that unwind and untwist DNA Step 2: single strand binding proteins bind to the upaired strands to keep them separated and stabilized The DNA can get twisted behind the replication fork so… Step 3: Topoisomerase binds and unbinds ahead of the replication fork to alleviate the tension

37 DNA replication in short
Step 4: primase starts a new strand – but it’s not a DNA strand…. Its an RNA strand – 5 – 10 nucleotides long. Step 5: the new DNA strand will start from the 3 prime end of this RNA primer. So, RNA polymerase starts the process but DNA polymerase takes over and adds a DNA strand onto the RNA primar

38 DNA Replication – in short
Step 6: Ligase attaches new nuceotides together at the sugar phosphate backbones

39 Table 13-1, p.212

40 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

41 DNA Replication Stepped Art Fig. 13-8a, p.213

42 DNA Replication DNA is antiparellel and undergoes semiconservative replication

43 synthesis occurs only in the 5´ to 3´
As Reiji Okazaki discovered, strand assembly is continuous on just one parent strand. This is because DNA synthesis occurs only in the 5´ to 3´ direction. On the other strand, assembly is discontinuous: short, separate stretches of nucleotides are added to the template, and then enzymes fill in the gaps between them. Fig. 13-8b, p.213

44 Why the discontinuous additions
Why the discontinuous additions? Nucleotides can only be joined to an exposed —OH group that is attached to the 3´ carbon of a growing strand. Fig. 13-8c, p.213


46 Lagging Strand uneven Okazaki fragments cause a problem at the end of the DNA strand


48 Antiparellel – semiconservative replication
DNA polymerases can only add nucleotides to the free 3 prime end – never 5 prime end – So, DNA can only elongate in the 5 – 3prime direction One strand is the leading strand – continuous replication

49 The other strand is the “lagging” strand – discontinuous replication
So it has to replicate in sections – away from the replication fork These fragments are called Okazaki fragments

50 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

51 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

52 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)

53 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

54 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

55 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

56 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

57 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

58 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

59 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

60 1 A microneedle is about to remove the nucleus from an unfertilized sheep egg (center).
2 The microneedle has now emptied the sheep egg of its own nucleus, which held the DNA. 3 DNA from a donor cell is about to be deposited in the egg. 4 An electric spark will stimulate the egg to enter mitotic cell division. the first cloned sheep Fig. 13-9, p.214

61 Table 13-2, p.214

62 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

63 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

64 Therapeutic Cloning Therapeutic cloning uses SCNT to produce human embryos for research purposes Researchers harvest undifferentiated (stem) cells from the cloned human embryos

65 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

66 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

67 Rosalind Franklin’s X-Ray Diffraction Image
Franklin died of cancer at age 37, possibly related to extensive exposure to x-rays

68 13.5 Key Concepts The Franklin Footnote
Science proceeds as a joint effort; many scientists contributed to the discovery of DNA’s structure

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