The Molecular Basis of Inheritance

1. The Molecular Basis of Inheritance Chapter 16 A. P. Biology Mr. Knowles Liberty Senior High

2. Concept 16.1: DNA is the genetic material Early in the 20th century The identification of the molecules of inheritance loomed as a major challenge to biologists

3. Hammerling�s Acetabularia Exp. Danish biologist, Joachim Hammerling in the 1930�s. Used a unicellular green algae, Acetabularia. Proved that the hereditary material is in the nucleus.

5. DNA is a �Transforming Principle� Frederick Griffith, 1928, showed that dead bacteria could be transformed into living cells. Used 2 strains of Pneumococcus bacteria, one pathogenic and the other nonpathogenic.

8. Identified the �Transforming Agent� Oswald Avery- in 1944, he separated and purified the different organic compounds of the bacteria and identified the DNA as responsible for the transformation effect.

11. Support for Avery Alfred Hershey and Martha Chase- in 1952, used bacteriophage (T2) as a model. They radiolabeled the protein coat with 35 S and the DNA with 32P in order to �see� which of the molecules actually entered the cell and produced more phage.

12. A Bacteriophage

15. Other Contributors Friedrich Miescher- 1869, extracted �nuclein�- nucleic acid from human cells and fish sperm. Erwin Chargaff- A = T and the G = C, called Chargaff�s Rule; equal proportion of purines and pyrimidines; amt. varied from species to species.

16. The Chemistry of DNA A nucleotide = 1. PO4, 2. a five carbon sugar, 3. a nitrogen- containing base. Phosphodiester Bonds- 2 P-O-C bonds link nucleotides.

17. What is a Nucleotide? Subunits of DNA/RNA are Nucleotides = nitrogenous base + deoxy- or ribose sugar (5 carbons) + PO4 Purines: Adenine and Guanine Pyrimidines: Cytosine, Thymine, Uracil (in RNA)

19. Monosaccharides of Nucleic Acids

20. Adenosine Monophosphate Base = adenine In DNA, sugar = deoxyribose (In RNA, sugar = ribose) A phosphate group, PO4 The Nucleotide = AMP

21. Adenosine Monophosphate

22. Adenosine Triphosphate (ATP)

24. Maurice Wilkins and Rosalind Franklin Were using a technique called X-ray crystallography to study molecular structure Rosalind Franklin Produced a picture of the DNA molecule using this technique

25. Franklin had concluded that DNA Was composed of two antiparallel sugar-phosphate backbones, with the nitrogenous bases paired in the molecule�s interior The nitrogenous bases Are paired in specific combinations: adenine with thymine, and cytosine with guanine

26. DNA is Antiparallel

27. Three Dimensional Structure of DNA Rosalind Franklin- X-ray crystallography of DNA- showed that DNA was in a helix with PO4 and sugars to the outside. James Watson and Francis Crick- took Franklin�s data- in April 23, 1953, and deduced the structure of DNA. Won the Nobel Prize along with Maurice Wilkins.

31. Watson and Crick reasoned that there must be additional specificity of pairing Dictated by the structure of the bases Each base pair forms a different number of hydrogen bonds Adenine and thymine form two bonds, cytosine and guanine form three bonds

32. Watson and Crick

35. Characteristics of DNA All chains of DNA and RNA have a 5� PO4 end and a 3� OH end. Base sequences are written in a 5� to 3� direction. Ex. 5� pGpTpCpCpApT-OH 3�

36. Characteristics of DNA Base pairs stabilize the molecule by forming H-bonds. Antiparallel Strands- 5�----------------3� 3�----------------5� Strands are complementary.

37. In DNA replication The parent molecule unwinds, and two new daughter strands are built based on base-pairing rules

38. DNA replication is semiconservative Each of the two new daughter molecules will have one old strand, derived from the parent molecule, and one newly made strand

40. Experiments performed by Meselson and Stahl Supported the semiconservative model of DNA replication

43. Getting Started: Origins of Replication The replication of a DNA molecule Begins at special sites called origins of replication (special sequence, AT rich), where the two strands are separated.

44. Bidirectional Replication in Bacteria- One Origin

45. Eukaryotic DNA Replication Replicates the DNA on a chromosome in a discrete section- Replication Unit. (about 100 kbp long). Prokaryotic Replication: 500 nucleotides/ second. Eukaryotic Replication: 50 nucleotides/second.

46. Problem? If eukaryotic replication is 100 N/ sec. and there are 3.0 X 108 N/ chromosome, how long would it take to replicate one human genome? Ans: 3 X 106 sec. = 34.7 days! How long does it actually take to go through S phase? S phase = 8 hours How?

47. Multiple Origins of Replication Each replication unit on a chromosome has its own origin of replication. Multiple units can be replicating at any given time. Each chromosome has numerous replication forks.

48. A eukaryotic chromosome May have hundreds or even thousands of replication origins

51. The Problem with DNA Replication DNA Polymerase can only build in the 5� to 3� direction. Since the parent strands are antiparallel, the new strands are synthesized in opposite directions.

52. DNA polymerases add nucleotides Only to the free 3??end of a growing strand. Along one template strand of DNA, the leading strand: DNA polymerase III can synthesize a complementary strand continuously, moving toward the replication fork

53. To elongate the other new strand of DNA, the lagging strand: DNA polymerase III must work in the direction away from the replication fork The lagging strand Is synthesized as a series of segments called Okazaki fragments, which are then joined together by DNA ligase

55. DNA Replication Leading Stand- elongates toward the fork, 5� to 3� Lagging Strand- elongates way from fork; synthesized discontinuously in short segments-Okazaki Fragments.

57. Priming DNA Synthesis DNA polymerases cannot initiate the synthesis of a polynucleotide They can only add nucleotides to the 3? end The initial nucleotide strand Is an RNA or DNA primer made by � Primase.

62. A summary of DNA replication

63. Replication Fork

65. Some Animations of DNA Replication

66. DNA Replication Several Enzymes (+12) involved: Helicase + ATP - unwinds the DNA and stabilizes it. Single-stranded binding Proteins-stabilize the DNA and prevent base pairing. Primase -makes an RNA primer. DNA Polymerase III- builds new complementrary strands in a 5�--->3� direction. DNA Polymerase I � removes the primer from the 5� end of Okazaki fragment and replaces it with DNA. DNA Ligase � bonds the 3� end of the second Okazaki fragment with the 5� end of the first, and so on.

68. Proofreading and Repairing DNA DNA polymerases proofread newly made DNA While the initial pairing errors are high as nucleotides come in. Proofreading replaces any incorrect nucleotides; error rate of about 1 in 10 billion nucleotides. In mismatch repair of DNA Repair enzymes correct errors in base pairing Defective repair enzymes � xeroderma pigmentosum

70. Replicating the Ends of DNA Molecules The ends of eukaryotic chromosomal DNA Get shorter with each round of replication

72. Eukaryotic chromosomal DNA molecules Have at their ends nucleotide sequences that repeat (100 � 1,000), called telomeres, that postpone the erosion of genes near the ends of DNA molecules

73. Show me the Replication!

74. DNA Packaging Eukaryotic DNA is packaged into nucleosomes- 146 bp of DNA wrapped around histones. Condenses the DNA into a nucleus. Protects the DNA from exposure.

75. How many genes are on a chromosome? Human Chromosome #22- composed of 33.4 megabases (Mb), identified 545 genes, 298 of these were unknown. 100-200 more genes may be identified! The rest is �junk� DNA.

76. Chromosomes have... Humans have about 3 X 109 bp of DNA. Conservative estimate is 30,000 genes. Therefore, the average gene is 100,000 bp long.

77. One Gene-One Enzyme Hypothesis 1941- Beadle and Tatum- created mutants in the fungus Neurospora using X-rays. Exposed wild-type spores to the mutagen. Change growth from a complete to minimal media. Mutants no longer able to synthesize essential organics (e.g. amino acids)

78. One Gene-One Enzyme Beadle and Tatum- found several mutants. Found a different site for each enzyme. Each mutant had a different mutation at a different site (enzyme) on the chromosome. Each mutant had a defect in a different enzyme. Concluded: one gene encodes one enzyme.

79. One Gene-One Enzyme Hypothesis Genetic traits are expressed as a result of the activities of enzymes. Many enzymes contain multiple subunits. Each subunit encoded by a differernt gene. Now, referred to as one gene-one polypeptide.

80. How does DNA Encode Proteins?

91. Some Differences between Prokaryotic and Eukaryotic Protein Synthesis

92. Prokaryotes Eukaryotes Lack introns- no mRNA processing. Begin translation before transcription is finished. Have introns- mRNA is spliced. mRNA completely formed before translation. mRNA has a 5� methyl G cap added

97. Prokaryotes Eukaryotes mRNA begins at an AUG start codon, no cap. Multiple genes on one mRNA. 70S ribosome used. mRNA has a poly A tail added at the 3�. A single gene on one mRNA. 80S ribosome used.

102. Other Differences in Gene Organization Tandem Clusters- several hundred genes organized together; Ex. rRNA genes. Multigene Families- genes very different from each other, but encode similar proteins; Ex. globin genes. Pseudogenes- silent copies of genes inactivated by mutations.

103. Transposons

108. Acute Promyelocytic Leukemia- Several Translocation Events; one is between #7 and #15.

113. DNA Replication Prokaryotes single, circular chromosome. one single origin on the chromosome. rate of over 1,000 nt/second. Eukaryotes multiple, linear chromosomes. several origins on each chromosome. rate of about 50-100 nt/second