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Ch. 16: Molecular Basis of Inheritance Thought Questions

Ch. 16: Molecular Basis of Inheritance Thought Questions. Ms. Whipple – Brethren Christian High School. 1. What are the two chemical components of a chromosome?.

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Ch. 16: Molecular Basis of Inheritance Thought Questions

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  1. Ch. 16: Molecular Basis of InheritanceThought Questions Ms. Whipple – Brethren Christian High School

  2. 1. What are the two chemical components of a chromosome? • When T. H. Morgan’s group showed that genes are located on chromosomes, the two components of chromosomes—DNA and protein—became candidates for the genetic material Section 16.1

  3. 2. How did Fredrick Griffith discover that DNA was the genetic material used in heredity? • The discovery of the genetic role of DNA began with research by Frederick Griffith in 1928 • Griffith worked with two strains of a bacterium, one pathogenic and one harmless • When he mixed heat-killed remains of the pathogenic strain with living cells of the harmless strain, some living cells became pathogenic • He called this phenomenon transformation, now defined as a change in genotype and phenotype due to assimilation of foreign DNA Section 16.1

  4. EXPERIMENT Mixture ofheat-killedS cells andliving R cells Heat-killedS cells(control) Figure 16.2 Living S cells(control) Living R cells(control) RESULTS Mouse dies Mouse healthy Mouse healthy Mouse dies Living S cells

  5. 2. How did Fredrick Griffith discover that DNA was the genetic material used in heredity? • In 1944, Oswald Avery, Maclyn McCarty, and Colin MacLeod announced that the transforming substance was DNA • Their conclusion was based on experimental evidence that only DNA worked in transforming harmless bacteria into pathogenic bacteria • Many biologists remained skeptical, mainly because little was known about DNA Section 16.1

  6. 3. Describe the process of Transformation in bacteria? • Transformation: The change in genotype and phenotype due to assimilation of foreign DNA from the external environment. Section 16.1

  7. 4. What does bacteriophage mean? • Bacteriophage means “Bacteria Eater” and it refers to any virus that attacks bacterial cells. Section 16.1

  8. 5. What is a virus and what does it consist of? • Viruses consist of DNA (or sometimes RNA) enclosed in a protective coat of protein. • To replicate, a virus infects a host cell and takes over the cell’s metabolic machinery. Section 16.1

  9. Figure 16.3 Phagehead Tailsheath Tail fiber DNA 100 nm Bacterialcell Section 16.1

  10. 6. Describe the Hershey-Chase experiment that helped in our understanding of DNA as genetic material? • In 1952, Alfred Hershey and Martha Chase performed experiments showing that DNA is the genetic material of a phage known as T2 • To determine this, they designed an experiment showing that only one of the two components of T2 (DNA or protein) enters an E. coli cell during infection • They concluded that the injected DNA of the phage provides the genetic information Section 16.1

  11. EXPERIMENT Figure 16.4-1 Radioactiveprotein Phage Bacterial cell Batch 1:Radioactivesulfur(35S) DNA RadioactiveDNA Batch 2:Radioactivephosphorus(32P) Section 16.1

  12. EXPERIMENT Emptyproteinshell Figure 16.4-2 Radioactiveprotein Phage Bacterial cell Batch 1:Radioactivesulfur(35S) DNA PhageDNA RadioactiveDNA Batch 2:Radioactivephosphorus(32P)

  13. EXPERIMENT Emptyproteinshell Figure 16.4-3 Radioactiveprotein Radioactivity(phage protein)in liquid Phage Bacterial cell Batch 1:Radioactivesulfur(35S) DNA PhageDNA Centrifuge Pellet (bacterialcells and contents) RadioactiveDNA Batch 2:Radioactivephosphorus(32P) Centrifuge Radioactivity(phage DNA)in pellet Pellet Section 16.1

  14. 7. How did E. Chargraff change our view of DNA as genetic material? What are his two rules? • In 1950, Erwin Chargaff reported that DNA composition varies from one species to the next • This evidence of diversity made DNA a more credible candidate for the genetic material • Twofindings became known as Chargaff’s rules • The base composition of DNA varies between species • In any species the number of A and T bases are equal and the number of G and C bases are equal • The basis for these rules was not understood until the discovery of the double helix Section 16.1

  15. 8. Describe how the structure of DNA was discovered? Please emphasize the process of x-ray crystallography and Rosalind Franklin’s contribution to the discovery. • After DNA was accepted as the genetic material, the challenge was to determine how its structure accounts for its role in heredity • Maurice Wilkins and Rosalind Franklin were using a technique called X-ray crystallography to study molecular structure • Franklin produced a picture of the DNA molecule using this technique • Franklin’s X-ray crystallographic images of DNA enabled Watson to deduce that DNA was helical • The X-ray images also enabled Watson to deduce the width of the helix and the spacing of the nitrogenous bases • The pattern in the photo suggested that the DNA molecule was made up of two strands, forming a double helix Section 16.1

  16. (b) Franklin’s X-ray diffractionphotograph of DNA Figure 16.6 (a) Rosalind Franklin Section 16.1

  17. 8. Describe how the structure of DNA was discovered? Please emphasize the process of x-ray crystallography and Rosalind Franklin’s contribution to the discovery. • Watson and Crick built models of a double helix to conform to the X-rays and chemistry of DNA • Franklin had concluded that there were two outer sugar-phosphate backbones, with the nitrogenous bases paired in the molecule’s interior • Watson built a model in which the backbones were antiparallel (their subunits run in opposite directions) Section 16.1

  18. 9. Describe & Draw the structure of DNA. How are the base pairs connected? Include the following words in your description: Double Helix, Antiparallel, and Nitrogenous Bases. Double Helix Antiparallel Nitrogenous Bases Section 16.1

  19. 1. In your own words, please describe Watson & Crick’s hypothesis for the replication of DNA. Why is this called a Semiconservative Model? • Since the two strands of DNA are complementary, each strand acts as a template for building a new strand in replication • In DNA replication, the parent molecule unwinds, and two new daughter strands are built based on base-pairing rules • Watson and Crick’s semiconservative model of replication predicts that when a double helix replicates, each daughter molecule will have one old strand (derived or “conserved” from the parent molecule) and one newly made strand • Competing models were the conservative model (the two parent strands rejoin) and the dispersive model (each strand is a mix of old and new) Section 16.2

  20. “Daughter” DNA molecules,each consisting of oneparental strand and onenew strand Separation ofstrands (b) (c) Figure 16.9-3 A A A T T T T A G C C G G G C C A T A A A T T T T A T A A T T A C C G G G C C G (a) Parent molecule Section 16.2

  21. Conservativemodel (a) Semiconservativemodel (b) Firstreplication Parentcell Secondreplication Figure 16.10 (c) Dispersive model Section 16.2

  22. 2. Briefly describe Meselson and Stahl’s experiment and their results showing the semiconservative replication of DNA. • Experiments by Matthew Meselson and Franklin Stahl supported the semiconservative model • They labeled the nucleotides of the old strands with a heavy isotope of nitrogen, while any new nucleotides were labeled with a lighter isotope • The first replication produced a band of hybrid DNA, eliminating the conservative model • A second replication produced both light and hybrid DNA, eliminating the dispersive model and supporting the semiconservative model Section 16.2

  23. 1 3 4 2 Figure 16.11a EXPERIMENT Bacteriacultured inmedium with15N (heavyisotope) Bacteriatransferred tomedium with14N (lighterisotope) RESULTS Less dense DNA samplecentrifugedafter firstreplication DNA samplecentrifugedafter secondreplication More dense Section 16.2

  24. CONCLUSION Figure 16.11b First replication Second replication Predictions: Conservativemodel Semiconservativemodel Dispersivemodel Section 16.2

  25. 3. About how many nucleotide pairs does an average human cell contain? How quickly is this base pair sequence replicated? How often do errors occur? • It takes E. coli less than an hour to copy each of the 4.6 million nucleotide pairs in its single chromosome and divide to form two identical daughter cells. • A human cell can copy its 6 billion nucleotide pairs and divide into daughter cells in only a few hours. • This process is remarkably accurate, with only one error per 10 billion nucleotides. Section 16.2

  26. 4. What are Origins of Replication? What occurs at these points? Is their only one for each strand of DNA? • Replication begins at particular sites called origins of replication, where the two DNA strands are separated, opening up a replication “bubble” • A eukaryotic chromosome may have hundreds or even thousands of origins of replication • Replication proceeds in both directions from each origin, until the entire molecule is copied Section 16.2

  27. Figure 16.12a (a) Origin of replication in an E. coli cell Origin ofreplication Parental (template) strand Daughter (new) strand Double-strandedDNA molecule Replication fork Replicationbubble Two daughterDNA molecules 0.5 m Section 16.2

  28. Figure 16.12b (b) Origins of replication in a eukaryotic cell Double-strandedDNA molecule Origin of replication Parental (template)strand Daughter (new)strand Replication fork Bubble Two daughter DNA molecules 0.25 m Section 16.2

  29. 5. Please define the following vocabulary words and their function during DNA replication: • At the end of each replication bubble is a replication fork, a Y-shaped region where new DNA strands are elongating • Helicases are enzymes that untwist the double helix at the replication forks • Single-strand binding proteins bind to and stabilize single-stranded DNA • Topoisomerase corrects “overwinding” ahead of replication forks by breaking, swiveling, and rejoining DNA strands Section 16.2

  30. 5. Please define the following vocabulary words and their function during DNA replication: • DNA polymerases cannot initiate synthesis of a polynucleotide; they can only add nucleotides to the 3 end • The initial nucleotide strand is a short RNA primer • An enzyme called primasecan start an RNA chain from scratch and adds RNA nucleotides one at a time using the parental DNA as a template • The primer is short (5–10 nucleotides long), and the 3 end serves as the starting point for the new DNA strand Section 16.2

  31. 5. Please define the following vocabulary words and their function during DNA replication: • Enzymes called DNA polymerases catalyze the elongation of new DNA at a replication fork • Most DNA polymerases require a primer and a DNA template strand • The rate of elongation is about 500 nucleotides per second in bacteria and 50 per second in human cells • Each nucleotide that is added to a growing DNA strand is a nucleoside triphosphate Section 16.2

  32. Figure 16.13 Primase 3 Topoisomerase RNAprimer 5 3 5 3 Helicase 5 Single-strand bindingproteins Section 16.2

  33. Template strand Figure 16.14 New strand 5 5 3 3 Sugar A T A T Base Phosphate C G C G C G G C DNApolymerase OH 3 A T A T OH P P P i P P 3 C C Pyrophosphate OH 2P i Nucleosidetriphosphate 5 5 Section 16.2

  34. 5. Please define the following vocabulary words and their function during DNA replication: • The antiparallel structure of the double helix affects replication • DNA polymerases add nucleotides only to the free 3end of a growing strand; therefore, a new DNA strand can elongate only in the 5to3direction • Along one template strand of DNA, the DNA polymerase synthesizes a leading strand continuously, moving toward the replication fork. Section 16.2

  35. Overview Leadingstrand Laggingstrand Figure 16.15 Origin of replication Primer Leadingstrand Laggingstrand Origin of replication Overall directionsof replication 3 5 RNA primer 5 3 Sliding clamp 3 DNA pol III Parental DNA 5 3 5 5 3 3 Section 16.2 5

  36. 5. Please define the following vocabulary words and their function during DNA replication: • To elongate the other new strand, called the lagging strand, DNA polymerase 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 joined together by DNA ligase Section 16.2

  37. 3 Overview 5 Origin of replication Laggingstrand Leadingstrand 3 Figure 16.16 Templatestrand 5 RNA primerfor fragment 1 Lagging strand 3 2 1 Leadingstrand 5 1 3 Overall directionsof replication 5 3 Okazakifragment 1 5 1 RNA primerfor fragment 2 3 5 5 Okazakifragment 2 3 2 1 3 5 5 3 2 1 3 5 5 3 2 1 3 Section 16.2 5 Overall direction of replication

  38. 6. Briefly compare & contrast the synthesis of the leading strand (5’-3’) & lagging strand (3’-5’) • Along one template strand, DNA polymerase III can synthesize a complementary strand continuously by elongating the new DNA in the mandatory 53 direction. • DNA pol III remains in the replication fork on that template strand and continuously adds nucleotides to the new complementary strand as the fork progresses. • The DNA strand made by this mechanism, called the leading strand, requires a single primer. • The other parental strand (53 into the fork), the lagging strand, is copied away from the fork. • Unlike the leading strand, which elongates continuously, the lagging stand is synthesized as a series of short segments called Okazaki fragments. • Okazaki fragments are about 1,000 to 2,000 nucleotides long in E. coli and 100 to 200 nucleotides long in eukaryotes. • Although only one primer is required on the leading strand, each Okazaki fragment on the lagging strand must be primed separately. Section 16.2

  39. Figure 16.16a Overview Origin of replication Leadingstrand Laggingstrand Lagging strand 2 1 Leadingstrand Overall directionsof replication Section 16.2

  40. Figure 16.17 Overview Origin of replication Leadingstrand Laggingstrand Leadingstrand Laggingstrand Overall directionsof replication Leading strand 5 DNA pol III 3 Primer Primase 5 3 3 ParentalDNA Lagging strand DNA pol III 5 DNA pol I DNA ligase 4 3 5 3 3 2 1 5 Section 16.2

  41. Overview Origin of replication Figure 16.17a Leadingstrand Laggingstrand Leadingstrand Laggingstrand Overall directionsof replication Leading strand 5 DNA pol III 3 Primer Primase 3 5 3 ParentalDNA Section 16.2

  42. Overview Origin of replication Leadingstrand Figure 16.17b Laggingstrand Leadingstrand Laggingstrand Overall directionsof replication Leading strand Primer Lagging strand DNA pol III 5 DNA pol I DNA ligase 4 3 5 3 3 3 2 1 Section 16.2 5

  43. Figure 16.18 DNA pol III Leading strand Parental DNA 5 5 3 3 3 5 3 5 Connectingprotein Helicase Laggingstrandtemplate 3 5 DNA pol III Lagging strand 3 5 Section 16.2

  44. 7. Since mutations (changes in the DNA) are usually detrimental or possible lethal. The synthesis of new DNA must be very accurate. How is the DNA strand proofread and repaired during and after synthesis? How many repair enzymes have been found in humans? • DNA polymerases proofread newly made DNA, replacing any incorrect nucleotides • In mismatch repair of DNA, repair enzymes correct errors in base pairing • DNA can be damaged by exposure to harmful chemical or physical agents such as cigarette smoke and X-rays; it can also undergo spontaneous changes • In nucleotide excision repair, a nuclease cuts out and replaces damaged stretches of DNA Section 16.2

  45. 3 5 Figure 16.19 3 5 Nuclease 5 3 5 3 DNA polymerase 3 5 3 5 DNA ligase 5 3 Section 16.2 3 5

  46. 8. Describe the function of Telomeres and Telomerase eukaryotic cells. Why are they needed? • Limitations of DNA polymerase create problems for the linear DNA of eukaryotic chromosomes • The usual replication machinery provides no way to complete the 5 ends, so repeated rounds of replication produce shorter DNA molecules with uneven ends • This is not a problem for prokaryotes, most of which have circular chromosomes • Eukaryotic chromosomal DNA molecules have special nucleotide sequences at their ends called telomeres • Telomeres do not prevent the shortening of DNA molecules, but they do postpone the erosion of genes near the ends of DNA molecules • It has been proposed that the shortening of telomeres is connected to aging Section 16.2

  47. 5 Leading strand Ends of parentalDNA strands Figure 16.20 Lagging strand 3 Last fragment Next-to-last fragment RNA primer Lagging strand 5 3 Parental strand Removal of primers andreplacement with DNAwhere a 3 end is available 5 3 Second roundof replication 5 New leading strand 3 New lagging strand 5 3 Further roundsof replication Section 16.2 Shorter and shorter daughter molecules

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