Presentation on theme: "Identifying the Substance of Genes"— Presentation transcript:
1 Identifying the Substance of Genes Chapter 12Identifying the Substance of Genes
2 Chapter Mystery Page 337 UV Light Why is UV light so dangerous? How can these particular wavelengths of light damage our cells to the point of causing cell death and cancer?Hypothesis……….
3 Section 12.1 Identifying the Substance of Genes Objectives:What clues did bacterial transformation yield about the gene?What role did bacterial viruses play in identifying genetic material?What is the role of DNA in heredityDefine:Transformationsbacteriophage
4 I. Bacterial Transformations If the molecule that carries genetic information could be identified, it might be possible to understand how genes actually control the inherited characteristics of living thingsFredrick Griffith was trying to figure out how bacteria make people sick (pneumonia)He isolated 2 very similar types of bacteria from the mice = 2 different varieties (strains) of the same bacterial speciesBoth grew very well in culture platesOnly 1 caused pneumoniaDiseased-causing bacteria (S-strain) grew into smooth colonies & harmless bacteria (R-strain) produced colonies w/ rough edgesDifferences in appearance made the 2 strains easy to tell apart
5 A. Griffith’s Experiments Griffith injected mice w/ disease-causing bacteria mice developed pneumonia mice diedInjected mice with harmless bacteria mice stayed healthy1. Heated S-strain bacteria killed bacteria then injected into mice mice survived = (suggested cause of pneumonia was not a toxin from disease-causing bacteria)Mixed heat-killed S-strain w/ live, harmless bacteria from R-strain injected into mice (by themselves, neither type should have made mice sick) injected mice developed pneumonia many died- Examined lungs and found them to be full of harmful bacteria!!!
6 Draw and Label a diagram of Griffith’s Experiments
7 B. TransformationHeat-killed bacteria passed disease-causing ability to harmless bacteriaGriffith’s hypothesis: when mixed, some chemical factor transferred from heat-killed cells of S strain into live cells of R strainTransformation – process by which one type of bacteria (harmless) had been changed permanently into another (disease-causing form)Ability to cause disease was inherited by the offspring of the transformed bacteriaGriffith concluded: transforming factor had to be a gene
8 C. The Molecular Cause of Transformation 1944 – Avery repeated Griffith’s experiments to determine which molecule in the heat-killed bacteria was most important for transformationIf they could find this particular molecule, it might reveal the chemical nature of the geneExtracted various molecules from heat-killed bacteria treated mixture w/ enzymes that destroyed proteins, lipids, carbs, nucleic acid RNA transformation still occurred (results)Since all those molecules were destroyed, they could not be responsible for the transformation (conclusion)Repeated experiment used enzymes that broke down DNA transformation did not occur (results) DNA was the transferring factor (conclusion)By observing bacterial transformation, Avery and other scientists discovered that the nucleic acid DNA stores and transmits genetic information from one generation of bacteria to the next
10 II. Bacterial Viruses Scientists are skeptical Takes several experiments to convince them of something as important as chemical nature of geneMost important = 1952 = Alfred Hershey and Martha ChaseStudied viruses – tiny, nonliving particles that can infect living cells
11 A. Bacteriophages Bacteriophage – kind of virus that infects bacteria Bacteriophage enters bacteria attaches to surface of cell injects genetic information into cellViral genes act to produce as many bacteriophages (destroys bacterium)When cell splits open hundreds of new viruses burst out
12 Draw a diagram of how bacteriophage infects a bacteria cell
13 B. The Hershey-Chase Experiment Studied bacteriophage composed of DNA core and protein coatWanted to determine which part of virus entered bacterial cell to support or disprove Avery’s finding that genes were made of DNAUsed radioactive markers to tell which molecules actually entered bacteria carrying genetic information of virusMixed marked viruses w/ bacterial cells waited few minutes for viruses to inject genetic material separated viruses from bacteria tested bacteria for radioactivityResults: Nearly all radioactivity in bacteria indicated DNAConcluded: genetic material of bacteriophage was DNA; not proteinHershey and Chase’s experiment w/ bacteriophages confirmed Avery’s results, convincing many scientists that DNA was the genetic material found in genes – not just in viruses and bacteria, but in all living cells.
15 III. The Role of DNAScientists wondered how DNA, or any molecule, could do critical things that genes were known to doThe DNA that makes up genes must be capable of storing, copying, and transmitting the genetic information in a cell
16 A. Storing Information Foremost job of DNA Genes that make a flower purple must carry info needed to produce purple pigmentGenes for blood type and eye color must have info needed for jobGenes control patterns of developmentInstructions that cause a single cell to develop into an oak tree, sea urchin, or dog must be written into DNA of each of these organisms
17 B. Copying InformationBefore a cell divides must make complete copy of every one of its genesMechanism for this process could not be proposed until structure of molecule was knownDiscussed in next section…
18 C. Transmitting Information Genes are transmitted from one generation to the next (per Mendel)DNA molecule must be carefully sorted and passed along during cell divisionEspecially important during formation of sex cells in meiosisChromosomes of eukaryotic cells contain genes made of DNALoss of any DNA during meiosis might mean loss of valuable genetic information from one generation to the next
19 Section 12.2 The Structure of DNA Objectives:What are the chemical components of DNA?What clues helped scientists solve the structure of DNA?What does the double-helix model tell us about DNA?Define:Base pairing
20 I. The Components of DNA Deoxyribonucleic Acid DNA is a nucleic acid made up of nucleotides joined into long strands or chains by covalent bonds
21 A. Nucleic Acids and Nucleotides Nucleic acids – long, slightly acidic molecules originally identified in cell nucleiMacromolecule made up of smaller subunits linked together to form long chainsNucleotides – building blocks of nucleic acidsMade up of 3 basic components:5-carbon sugar (deoxyribose)Phosphate groupNitrogenous base
22 B. Nitrogenous Bases and Covalent Bonds Nitrogenous bases – bases that contain nitrogen4 types:Adenine (A)Guanine (G)Cytosine (C)Thymine (T)Nucleotides joined by covalent bonds formed b/w sugar of one nucleotide and phosphate group of nextNitrogenous bases stick out sideways from nucleotide chainNucleotides can be joined together in any order any sequence of bases is possibleCould carry coded genetic informationBases have chemical that makes them especially good at absorbing UV (ultraviolet) light
23 Mystery CluePage 344…The energy from UV light can excite electrons in the absorbing substance to the point where the electrons cause chemical changes.What chemical changes might occur in the nitrogenous bases of DNA?
24 II. Solving the Structure of DNA Know DNA is made from long chains of nucleotidesMust find out the way in which those chains are arranged in 3 dimensions
25 A. Chargaff’s RulePercentages of adenine and thymine bases are almost equal in any sample of DNASame thing is true for guanine and cytosineChargaff’s Rule: A=T and G=CSamples from all organisms obey this rule
26 B. Franklin’s X-RaysRosalind Franklin (1950s) – used X-ray diffraction to get info about DNA structurePurified large amount of DNAStretched DNA fibers in thin glass tube (so most of strands were parallel)Aimed powerful X-ray beam at concentrated DNARecorded scattering pattern of X-rays on filmRepeated until she obtained clear patternsClues from X-ray:X-shaped pattern strands in DNA are twisted around each other like coils of a springShape = helixAngle of X there are 2 strands in structureOther clues nitrogenous bases near center of DNA molecule
27 C. The work of Watson and Crick James Watson & Francis CrickBuilt 3 dimensional models of DNA made of cardboard and wireThe clues in Franklin’s X-ray pattern enabled Watson and Crick to build a model that explained the specific structure and properties of DNA
28 III. The Double-Helix Model Double helix looks like a twisted ladder2 strands twist around each other like a spiral staircaseThe double-helix model explains Chargaff’s rule of base pairing and how the two strands of DNA are held togetherAlso tells us how DNA can function as a carrier of genetic information
29 A. Antiparallel Strands 2 strands of DNA run in opposite directions = “antiparallel”Enables nitrogenous bases on both strands to come into contact at the center of moleculeAllows each strand to carry a sequence of nucleotidesArranged almost like letters in a four-lettered alphabet
30 Mystery CluePage 347…Our skin cells are exposed to UV light whenever they are in direct sunlight.How might this exposure affect base pairing in the DNA of our skin cells?
31 B. Hydrogen Bonding Relatively weak chemical forces Could form b/w certain nitrogenous basesProvides just enough force to hold 2 strands togetherDoes it make sense that a molecule as important as DNA should be held together by such weak bonds?Yes!!If 2 strands of helix were held together by strong bonds it might be impossible to separate themSeparation is critical to DNA’s functions
32 C. Base PairingWatson and Crick’s model showed that hydrogen bonds could create a nearly perfect fit b/w nitrogenous bases along the center of moleculeBonds only form b/w certain base pairsA with TG with CBase pairing – nearly perfect fit b/w A-T and G-CExplains Chargaff’s ruleFor every adenine exactly one thymineFor each cytosine exactly one guanine
33 Section 12.3 DNA Replication Objectives:What role does DNA polymerase play in copying DNA?How does DNA replication differ in prokaryotic cells and eukaryotic cells?Define:ReplicationDNA polymeraseTelomere
34 I. Carrying the CodeBase pairing in double helix explains how DNA can be copied/replicatedEach base on one strand pairs w/ 1 (and only 1) base on the opposite strandEach strand has all the information needed to reconstruct the other half by mechanism of base pairingEach strand can be used to make the other strand strands are complementary
35 A. The Replication Process Before a cell divides it duplicates its DNAProcess is called replicationOccurs during late interphaseEnsures that each resulting cell has same set of DNA moleculesDNA separates into 2 strandsProduces 2 new complementary strandsFollows rules of base pairingEach strand serves as template/model for new strand
36 2 strands of double helix unzip Allow 2 replication forks to formAs each new strand forms, new bases are added following rules of base pairingIf base pair is A then T is added to new strandG always pairs with CExample: strand w/ base sequence: TACGTT produces a complementary base sequence: ATGCAAResults: 2 DNA molecules identical to each other and to the original moleculesEach new molecule has one new strand and one old strand
37 B. The Role of EnzymesEnzymes – unzip molecule of DNA by breaking hydrogen bonds b/w base pairs & unwinding the 2 strandsEach strand serves as template for attachment of complementary basesDNA polymerase – principal enzyme involved in DNA replicationDNA polymerase is an enzyme that joins individual nucleotides to produce a new strand of DNAProduces sugar-phosphate bonds that join nucleotides together“Proofreads” each new DNA strand, so that each molecule is a near-perfect copy of original
38 Mystery CluePage 351…How might UV-induced chemical changes in bases affect the process of DNA replication?
39 C. TelomeresTelomeres – DNA at tips of chromosomesDifficult to replicateCells use special enzyme telomeraseAdds short, repeated DNA sequences to telomeresIn rapidly dividing cells (stem cells) telomerase helps to prevent genes from being damaged or lost during replicationOften switched off in adult cellsMay be activated in cancer cells enabling cells to grow and proliferate rapidly
40 II. Replication in Living Cells DNA replication occurs during S phaseCarefully regulated so it is completed before cell enters mitosis or meiosisProkaryotes = single, circular DNA molecule in cytoplasm – contains nearly all genetic informationEukaryotes = 1000 times more DNA in nucleus, packaged into chromosomesChromosomes consist of DNA tightly packed together w/ proteins = chromatinDNA + histones = beadlike nucleosomesHistones = proteins chromatin is coiled around
41 A. Prokaryotic DNA Replication DNA replication does not start until regulatory proteins bind to a single starting point on the chromosomeThen proteins trigger beginning of S phaseThen DNA replication beginsReplication in most prokaryotic cells starts from a single point and proceeds in 2 directions until entire chromosome is copied2 chromosomes are attached to different points inside the cell membrane and separated when cell splits
42 B. Eukaryotic DNA Replication Chromosomes = much biggerReplication may begin at dozens or even hundreds of places on the DNA molecule, proceeding in both directions until each chromosome is completely copiedAlthough proteins check DNA for chemical damage or base pair mismatches prior to replication system is not foolproofDamaged regions of DNA are sometimes replicated result in changes of DNA sequences that may alter certain genes and produce serious consequences
43 2 copies of DNA produced by replication in each chromosome remain closely associated until cell enters prophase of mitosisThen chromosomes condense 2 chromatids in each chromosome become clearly visibleChromatids separate from each other in anaphase of mitosisProduce 2 new cells each w/ complete set of genes coded in DNA
44 Solve the Chapter Mystery Page 357…Use your understanding of the structure of DNA to predict what sorts of problems excessive UV light might produce in the DNA molecule. How might these changes affect the functions of DNA?All cells have systems of enzymes that repair UV-induced damage to their DNA. Some cellular systems block DNA replication if there are base pairing problems in the double helix. Why are these systems important? How might they work?Analyze the effects that UV light might have on skin cells. What is UV light so dangerous? Why is the skin particularly vulnerable to it?Among humans who inherit genetic defects in their DNA-repair systems, the incidence of skin cancer is as much as 1000 times greater than average. Based on this information, what can you infer about the effect of UV light on DNA?