Chapter 12: DNA and RNA

Chapter 12: DNA and RNASCCS Honors Biology1DNADeoxyribonucleic acid

A large polymer used to carry the genetic code of all living organisms

2DNA Heredity & StructureWhat we know about DNA was not discovered overnight! Many different scientists contributed information. Because of the efforts of all these scientists, we now have a model of DNA that consistently fits the observations we make. It also allows us to make useful predictions!

3Griffiths ExperimentWas trying to develop a vaccination for the pneumococcus bacteria. Vaccine- a prepared substance from killed or weakened disease-causing agents used to prevent future infectionsHe was working with two strains of bacteria. Rough - bacteria had a rough appearance in culture, non-virulent (doesn't kill) Smooth - bacteria had a smooth appearance in culture, virulent (kills)

Control(no growth)Dies of pneumoniaLivesLivesDies of pneumoniaDisease-causing bacteria (smooth colonies)Harmless bacteria (rough coloniesHeat-killed, disease-causing bacteria (smooth colonies)Heat-killed, disease-causing bacteria (smooth colonies)Harmless bacteria (rough colonies)Live, disease-causingbacteria (smooth colonies)DNA as hereditary materialThe Genetic Material is DNA Alfred Hershey and Martha Chase, 1952Previously, scientists thought that proteins were the hereditary moleculeHershey and Chase worked with viruses that infect bacteria called bacteriophagesThrough a series of experiments, they were able to show that DNA, not protein, is the hereditary molecule.

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Martha Chase (left) & Alfred Hershey (right)7Virus StructureDNA is locatedin the head.

The outside and tailof the virus ismade out of protein.

Virus ATTACKS!!

Bacteriophages ATTACK!!

Hershey Chase Experiment DNA in VirusesBacteriophage with phosphorus-32 in DNABacteriophage with sulfur-35 in protein coatPhage infectsbacteriumRadioactivity inside bacteriumPhage infectsbacteriumNo radioactivity inside bacteriumHershey & Chase ExperimentConcluded that the DNA of viruses is injected into the bacterial cells, while the viral proteins remain outsideThe injected DNA molecules cause the bacterial cells to produce more virusesDNA is the hereditary material not proteins.Wilkins & FranklinM.H.F. Wilkins and Rosalind Franklin, early 50sWilkins and Franklin studied the structure of DNA crystals using X-rays.They found that the crystals contain regularly repeating subunits.The X pattern produced by DNA suggested that DNA contains structures with dimensions of 2 nm, 0.34 nm, and 3.4 nm. The dark structures at the top and bottom indicate that some structure was repeated, suggesting a helix.

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Rosalind FranklinX-ray diffraction image of DNA14Watson & CrickJames Watson and Francis H.C. Crick, 1953Watson and Crick used Chargaff's base data and Franklins X-ray diffraction data to construct a model of DNA.The model showed that DNA is a double helix with sugar-phosphate backbones on the outside and the paired nucleotide bases on the inside, in a structure that fit the spacing estimates from the X-ray diffraction data.Chargaff's rules showed that A = T and G = C, so there was complementary base pairing of a purine with a pyrimidine, giving the correct width for the helix.The paired bases can occur in any order, giving an overwhelming diversity of sequences.

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Watson & Crick with their model of DNA16DNA StructureDouble Helix - twisted ladder

Made up of monomers called nucleotides

Nucleotides are composed of:Deoxyribose sugarPhosphate groupNitrogenous base

17Nitrogenous BasesTwo types:Purines (two rings)Pyrimidines (one ring)

PurinesAdenine and GuaninePyrimidinesThymine and Cytosine

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PurinesPyrimidinesAdenineGuanineCytosineThyminePhosphate groupDeoxyribosePractice PairingACGGTATEMPLATE STRANDTTGCCABondingACGGTATEMPLATE STRANDTTGCCAWeak HYDROGEN bonds form between the Nitrogen Base Pairs.Chargaffs rules:Base pairing rule is A-T and G-CThymine is replaced by Uracil in RNABases are bonded to each other by Hydrogen bondsDiscovered because of the relative percent of each base; (notice that A-T is similar and C-G are similar)22

Erwin Chargaff23Source of DNAATCGStreptococcus29.831.620.518Yeast31.332.918.717.1Herring27.827.522.222.6Human30.929.419.919.824DNA Structure

Backbone alternates with phosphate and sugar (deoxyribose)with the nucleotides formingthe rungs or steps of the ladder

The backbone of it allACGGTATEMPLATE STRANDTTGCCAThe backbone is made of alternating sugars and phosphates.- Remember: Sugar ALWAYS attaches to the Nitrogen base

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Hydrogen bondsNucleotideSugar-phosphate backboneKeyAdenine (A)Thymine (T)Cytosine (C)Guanine (G)DNA (contd)DNA is found packed in the nucleus of eukaryotic organisms; it is found in the cytoplasm of prokaryotic organismsDNA is packed together and wrapped around special proteins called HISTONESDNA bound protein is called CHROMATINWhen chromatin condenses (gets thicker) it forms CHROMOSOMES29So Remember:DNA Chromatin Chromosomes30

ChromosomeSupercoilsCoilsNucleosomeHistonesDNAdoublehelixDNA Replication Part 2DNA & RNA continued!Before mitosis (during S phase of interphase) , a complete copy of a cells DNA is made through a process called replication.When a cell divides, each daughter cell gets one complete copy of the DNA.Similar to photocopying a document the end result is two identical documents that contain the same information.Now that we know something about DNAs structure, lets look at how it replicates.

33Steps of DNA ReplicationDNA must unwind and break the hydrogen bondsEach strand is used as a template (blueprint)Two new strands of DNA are formed from the original strand by the enzyme DNA Polymerase34During replication, an enzyme called helicase unzips the DNA molecule along the base pairing, straight down the middle.Another enzyme, called DNA polymerase, moves along the bases on each of the unzipped halves and connects complementary nucleotides.What do we mean by complementary nucleotides?

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GrowthGrowthReplication forkDNA polymeraseNew strandOriginal strandDNA polymeraseReplication forkOriginal strandNew strandNitrogenous basesBecause of Chargaffs rule, only the correct, complementary bases will fit, so chances are good that the DNA polymerase will make a perfect copy.Mistakes happen! Mutation!

Is this frog likely to survive long in the wild?

37DNA Replication AnimationWhen DNA polymerase finishes, the nucleus has two identical (excluding mutations) copies of the original DNA molecule. Now the cell can proceed with mitosis.The long DNA molecules condense into chromosomes. (prophase)Chromosomes line up in center of cell (metaphase)Spindle fibers attach to centromeres (anaphase)Sister chromatids move to opposite sides of cell (telophase)Cytokinesis happens, and BOOM two new cells.

PMAT

39Protein SynthesisPart 3Transcription and TranslationDNAs PurposeDNA has genes that code for the synthesis (creation) of specific PROTEINSHeres the problemWhere is DNA located?NucleusWhere does Protein Synthesis occur?At ribosomes in the cytoplasmCan DNA ever leave the nucleus?No.

RNARibonucleic acidSingle-strandedSugar is riboseThymine is replaced by URACIL42Differences between DNA and RNA DNAStructure: Double strandedSugar: DeoxyriboseBases:AdenineGuanineCytosineThymineRNAStructure: Single-strandedSugar: RiboseBases:AdenineGuanineCytosineUracil

43Transcription- how RNA is madeJust as DNA polymerase makes new DNA, a similar enzyme called RNA polymerase makes new RNA.

RNA polymerase temporarily separates the strands of a small section of the DNA molecule.This exposes some of the bases of the DNA molecule.

Along one strand, the RNA polymerase binds complementary RNA nucleotides to the exposed DNA bases.An exposed thymine on the DNA strand matches up with an adenine; an exposed cytosine on the DNA matches up with a guanine base; an exposed adenine DNA base matches up with URACIL!44As the RNA polymerase moves along, it makes a strand of messenger RNA (mRNA).It is called messenger RNA because it carries DNAs message out of the nucleus and into the cytoplasm.mRNA is SINGLE STRANDED!When the RNA polymerase is done reading the gene in the DNA, it leaves.The separated DNA strands reconnect, ready to be read again when necessary. mRNA moves out of the nucleus and finds a ribosomeOn the ribosome, amino acids are assembled to form proteins in the process called translation.45DNA TranscriptionTypes of RNA:Messenger RNA (mRNA)carries information from DNA to ribosomeTransfer RNA (tRNA)Carries amino acidsRibosomal RNA (rRNA)Makes up ribosomes47fromtoto make upalso calledwhich functions toalso calledalso calledwhich functions tocan beRNAMessenger RNARibosomal RNATransfer RNAmRNACarry instructionsrRNACombine with proteinstRNABringamino acids toribosomeDNARibosomeRibosomesPart 4- Translation- the Ultimate Goal!

Going from mRNA to the final product

Decoding the Information in DNAHow does DNA (a twisted latter of atoms) control everything in a cell and ultimately an organism?DNA controls the manufacture of all cellular proteins including enzymesA gene is a region of DNA that contains the instructions for the manufacture of on particular polypeptide chain (chain of amino acids)DNA is a set of blueprints or code from making proteinsHow do you get from DNA to Proteins?

TRANSCRIPTION the synthesis of RNA under the direction of DNATRANSLATION the actual synthesis of a protein, which occurs under the direction of mRNA Where does this happen?Where is the DNA?

Protein synthesis the manufacture of proteins

Where are proteins made in the cell?

Genetic CodeGenetic code the language of mRNA instructions (blueprints)Read in three letters at a timeEach letter represents one of the nitrogenous bases: A, U, C, G

Codon found on mRNA; consists of three bases (one right after the other)64 codons for 20 amino acids53Codon (contd)For example, consider the following RNA sequence:UCGCACGGU

The sequence would be read three base pairs at a time:UCG CAC GGU

The codons represent the amino acids:Serine Histidine - Glycine54Codons (contd)AUG start codon or Methionine

UAA, UAG, UGA stop codons; code for nothing; like the period at the end of a sentence55

Translation: Protein SynthesismRNA combines with a ribosome and tRNA and makes a protein

Remember:mRNA carries the codon (three base sequence that codes for an amino acid)tRNA carries the anticodon which pairs up with the codontRNA brings the correct amino acid by reading the genetic code57tRNA (transfer RNA)tRNA carries (or transfers) the correct amino acid to the codon on the mRNA.tRNA has an ANTICODON that can attach to mRNAs codon.

TranslationGUA UCU GUU ACC GUAmRNAmRNA carries the same message as DNA but rewritten with different nitrogen bases.This message codes for a specific sequence of amino acidsReview..Amino acids are the building blocks ofPROTEINSTranslationGUA UCU GUU ACC GUAmRNACodon: a sequence of 3 nitrogen bases on mRNA that code for 1 amino acidIts a TRIPLET codeExample: This strand of mRNA has 5 codons, so it would code for 5 amino acids.TranslationGUA UCU GUU ACC GUAmRNAThese codons are universal for every bacteria, plant and animal on earth

There are 64 codons which code for all 20 amino acids on earth.

TranslationGUA UCU GUU ACC GUAmRNAThe mRNA molecule travels to the ribosomes where the mRNA codes are read by the ribosomesRibosomes hold the mRNA so another type of RNA, transfer RNA (tRNA) can attach to the mRNARibosomeTranslationGUA UCU GUU ACC GUAmRNARibosomeCUAAG ATranslationGUA UCU GUU ACC GUAmRNACUAAG ACA ACodons match up with anticodons to create a protein

Protein formationAmino acids link together to form a proteinThe new protein could become cell part, an enzyme, a hormone etc.

SO:Say the mRNA strand reads:mRNA (codon) AUGGACCAG-UGAtRNA (anticodon) UAC-CUG-GUC-ACU

tRNA would bring the amino acids:Methionine-Aspartic acid-Glutamine-stop691)mRNA is transcribed in the nucleus and leaves the nucleus to the cytoplasm

2) mRNA attaches to the ribosome3)The codon on the mRNA is read by the anticodon on the tRNA4) tRNA brings the amino acid as it reads mRNA5) The amino acids are joined together to form a polypeptide (protein)6) When a stop codon is reached (UAA, UAG, UGA) protein synthesis stops70Transcription and translationWhat if things go wrong?MUTATION!!!If transcription or translation were to copy the wrong sequence, the incorrect amino acid could be addedThis would change the overall protein structure and could make the protein ineffectiveSickle cell anemia is caused by a single amino acid difference in the hemoglobin protein sequence

Kinds of MutationsPoint Mutations only occur at a single point in the DNA sequence

Substitution/ Insertion / Deletion

Frameshift Mutations shift the entire reading frame change ALL the amino acidsMutationsSubstitution one base replaces another

Insertion an extra base is inserted

Deletion loss of a single letter (makes entire base disappear!)

SubstitutionInsertionDeletionChromosomal MutationsChange in the number or structure of chromosomes

Ex. Deletion, Duplication, Inversion, and Translocation

DeletionDuplicationInversionTranslocation