Biology 102 Lecture 12: From DNA To Proteins

11/1/2015GenesBiology 102 Sequence of nucleotide bases in DNA is a codefor making proteins To uniquely code for each of the 20 amino acids,how many bases must code for a single aminoacid?Lecture 12: From DNA toProteinsTotal number of unique codes1 base forms code pattern4 unique codes - A, T, C, G (41)2 bases form code pattern42 16 unique codes (too few)3 bases form code pattern43 64 unique codes (just right)4 bases form code pattern44 256 unique codes (too many)Genes TranscriptionA group of 3 DNA bases is called a triplet Each triplet codes for one amino acid in aprotein’s primary structure (more on this later)DNA is too large, too valuable to leave thenucleus Can’t get to the ribosomes to give instructionsfor making proteins Sends a temporary copy made of RNA Process of making this copy is calledtranscription Specific type of RNA produced in transcriptionis messenger RNA (mRNA)Steps in Transcription1. Initiation Chromatin unwinds to expose DNA DNA “unzips” Enzymes bind to gene of interestSteps in Transcription2. Elongation RNA nucleotides are added to compliment theDNA base pairs on the template strand(elongation)1

11/1/2015Steps in Transcription3. Termination Enzymes and mRNA strand releasedRNA nucleotidesTranscription DNA triplets have been rere-coded as mRNAcodons But in mRNA, T is replaced with UmRNA ProcessingmRNA Processing mRNA is not yet ready to direct proteinsynthesis Protein--coding DNA sections of genes are calledProtein Introns must be removed and exons joinedtogetherCalled RNA splicingexons Genes are further protected with interspersednon--coding regions called intronsnon2

11/1/2015mRNA Processing Some genes can be spliced together in multipleways Called alternative splice products orsplice isoforms One gene can code for more than one protein One protein may have multiple formsProtein Synthesis Once a strand of mRNA has been spliced, it isready to be decoded to build a proteinGenetic Linguistics Think about different written languages Do the same letter combinations mean the samething in all languages?This process is called translationThe genetic code is translated from the languageof nucleic acids to the language of proteinsExamplesFrenchOursPainPetGenetic LinguisticsExperiment What about the language of DNA and proteins? Jellyfish are cool Will every species read DNA the same way? If we put the gene for a human protein intoanother species, will it make the same protein?Some of them make a protein known as GFP thatglows green when exposed to UV light3

11/1/2015ExperimentExperiment What if we take the gene for this protein andput it into bacteria? Will the bacteria make GFP? Or will theytranslate the genetic code differently? They make GFP! ?Experiment Experiment What about other species? They make the same protein! How will they translate jellyfish DNA? What does this tell you about the genetic code?ExperimentTranslation They make the same protein! How do we get from DNA to protein? What does this tell you about the genetic code? It’s universal! An mRNA codon will be translatedinto the same protein, no matter the speciesThe process is pretty complicated, but let’s takea look We exploit this is so many, very cool ways Recombinant DNA technology4

11/1/2015Protein Synthesis All 3 types of RNA are involved in this processProtein Synthesis Let’s take a closer look at tRNA Note the anticodon loop at the bottom Note the amino acid attached to the other end Each tRNA will carry a specific amino aciddetermined by the specifc anticodonAnticodonloopProtein Synthesis 3 exosed bases on the anticodon loop bind to acodon in mRNA through complimentary basebase-pairing Protein Synthesis Ribosome moves long mRNA to sequentiallyexpose each codon Base-pairing brings amino acids to the ribosomeBasein the correct orderHydrogen bondsProtein Synthesis Amino acids joined together to form a protein’sprimary structure Takes about 20 seconds to form the wholeproteinProtein Synthesis The signal for where to begin translation is thestart codoncodon:: AUG Codes for the amino acid methionine All proteins begin with methionineTranslation ends when a STOP codon is reached Codes for no amino acid No corresponding tRNA Ribosome comes apart and releases mRNA,protein5

11/1/2015Universal Genetic CodeDNA DNA is PRECIOUS!! If the code is changed, another amino acid couldbe made A change in just one nucleotide base can bedevastatingDNA Damage, Repair, and MutationDNA Damage, Repair, and Mutation Many factors lead to DNA damage Many different DNA repair enzymes exist Happens CONSTANTLY – yet usually we’re fine! Replace damaged piece by Direct repair Referencing complimentary strand Rarely, DNA damage is undetected or improperlyrepaired This is called a mutation Several types of mutations existSingle base or nucleotide excision,mismatch repairReferencing homologous chromosome DNA Damage, Repair, and MutationMethylation defectsDouble--strand breaksDoublePoint MutationsChanging one baseCould change an amino acid Lots of other things could happen Nothing Example: ACC to ACA Both code for THR Could cause truncation (protein gets cut short) Example: UGG to UGA UGG TRP UGA Stop Could make a protein too long, could miss aStart codoncodon,, etc 6

11/1/2015Frameshift Mutations When a mutation changes how codons aregrouped and read Changes every amino acid following the mutation Example: Take a sentence with all 33-letter wordsOther Mutations Additions Deletions Inversions and translocations Large pieces of DNA (sometimes most of achromosome) broken apart and reattached Sometimes within a chromosome Sometimes to another chromosomeTHE DOG SAW THE CAT If we delete the E at position 3 and rere-group thewords, it no longer makes senseTHD OGS AWT HEC AT Which mutations in our previous examplesproduced a frameshift?frameshift?Other Mutations May be benign Entire genes with their promoters simplymoved from one place to anotherGene may split into pieces No longer codes for a functional protein Example: most severe hemophilia is caused byan inversion of the gene for a bloodblood-clottingprotein7

Protein Synthesis Amino acids joined together to form a protein’s primary structure Takes about 20 seconds to form the whole protein Protein Synthesis The signal for where to begin translation is the start start codoncodon: AUG: AUG Codes for Cod