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Chapter 17
From Gene to Protein
Gene Expression
The process by which DNA directs the synthesis of proteins2 stages: transcription and translation
Detailed flow of info from gene to proteinExplain how genetic mutations affect
organisms through their proteinsCentral Dogma (Francis Crick)
DNA transcribed RNA translated protein
Proteins link DNA to phenotype
Decoding Genes to PhenotypeArchibald Garrod
Inborn error of metabolism Inherited disease from inability to make a particular enzyme
Suggested that genes dictate phenotype through enzymes that catalyze specific reactions
George Beadle and Edward TatumOne gene – one enzyme hypothesis
A single gene specifies synthesis of a single enzyme in the body
Shared Nobel PrizeAdditional Scientists
Not all proteins are enzymes and many proteins are 1+ polypeptides
Revised to one gene – one polypeptideEach gene codes for 1 polypeptide of a protein
From DNA to ProteinGenes instruct, but
don’t buildNucleotides and
amino acids are different ‘languages’
RNA connects themTranscription:
same languageTranslation:
different languageOccurs in all
organisms
Reviewing DNA and RNA
DNA RNASugar is deoxyribose
Has –HBases are A,C, G, and
TDouble-stranded
helixOnly in nucleusModified only by
mutations1 type
Sugar is riboseHas -OH
Bases are A, C, G, and USingle-strandedNot confined to nucleusLots of processing and
modifications3 types (mRNA, tRNA,
rRNA)
Summary of Protein Synthesis
Genes determine the sequence of bases along an mRNAOnly template strand is
transcribedSimilar to DNA replicationmRNA is complementary
to template strandNon-template strand is
‘identical’ to mRNA = coding strand
Sequence of mRNA as codons translated to amino acids
TranscriptionIn the nucleusInitiation
RNA polymerase binds to promoter
Many work at once = efficiencyElongation
Builds 5’ 3’ = downstreamUnstable complex so mRNA
immediately released and DNA rejoins
TerminationTerminator reached =
releasing transcript (pre-mRNA)
mRNA ProcessingBefore mRNA leaves the nucleusAlteration of 5’ and 3’ ends
5’ cap, modified G, directs ribosome attachment for protein synthesis
Poly-A tail, addition of 5-250 adenines (A), inhibits degradation as leaves nucleus
RNA splicingPre-mRNA transcript contains exons and introns
RNA sequences and DNA sequences that encode themSpliceosomes splice out introns and rejoin exons = true
mRNA
TranslationIn the cytoplasm within
ribosomesmRNA as codon
message from DNATranslated by tRNA
Anticodons and amino acid ends
Ribosomes facilitate addition of tRNA to mRNA
3 steps like transcription
Ribosomes
Facilitate coupling of tRNA anticodons and mRNA codons
Large and a small subunitsFunctional when subunits
join with mRNA3 unique binding sites
facilitateMore than 1 can bind to a
single mRNA
Initiation
Small ribosome subunit binds to mRNA and moves to start codon
1st tRNA enters the P site carrying the AA metAnticodon is what?
Large subunit bindsInitiation factors facilitate and GTP supplies energy
Elongation2nd tRNA molecule
enters the A siteOnce matched the
growing polypeptide chain binds to the new AA
Ribosome shifts 5’ 3’, changing P site tRNA to E site, A site to P site, and freeing A site = translocation
Termination
Stop codon sequence that signifies the end of a polypeptide chainSequences are UAG, UAA, and UGA Don’t code for AA’s
Polypeptide cleaved from last tRNA (P site) and leaves the ribosome
Folds into quaternary structure = protein
Decoding CodonsOnly 4 nucleotide
bases to specify 20 amino acids
Genetic instructions are based on codons42 = 16 (not enough);
43 = 64 (plenty)
Demonstrates redundancy, but not ambiguity3rd base is wobble
baseNearly universal
across species
MutationsChanges to the genetic information of a cell, or
virusUltimate source of diversity because ultimate
source of new genesChapter 15 was large scale mutations which
effect long DNA segmentsWhat were the 4 types?
Point mutations change single nucleotides and causes changes in single specific codonsBase-pair substitution
Frameshift mutations result from altering the reading frameBase-pair insertions/deletions
Base-pair SubstitutionsReplaces 1 nucleotide pair with anotherEffect depends on particular codon change and
locationSilent mutations occur at wobble bases so
have no effect on the encoded proteinMissense mutations change 1 AA to anotherNonsense mutation codes for a stop codon
rather than an AAStops translation early
Can result in wrong AA being added, abnormal protein shape, or shortened proteins
Alters active site and can make non-functional
Frameshift MutationsInsertions and deletions add or remove
nucleotide pairs of a geneOften more deleterious when changes
don’t occur in groups of threeAny downstream bases will be affectedAlmost always causes protein to be
nonfunctionalTHE CAT ATE THE RATTHE ATA TET HER AT (delete C)THE CAT GAT ETH ERA T (insert G)