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)