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Chapter 17: Chapter 17: From Gene to ProteinFrom Gene to Protein(Protein Synthesis)(Protein Synthesis)
Essential Knowledge
3.a.1 – DNA, and in some cases RNA, is the primary source of heritable information (17.1-17.4).
3.c.1 – Changes in genotype can result in changes in phenotype (17.5).
Question? Question? How does DNA control a cell? (or identify a
phenotype) By controlling protein synthesis (otherwise
known as gene expression) Proteins are the link between genotypegenotype and
phenotypephenotype
1909 - Archibald 1909 - Archibald GarrodGarrod Suggested genes control enzymes that
catalyze chemical processes in cells Inherited Diseases - “inborn errors of
metabolism” where a person can’t make an enzyme Symptoms reflect person’s inability to make proteins/enzymes
ExampleExample Alkaptonuria - where urine turns black
after exposure to air Lacks - an enzyme to metabolize/break
down alkapton
George Beadle and Edward George Beadle and Edward TatumTatum
Worked with Neurospora and proved the link between genes and enzymes
Grew Neurospora on agar Varied the nutrients in the agar Looked for mutants that failed to grow
on minimum agar
ConclusionConclusion
Mutations were abnormal genes Each gene dictated the synthesis/production
of one enzyme One Gene - One Enzyme Hypothesis
Current HypothesisCurrent Hypothesis One Gene - One Polypeptide
Hypothesis.Why change? Not all proteins are enzymes
We now know proteins may have 4th degree structure.
Central DogmaCentral DogmaDNA
Transcription
RNA Translation
Polypeptide chain (will become protein)
ExplanationExplanation DNADNA – the genetic code or genotype RNARNA - the message or instructions PolypeptidePolypeptide - the end product for the
phenotype
Why is there an RNA Why is there an RNA intermediate?intermediate? Evolutionary adaptation:
Check-point in processProvides protection for DNA codeMore copies can be made simultaneously
Genetic CodeGenetic Code
Sequence of DNA bases that describe which amino acid to place in what order in a polypeptide chain
The genetic code gives ONLY the primary protein structure All other protein structures result from chemical interactions
amongst primary protein structure
Genetic CodeGenetic Code
Is based on triplets of bases (called codonscodons)
Has redundancy; some AA's have more than 1 code/3-base codon
ProofProof - make artificial RNA and see what AAs are used in protein synthesis (early 1960’s)
Codon Codon A 3-nucleotide “word” in the Genetic
Code 64 possible codons known
Codon
Amino
acid
Codon DictionaryCodon Dictionary Start- AUG (Met) Stop- UAA
UAG UGA
60 codons for the other 19 AAs
Code RedundancyCode Redundancy
Third base in a codon shows "wobble” effect
First two bases are the most important in reading the code and giving the correct AA The third base often doesn’t matterThis allows for mistakes during DNA
replication
Reading FrameReading Frame The “reading” of the code is every three
basesEx: the red cat ate the ratEx: ATT GAT TAC ATT
The “words” (codons) only make sense if “read” in this grouping of three (in correct “letter” order)
Code EvolutionCode Evolution
The genetic code is nearly universal Ex: CCG = proline (all life) Reason:
Code must have evolved early Life on earth must share a common ancestor
Protein Synthesis IntroProtein Synthesis Intro Intro movie
Protein Synthesis Protein Synthesis IntroIntro Step 1: Transcription:
DNA mRNA Step 2: Translation:
mRNA tRNA Am. Acid Polypep. chain
Polypeptide chain then becomes protein
TranscriptionTranscription Process of making RNA from a DNA
templateRNA type: mRNA (messenger)Intermediate type
Takes place in nucleus (in eukaryotes)
Transcription StepsTranscription Steps1. RNA Polymerase Binding
2. Initiation
3. Elongation
4. Termination
RNA PolymeraseRNA Polymerase
Enzyme for building RNA from RNA nucleotides Prokaryotes - 1 type Eukaroyotes- 3 types
Splits two DNA strands apart Hooks RNA nucleotides together (as they
pair with DNA)
11stst Step Step: RNA Polymerase : RNA Polymerase BindingBinding
Requires that the enzyme find the “proper” place on the DNA to attach and start transcription
Different from DNA polymerase Doesn’t require an RNA primer
RNA Polymerase Binding RNA Polymerase Binding Needs:Needs:
Promoter RegionsPromoter Regions (on the DNA)Special sequences of DNA nucleotides that
“tell” cell where transcriptiontranscription begins
Transcription FactorsTranscription FactorsProteins
PromotersPromoters
Regions of DNA where RNA Polymerases can bind
About 100 nucleotides long. Include initiation site and recognition areas for RNA Polymerase
Also “decide” which DNA strand to use
TATA BoxTATA Box
ONLY in eukaryotes Short segment of T,A,T,A Located 25 nucleotides upstream from the
initiation site Recognition site for transcription factors to bind
to the DNA
Transcription FactorsTranscription Factors Proteins that bind to DNA before RNA
Polymerase Recognizes TATA box, attaches, and
“flags” the spot for RNA PolymeraseRNA poly won’t attach unless these are
present
Transcription Initiation Transcription Initiation ComplexComplex
The complete assembly of 1) transcription factors and 2) RNA Polymerase
Bound to the promoter area of the DNA to be transcribed
22ndnd Step Step: Initiation: Initiation 2nd step of transcription Actual unwinding of DNA to start RNA
synthesis. Requires Initiation Factors
CommentComment Getting Transcription started is
complicated Gives many ways to control which
genes are decoded and which proteins are synthesized
33rdrd Step Step: Elongation: Elongation 3rd step in transcription RNA Polymerase untwists DNA 1 turn at a time Exposes 10 DNA bases for pairing with RNA
nucleotides
ElongationElongation Adds nucleotides to 3` end of growing
RNA strand Enzyme moves 5` 3` (of RNA strand) Rate is about 60 nucleotides per second
CommentComment Each gene can be read by sequential
RNA Polymerases giving several copies of RNA
Result - several copies of the protein can be made
44thth Step Step: Termination: Termination DNA sequence that tells RNA Polymerase
to stop Ex: AATAAA RNA Polymerase detaches from DNA
after closing the helix
Final ProductFinal Product Pre-mRNA This is a “raw” RNA that will need
processing and modifications
Modifications of RNAModifications of RNA1. 5’ Cap
2. Poly-A Tail
3. Splicing
5' Cap5' Cap Modified Guanine nucleotide added to
the 5' end Protects mRNA from digestive enzymes Recognition sign for ribosome
attachment
Poly-A TailPoly-A Tail 150-200 Adenine nucleotides added to
the 3' tail Protects mRNA from digestive enzymes. Aids in mRNA transport from nucleus.
RNARNA SplicingSplicing Removal of non-protein coding regions
of RNA Coding regions are then spliced back
together
Introns and ExonsIntrons and Exons Introns:
Intervening sequencesRemoved from RNA.
Exons:Expressed sequences of RNATranslated into AAs
Introns - FunctionIntrons - Function Left-over DNA (?) Way to lengthen genetic message Old virus inserts (?) Way to create new proteins
Translation Poster Requirements 1. What is translation? (definition) 2. What is needed? 3. Specifics & Structure of tRNA 4. Where does it occur? 5. Ribosome specifics – be sure to include the
specifics of each subunit 6. Steps of translation & details of each step 7. What bonds are formed? 8. Illustration
22ndnd step of Protein step of Protein Synthesis: Synthesis: TranslationTranslation Process by which a cell interprets a
genetic message and builds a polypeptide
Location: mRNA moves from nucleus to cytoplasm and ribosomes
Materials Required for Materials Required for translationtranslation tRNA Ribosomes mRNA
Transfer RNA = tRNATransfer RNA = tRNA Made by transcription About 80 nucleotides long Carries AA for polypeptide synthesis
Structure of tRNAStructure of tRNA Has double stranded regions and 3
loops. AA attachment site at the 3' end. 1 loop serves as the AnticodonAnticodon.
AnticodonAnticodon Region of tRNA that base pairs to mRNA codon Usually is a compliment to the mRNA bases, so
reads the same as the DNA codon Example:
DNA- GAC mRNA – CUGtRNA anticodon - GAC
RibosomesRibosomes Two subunits (large and small) made in
the nucleolus Made of rRNA (60%)and protein (40%) rRNA is the most abundant type of RNA
in a cell
Large SubunitLarge Subunit Has 3 sites for tRNA. P site: PPeptidyl-tRNA site - carries the
growing polypeptide chain A site: AAminoacyl-tRNA site -holds the tRNA
carrying the next AA to be added E site: EExit site
Translation StepsTranslation Steps1. Initiation
2. Elongation
3. Termination
InitiationInitiation Brings together:
mRNAA tRNA carrying the 1st AA2 subunits of the ribosome
Initiation Steps:Initiation Steps:1. Small subunit binds to the
mRNA
2. Initiator tRNA (Met, AUG) binds to mRNA
3. Large subunit binds to mRNA
Initiator tRNA is in the P-site
InitiationInitiation Requires other proteins called "Initiation
Factors” GTP used as energy source
Elongation Steps:Elongation Steps:1. Codon Recognition
2. Peptide Bond Formation
3. Translocation
Codon RecognitionCodon Recognition tRNA anticodon matched to mRNA
codon in the A site
Peptide Bond Peptide Bond FormationFormation
A peptide bond is formed between the new AA and the polypeptide chain in the P-site
Bond formation is by rRNA acting as a ribozyme
After bond formation:The polypeptide is now transferred from the tRNA
in the P-site to the tRNA in the A-site
TranslocationTranslocation tRNA in P-site is released Ribosome advances 1 codon, 5’ 3’ tRNA in A-site is now in the P-site Process repeats with the next codon
TerminationTermination Triggered by stop codons Release factor binds in the A-site
instead of a tRNA H2O is added instead of AA, freeing the
polypeptide Ribosome separates
PolyribosomesPolyribosomes Cluster of ribosomes all reading the
same mRNA Another way to make multiple copies of
a protein
Prokaryotes: Prok. vs. Euk. Prokaryotes: Prok. vs. Euk. Protein Synthesis VideoProtein Synthesis Video
Polypeptide vs. ProteinPolypeptide vs. Protein Polypeptide usually needs to be
modified before it becomes functional Ex:
Sugars, lipids, phosphate groups addedSome AAs removedProtein may be cleavedJoin polypeptides together (Quaternary
Structure)
MutationsMutations Changes in the genetic make-up of a
cell Chapter 15 covered large-scale
chromosomal mutations (Hint - review these!)
Mutation types - Mutation types - CellsCells Somatic cells or body cells – not
inherited Germ Cells or gametes - inherited
Point or Spot Point or Spot MutationsMutations Changes in one or a few nucleotides in the genetic code
Effects - none to fatal
Types of Point Types of Point MutationsMutations
1. Base-Pair Substitutions
2. Insertions
3. Deletions
Base-Pair SubstitutionBase-Pair Substitution The replacement of 1 pair of nucleotides
by another pair Ex: Sickle cell anemia
Types of SubstitutionsTypes of Substitutions1. MissenseMissense - altered codons, still code for
AAs but not the right ones
2. NonsenseNonsense - changed codon becomes a stop codon
Question?Question? What will the "Wobble" Effect have on
Missense? If the 3rd base is changed, the AA may
still be the same and the mutation is “silent”
Missense EffectMissense Effect Can be none to fatal depending on
where the AA was in the protein Ex:
If in an active site - major effectIf in another part of the enzyme - no effect
Nonsense EffectNonsense Effect Stops protein synthesis Leads to nonfunctional proteins unless
the mutation was near the very end of the polypeptide
Sense MutationsSense Mutations The changing of a stop codon to a
reading codon Result - longer polypeptides which may
not be functional Ex. “heavy” hemoglobin
Insertions & DeletionsInsertions & Deletions The addition or loss of a base in the
DNA Cause frame shifts and extensive
missense, nonsense or sense mutations
Frame ShiftFrame Shift The “reading” of the code is every three
bases Ex: the red cat ate the rat Ex: thr edc ata tat her at The “words” only make sense if “read” in this
grouping of three
Question?Question? Loss of 3 nucleotides is often not a
problem Why?
Because the loss of a 3 bases or one codon restores the reading frame
MutagensMutagens Materials that cause DNA changes
1. Radiationex: UV light, X-rays
2. Chemicalsex: 5-bromouracil
Chernobyl video
SummarySummary Recognize the relationship between genes
and enzymes (proteins) as demonstrated by the experiments of Beadle and Tatum.
Identify the flow of genetic information from DNA to RNA to polypeptide (the “Central Dogma”).
Read DNA or RNA messages using the genetic code.
Recognize the steps and procedures in transcription.
SummarySummary Identify methods of RNA modification. Recognize the steps and procedures
in translation. Recognize categories and
consequences of base-pair mutations. Identify causes of mutations. Be able to recognize and discuss
“What is a gene?”