protein synthesis chapter 17. protein synthesis dna responsible for hereditary information dna...
TRANSCRIPT
Protein Synthesis
Chapter 17
Protein synthesis
DNA Responsible for hereditary information DNA divided into genes Gene: Sequence of nucleotides Determines amino acid sequence in
proteins Genes provide information to make
proteins
Protein synthesis
DNA RNA protein
Central Dogma
Mechanism of reading & expressing genes
Information passes from the genes (DNA) to an RNA copy
Directs sequence of amino acids to make proteins
Protein synthesis
Transcription: DNA sequence is copied into an RNA Translation: Information from the RNA is turned
into an amino acid sequence
RNA
RNA (ribonucleic acid) Single strand Sugar –ribose (-OH on 2’ carbon) Uracil instead of thymine
RNA
mRNA: Messenger RNA Transcribes information from DNA Codons (3 nucleotides) CGU mRNA Codes for amino acids rRNA: Ribosomal RNA Polypeptides are assembled
RNA
tRNA: Transfer RNA Transports aa to build proteins Positions aa on rRNA Anticodons (3 complementary nucleotides) GCA
Cracking the code
Francis Crick Codons (Triplet code)-mRNA Each codon corresponds to an aa 20 amino acids Reading frame Reading symbols in correct
groupings
Cracking the code
1 or 2 deletions or additions Gene was transcribed incorrectly 3 deletions Reading frame would shift Gene was transcribed correctly
WHYDIDTHEREDCATEATTHEFATRAT
WHYIDTHEREDCATEATTHEFATRAT
WHYDTHEREDCATEATTHEFATRAT
WHYTHEREDCATEATTHEFATRAT
The code
Universal code AGA codes for amino acid Arginine Humans & bacteria Genes from humans can be
transcribed by mRNA from bacteria Produce human proteins Insulin
Protein synthesis
DNA RNA Protein
Transcription Translation
Prokaryotes
Transcription Getting the code from DNA Template strand Strand of DNA that is transcribed or
read Transcribed RNA is complementary
to the DNA
Prokaryotes
Coding strand DNA strand not coded Same sequence of nucleotides as
the RNA transcript Only T instead of U.
Prokaryotes
RNA polymerase Enzyme Adds nucleotides to the 3’end 5’to3’ direction Does not need a primer to start
Prokaryotes
Stages of transcription Initiation Elongation Termination
Prokaryotes
Initiation Promoters: Sequence on DNA where
transcription starts -35 sequence TTGACA -10 sequence TATAAT Sequences are not transcribed
Prokaryotes
RNA polymerase binds promoter Unwinds DNA Uses an ATP or GTP to start Uses phosphate group Transcription bubble: RNA polymerase, DNA & growing
RNA strand
Prokaryotes
Termination Stop signal Sequence on DNA RNA transcript signals polymerase
to detach from DNA RNA strand separates from the DNA
Prokaryotes
Translation Passing the code to make a
polypeptide mRNA binds to rRNA on the
ribosome mRNA attaches so only one codon is
exposed at a time
Ribosome
Located in the cytoplasm Site of translation 2 subunits composed of protein &
RNA Small (20 proteins and 1 RNA) Large (30 proteins and 2 RNA) 3 sites on ribosome surface involved
in protein synthesis E, P, and A sites
Ribosome
Ribosome
Prokaryotes
tRNA (anti-codon) Complementary sequence Binds to mRNA tRNA carries a specific amino acid Adds to growing polypeptide 45 tRNA’s
Prokaryotes
Aminoacyl-t-RNA synthetases Activating enzymes Link correct tRNA code to correct aa One for each 20 amino acids Some read one code, some read
several codes
Prokaryotes
Nonsense codes UAA, UAG, UGA code to stop AUG codes for start as well as
methionine Ribosome starts at the first AUG it
comes across in the code
Prokaryotes
Translation 1. Initiation 2. Elongation 3. Termination
Prokaryotes
Initiation Initiation complex 1. tRNA with formylmethionine attached
binds to a small ribosome 2. Initiation factors position the tRNA on
the P site 3. A site (aminoacyl) where other tRNA’s
form
Prokaryotes
4. tRNA is positioned on to the mRNA at AUG
5. Attachment of large ribosomal unit
Prokaryotes
Elongation factors Help second tRNA bind to the A-site Two amino acids bind (peptide
bond) Translocation: Ribosome moves 3 more
nucleotides along mRNA in the 5’to 3’ direction
Prokaryotes
Initial tRNA moves to E site Released New tRNA moves into A site Continues to add more aa to form
the polypeptide
Prokaryotes
Release factors: Proteins that release newly made
polypeptides Codon (UAG, UAA, UGA) Release factor binds to the codon Polypeptide chain is released from A
site
Eukaryotes
Transcription (nucleus) Initiation Elongation Termination
Eukaryotes
Initiation Transcription Initiation Complex
is formed Transcription factors bind first to the
promoter RNA pol II binds DNA Starts to transcribe
Fig. 17-7b
Elongation
RNApolymerase
Nontemplatestrand of DNA
RNA nucleotides
3 end
Direction oftranscription(“downstream”) Template
strand of DNA
Newly madeRNA
3
5
5
Fig. 17-UN1
Transcription unit
Promoter
RNA transcriptRNA polymerase
Template strandof DNA
5
553
3 3
Eukaryotes
Termination Polyadenylation signal sequence Recognized by RNA polymerase II mRNA is released
Transcription
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Eukaryotes
mRNA is modified Nucleus RNA processing
Eukaryotes
5’ cap Addition of a GTP 5’ phosphate of the first base of
mRNA Methyl group is added to the GTP 3’poly-A-tail Several A’s on the end of the mRNA
Eukaryotes
Introns: non-coding sequences of nucleic
acids Exons: coding sequences of nucleic acids
Euraryotes
RNA splicing Cut out introns Reconnect exons snRNP’s (small nuclear RNA’s) Spliceosome: Many snRNP’s come together &
remove introns
Eukaryotes
Translation 1. Initiating aa is methionine 2. Initiation complex is more
detailed
Fig. 17-16bP site (Peptidyl-tRNAbinding site) A site (Aminoacyl-
tRNA binding site)E site(Exit site)
mRNAbinding site
Largesubunit
Smallsubunit
(b) Schematic model showing binding sites
Next amino acidto be added topolypeptide chain
Amino end Growing polypeptide
mRNAtRNA
E P A
E
Codons
(c) Schematic model with mRNA and tRNA
5
3
<>
Fig. 17-UN3
mRNA Ribosome
Polypeptide
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Similarities
DNA RNA Protein
Transcription Translation
Differences in gene expression
Transcription 1. Prokaryotes one RNA polymerase Eukaryotes 3 RNA polymerases (poli-II
mRNA synthesis) 2. Prokaryotes mRNA contain transcripts
of several genes Eukaryotes only one gene 3. Prokaryotes no nucleus so start
translation before transcription is done
Differences in gene expression
3. Eukaryotes complete transcription before leaving the nucleus
4. Eukaryotes modify RNA Introns/exons
5. Prokaryotes Polymerase binds promoters
Eukaryotes transcription factors bind first then enzyme
6. Termination
Differences in gene expression
Translation 1. Prokaryotes start translation with
AUG Eukaryotes 5’cap initiates
translation 2. Prokaryotes smaller ribosomes
Mutations
Changes in genetic information Point mutations: Change in a single base pair Sickle cell mutation
Mutations
Two types 1. Base-pair substitution Exchange one nucleotide and base
pair with another Silent mutations No effect on proteins
Mutations
Missense mutations: Substitutions that change one aa for
another Little effect
Mutations
Nonsense mutations Point mutation codes for stop codon Stops translation too soon Shortens protein Non-functional proteins
Mutations
2. Insertions or deletions Additions or losses of nucleotides Frameshift mutations Improperly grouped codons Nonfuctional proteins
Fig. 17-23Wild-type
3DNA template strand
5
5
53
3
Stop
Carboxyl endAmino end
Protein
mRNA
33
3
55
5
A instead of G
U instead of C
Silent (no effect on amino acid sequence)
Stop
T instead of C
33
3
55
5
A instead of G
Stop
Missense
A instead of T
U instead of A
33
3
5
5
5
Stop
Nonsense No frameshift, but one amino acid missing (3 base-pair deletion)
Frameshift causing extensive missense (1 base-pair deletion)
Frameshift causing immediate nonsense (1 base-pair insertion)
5
5
533
3
Stop
missing
missing
3
3
3
5
55
missing
missing
Stop
5
5533
3
Extra U
Extra A
(a) Base-pair substitution (b) Base-pair insertion or deletion
Mutagens
Chemical or physical agents Mutations in DNA