protein synthesis chapter 17. protein synthesis dna responsible for hereditary information dna...

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