PROTEIN SYNTHESIS

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PROTEIN PROTEIN SYNTHESISSYNTHESIS

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Protein SynthesisProtein Synthesis The production (synthesis) of The production (synthesis) of

polypeptide chains polypeptide chains (proteins)(proteins) Two phases:Two phases:

Transcription Transcription & & TranslationTranslation mRNA must be mRNA must be processed before processed before

it leaves the nucleusit leaves the nucleus of of eukaryotic cellseukaryotic cells

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TranscriptionTranscription

TranslationTranslation

DNA

mRNA

Ribosome

Protein

Prokaryotic CellProkaryotic Cell

DNA DNA RNA RNA ProteinProtein

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DNA DNA RNA RNA ProteinProtein

Nuclearmembrane


RNA ProcessingRNA Processing


DNA

Pre-mRNA

mRNA

Ribosome

Protein

EukaryotEukaryotic Cellic Cell

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Pathway to Making a Pathway to Making a ProteinProtein

DNADNA

mRNAmRNA

tRNA (ribosomes)tRNA (ribosomes)

ProteinProtein

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RNARNA

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RNA Differs from DNARNA Differs from DNA1.1. RNARNA has a sugar has a sugar riboseribose

DNADNA has a sugar has a sugar deoxyribosedeoxyribose

2.2. RNARNA contains the base contains the base uracil uracil (U)(U)

DNADNA has has thymine (T)thymine (T)

3.3. RNARNA molecule is molecule is single-single-strandedstranded

DNADNA is is double-strandeddouble-stranded

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Structure of RNAStructure of RNA

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Three Types of RNAThree Types of RNA•Messenger RNA (mRNA)Messenger RNA (mRNA)

carries genetic information carries genetic information to the ribosomesto the ribosomes

•Ribosomal RNA (rRNA)Ribosomal RNA (rRNA),, along with protein, makes along with protein, makes up the up the ribosomesribosomes

• Transfer RNA (tRNA)Transfer RNA (tRNA) transfers amino acids to the transfers amino acids to the ribosomes where proteins ribosomes where proteins are synthesizedare synthesized

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Making a Making a ProteinProtein

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Genes & ProteinsGenes & Proteins ProteinsProteins are made of are made of amino amino

acidsacids linked together by linked together by peptide bondspeptide bonds

2020 different amino acids different amino acids existexist Amino acids chains are called Amino acids chains are called

polypeptidespolypeptides Segment of DNA that codes Segment of DNA that codes

for the amino acid sequence for the amino acid sequence in a protein are called in a protein are called genesgenes

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Two Parts of Protein Two Parts of Protein SynthesisSynthesis

TranscriptionTranscription makes an RNA makes an RNA molecule complementary to molecule complementary to a portion of DNAa portion of DNA

TranslationTranslation occurs when the occurs when the sequence of bases of mRNA sequence of bases of mRNA DIRECTSDIRECTS the the sequence of sequence of amino acidsamino acids in a polypeptide in a polypeptide

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Genetic CodeGenetic Code DNA contains a DNA contains a triplet codetriplet code Every three bases on DNA Every three bases on DNA

stands for stands for ONE amino acidONE amino acid Each three-letter unit on Each three-letter unit on mRNAmRNA

is called a is called a codoncodon Most amino acids have more Most amino acids have more

than one codon!than one codon! There are There are 20 amino acids20 amino acids with with

a possible 64 different tripletsa possible 64 different triplets The code is nearly The code is nearly universal universal

among living organismsamong living organisms

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TranscriptioTranscriptionn

TranslatiTranslationon

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Overview of Overview of TranscriptionTranscription

During During transcriptiontranscription in the in the nucleus, a segment of DNA nucleus, a segment of DNA unwinds and unzips, and the unwinds and unzips, and the DNA DNA serves as aserves as a template for template for mRNA formationmRNA formation

RNA polymeraseRNA polymerase joins the RNA joins the RNA nucleotides so that the nucleotides so that the codons codons in mRNA are complementaryin mRNA are complementary to to the triplet code in DNAthe triplet code in DNA

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Steps in Steps in TranscriptionTranscription

The transfer of information in the The transfer of information in the nucleusnucleus from a from a DNADNA molecule to an molecule to an RNARNA molecule molecule

Only 1 Only 1 DNADNA strand serves as the strand serves as the templatetemplate

Starts at promoter Starts at promoter DNADNA (TATA box) (TATA box) Ends at terminator Ends at terminator DNADNA (stop) (stop) When complete, When complete, pre-RNApre-RNA molecule is molecule is

releasedreleased

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What is the What is the enzymeenzyme

responsible for responsible for the production of the production of

the mRNA the mRNA molecule?molecule?

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RNA PolymeraseRNA Polymerase

EnzymeEnzyme found in the found in the nucleusnucleus

Separates Separates the two DNA the two DNA strands by strands by breaking the breaking the hydrogen bondshydrogen bonds between between the basesthe bases

Then moves along one of Then moves along one of the DNA strands and the DNA strands and links links RNA nucleotidesRNA nucleotides together together

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DNADNA

pre-mRNApre-mRNA

RNA PolymeraseRNA Polymerase

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Question:Question:

What would be the What would be the complementary RNA complementary RNA strand for the following strand for the following DNA sequence?DNA sequence?

DNA 5’-DNA 5’-GCGTATGGCGTATG-3’-3’

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Processing Pre-mRNAProcessing Pre-mRNA

• Also occurs in the Also occurs in the nucleusnucleus• Pre-mRNA Pre-mRNA made up of segments made up of segments

called called introns & exonsintrons & exons• Exons code for proteins, while Exons code for proteins, while

introns do NOT!introns do NOT!• Introns spliced out by Introns spliced out by splicesome-splicesome-

enzymeenzyme and exons re-join and exons re-join

• End product is a End product is a mature RNAmature RNA molecule that leaves the nucleus molecule that leaves the nucleus to the cytoplasmto the cytoplasm

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RNA ProcessingRNA Processing

pre-RNA molecule

intron

intronexon exon exon

exon exon exon

Mature RNA moleculeMature RNA molecule

exon exon exon

intron intron

splicesome splicesome

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Messenger RNA Messenger RNA (mRNA)(mRNA)

•Carries the information for a Carries the information for a specific proteinspecific protein

•Made up of Made up of 500 to 1000500 to 1000 nucleotides longnucleotides long

•Sequence of 3 bases called Sequence of 3 bases called codoncodon•AUGAUG – methionine or – methionine or start start codoncodon•UAA, UAG, or UGAUAA, UAG, or UGA – – stopstop codons codons

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Messenger RNA Messenger RNA (mRNA)(mRNA)

methionine glycine serine isoleucine glycine alanine stopcodon

proteinprotein

A U G G G C U C C A U C G G C G C A U A AmRNAmRNA

startcodon

Primary structure of a proteinPrimary structure of a protein

aa1 aa2 aa3 aa4 aa5 aa6

peptide bonds

codon 2 codon 3 codon 4 codon 5 codon 6 codon 7codon 1

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Transfer RNA (tRNA)Transfer RNA (tRNA)• Made up of Made up of 75 to 80 nucleotides75 to 80 nucleotides

longlong• Picks up the appropriate Picks up the appropriate amino amino

acidacid floating in the cytoplasm floating in the cytoplasm • Transports Transports amino acidsamino acids to the to the

mRNAmRNA• Have Have anticodonsanticodons that are that are

complementary to complementary to mRNA codonsmRNA codons• Recognizes the appropriate Recognizes the appropriate codonscodons

on the on the mRNAmRNA and bonds to them and bonds to them with H-bondswith H-bonds

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Transfer RNA (tRNA)Transfer RNA (tRNA)

amino acidamino acidattachment siteattachment site

U A C

anticodonanticodon

methionine amino acidamino acid

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Ribosomal RNA (rRNA)Ribosomal RNA (rRNA)•Made up of Made up of

rRNA is rRNA is 100 to 100 to 3000 3000 nucleotidesnucleotides long long

•Made inside the Made inside the nucleusnucleus of a cell of a cell

•Associates with Associates with proteins to form proteins to form ribosomesribosomes

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RibosomesRibosomes

•Made of a large and small subunit

•Composed of rRNA (40%) and proteins (60%)

•Have two sites for tRNA attachment --- P and A

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RibosomesRibosomes

PSite

ASite

Largesubunit

Small subunit

mRNAmRNA

A U G C U A C U U C G

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•Synthesis of proteinsSynthesis of proteins in the in the cytoplasmcytoplasm

• Involves the following:Involves the following:

1.1. mRNA (codons)mRNA (codons)

2.2. tRNA (anticodons)tRNA (anticodons)

3.3. ribosomesribosomes

4.4. amino acidsamino acids

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TranslationTranslation• Three steps:Three steps:

1.1. initiationinitiation: start codon (AUG): start codon (AUG)

2.2. elongationelongation: amino acids : amino acids linkedlinked

3.3. terminationtermination: stop codon : stop codon (UAG, UAA, or UGA).(UAG, UAA, or UGA).

Let’s Make a Protein !Let’s Make a Protein !

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mRNA Codons Join the mRNA Codons Join the RibosomeRibosome

PSite

ASite

Largesubunit

Small subunit

mRNAmRNA


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InitiationInitiation

mRNAmRNA


2-tRNA

G

aa2

A U

A

1-tRNA

U A C

aa1

anticodon

hydrogenbonds codon

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mRNAmRNA


1-tRNA 2-tRNA

U A C G

aa1 aa2

A UA

anticodon

hydrogenbonds codon

peptide bond

3-tRNA

G A A

aa3

ElongationElongation

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mRNAmRNA


1-tRNA

2-tRNA

U A C

G

aa1

aa2

A UA

peptide bond

3-tRNA

G A A

aa3

Ribosomes move over one codon

(leaves)

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mRNAmRNA


2-tRNA

G

aa1

aa2

A UA

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

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mRNAmRNA


2-tRNA

G

aa1aa2

A U

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

(leaves)


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mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5

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mRNAmRNA

G C U A C U U C G

aa1aa2

A

peptide bonds

3-tRNA

G A A

aa3

4-tRNA

G C U

aa4

A C U

U G A

5-tRNA

aa5


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mRNAmRNA

A C A U G U

aa1

aa2

U

primaryprimarystructurestructureof a proteinof a protein

aa3

200-tRNA

aa4

U A G

aa5

C U

aa200

aa199

terminatorterminator or stopor stop codoncodon

TerminationTermination

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End Product –The End Product –The Protein!Protein!

• The end products of protein The end products of protein synthesis is a synthesis is a primary structureprimary structure of a proteinof a protein

• A A sequence of amino acidsequence of amino acid bonded together by peptide bonded together by peptide bondsbonds

aa1

aa2 aa3 aa4aa5

aa200

aa199

PROTEIN SYNTHESIS

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