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Protein Translation
From Gene to ProteinHonors Biology
Ms. Kim
Central Dogma
DNA RNA Protein• Protein synthesis consists of 2 mains parts:– Transcription – DNA is copied in the nucleus, the
result is the formation of mRNA– Translation – mRNA travels to the cytoplasm and
attaches to rRNA and with the help of tRNA a protein is made
Translation
• mRNA polypeptide (protein)• Converting mRNA code (from DNA) to protein!– Protein = polypeptide
How is mRNA used to make protein? The genetic code is read 3 letters (nucleotides) at a
time 3 nucleotides= codon
One codon codes for an amino acid
tRNA
• A cell translates mRNA message into protein with help from transfer RNA (tRNA)–Type of RNA –~80 nucleotides–“t” shape–Carries amino acids–Matches codons to anticodons
Transfer RNATransfer RNA (tRNA)
Reads the mRNA codeCarries the amino acid that will be added to the growing protein chain
Notice the 3 bases at the bottom of the tRNA make up the anticodon
The anticodon base pairs with the mRNA codon to make sure that each AA is delivered to the correct place on mRNA
At the top of tRNA is an amino acid
tRNA
• Molecules of tRNA are not all identical–Each carries a specific amino acid–Each has an specific anticodon on the
other end
Ribosomes• Help bind tRNA anticodons with mRNA codons during
translation
• Found on ROUGH Endoplasmic Reticulum RER (“bound”) or in cytoplasm (“free”)
Anticodon (tRNA) = codon (mRNA)
tRNA(with
anticodons)
mRNA(with
codons)
Ribosomes• There are 2 ribosomal subunits
• Constructed of proteins and ribosomal RNA or rRNA
Ribosomal RNA Ribosomal RNA (rRNA)
Ribosome attaches to mRNAContains the enzymes necessary for protein synthesis
E P A
Ribosome: 3 tRNA binding spots:
E – exitP – current amino acidA – on deck amino acid
Has a large & small subunit
Binding Sites in a Ribosome
• The ribosome has three binding sites for tRNA– The P site– The A site– The E site
E P A
P site (Peptide Bond-tRNAbinding site)
E site (Exit site)
mRNAbinding site
A site (Amino acid-tRNA binding site)
Largesubunit
Smallsubunit
\
Amino acid Growing polypeptide
Next amino acidto be added topolypeptide chain
tRNA
mRNA
Codons
3
5
What is Translation?
• Process of building a protein chain by reading the mRNA code
• Occurs in ribosomes• Uses codons
Building a Polypeptide
• Translation can be divided into 3 phases–Initiation–Elongation–Termination
Initiation of Translation
• initiation stage:1. mRNA binds to small subunit of
ribosome2. tRNA bearing 1st amino acid (“start”)
called MET (codon AUG) comes3. tRNA binds to start codon (AUG)
anticodon (UAC)4. Large ribosomal subunit binds working
ribosome5. Initiator tRNA fits into the P site and
holds the growing protein6. The A site is empty and ready for the
next A.A1. 2 subunits of a ribosome come
together
Every codon has a complementary anticodon which pairs with it AUG of mRNA pairs with UAC
of tRNA
Translation: Initiation• mRNA is organized into a reading frame • The reading frame is made of codons that specify
specific amino acids– Every 3 bases (triplet) make up a codon– Codons are arranged following the initiation codon AUG
Large ribosomal subunit
Initiator tRNA
mRNA
mRNA binding site Smallribosomalsubunit
Translation initiation complex
P site
GDPGTP
Start codon
MetMet
U A C
A U G
E A
3
5
5
3
35 35
mRNA codons are what create the amino acids (aka – use the chart) NOT tRNA
Elongation
• Amino acids are added one by one to the preceding amino acid
• Peptide bonds are formed
Amino acid
ElongationAfter initiation, A.A. are added to the first A.A…•3 step process:• 1) Incoming tRNA anticodon pairs with mRNA codon• 2) A new polypeptide bond is formed• 3) tRNA shift (P leaves, A moves to the P spot)
Translation: Elongation
• A chain of amino acids is called a polypeptide chain
• This chain continues to form as long as tRNA is added to the mRNA strand
Termination of Translation
• When the ribosome reaches a stop codon on the mRNA–There are 3 stop codons•UAA, UAG, and UGA
• Release factor (protein) binds to stop codon in A site (NOT tRNA) polypeptide (protein) released
At this point the amino acid or polypeptide will fold into a specific structure forming a working protein
Termination of Translation
Release factor
Freepolypeptide
Stop codon(UAG, UAA, or UGA)
5
3 35
35
Figure 17.13
TRANSCRIPTION
TRANSLATION
DNA
mRNA
Ribosome
Polypeptide
Polypeptide
Aminoacids
tRNA with amino acid attachedRibosome
tRNA
Anticodon
mRNA
Trp
Phe Gly
A G C
A A A
C C G
U G G U U U G G C
Codons5 3
“EMPTY”tRNA
Translation (Protein Synthesis) Animation
• http://www.wisc-online.com/objects/index_tj.asp?objID=AP1302
• http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter15/animations.html#
• http://www.ucopenaccess.org/courses/APBiologyI/course%20files/multimedia/lesson13/lessonp.html
Summary of Protein Synthesis
• Replication → DNA to DNA, occurs in nucleus• Transcription → DNA to RNA, occurs in
nucleus• Translation → RNA to Protein, occurs in
ribosome DNA RNA PROTEIN!
Example• DNA sequence (template): 3’ TACGCTAGTACGATG 5’
• mRNA sequence: 5’ AUGCGAUCAUGCUAC 3’
• Codons: AUG CGA UCA UGC UAC
• Amino Acids: met (start)-arg-ser-cys-tyr
• Codons determine which of the 20 possible amino acids is needed