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