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)

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 anticodonThe 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 subunit2. 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.A

1. 2 subunits of a ribosome come together

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

Elongation

After 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)

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

Termination of TranslationRelease 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

Polypeptide 3D Protein• Primary Structure– Amino acids is a row (LINEAR)

• Secondary Structure– Amino Acids BEND and FOLD towards

each other• Tertiary Structure– Amino acids BIND to each other – 3D shape made

• Quartnary structure (only some)– 2 or more polypeptides BIND to each

other

1 2

3 4

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!