Ch 17 part 1 Protein Synthesis (Translation)

How does an mRNA molecule produce a

protein?

Things you will need to know in order to understand translation:

• mRNA codon chart

• Redundancy of the code

• Structure of tRNA

• Ribosome structure

mRNA Codons:

  codon = group of 3 mRNA nucleotides 

**1 codon codes for 1 amino acid

  During translation, proteins are synthesized according to the genetic message of sequential codons along the mRNA

Figure 17.5 shows the “dictionary” for the mRNA codons and their corresponding amino acids

Redundant but not Ambiguous

• Codons GAA and GAG both specify glutamic acid (redundancy)

• Neither of them specifies any other amino acid (no ambiguity)

*FYI: Redundancy can also be called “degeneracy”

  Transfer RNA (tRNA) is the interpreter between the 2 forms of information: base sequence in mRNA and amino acid sequence in polypeptides

tRNA Structure

Amino acid location (CCA)

3 exposed bases

-an enzyme links a specific amino acid from the cytosol/cytoplasm to each tRNA molecule on the 3’ end (CCA) using ATP for energy

LE 17-14b

Hydrogenbonds

Amino acidattachment site5

3

3 5

Anticodon

Symbol used in this book

Anticodon

Three-dimensional structure

LE 17-15Amino acid Aminoacyl-tRNA

synthetase (enzyme)

Pyrophosphate

Phosphates

tRNA

AMP

Aminoacyl tRNA(an “activatedamino acid”)

  Molecules of tRNA are specific for only 1 amino acid

-one end of tRNA attaches to a specific amino acid

-the other end Hydrogen bonds to mRNA codon by base pairing (anticodon = a sequence of 3 bases on tRNA)

Amino acid

Ribosome Structure• large ribosomal subunit (50s)

• small ribosomal subunit (30s)– each subunit = ribosome/protein complex

• an mRNA binding site

• 3 tRNA binding sites

E P A

LE 17-16b

P site (Peptidyl-tRNAbinding site)

E site (Exit site)

mRNAbinding site

A site (Aminoacyl-tRNA binding site)

Largesubunit

Smallsubunit

Schematic model showing binding sites

E P A

LE 17-16a

tRNAmolecules

Exit tunnelGrowingpolypeptide

Largesubunit

mRNA 3

Computer model of functioning ribosome

Smallsubunit

5

E P A

Processes of Translation

• Translation includes 4 processes: initiation, elongation, translocation, and termination

Initiation of Translation

• mRNA binds to small ribosomal subunit

• Initiator tRNA brings1st amino acid, Methionine (Met)

• Large ribosomal subunit binds– Met is in the “P” site– “A” site is available for the

next tRNA

LE 17-17

Met

GTPInitiator tRNA

mRNA

53

mRNA binding site

Smallribosomalsubunit

Start codon

P site

5 3

Translation initiation complex

E A

Largeribosomalsubunit

GDP

Met

Elongation

• The next tRNA anticodon complementary base pairs with the mRNA codon in the “A” site aligning the appropriate amino acid next to “Met.”

• Ribosome forms a peptide bond between “Met” and the 2nd amino acid and it passes the elongating polypetide chain to the tRNA in the “A” site

Translocation

• Ribosome moves (translocates) the tRNA in the “A” site to the “P” site

• The empty tRNA moves to the “E” site where it is released

• mRNA moves along/through ribosome to expose the next mRNA codon to the “A” site

• Elongation continues

LE 17-18

Ribosome ready fornext aminoacyl tRNA

mRNA

5

Amino endof polypeptide

E

Psite

Asite

3

2

2 GDP

E

P A

GTP

GTP

GDP

E

P A

E

P A

Termination• When a “STOP” codon is

reached, there isn’t a complementary tRNA

• A “release factor” binds to the “A” site – Causes the addition of a

water molecule to the polypeptide which hydrolyzes the completed polypeptide from the tRNA

• Ribsomal subunits dissociate

LE 17-19

Releasefactor

Stop codon(UAG, UAA, or UGA)

5

3

5

3

5

Freepolypeptide

3

When a ribosome reaches a stopcodon on mRNA, the A site of theribosome accepts a protein calleda release factor instead of tRNA.

The release factor hydrolyzes thebond between the tRNA in theP site and the last amino acid of thepolypeptide chain. The polypeptideis thus freed from the ribosome.

The two ribosomal subunitsand the other componentsof the assembly dissociate.

Polyribsomes• Typically, a single

mRNA is used to make many copies of a protein simultaneously

• Also called “polysomes”

LE 17-20a

Incomingribosomalsubunits

Growingpolypeptides

Completedpolypeptide

Start ofmRNA(5 end)

End ofmRNA(3 end)

Polyribosome

An mRNA molecule is generally translated simultaneouslyby several ribosomes in clusters called polyribosomes.

LE 17-21

Ribosomes

mRNASignalpeptide

Signal-recognitionparticle(SRP)

SRPreceptorprotein

CYTOSOL

ER LUMEN Translocationcomplex

Signalpeptideremoved

ERmembrane

Protein

SRP: Signal-Recognition Particle- if a protein is to be secreted from the cell (or used in a lysosome), an SRP will attach to the growing polypeptide chain and “drag” it and the ribosome to the ER (aka rough ER)

http://www.brookscole.com/chemistry_d/templates/student_resources/shared_resources/animations/protein_synthesis/protein_synthesis.html

What is the DNA strand called that gets transcribed by RNA?

A. Antisense

B. Sense

C. Leading

D. lagging

Which RNA are involved in Translation?A. tRNA

B. rRNA

C. mRNA

D. All of the above

E. A and B

Where does tRNA pick up new amino acids?

A. Nucleus

B. Rough ER

C. Golgi apparatus

D. cytosol

When the 1st tRNA is finished, what is the name of the site where it will

leave? A. A site

B. P site

C. E site

D. Methionine site

What happens at the promoter region of DNA in Transcription?

A. RNA polymerase reaches a specific sequence of DNA that causes transcription to end

B. RNA polymerase promotes the synthesis of an RNA primer

C. DNA polymerase reaches a specific sequence of DNA that causes transcription to end

D. RNA polymerase binds to a specific DNA sequence to begin transcription

What is the name of the covalent bond created between amino acids?A. Glycosidic linkage

B. Ester

C. Peptide

D. phosphodiester

What are the differences between introns & exons?A Introns are translated but

exons are notIntrons are excised from mRNA but exons remain part of it

B Exons are translated but introns are not

Exons are excised from mRNA but introns remain part of it

C Introns are translated but exons are not

Exons are excised from mRNA but introns remain part of it

D Exons are translated but introns are not

Introns are excised from mRNA but exons remain part of it