Translation is the RNA-directed

synthesis of a polypeptide

In the process of translation, a cell “reads” a genetic message and builds a polypeptide accordingly.

Polypeptide

RibosomeTrp

Phe

tRNA withamino acidattached

Aminoacids

tRNA

Anticodon

Codons

U U U UG G G G C

AC C

C

A A A

CC

G

5 3mRNA

Molecular Components tRNA The function of tRNA is to

transfer amino acids from the cytoplasm pool of amino acids to a growing polypeptide in a ribosome.

A tRNA molecule translates a given mRNA codon into a certain amino acid. This is possible because a tRNA

bears a specific amino acid sequence at one end, while at the other end is a nucleotide triplet that can base-pair with the complementary codon on mRNA (anti-codon).

tRNA Structure A tRNA consists of a single

RNA strand (about 80 nucleotides).

This single strand can fold back on itself and create a 3-dimensional structure. Flattened on a plane:

cloverleaf shape. 3-D shape: roughly L-shaped.

A loop extending from one end contains the anti-codon.

The 3’ end serves as the attachment site for an amino acid.

Amino acidattachmentsite

Hydrogenbonds

Anticodon

tRNA Production tRNA molecules are transcribed from DNA.

Occurs in nucleus of eukaryotes. After transcription, tRNA leaves nucleus.

Occurs in cytoplasm of prokaryotes. The correct matching up of tRNA and amino

acid is carried out by enzymes called aminoacyl-tRNA synthetases. The active site of each type of aminoacyl-tRNA

synthetase fits only a specific combination of amino acid and tRNA.

About 20 different synthesases. These enzymes require energy from the hydrolysis

of ATP.

Aminoacyl-tRNAsynthetase (enzyme)

Amino acid

P P P Adenosine

ATP

P

P

P

PP

Adenosine

tRNA

AdenosineP

tRNA

AMP

Computer model

Aminoacid

Aminoacyl-tRNAsynthetase

Aminoacyl tRNA(“charged tRNA”)

Numbers of tRNA There are only about 45 different tRNA

molecules. Some tRNAs must be able to bind to more than

one codon! tRNAs are versatile!

Such versatility is possible because the rules for base pairing between the third nucleotide base of a codon and the corresponding base of the tRNA anticodon are relaxed. Wobble! Wobble explains why the synonymous codons for

a given amino acid more often differ in their third nucleotide base.

Ribosome Basics Ribosomes consist of a large subunit and a small

subunit, each made up of proteins and one or more ribosomal RNAs (rRNA). The ribosome is only functional when these subunits

join together and attach to a mRNA molecule. Ribosomes are constructed in the nucleolus.

Ribosomal protein translated in cytoplasm. rRNA transcribed in nucleus. Ribosomal protein imported from cytoplasm, and then

assembly occurs. One-third of the ribosomes mass is rRNA.

Three rRNA molecules in bacterial ribosomes. Four rRNA molecules in eukaryotic ribosomes.

Ribosome Anatomy P site: holds the tRNA carrying the growing polypeptide

chain. A site: holds the tRNA carrying the next amino acid t be

added to the chain. E site: where discharged tRNAs leave the nucleus. Exit tunnel: area that growing polypeptide passes through.

Ribosome Association and Initiation of Translation Initiation of translation brings together mRNA, a tRNA bearing

the first amino acid of the polypeptide (methionine), and the two ribosomal subunits.

In eukaryotes… Small subunit, with the initiator tRNA already bound, binds to the 5’

cap of the mRNA. The small subunit scans downstream along the mRNA unit it reaches

the start codon (AUG). Finding the start codon established the correct codon reading frame.

The union of mRNA, initiator tRNA, and small ribosomal subunit is followed by the attachment of a large ribosomal subunit, completing the translation initiation complex. Initiation factors bring these components together. Cell expends energy obtained by hydrolysis of GTP.

The initiator tRNA then sites in the P site of the ribosome. Polypeptide then is synthesized in the N-terminus to C-terminus

direction.

InitiatortRNA

mRNA

5

53Start codon

Smallribosomalsubunit

mRNA binding site

3

Translation initiation complex

5 33U

UA

A GC

P

P site

i

GTP GDP

Met Met

Largeribosomalsubunit

E A

5

Ribosome Association and Initiation

Elongation of the Polypeptide Chain In the elongation stage, amino acids are

added one by one to the previous amino acid at the C-terminus of the growing chain.

Elongation factors aid in the addition of amino acids to the growing polypeptide chain.

Energy is expended in first and third step. Codon recognition requires hydrolysis of one

molecule of GTP, which increases accuracy and efficiency of this step.

One more GTP is hydrolyzed in the translocation step.

The ribosome moves in the 5’3’ direction along the mRNA.

Amino end ofpolypeptide

mRNA

5

E

Asite

3

E

GTP

GDP P i

P A

E

P A

GTP

GDP P i

P A

E

Ribosome ready fornext aminoacyl tRNA

Psite

Elongation

Termination of Translation Elongation continues until a stop codon in the mRNA

reaches the A site of the ribosome. A release factor, a protein shaped like an aminoacyl

tRNA, binds directly to the stop codon in the A site. The release factor causes the addition of a water

molecule. This reaction breaks the bond between the complete

polypeptide and the tRNA in the P site. The polypeptide is then released through the exit tunnel.

The breakdown of the translation assembly then occurs, which requires the hydrolysis of two more GTP molecules.

Releasefactor

Stop codon(UAG, UAA, or UGA)

3

5

3

5

Freepolypeptide

2 GTP

5

3

2 GDP 2 iP

Termination

Completing and Targeting the Functional ProteinProtein Folding and

Post-translational Modifications

Targeting Polypeptides to

Specific Locations

During synthesis, polypeptide begins to fold spontaneously. Due to primary structure. Chaperons assist with folding.

Post-translational Modifications: Amino acids may be

chemically modified. Enzymes may remove one or

two amino acids from leading end.

Polypeptide chain may be cleaved.

Polypeptide synthesis always begins at a free ribosome. Polypeptides bound for

excretion or the endomembrane system signal for the ribosome to attach to ER.

Signal peptide is recognized by signal-recognition particle. This particle escorts the

ribosome to the ER. Proteins are either inserted

into the ER or embedded in the ER membrane.

Ribosome

mRNA

Signalpeptide

SRP

1

SRPreceptorprotein

Translocationcomplex

ERLUMEN

2

3

45

6

Signalpeptideremoved

CYTOSOL

Protein

ERmembrane

Making Multiple Polypeptides

In both bacteria and eukaryotes multiple ribosomes translate an mRNA at the same time.

Once a ribosome is fare enough past the start codon, a second ribosome can attach to the mRNA.

Polyribosome!

Concept 17.5: Mutations of one or a few nucleotides can affect protein structure and function

Mutations are changes in the genetic material of a cell or virus.

Point mutations are chemical changes in just one base pair of a gene.

The change of a single nucleotide in a DNA template strand can lead to the production of an abnormal protein.

Types of Small-Scale Mutations Point mutations within a gene can be divided

into two general categories: Nucleotide-pair substitutions One or more nucleotide-pair insertions or

deletions

Substitutions A nucleotide-pair substitution replaces

one nucleotide and its partner with another pair of nucleotides.

Silent mutations have no effect on the amino acid produced by a codon because of redundancy in the genetic code.

Missense mutations still code for an amino acid, but not the correct amino acid.

Nonsense mutations change an amino acid codon into a stop codon, nearly always leading to a nonfunctional protein.

Insertions and Deletions Insertions and deletions are additions or

losses of nucleotide pairs in a gene. These mutations have a disastrous effect on

the resulting protein more often than substitutions do. May cause a premature stop. May cause extensive missense.

Insertion or deletion of nucleotides may alter the reading frame, producing a frameshift mutation.

Wild type

DNA template strand

mRNA5

5

3

Protein

Amino end

A instead of G

(a) Nucleotide-pair substitution

3

3

5

Met Lys Phe Gly StopCarboxyl end

T T T T TTTTTTA A A A AAAAACC

C

C

A

A A A A A

G G G G

GC C

G GGU U U U UG

(b) Nucleotide-pair insertion or deletionExtra A

35

53

Extra U5 3

T T T TT T T T

AA A A

AAT G G G G

GAAAAC

CCCC AT35

5 3

5T T T T TAAAACCA AC C

TTTTTA A A A ATG G G G

U instead of C

Stop

UA A A A AG GGU U U U UGMetLys Phe Gly

Silent (no effect on amino acid sequence)

T instead of C

T T T T TAAAACCA GT C

T A T T TAAAACCA GC C

A instead of G

CA A A A AG AGU U U U UG UA A A AG GGU U U G AC

AA U U A AU UGU G G C UA

GA U A U AA UGU G U U CG

Met Lys Phe Ser

Stop

Stop Met Lys

missing

missing

Frameshift causing immediate nonsense(1 nucleotide-pair insertion)

Frameshift causing extensive missense (1 nucleotide-pair deletion)

missing

T T T T TTCAACCA AC G

AGTTTA A A A ATG G G C

Leu Ala

Missense

A instead of T

TTTTTA A A A ACG G A G

A

CA U A A AG GGU U U U UG

TTTTTA T A A ACG G G G

Met

Nonsense

Stop

U instead of A

35

35

53

35

53

35 3Met Phe Gly

No frameshift, but one amino acid missing(3 nucleotide-pair deletion)

missing

35

53

5 3U

T CA AA CA TTAC G

TA G T T T G G A ATC

T T C

A A G

Met

3

T

A

Stop

35

53

5 3

Figure 17.24