Translation is the RNA-Directed Synthesis of a PolypeptideWilliam ChenMaggie McCormickEsther Park

Molecular Components of Translation

•tRNA (transfer RNA) the function is to literally transfer amino acids.

•The one end of the tRNA is a anticodon (a nucleotide triplet).TRANSCRIPTION

TRANSLATION

DNA

mRNARibosome

Polypeptide

PolypeptideAminoacidstRNA withamino acidattachedRibosome

tRNAAnticodon

mRNA

TrpPhe Gly

A G C

A A A

C C G

U G G U U U G G C

Codons5 3

tRNA is Form = Function

•Transfer RNA (tRNA) molecules are transcribed from DNA templates.

•Two dimensional structure.

Figure 17.14a

3C

CACGCUUA

A

GACACCU*GC

* *GUGU*CU

*GAGGU**A

*A A GUC

AGACC*

CGAGAGGG**GACUC*A

U UUAGGCG5

Amino acidattachment site

Hydrogenbonds

Anticodon

A

•Three dimensional structure

(b) Three-dimensional structureSymbol used in this book

Amino acidattachment site

Hydrogen bonds

Anticodon Anticodon

AAG

53

3 5

(c)

Accurate Translation

•There must be a correct match between a tRNA and an amino acid.

•Then there is an involvement of a correct match between the tRNA anticodon and a mRNA codon.

•The base pairing for tRNAs are not as strict as DNA or mRNA codons. This is called “wobble” where the third base pairing is more flexible than any other bases.

Ribosomes

•Facilitate the specific coupling of tRNA anticodons with mRNA codons during protein synthesis.

•There are tRNA, mRNA, and rRNA.

Translation : Building a Polypeptide

•Translation has 3 stages:▫Initiation▫Elongation

Codon Recognition Peptide bond formation Translocation

▫Termination

Initiation (requires GTP)

•During Initiation, Initiation factors bring mRNA, tRNA, and two ribosomes together.

•First, a ribosome binds to mRNA to a initiator tRNA.

•Then, the initiator tRNA hydrogen bonds with the start codon(AUG).

•After this, a large ribosomal subunit is attached and completes initiation.

Initiation(Pg.323)

Elongation

•Amino acids are added one by one. For every addition, a protein called elognation factors are involved.

•Codon Recognition(requires GTP) •Peptide bond formation•Translocation (requires GTP)

Elongation (Page 324)

A siteE site

P site

Termination

•Elongation stops when the ribosome reaches a stop codon in the mRNA.

•UAG, UAA, and UGA are the triplets for stop codons.

•A protein called a release factor goes on the A site instead of a tRNA.

•The factor hydrolyzes the polypeptide from the tRNA and makes the polypeptide go to the exit tunnel.

•The tunnel then breaks down.

Termination(Pg.325)

Polyribosomes

•Once a ribosome moves past the start codon, a second ribosome may attach more than one ribosome. These are found in both eukaryotes and prokaryotes.

Completing and targeting the functional protien•Translation isnt sufficent enough to make

a functional protien

•What it uses is… ▫Protien folding▫Post-translational modifications

Protein Folding

•Polypeptide chains coil and fold to make a tertiary structure

Post-translational modifications

•Certain amino acids may be chemically modified by the attatchment of sugars, lipids phosphate groups and others

Targeting polypeptides to specific locations•Protien synthesis happens in both

▫Free ribosomes (synthesize proteins that dissolve in the cytosol and function there)

▫Bound ribosomes (synthesize proteins of the endomembrane system as well as proteins secreted from the cell)

Vocabulary

•Signal peptide – targets the protein to the ER (see figure 17.21)

•Signal-recognition particle (SRP) – a protein-RNA complex that recognizes a signal peptide as it emerges from the ribosome.

tRNA YOUTUBE IT!!!!

•http://www.youtube.com/watch?v=gE46bMA0Z14

•http://www.youtube.com/watch?v=Deie92DylRM