PROTEIN SYNTHESIS:

TRANSLATION

What is the genetic code?

• the sequence of nt in mRNA• the order of mRNA n.t. determines the order

of amino acids that will bond to build a polypeptide

• the code is a triplet code: in mRNA, every 3 nucleotides codes for an aa– called a CODON

Recall transcription:

codon

• 64 codons and only 20 corresponding amino acids

• The near-universality of the genetic code supports the idea that all organisms are evolutionarily related

• Note the chart on p. 207

• several codons code for the same amino acid

• codons often differ only by the 3rd nucleotide

All about codons:

• START Codon: AUG • also codes for methionine • engages a ribosome to start

translating• always found at the beginning of

the mRNA transcript but can be found throughout the transcript as well

• STOP Codons: UAA, UAG, UGA• cause a ribosome to stop

translating• do not code for any amino acids

Translation Basics

• Translation = the process of assembling polypeptides from info encoded in mRNA

What’s needed?

1. mRNA2. tRNA3. ribosomes

1. mRNA

• mRNA – it has the instructions for protein synthesis from DNA!

2. tRNA

• tRNA - binds specifically to amino acids in the cytoplasm and transports them to the ribosomes

• the bottom loop of tRNA has a 3 nt sequence that bonds with mRNA nt

• this is the ANTICODON

• ANTICODON is complementary to its paired mRNA CODON

• Same base pairing rules apply

• pairing of anticodon with codon ensures that amino acids are added in the correct order

• attachments of amino acid, ATP, and tRNA are all controlled by enzymes

3. Ribosomes

• Composed of 1. rRNA2. Proteins

• Found in 2 places:1. free in cytoplasm (to produce proteins used by the

cell)2. attached to ER (to produce exported proteins)

1. mRNA site2. P site3. A site

• mRNA site- holds mRNA transcript so that its codons are available to bond to tRNA molecules

Ribosomes have 3 binding sites that are key to translation

• other 2 sites = P site and A site – each holds a tRNA whose anticodons pair with

mRNA codons

• P site = holds tRNA with growing polypeptide chain

• A site = holds tRNA that is transferring the next amino acid to be added to the chain

Translation

• Also called protein synthesis– Synthesis = to put together

• 3 steps1. Initiation2. Elongation3. Termination

Protein Assembly1. Initiation

• Begins when a ribosome attaches to the start codon (AUG) on a mRNA transcript.

• This pairs with UAC on t-RNA.

• Remember… AUG codes for methionine– The first amino acid in every polypeptide is initially

methionine (it can later be removed).

Translates in one direction 5’ to 3’!!!

2. Elongation

• As ribosome moves along mRNA transcript, each mRNA codon is paired with its tRNA anticodon.

• This causes a specific amino acid to attach to previous amino acid with a covalent bond (called a peptide bond!)

• This continues until ribosome reaches a stop codon, ending translation.

Dehydration Synthesis of Proteins

3. Termination

• mRNA is released • the polypeptide is complete.

Result of Protein Synthesis

• Polypeptide chain represents the protein’s 1° structure.

• As it folds and (or binds) to other polypeptides, it assumes its functional (4°) structure of a completed protein.

• What organelle folds and modifies the polypeptide chain into a function protein?

Other info…

• Several ribosomes may simultaneously translate the same mRNA transcript = polysomes

• usually about 80 nucleotides apart– 1st one starts, gets about 80 nt away from start codon

before a 2nd ribosome starts at start codon and follows behind 1st

• This speeds up translation of one transcript and results in larger quantities of one particular protein.

• https://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html

• http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/

• http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1302