Protein Synthesis

Translation

Is the process of a ribosome using a mRNA template to guide the synthesis of a polypeptide.

Translating

From a language written in nucleotides to a language written in amino acids

Key players in translation

• Ribosomes

• mRNA

• tRNA

Many additional proteins

Ribosomes

Carry out the process of polypeptide synthesis

mRNA

Molecules encode the amino acid sequence and are ‘read’ by the ribosomes.

tRNA

Are RNAs used to deliver amino acids to the ribosome.

Has an anticodon that is complementary to a codon

Ribosomes (Structures)

• Subcellular structure made of protein and rRNA

• Built from two dissociable subunits (large/small)

Some schematics use S values for bacterial ribosomes, which are smaller

• Bacterial 50S — Eukaryotic 60S; Bacterial 30S — Eukaryotic 40S

Ribosome (3 sites)

1) A site: binds the newly arriving tRNA with its amino acid

2) P site: binds the tRNA that carries the growing polypeptide chain

3) E site: Where the tRNA leaves the ribosome after depositing an amino acid.

Aminoacyl tRNA

Attachs amino acid to the tRNA.

Aminoacly tRNA

tRNA with an amino acid on it, with the help of ATP. Brings a new amino acid into the position in the ribosome.

• tRNA anticodon base-pairs with codon

Wobble Base Pairs

Can form between the third base of a condon and the corresponding base in its anticondon.

• Wobble base pair = RNA base pairs that does not follow Watson/crick base pair

• The thrid nucleotide of a codon is called the wobble position

Example of a wobble

Inosine can form a wobble base pair with C, U, or A

Translation (Steps)

1) During initiation, the components of the translational apparatus come together with an mRNA, and a tRNA carrying the first amino acid (AA) binds to the start codon (AUG)

2) During elongation, amino acids are brought to the mRNA by tRNAs and are added, one by one, to a growing polypeptide chain.

3) During termination, a stop codon in the mRNA is recognized by a protein release factor, and the translation apparatus comes apart, releasing a completed polypeptide.

Mutations (types)

• Missense Mutations: Change 1 amino acid

• Nonsense Mutations: Create stop codons

• Frameshift Mutations: Change all subsequent codons.

mRNAs contain translated/untranslated region

• Untranslated region (UTR): (5’-UTR or 3’-UTR)

• Binding sites for proteins that regulate mRNA translation and stability.

Coding sequence

The mRNA encodes that amino acid sequence of the polypeptide

• Begins with a start codon and ends with a stop codon

• The length of the codon region is always a multiple of 3.

5’ caps and 3’ poly(A) tails do what

Protect RNA from degradation, aid in nuclear export, and promote translation.

• Help recruit translation initiation factors (IFs), which recruit the small ribosomal subunit (with tRNA that binds the AUG codon)

• The small subunit scans until an AUG is found.

Cap-Dependent translation

• After recruitment to the 5’ cap, the small ribosome subunit scans the mRNA until it encounters a start codon.

• Start codons more recognized when apart of the Kozak sequence.

• After finding the AUG, initiation factors are released and the full ribosome is assembled.

Peptide bond

Formation links this amino acid to the growing polypeptide.

Release factors

• Stop codons are recognized by these proteins.

• Once released factos bind to the stop codons, translation is terminated through release of the completed polypeptide.