Translation

Protein Biosynthesis

Central Dogma

DNA

RNA

protein

transcription

translation

mRNAs are exported for translation

Through nuclear pore complex Recognizes and transports ONLY

completed mRNA

Translation

Process by which the sequence of nucleotides in an mRNA directs the incorporation of amino acids into a protein

Necessary Components for Translation mRNA tRNAs covalently linked to amino acids Ribosome

Three phases of Translation Initiation Elongation Termination

The mRNA strand is “read” and amino acids are linked together to make a protein by the ribosome

mRNA

Carries the genetic information from the chromosomes to the ribosomes

How is the language of nucleic acid sequences translated into the amino acid language of proteins?

mRNA is decoded in sets of three nucleotides = codons

Genetic Code

Each codon specifies either an amino acid or stop signal to translation

There are only 20 amino acids and there are 64 possible codons

The genetic code is degenerate - i.e. there are "synonyms" (multiple codons) for some amino acids

Three codons (UAG, UGA, and UAA) encode translation "stop" signals rather than amino acids

mRNA must be read in the correct reading frame to be decoded into the protein

Redundancy or degenerate coding Reference page 367

Amino acids

Organic molecule containing both an amino group and a carboxyl group

Building blocks of proteins Are added to the C-terminal end of a

growing polypeptide chain by the formation of a peptide bond

Peptide bonds – between the carboxyl group at the end of growing chain and a free amino group of incoming amino acid

Proteins are synthesized from its N-terminus to its C-terminus

tRNA

Adapter molecule that mediates recognition of the codon sequence in mRNA and allows its translation into the appropriate amino acid.

~ 80 nucleotides long Folds into 3D structure It has sites for amino-acid attachment and codon

recognition The codon recognition is different for each tRNA and

is determined by the anticodon region, which contains the complementary bases to the ones encountered on the mRNA.

Each tRNA molecule binds only one type of amino acid, but because the genetic code is degenerate, more than one codon exists for each amino acid.

Ribosomes

Small and Large subunits The site of translation Helps to maintain the correct reading

frame and to ensure accuracy Complex catalytic machine made up of 50

different proteins and several RNA molecules (rRNAs)

Produced in nucleolus Millions exist in cell

tRNA structuretRNA Molecule

Aminoacyl tRNA Synthetase

In Eukaryotes Unique synthetase for each amino acid

Proper Amino Acid by affinity or fit

Corrected by Hydrolytic editing

EP A

mRNA binding• two subunits, a large and a small

• the mRNA binds to the small subunit

• there are three sites of activity and tRNA binding within the large subunit

The Ribosome

How do Ribosomes work?Via 4 binding sites for RNA molecules

C G A U C A A U A CC G A U C A A U G C G

codon codon codon codon codon codon

The ribosome attaches to the RNA and scans for AUG,the start codon

The ribosome reads the mRNA three nucleotides at a time

Each group of three nucleotides is a single codon

Each codon specifies an particular amino acid

Start codon

TRANSLATION INITIATION

Initiation

Translation begins with the codon AUG A special tRNA is required to initiate

translation Initiator tRNA always carries the amino

acid methionine Initiator tRNA is loaded onto the small

subunit of the ribosome with the aid of additional proteins (eIFs) which are attached to GTPs

Initiation, cont.

Initiator tRNA binds small ribosomal subunit Small subunit then binds to 5’ end of an

mRNA molecule (recognized by 5’ cap) The small subunit then moves along mRNA

(5’-3’) searching for the first AUG eIF2 hydrolyzes GTP to GDP and detaches Large Subunit then assembles and

elongation can begin Bacteria use Shine-Dalgarno sequences to

initiate translation at any point on the mRNA.

Shine Dalgarno sequence Ribosome docking sequences

Upstream of AUG consensus sequence

Once the ribosome recognizes the start codon, protein synthesis begins

The ribosome promotes a chemical reaction to occur that joins two amino acids with a peptide bond

Amino acids are transferred to the ribosomes by tRNA molecules

tRNAs have an anticodon on one end and an amino acid on the other

The anticodon is a sequence of three nucleotides that complement a codon

Cont.

The anticodon determines which amino acid it carries to the ribosome

EF-Tu (EF-1) helps the fidelity of the process.

Each of the twenty amino acids pairs up with between 1 and 4 anticodons

The process continues, the ribosome moves along the mRNA to the next codon with the help of EF-G (EF-2)

A new tRNA recognizes the next codon.

U C A G C A A G A C

Met

U C A G U A A U G U C

Anti-codon

tRNA

Amino acid

• This continues until the ribosome reaches a STOP codon, which indicates the end of the gene•The ribosome & last tRNA fall off the mRNA & the amino acid chain is complete!

TRANSLATION ELONGATION

Elongation

Termination

One of the three STOP codons mark the end of translation

The stop codons are recognized by proteins known as release factors that do not specify any amino acids

The release factor triggers an addition of water to the end of the polypeptide chain the release of the new protein.

Protein Folding

Begins while Protein is still being synthesized

Guided by and made more efficient by molecular chaperones

The amino acid chain folds up into a 3-dimensional structure dictated by the order of the amino acids.

This unique structure gives the protein its unique function and allows it to do its work

Every protein has a unique order of amino acids

Proteins have many functions

Protein example: Antibiotics

Some antibiotics are peptides, others glycopeptides, others are amino acid derivatives

Inhibitors of prokaryotic translation, allowing for discrimination between prokaryotic and eukaryotic cells

Examples: Tetracycline, Streptomycin, Chloramphenicol, Erythromycin