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 structure tRNA Molecule

Aminoacyl tRNA Synthetase

In Eukaryotes Unique synthetase for each amino acid  Proper Amino Acid by affinity or fit

Corrected by Hydrolytic editing

E PA 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 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