Protein Synthesis Translation

Protein Synthesis

Translation Translation InitiationElongationTermination

Translation From DNA blueprint  Protein Once mRNA enters the cytoplasm, it is ready to be translated Translation is made up three stages

Translation- Initiation Initiation occurs when a ribosome recognizes a specific sequence, the 5 ' cap, on the mRNA and binds to that site This process is the initiation of translation of nucleic acid into protein

Ribosomes In eukaryotes, the ribosome consists of two subunits A large subunit of 60S A small subunit of 40S The S refers to the rates at which various components sediment when centrifuged The rates of a measure of size Intact ribosome (80S)

Structure of RNA E site (Exit site) P site (Peptidyl tRNA binding site) A site (Aminoacyl tRNA binding site) mRNA binding site Small subunit Large subunit Ribosomal subunits are a combination of ribosomal RNA (rRNA) and protein

Translation-Initiation The ribosomes move along the mRNA in the 5 ' -3 ' direction, adding a new amino acid to the growing polypeptide chain The two ribosomal subunits bind to the mRNA, clamping the mRNA between them

Ribosomes Ribosomes synthesize different proteins by associating with different mRNAs and reading their coding sequence Reading frame: a contiguous and non- overlapping set of three-nucleotide codonsnucleotide codons There are 3 possible reading frames in an mRNA strand mRNA

Reading Frame In theory any mRNA can be translated in any of three different "reading frames" depending on the nucleotide sequence at which translation starts. Thus 3 different amino acid sequences can be obtained from a single mRNA

The Role of Transfer RNA (tRNA) The ribosome alone cannot synthesize the polypeptide chain The correct amino acids must be delivered to the ribosome binding site The molecule that delivers the amino acids is tRNA

tRNA A small single-stranded nucleic acid whose structure resembles a cloverleaf At one arm of tRNA is the anticodon Anticodon is a sequence of three bases that recognizes the codon of the mRNA The opposite arm carries the corresponding amino acid mRNA 5'5' 3'

tRNA tRNA and mRNA recognize each other by complementary base pairing

tRNA Every tRNA carries only a specific amino acid, which means that at least 20 different tRNAs are required Recall: there is 64 possible codons In reality, anywhere between types of tRNA molecules are available depending on the organism

Wobble Hypothesis Proposed by Francis Crick The theory proposes that the first two bases in the codon and anticodon will form complementary pairs in the normal antiparallel fashion.anticodon However, a degree of ‘wobble’ is allowed in the base-pairing at the third position. This hypothesis accounts for the number of tRNAs that are observed It also accounts for the degeneracy that is observed in the Genetic Code. The degenerate base is that in the wobble position.

Wobble Theory Crick recognized that the following base- pair schemes were possible: 5' anticodon base3' codon base AU CG GC or U UA or G IA or C or U

Aminoacyl-tRNA tRNA molecule with its corresponding amino acid attached to its acceptor site at the 3 ' end Acceptor site of amino acid

Translation-Initiation The mRNA joins to the small ribosomal unit at the 5' untranslated region. This binds to a special binding site on the small ribosomal subunit.

Translation-Initiation The large ribosomal subunit attaches to the small subunit such that the first codon is aligned at the P binding site.

Translation-Initiation A tRNA carrying the amino acid methionine attaches to the start codon (AUG) on the messenger RNA. This initiates elongation. The AUG codon codes for methionine, i.e, every protein initially starts with this amino acid The tRNA that carries methionine enters the P site

Translation-Initiation

Translation-Elongation The next tRNA carrying the required amino acid enters the A site The tRNA and its amino acid attaches to the A binding site. Amino end of polypeptide mRNA

Translation-Elongation The methionine amino acid at the P site is bonded to the amino acid at the A site There is a Peptide bond formation between the two amino acids Peptide bond formation

Translation- Elongation The tRNA at the P site is now free of amino acids

Translation-Elongation Ribosome shifts over one codon (three bases) Ribosome moves in the 3' direction down the messenger RNA The A binding site is now open and a vacant tRNA is in the E binding site. Ribosome ready for next aminoacyl tRNA

Translation- Elongation A new tRNA attached to an amino acid enters the vacant A site on the ribosome and the process repeats again Amino end of polypeptide mRNA

Translation-Termination The ribosome will eventually reach a stop codon The three stop codons are: UGAUAGUAA Since these codons do not code for an amino acid, there are no corresponding tRNAs

Translation- Termination Since, the stop codons do not code for an amino acid, there are no corresponding tRNAs

Translation- Termination A protein known as a release factor recognizes that the ribosome has stopped translating The release factor helps in the release of the polypeptide chain from the ribosome The two subunits fall off the mRNA and translation officially stops