Translation The sequence of nucleotide bases in an
Translation The sequence of nucleotide bases in an m. RNA molecule is a set of instructions that gives the order in which amino acids should be joined to produce a polypeptide. The forming of a protein requires the folding of one or more polypeptide chains. Ribosomes use the sequence of codons in m. RNA to assemble amino acids into polypeptide chains. The decoding of an m. RNA message into a protein is a process known as translation.
Steps in Translation Messenger RNA is transcribed in the nucleus and then enters the cytoplasm for translation. The m. RNA then goes to the ribosomes for translation to begin.
The Roles of t. RNA and r. RNA in Translation Ribosomes are composed of roughly 80 proteins and three or four different r. RNA molecules. These r. RNA molecules help hold ribosomal proteins in place and help locate the beginning of the m. RNA message. They may even carry out the chemical reaction that joins amino acids together.
• Ribosomes consist of two subunits. – The large subunit has three binding sites for t. RNA. – The small subunit binds to m. RNA.
Ribosome Structure • The ribosome is a very complicated organelle made of two separate pieces that come together to get the job done. • The large subunit, which has three sites for the t. RNAs, named the E site, P site, and A site (remember “EPA” or "APE") • A: Arrival • P: Polypeptide (is created) • E: Exit
Steps in Translation Each t. RNA molecule carries just one kind of amino acid. In addition, each t. RNA molecule has three unpaired bases, collectively called the anticodon —which is complementary to one m. RNA codon. The t. RNA molecule for methionine has the anticodon UAC, which pairs with the methionine codon, AUG.
Amino acids are linked to become a protein. • A t. RNA carrying an amino acid is charged • When the t. RNA picks up an amino acid, it is charging
Steps to Translation • • • Translation, like transcription, is a 3 step process: • Initiation • Elongation • Termination Goal: To translate the m. RNA code into a protein. Each triplet codon must be read from the m. RNA and used to add the appropriate amino acid to the polypeptide.
Steps in Translation begins when a ribosome attaches to an m. RNA molecule in the cytoplasm. As the ribosome reads each codon of m. RNA, it directs t. RNA to bring the specified amino acid into the ribosome. One at a time, the ribosome then attaches each amino acid to the growing chain.
Initiation • • • The small ribosomal subunit binds to the m. RNA at the 5' end of the m. RNA molecule. The initiator t. RNA • Has methionine as an amino acid • Has an anticodon that is complementary to the start codon on m. RNA (normally AUG). • This anticodon (normally UAC) binds to the m. RNA. The large ribosomal subunit binds to the t. RNA at the P site. • This creates the initiation complex.
Initiation
Elongation The ribosome has a second binding site for a t. RNA molecule for the next codon. If that next codon is UUC, a t. RNA molecule with an AAG anticodon brings the amino acid phenylalanine into the ribosome.
Elongation The ribosome helps form a peptide bond between the first and second amino acids— methionine and phenylalanine. At the same time, the bond holding the first t. RNA molecule to its amino acid is broken.
Elongation • Codon recognition: the codon in the A site of the ribosome pairs with the appropriate t. RNA molecule. • Peptide bond formation: the two amino acids are linked. • Translocation: The t. RNAs in the P and A site move left. �A moves to P. • P moves to E. • The t. RNA in the E site exits the ribosome.
Elongation That t. RNA then moves into a third binding site, from which it exits the ribosome. The ribosome then moves to the third codon, where t. RNA brings it the amino acid specified by the third codon.
Termination The polypeptide chain continues to grow until the ribosome reaches a “stop” codon on the m. RNA molecule. When the ribosome reaches a stop codon, it releases both the newly formed polypeptide and the m. RNA molecule, completing the process of translation.
Termination • • Occurs when one of the three stop codons (UAA, UAG, UGA) arrives in the P site. A release factor binds to the A site, causing the whole thing to fall apart. • The large and small ribosomal subunits separate from each other and from the m. RNA. • The new protein is released and can now fold into its 3 D shape and get to work in the cell
Translation Errors • The ribosome moves along the m. RNA exposing the codons to be translated – this is the reading frame • Frame-shift mutation results if initiation is shifted over from the reading frame by one or two nucleotides (alternate initiation site) or due to errors during splicing of the exons (insertion or deletion)
Signal Sequence • signal sequence - short (5 -30 amino acids long) at the end of the newly synthesized polypeptide chain • Directs the polypeptide chain into the endoplasmic reticulum or other organelle for processing, packaging, and transport around or out of the cell
Protein Processing • After translation on ribosomes in the cytosolic compartment all proteins are processed either in the cytosol or in the ER/Golgi system. • The initial stages of protein processing involving folding. • folding of proteins takes place through interaction with chaperone proteins (they hold the polypeptide chain and aide in folding) • Proteins that are not properly folded are destroyed. In the cytosol compartment they are tagged with ubiquitin and destroyed by proteasomes
Protein Processing
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