ABEEHA SARWAR

Translation:

• Translation is the first stage of protein biosynthesis (part of the overall process of gene expression)

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• Translation is the production of proteins by decoding mRNA produced in transcription

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• It occurs in the cytoplasm where the ribosomes are located

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• Ribosomes are made of a small and large subunit which surrounds the mRNA

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• In translation, messenger RNA (mRNA) is decoded to produce a specific polypeptide according to the rules specified by the genetic code

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• This uses a mRNA sequence as a template to guide the synthesis of a chain of amino acid that form a protein

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• Many types of transcribed RNA, such as transfer RNA, ribosomal RNA, and small nuclear RNA are not necessarily translated into an amino acid sequence

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Types of RNA:

• There are three major types of RNA molecules and each one plays its own unique role
• Messenger RNA:
• It carries the genetic code to the cytoplasm, where it is translated by the ribosomes
• Ribosomal RNA:
• It forms an integral part of the ribosomes, without which translation would be impossible

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• Transfer RNA:
• It collects the proper amino acids and brings them to the ribosomes, where they are used to synthesize the polypeptide chain

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Ribosomes:

• Translation involves special machinery known as ribosomes
• A given ribosome consist of two subunits , the small subunit and a large subunit
• These subunits consist of a combination of ribosomal RNA and protein
• Ribosomal RNA is synthesized in the nucleolus and travel to the cytoplasm, where thy combine with the mRNA to form the ribosomes

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• Prokaryotes and eukaryotes have ribosomes that are composed of slightly different subunit

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• S stands for Svedberg units, which are the units that measure the rate at which a given particle/molecule sediments

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• The Svedberg unit really refers to how quickly something travels down a test tube when it is rotating with some angular velocity

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• It is directly related to mass and also depends on size of the molecule

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• Heavier molecules (large subunits) will be found farther to down the test tube than lighter (small subunits) ones

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• The Large and small subunits only come together during the process of translation

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• The purpose of Ribosomes is to use the genetic code to translate the sequence of nucleotide on the mRNA into the correct sequence of amino acids, thereby synthesizing the protein

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Stages of translation:

• Translation proceeds in three phases: 

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• Initiation

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• Elongation

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• Termination

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Initiation:

• Once the pre-mRNA undergoes, the proper modification in the nucleus, it becomes mRNA

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• mRNA travels through the nuclear pores and into the cytoplasm of the cell

• Once inside the cytoplasm with the help from protein called Initiation Factor, the small subunit binds to the 5’end of the mRNA

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• The subunit slides along the mRNA until it reaches the nucleotide sequence 5’-AUG-3’

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• This is the start codon and it signals for tRNA to bring the amino acid Methionine

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• Once the tRNA with the methionine binds to the start codon, the large subunit binds to the small subunit, thereby completing the ribosome

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• The structure of tRNA is especially suitable for its function

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• Towards the middle of the RNA strand is a sequence of nucleotides that is complementary to the start codon sequence; this is known as the anti-codon of the tRNA

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• The 3’- end of the tRNA contains the nucleotide sequence that binds the appropriate amino acid

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• An enzyme called tRNA synthase catalyze the binding of the tRNA to the appropriate amino acid

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Elongation:

• Elongation is divided into three steps:

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• Amino acyl tRNA delivery

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• Peptide bond formation

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• Translocation

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Elongation:

• Once the ribosomes complex has been formed and the tRNA that carries the methionine is in the P-site, the ribosomes can now begin sliding along the mRNA

• The next amino acyl tRNA that carries the next amino acid is brought into the A-site

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• Once the P-and A-sites are filled an enzyme called peptidyl transferase catalyze the formation of peptidyl bond between amino acid in the P site and A-site

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• When the peptide bond is formed the amino acid detaches from the tRNA in the P-site

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• In the next step the ribosomes complex moves three nucleotide from 5՜ to 3՜ direction along mRNA

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• This moves the tRNA in the P-site into the E-site and it rotates the tRNA carrying the growing polypeptide into the P-site

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• Once the tRNA moves into the E-site, it is then expelled out of the ribosome complex

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• This sliding process is known as translocation

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• Once the A-site is empty, process can be repeated itself to add another amino acid

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• This continues until the polypeptide chain is complete

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Termination:

• Just like there is a start codon that signals initiation, there is also a stop codon that signals termination

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• If the ribosome reads either UAA, UGA and UAG a protein called release factor will bind to the A-site

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• This will ultimately cause the polypeptide chain to break off of the tRNA in the P-site

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• The ribosome complex will then dissociates and this will then end termination

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Polyribosomes:

• A single strand of mRNA is translated simultaneously by many ribosomes, spaced closely together

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• Such clusters of ribosomes are called Polysomes or polyribosome

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• The simultaneous translation of a single mRNA by many ribosomes allow highly efficient use of the mRNA

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Polyribosome figure:

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