PROTEIN SYNTHESIS TRANSCRIPTION: DNA m RNA TRANSLATION: m RNA Protein.
PROTEIN SYNTHESIS: TRANSLATION
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Transcript of PROTEIN SYNTHESIS: TRANSLATION
PROTEIN SYNTHESIS:
TRANSLATION
What is the genetic code?
• the sequence of nt in mRNA• the order of mRNA n.t. determines the order
of amino acids that will bond to build a polypeptide
• the code is a triplet code: in mRNA, every 3 nucleotides codes for an aa– called a CODON
Recall transcription:
codon
• 64 codons and only 20 corresponding amino acids
• The near-universality of the genetic code supports the idea that all organisms are evolutionarily related
• Note the chart on p. 207
• several codons code for the same amino acid
• codons often differ only by the 3rd nucleotide
All about codons:
• START Codon: AUG • also codes for methionine • engages a ribosome to start
translating• always found at the beginning of
the mRNA transcript but can be found throughout the transcript as well
• STOP Codons: UAA, UAG, UGA• cause a ribosome to stop
translating• do not code for any amino acids
Translation Basics
• Translation = the process of assembling polypeptides from info encoded in mRNA
What’s needed?
1. mRNA2. tRNA3. ribosomes
1. mRNA
• mRNA – it has the instructions for protein synthesis from DNA!
2. tRNA
• tRNA - binds specifically to amino acids in the cytoplasm and transports them to the ribosomes
• the bottom loop of tRNA has a 3 nt sequence that bonds with mRNA nt
• this is the ANTICODON
• ANTICODON is complementary to its paired mRNA CODON
• Same base pairing rules apply
• pairing of anticodon with codon ensures that amino acids are added in the correct order
• attachments of amino acid, ATP, and tRNA are all controlled by enzymes
3. Ribosomes
• Composed of 1. rRNA2. Proteins
• Found in 2 places:1. free in cytoplasm (to produce proteins used by the
cell)2. attached to ER (to produce exported proteins)
1. mRNA site2. P site3. A site
• mRNA site- holds mRNA transcript so that its codons are available to bond to tRNA molecules
Ribosomes have 3 binding sites that are key to translation
• other 2 sites = P site and A site – each holds a tRNA whose anticodons pair with
mRNA codons
• P site = holds tRNA with growing polypeptide chain
• A site = holds tRNA that is transferring the next amino acid to be added to the chain
Translation
• Also called protein synthesis– Synthesis = to put together
• 3 steps1. Initiation2. Elongation3. Termination
Protein Assembly1. Initiation
• Begins when a ribosome attaches to the start codon (AUG) on a mRNA transcript.
• This pairs with UAC on t-RNA.
• Remember… AUG codes for methionine– The first amino acid in every polypeptide is initially
methionine (it can later be removed).
Translates in one direction 5’ to 3’!!!
2. Elongation
• As ribosome moves along mRNA transcript, each mRNA codon is paired with its tRNA anticodon.
• This causes a specific amino acid to attach to previous amino acid with a covalent bond (called a peptide bond!)
• This continues until ribosome reaches a stop codon, ending translation.
Dehydration Synthesis of Proteins
3. Termination
• mRNA is released • the polypeptide is complete.
Result of Protein Synthesis
• Polypeptide chain represents the protein’s 1° structure.
• As it folds and (or binds) to other polypeptides, it assumes its functional (4°) structure of a completed protein.
• What organelle folds and modifies the polypeptide chain into a function protein?
Other info…
• Several ribosomes may simultaneously translate the same mRNA transcript = polysomes
• usually about 80 nucleotides apart– 1st one starts, gets about 80 nt away from start codon
before a 2nd ribosome starts at start codon and follows behind 1st
• This speeds up translation of one transcript and results in larger quantities of one particular protein.
• https://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter3/animation__how_translation_works.html
• http://www.pbslearningmedia.org/asset/lsps07_int_celltrans/
• http://www.wisc-online.com/objects/ViewObject.aspx?ID=AP1302