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Polypeptide Synthesis

Polypeptide Synthesis. From Genes to Proteins. Transcription in prokaryotes. Simple process No nucleus; ribosomes lie close to the DNA in cytoplasm 1 st part of the mRNA attaches directly to ribosomes The rest is being transcribed Assembly of aa into proteins begins right away.

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Polypeptide Synthesis

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  1. Polypeptide Synthesis From Genes to Proteins

  2. Transcription in prokaryotes • Simple process • No nucleus; ribosomes lie close to the DNA in cytoplasm • 1st part of the mRNA attaches directly to ribosomes • The rest is being transcribed • Assembly of aa into proteins begins right away

  3. Transcription in eukaryotes • Transcription MUCH MORE complex!!!! • DNA wound around protein spools • Before transcription happens… • Spools unwound • Enzyme cannot begin process-able to attach to start signal region(replication fork) • Once new mRNA is transcribed from DNA template mRNA breaks into pieces. Why?

  4. Eukaryotic transcription • mRNA reassembles to form finished mRNA • Portions of the original is left out. • Finished strand is shorter • Why does the original mRNA molecule break up into pieces? • What is discarded when the pieces are spliced back together?

  5. What is happening?

  6. What next? • During mRNA processing • Introns are cut out and exons are spliced together • Events occur in the cell • Introns: intervening sets of nucleotides (junk) • Exons: code for aa, because they are expressed • Once mRNA is processed, where do it go? • What happens to the mRNA molecule after processing is complete?

  7. Translation • Removing introns from mRNA is known as mRNA processing. • During Translation-processed mRNA attaches to free ribosomes or attached ribosomes on RER • Polypeptide synthesis begins • mRNA carries genetic info in sequence of codons • Particular order of ofaa

  8. Protein synthesis • To manufacture protein requires deciphering the info and assembling in an particular order • Synthesis of a polypeptide chain (small protein) from info carried by mRNA is called TRANSLATION!!!!! • How is mRNA “language” translated into protein “language”

  9. 20 amino acids/64 possible • Combinations • Job bringing aa to ribosomes is performed by tRNA • 2 parts[ anticodon & aa] • Anticodon-a set of 3 bases that is specific to the type of aa the tRNA carries. • Each have a different anticodon • Binds ONLY to the complementary mRNA codon

  10. Codons & anticodons joining

  11. mRNA codons  amino acids

  12. Joining codons & anticodons • Depends on proper translation • Ribosome attaches to 1st codon on mRNA strand • tRNA approaches carrying aa • If tRNA recognizes the 1st mRNA codon, 2 join • tRNA w/aa moves along mRNA strand, exposing the next mRNA codon • Again, proper tRNA w/its aa joins mRNA strand

  13. Forming a polypeptide • Once 1st two aa are in place, 1sttRNA is then released • Leaving it free to pick up another aa • Process continues-ribosomes moves along mRNA • Polypeptide chain grows • Aa added to a chain at 15/s • One of 3 codons stop the translation process • No tRNA to match up • Translation stops polypeptide complete • Breaks away from the assembly line

  14. summary • DNA stores instructions for polypeptide synthesis • The DNA instructions transcribed mRNA, carrying info from the nucleus into cytoplasm • Ribosomes & tRNA interact w/mRNA to translate the info & build polypeptides

  15. proteins • They are made to order • Synthesized when needed, in quantity needed • Only certain genes are expressed at one time • Kinds of proteins synthesized by a cell differentiate it from other kinds of cell in an organism • Protein in a muscle cell is different in nerve cell. Get it?

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