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Protein Synthesis

Protein Synthesis. How Proteins are Produced. An Analogy of Gene Expression. Transcribe (copy) a set of ingredients from the cookbook to make a recipe. Translate the ingredients into a dish. The Key Players…. DNA RNA Ribosomes Amino acids. RNA- ribonucleic acid.

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Protein Synthesis

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  1. Protein Synthesis How Proteins are Produced

  2. An Analogy of Gene Expression Transcribe (copy) a set of ingredients from the cookbook to make a recipe Translate the ingredients into a dish

  3. The Key Players…. • DNA • RNA • Ribosomes • Amino acids

  4. RNA- ribonucleic acid Structural Differences with DNA: 1) Ribose sugar instead of Deoxyribose 2) Single Stranded instead of double 3) Uracil instead of Thymine (GCAU) Uracil Thymine

  5. Functional Differences: 1) Deliver instructions/blueprints from DNA to ribosomes for protein-building

  6. 3 different types of RNA • Messenger RNA (mRNA) - made from a DNAtemplate. The sequence of bases in mRNA determines the sequence of amino acids in a protein. It carries the protein-building instructions out of the nucleus to the ribosomes - acts as a template for protein construction • Transfer RNA (tRNA) - translates mRNA code by attaching to the specific amino acid that is indicated in the code and bringing the amino acid to the ribosome to help build the required protein. • Ribosomal RNA (rRNA) -structural component of a ribosome, involved in synthesis of ribosomes

  7. mRNA

  8. protein synthesis begins in the nucleus with the production of mRNA (Transcription) • mRNA code is translated into polypeptide (Translation) • DNA  RNA  protein transcription translation • known as the CENTRAL DOGMA of MOLECULAR GENETICS

  9. Part One: Transcription • process of converting DNA to messenger RNA • mRNA is made inside the nucleus and has the job of carrying the instructions from the DNA out into the cytoplasm. DNA never leaves the nucleus! • divided into three sequential processes: initiation,elongation and termination (Fig.2 page 243)

  10. INITIATION • How is mRNA made? • RNA polymerase binds to the DNA molecule in a region upstream of the gene to be transcribed – called promoter region • This region is rich in A’s • and T’s

  11. ELONGATION Coding strand Template strand To build the chain of mRNA ( from the 5’  3’ end), RNA polymerase adds free-floating nucleotides that are complimentary to the strand of DNA that is being transcribed (template strand)

  12. TERMINATION RNA polymerase recognizes the end of the gene when it reaches the terminator sequence The newly synthesized mRNA detaches from the template DNA, and DNA zips back up. mRNA takes the coded message to the ribosomes in the cytoplasm or attached to the ER

  13. Posttranscriptional Modifications Before exiting the nucleus, the mRNA strand needs to be modified: • Capping - A 5’ cap is added– made of 7-methyl guanosine – protects the mRNA from digestion by enzymes & helps in the initiation of translation. • Tailing - A poly-A tail (string of ~ 200 adenines) is added to the 3’ end, protects the strand from degradation

  14. Removal of introns and joining of exons Introns (noncoding regions) must be removed by splicesomes and the remaining exons (coding regions) must be joined • mRNA leaves nucleus, spliced-out introns stay and get degraded - mRNA transcript is now ready to leave the nucleus

  15. How can we have over 120 000 proteins, and only about 25 000 genes?

  16. Question • What would be the mRNA strand for the following DNA sequence? DNA: 3’ T G G C A T G 5’ mRNA: 5’ A C C G U A C 3’

  17. So far: The order of bases in the DNA specifies the order of bases in the mRNA Now, The order of bases in the mRNA specifies the order of amino acids in the protein • A sequence of 3 mRNA bases is called a codon (codes for an amino acid)

  18. Messenger RNA (mRNA) start codon A U G G G C U C C A U C G G C G C A U A A mRNA codon 1 codon 2 codon 3 codon 4 codon 5 codon 6 codon 7 stop codon protein methionine glycine serine isoleucine glycine alanine Primary structure of a protein aa2 aa3 aa4 aa5 aa6 aa1 peptide bonds 43 = 64 amino acids 41 = 4 amino acids 42 = 16 amino acids

  19. Genetic Code

  20. Now that you have the mRNA (carrying the code), the code needs to be translated (into a protein). This process is called TRANSLATION • Uses the mRNA’s code to build proteins • Occurs in the cytoplasm on ribosomes • Need tRNA totransport amino acids to the ribosome • tRNA have anticodons that are complimentary to the mRNA codons • Bonds to corresponding mRNA sites on ribosome • **Note – the amino acid picked up matches the mRNA codon, not the anticodon

  21. A closer look at ribosomes:

  22. Transfer RNA (tRNA) amino acid attachment site amino acid U A C anticodon methionine

  23. Translation – 3 Steps Initiation – ribosome binds to mRNA. Start codon read (AUG). Special initiator tRNA brings in first amino acid- enters P-site Elongation – another tRNA molecule with anticodon complementary to next mRNA codon enters A-site. Peptide bonds forms between adjacent amino acids. Initiator tRNA exits ribosome. Ribosome moves along the mRNA – one codon. tRNA that was previously in the A-site moves into the P-site (carrying it’s growing peptide chain), opening up the A-site for another tRNA. Process continues until stop codon is reached Termination - stop codon reached. No tRNA for these codons. Release factor proteinbinds to stop codons causing polypeptide to be released from the ribosome. Ribosome disassembles.

  24. Initiation aa2 aa1 2-tRNA 1-tRNA G A U U A C anticodon A U G C U A C U U C G A hydrogen bonds codon mRNA

  25. aa3 3-tRNA G A A Elongation peptide bond aa1 aa2 1-tRNA 2-tRNA anticodon U A C G A U A U G C U A C U U C G A hydrogen bonds codon mRNA

  26. aa3 3-tRNA G A A aa1 peptide bond aa2 1-tRNA U A C (leaves) 2-tRNA G A U A U G C U A C U U C G A mRNA Ribosomes move over one codon

  27. aa4 4-tRNA G C U peptide bonds aa1 aa2 aa3 2-tRNA G A U (leaves) 3-tRNA G A A A U G C U A C U U C G A A C U mRNA Ribosomes move over one codon

  28. aa5 5-tRNA U G A peptide bonds aa1 aa2 aa3 aa4 3-tRNA G A A 4-tRNA G C U G C U A C U U C G A A C U mRNA Ribosomes move over one codon

  29. aa5 aa4 Termination aa199 aa200 aa3 primary structure of a protein aa2 aa1 terminator or stop codon 200-tRNA A C U C A U G U U U A G mRNA

  30. Protein synthesis animation: • http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120077/micro06.swf::Protein%20Synthesis

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