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Protein Synthesis – Review

Protein Synthesis – Review

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Protein Synthesis – Review

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  1. Protein Synthesis – Review

  2. Review!! •

  3. An Overview… • Genes on chromosomes need to be transcribed into RNA, and then translated by ribosomes into protein. • Genes act by directing the production of only ONE protein each. • This is known as the one gene-one enzyme hypothesis. • An error in gene sequence can result in a change in protein sequence, shape, and therefore function. • TRY THIS: Activity p 233, Translating the Code.

  4. Overview… • DNA in the nucleus is TRANSCRIBED into a complementary RNA message that moves to the cytoplasm. (copied from one medium to another)… • Numerous RNA copies can move to different ribosomes.

  5. Overview • The ribosomes TRANSLATE the message into Polypeptide chains and process into proteins (convert into a different language). • Transcription involves the copying of information in DNA into mRNA • Translation involves ribosomes using mRNA as blueprint to synthesize a protein composed of amino acids.

  6. A Cool Visual •

  7. ribose sugar with a hydroxyl on 2’ Carbon uracil replaces the base thymine and pairs with adenine single stranded RNA

  8. 3 Types of RNA • Messenger RNA (mRNA): varies in length, end product of transcription of a gene, moves to the ribosome • Transfer RNA (tRNA): delivers amino acids to the ribosomes for translation. Shaped like a cloverleaf; very short. • Ribosomal RNA (rRNA): binds with protein to make ribosomes

  9. Step 1 - Transcription Within the nucleus, the DNA of a particular gene unwinds and separates. One strand of the DNA is transcribed into messenger RNA (mRNA) in a process called transcription.

  10. Transcription – Making mRNA

  11. Transcription divided into three sequential processes… • Initiation • RNA polymerase bindsto DNA at specific site known as promoter, near the beginning of the gene. (usually high [] of A & T…only 2 bonds to break!) • Elongation • Enzyme RNA polymerase puts together the appropriate ribonucleotides and builds the mRNA transcript

  12. Termination • Shortly after RNA polymerase passes the end of a gene, recognizes signal to stop transcribing • mRNA transcript then is completely released from the DNA and will exit the nucleus •

  13. The Final Touches… • the mRNA must be prepared to leave the nucleus. A 5’ cap is added (for protection), and a poly A tail is added to the 3’ end • The mRNA contains regions of code that do not make the protein (introns). Introns are removed by spliceosomes. • Spliceosomes join exons (coding regions) together. • The mRNA strand leaves the nucleus and binds with ribosomes in the cytoplasm.

  14. P 249, Q 1-6

  15. Step 2 Translation • Once mRNA enters cytoplasm, it is ready to be translation • Ribosomes: made up of two parts: a large subunit (60s) and a small subunit (40s). Bind to the mRNA (recognizing the 5’ cap) • Must be lined up correctly to the proper reading frame (reading triplets correctly)

  16. tRNA • tRNA delivers amino acids to the polypeptide building site (ribosome) • tRNA is a small single stranded nucleic acid, resembles a cloverleaf • one arm: anticodon (sequence of three bases complementary to mRNA) • 3’ end has acceptor site for a particular amino acid • this recognition by tRNA of mRNA is facilitated through complimentary base pairing. • every tRNA carries only one specific amino acid • therefore there must be at least 20 (20-64) different tRNA’s

  17. Translation

  18. Translation • The mRNA consists of nucleotides that code for a specific amino acid. The code is in triplet called a CODON (3 nucleotides = 1 amino acid). Amino acids are brought into place by a molecule known as transfer RNA (tRNA). This process is known as translation. Peptide bonds occur b/w amino acids.

  19. Building the Chain… First codon recognized is start (AUG-methionine)- aligns the reading frame ribosome contains two sites for tRNA: an A (acceptor) site and a P (peptide) site. tRNA (Met- AUG) enters the P site. the rest of the tRNA enter at the A site and form peptide bonds between the amino acids as the chain forms.

  20. Ribosome translocates one codon over and continues the chain until it reaches a stop codon. (UGA, UAG, UAA) A release factor protein dismantles the complex, releasing the polypeptide chain. Enzymes may cleave the chain at specific places


  22. Protein Synthesis • Once all the triplet codes (Codons) are read and the amino acids are brought into place, the completed protein travels through the ER to the Golgi. • The Golgi modifies the structure and packages it into a vesicle. • Vesicle moves to the membrane and is released by exocytosis.

  23. More Homework... P 241 Q1-9 P 254 Q1-4