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Cell Division, Genetics, Molecular Biology

Cell Division, Genetics, Molecular Biology. 20.2 Gene Expression Transcription/Translation Protein Synthesis. Amino Acids.

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Cell Division, Genetics, Molecular Biology

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  1. Cell Division, Genetics, Molecular Biology 20.2 Gene Expression Transcription/Translation Protein Synthesis

  2. Amino Acids • Frederick Sanger 1952: - proteins consist of sequence of molecules called amino acids- specific sequence of amino acids determines chemical properties of each protein- proteins produced by cell determine structure, function & development of cell • Proteins responsible for inherited traits • Sequence of base pairs in DNA makes up genetic code of organism • Genetic code determines how amino acids strung together (how proteins are made)

  3. Genetic Code • In a gene: each set of 3 bases is a CODON • Genetic code always interpreted in terms of mRNA codon rather than original DNA sequence • Each mRNA codon codes for a specific amino acid • Only 20 amino acids found in proteins, depend on combination of bases in codon • Start and stop codons initiate or terminate protein synthesis

  4. Ex) A U G C U U A A A G C C U G A

  5. Gene Expression • Way information in a gene is converted into a specific trait through production of a polypeptide (protein)- products of all genes = polypeptides • RNA is utilized to convert genes into proteins- messenger RNA (mRNA)- transfer RNA (tRNA)- ribosomal RNA (rRNA) • 2 stages of gene expression: transcription and translation • Transcription: genetic information converted from DNA sequence into mRNA, carries information from nucleus to cytoplasm • Translation: genetic information from mRNA used to synthesize polypeptide chains of specific amino acids

  6. Gene Expression • Central Dogma: 2 step process of transferring genetic information from DNA to RNA, then from RNA to protein

  7. Transcription Overview • DNA sequence copied (transcribed) into the sequence of a single stranded mRNA molecule • THREE PROCESSES:1. Initiation: RNA polymerase binds to DNA at specific site near beginning of gene2. Elongation: RNA polymerase uses DNA as template to build mRNA molecule3. Termination: RNA polymerase passes the end of gene and stops • mRNA then released from template strand • Carried through nuclear pores, into cytoplasm of cell

  8. Transcription – Initiation • Only one strand of the double-stranded DNA is transcribed for each gene- template strand: one that is transcribed- either strand can serve as the template strand for different genes • RNA polymerase binds to segment of DNA)- opens double helix- binds in front of gene to be transcribed

  9. Transcription - Elongation • DNA strand to be transcribed = template • RNA polymerase moves along template strand of DNA and begins building mRNA in 5’ to 3’ direction • Promoter not transcribed • mRNA strand complementary to DNA template strand EXCEPT thymine is replaced with URACIL

  10. Transcription - Termination • RNA polymerase reaches end of gene • Termination sequence: RNA polymerase recognizes it as end • Transcription stops, mRNA disconnects from DNA template strand • RNA polymerase free to bind to another promoter region- transcribe another gene • mRNA free and diffuses out of nucleus into cytoplasm

  11. Review Transcription

  12. Translation Overview • mRNA now in the cytoplasm needs to be translated - puts amino acids together according to genetic information and creates proteins • Ribosomes: site of translation- composed of 2 subunits: clamp together around mRNA, moves along mRNA • THREE PROCESSES:1. Initiation2. Elongation3. Termination

  13. Translation - Initiation • Occurs when ribosome recognizes specific sequence on mRNA – binds to it • Ribosome moves along mRNA in 5’ to 3’ direction- adds amino acids to polypeptide chain once it reads a codon • Must start reading in correct spot on mRNA- START codon (AUG)- ensures ribosome translates code using reading frame of mRNA molecule- results in correct sequence of amino acids • Transfer RNA (tRNA) carries amino acids back to ribosome • structure of tRNA contains an ANTICODON- complementary to codon of mRNA- tells tRNA which amino acid to bind to- amino acid binds to opposite end- tRNA delivers amino acid to ribosome

  14. tRNA Structure

  15. Translation - Elongation • Start codon recognized by ribosome- codes for methionine • 2 sites for tRNA to attach- A (aminoacyl)- P (peptidyl) • tRNA with anticodon complementary to start codon enters P site • Next tRNA carrying required amino acid enters the A site- peptide bond formed between both amino acids • Ribosome shifts over one codon so that SECOND tRNA is now in P site • Released first tRNA from P site and allowed THIRD tRNA to enter empty A site- like a tickertape! • Released tRNA’s recycled back into cytoplasm

  16. Translation - Termination • Ribosome reaches one of THREE STOP CODONS- UGA, UAG, UAA • Do not code for an amino acid, no corresponding tRNA’s • Protein release factor recognizes that ribosome has stalled and helps release polypeptide chain from ribosome • TERMINATION. • Translation • Transcribe & Translate!

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