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Chapter 17: From Gene to Protein

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Chapter 17: From Gene to Protein

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  1. Chapter 17: From Gene to Protein

  2. DNA makes RNA makes Protein • Summary of Transcription and Translation • A triplet code in DNA is transcribed into a codon sequence in mRNA • This pre-RNA is processed in the nucleus • The codon sequence is then translated into an amino acid sequence at the ribosome

  3. DNA makes RNA makes Protein • If the strand of DNA triplets to be transcribed is: 5’ – AAA TAA CCG GAC – 3’ • Then the strand of mRNA codons that forms is: 3’ – UUU AUU GGC CUG – 5’ • The tRNAanticodon strand complementary to the mRNA strand is: AAA UAA CCG GAC

  4. Transcription • Transcription is the process by which DNA makes RNA • mRNA: messenger; carries messages directly from DNA to cytoplasm • tRNA: shaped like cloverleaf; carries amino acids to the mRNA at the ribosome • rRNA: ribosomal; makes up ribosome

  5. 3 Stages: initiation, elongation & termination • Initiation • RNA polymerase recognizes and binds to DNA at the promoter region (facilitates transcription of a gene) • A collection of proteins called transcription factors recognize a key area within the promoter called the TATA boxand mediates the binding of RNA polymerase to DNA • Transcription factors + RNA polymerase + promoter = transcription initiation complex

  6. 3 Stages: initiation, elongation & termination • Now that RNA polymerase has attached to the promoter, transcription of the DNA template can begin

  7. 3 Stages: initiation, elongation & termination • Elongation • RNA polymerase adds nucleotides to the 3’ end of a growing chain • RNA polymerase pries the two strands of DNA apart and attaches RNA nucleotides according to the base pairing rules: C with G, A with U • Stretch of DNA transcribed into mRNA molecule is called a transcription unit

  8. 3 Stages: initiation, elongation & termination • Each unit consists of triplets of bases called codons(AAU, CGA), which code for specific amino acids

  9. 3 Stages: initiation, elongation & termination • Termination • Final stage of transcription • Elongation continues for a short distance after the RNA polymerase transcribes the termination sequence (AAUAAA) • mRNA is now cut free from the DNA template

  10. RNA Processing • Pre-RNA that has just formed cannot be shipped to the ribosome until it is processed by a series of enzymes • A 5’ cap consisting of modified guanine nucleotide is added to the 5’ end and helps RNA bind to the ribosome

  11. RNA Processing • A poly (A) tail, consisting of a string of adenine nucleotides, is added to the 3’ strand • Protects RNA from degradation, helps ribosomes attach to RNA, facilitates release of RNA into cytoplasm • Noncoding regions of mRNA called introns or intervening sequences are removed by snRNPsand spliceosomes • Allows exons, or expressed sequences, to only leave the nucleus

  12. Translation • Translation is the process by which the codons of an mRNA sequence are changed into an amino acid sequence • Amino acids are present in a pool in the cytoplasm • They are carried by tRNA molecules to the codons of the mRNA strand at the ribosome according to base pairing rules

  13. Translation • One end of the tRNA molecules holds the specific amino acid; the other end bears a nucleotide triplet called the anticodon • Energy for this process is provided by GTP • AUG is the start codon; UAA, UGA and UAG are the stop codons

  14. Translation • Some tRNA molecules have anticodons that can recognize two or more different codons • This happens because pairing rules for the third base of a codon are not as strict as they are for the first two bases ; this is called wobble • Example: UCU, UCC, UCA and UCG all code for serine

  15. Initiation, Elongation & Termination • Initation • mRNA attaches to a subunit of the ribosome • The first codon is always AUG, which codes for methionine • Elongation • tRNA brings amino acids to the ribosome and a polypeptide chain is formed

  16. Initiation, Elongation & Termination • Termination • The ribosome reaches one of three termination or stop codons • A release factor breaks the bond between the tRNA and the last amino acid of the polypeptide chain • Polypeptide is then freed and mRNA is broken down

  17. Gene Mutation • Mutations are changes in genetic material that occur spontaneously and randomly • Mutagenic agents include toxic chemicals and radiation • Genetic disorders or hereditary diseases occur when mutations have adverse effects in somatic cells

  18. Gene Mutation • Mutations occurring in gametes can be passed on to offspring and change the gene pool of a population • Mutations are the raw material for natural selection

  19. Point Mutation • Simples mutation caused by a base-pair substitution • Example: • THE FAT CAT SAW THE DOG • THE FAT CAT SAW THE HOG • Remember wobble • If the final base in a sequence is different, it may not call for a harmful mutation

  20. Insertion or Deletion • Deletion = loss of a letter • Insertion = addition of a letter • Both result in a frameshift • Example: Deletion • THE FAT CAT SAW THE DOG • THF ATC ATS AWT HED OG

  21. Insertion or Deletion • Example: Insertion • THE FAT CAT SAW THE DOG • THE FTA TCA TSA WTH EDO G • A mutated polypeptide is formed or no polypeptide is formed • When point mutations or frameshifts change a codonwithin a geneinto a stop codon, a missenseor nonsense mutation occurs