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From DNA to Proteins

From DNA to Proteins. Chapter 15. Functions of DNA. Heredity: passing on traits from parents to offspring Replication Coding for our traits by containing the information to make proteins Protein Synthesis Transcription Translation. Genes.

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From DNA to Proteins

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  1. From DNA to Proteins Chapter 15

  2. Functions of DNA • Heredity: passing on traits from parents to offspring • Replication • Coding for our traits by containing the information to make proteins • Protein Synthesis • Transcription • Translation

  3. Genes • Genes are units of DNA that code to make a single polypeptide (protein) • Found within specific location on the chromosomes (loci) • Humans have >30,000 genes • How do we make a protein from the information in a gene?

  4. Steps of Protein synthesis Same two steps produce all proteins: • Transcription: • DNA (Gene) is transcribed to form messenger RNA (mRNA) • Occurs in the nucleus 2) Translation: • mRNA is translated to form polypeptide chains, which fold to form proteins • Occurs in ribosomes which are in the cytoplasm

  5. Transcription and Translation

  6. RNA vs. DNA

  7. Three Classes of RNAs • Messenger RNA (mRNA) • Carries protein-building instruction • Ribosomal RNA (rRNA) • Major component of ribosomes • Transfer RNA (tRNA) • Delivers amino acids to ribosomes

  8. A Nucleotide Subunit of RNA uracil (base) phosphate group sugar (ribose) Figure 14.2Page 228

  9. Transcription • DNA  RNA • Occurs in the nucleus • Requires the enzyme RNA Polymerase • Consists of 3 steps: • Initiation • Elongation • Termination

  10. RNA Polymerases • No primers needed to start complementary copy • RNA is made in the 5´→ 3´ direction • DNA template strand is 3´→ 5´

  11. Steps of Transcription: Initiation • RNA Polymerase binds to Promoter • Promoter: A base sequence in the DNA that signals the start of a gene • DNA is unwound • i.e. hydrogen bonds are broken

  12. Transcription: Initiation

  13. Steps of Transctription: Elongation • RNA ploymerase adds complementary RNA nucleotides to one strand of DNA – Template strand • Forms Pre-mRNA

  14. Transcription: Elongation

  15. Steps of Transcription: Termination • When mRNA synthesis is complete, RNA Polymerase falls off of DNA, RNA is released from DNA, and DNA rewinds

  16. Transcription: Termination

  17. Transcription vs. DNA Replication • Like DNA replication • Nucleotides added in 5’ to 3’ direction • Unlike DNA replication • Only small stretch is template • RNA polymerase catalyzes nucleotide addition • Product is a single strand of RNA

  18. Production of mRNAs in Eukaryotes • Eukaryotic protein-coding genes are transcribed into precursor-mRNAs that are modified in the nucleus • Introns are removed during pre-mRNA processing to produce the translatable mRNA • Introns contribute to protein variability

  19. Messenger RNA • Prokaryotes • Coding region flanked by 5´ and 3´ untranslated regions • Eukaryotes • Coding region flanked by 5´ and 3´ untranslated regions (as in prokaryotes) • Additional noncoding elements

  20. Eukaryotic Pre-mRNA • Precursor-mRNA (pre-mRNA) • Must be processed in nucleus to produce translatable mRNA • 5´ cap • Reversed guanine-containing nucleotide • Site where ribosome attaches to mRNA • Poly(A) tail • 50 to 250 adenine nucleotides added to 3´ end • Protects mRNA from RNA-digesting enzymes

  21. Eukaryotic Pre-mRNA • Introns • Non-protein-coding sequences in the pre-mRNA • Must be removed before translation • Exons • Amino acid coding sequences in pre-mRNA • Joined together sequentially in final mRNA

  22. RNA Processing

  23. mRNA Splicing • Introns in pre-mRNAs removed • Spliceosome • Pre-mRNA • Small ribonucleoprotein particles (snRNP) • Small nuclear RNA (snRNA) + several proteins • Bind to introns • Loop introns out of the pre-mRNA, • Clip the intron at each exon boundary • Join adjacent exons together

  24. mRNA Splicing

  25. Why are Introns Present? • Alternative splicing • Different versions of mRNA can be produced • Exon shuffling • Generates new proteins

  26. Alternative Splicing • Exons joined in different combinations to produce different mRNAs from the same gene • Different mRNA versions translated into different proteins with different functions • More information can be stored in the DNA

  27. Alternative mRNA Splicing • α-tropomyosin in smooth and striated muscle

  28. The next step: Translation • “Translating” from nucleic acid (DNA/RNA) “language” (nucleotides) to protein “language” (amino acids) • Occurs in the ribosome within the cytoplasm • Requires tRNA – transfer RNA • How does the mRNA (and DNA) code for proteins? The Genetic Code

  29. Genetic Code • Information • 4 nucleotide bases in DNA or RNA sequences • DNA: A,T,G,C RNA: A,U,G,C • 20 different amino acids in polypeptides • Code • One-letter words: only 4 combinations • Two-letter words: only 16 combinations • Three-letter words: 64 combinations

  30. Genetic Code • DNA • Three-letter code: triplet • RNA • Three-letter code: codon

  31. Genetic Code

  32. Features of the Genetic Code • Sense codons • 61 codons specify amino acids • Most amino acids specified by several codons (degeneracy or redundancy) • Ex: CCU, CCC, CCA, CCG all specify proline • Start codon or initiator codon • First amino acid recognized during translation • Specifies amino acid methionine

  33. Features of the Genetic Code • Stop codons or termination codons • End of a polypeptide-encoding mRNA sequence • UAA, UAG, UGA • Commaless • Nucleic acid codes are sequential • No commas or spaces between codons • Start codon AUG establishes the reading frame

  34. The Genetic Code

  35. Genetic Code is Universal • Same codons specify the same amino acids in all living organisms and viruses • Only a few minor exceptions • Genetic code was established very early in the evolution of life and has remained unchanged

  36. Translation Overview

  37. Translation Purpose • To “translate” from nucleic acid “language” to protein “language” • RNAprotein What is needed for translation? • mRNA transcript (processed) • tRNAs • Ribosomes

  38. tRNAs • Transfer RNAs (tRNA) • Bring specific amino acids to ribosome • Cloverleaf shape • Bottom end of tRNA contains anticodon sequence that pairs with codon in mRNAs

  39. tRNA Structure

  40. Ribosomes • Made of ribosomal RNA (rRNA) and proteins • Two subunits: large and small

  41. Translation Stages • Initiation • Ribosome assembled with mRNA molecule and initiator methionine-tRNA • Elongation • Amino acids linked to tRNAs added one at a time to growing polypeptide chain • Termination • New polypeptide released from ribosome • Ribosomal subunits separate from mRNA

  42. Initiation • Initiator tRNA (Met-tRNA) binds to small subunit

  43. Initiation • Complex binds to 5´ cap of mRNA, scans along mRNA to find AUG start codon

  44. Initiation • Large ribosomal subunit binds to complete initiation

  45. Elongation • tRNA matching the next codon enters A site carrying its amino acid • A peptide bond forms between the first and second amino acids, which breaks the bond between the first amino acid and its tRNA • Ribosome moves along mRNA to next codon • Empty tRNA moves from P site to E site, then released • Newly formed peptidyl-tRNA moves from A site to P site • A site empty again

  46. Elongation

  47. Termination • Begins when A site reaches stop codon • Release factor (RF) or termination factor binds to A site • Polypeptide chain released from P site • Remaining parts of complex separated

  48. Termination

  49. What Happens to the New Polypeptides? • Some just enter the cytoplasm • Many enter the endoplasmic reticulum and move through the cytomembrane system where they are modified

  50. Transcription Gene ExpressionSummary: rRNA tRNA mRNA Mature mRNA transcripts ribosomal subunits mature tRNA Translation

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