1 / 59

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.

miriam
Download Presentation

From DNA to Proteins

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  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

More Related