27 28 105 fa13 transcription and translation skel

3,930 views
3,448 views

Published on

Published in: Technology, Health & Medicine
0 Comments
6 Likes
Statistics
Notes
  • Be the first to comment

No Downloads
Views
Total views
3,930
On SlideShare
0
From Embeds
0
Number of Embeds
2
Actions
Shares
0
Downloads
411
Comments
0
Likes
6
Embeds 0
No embeds

No notes for slide
  • Don’t memorize stuff on the right side
  • (Trna)AAG----(MRNA)-uuc and uuu
  • A-AMINOACIL tRNA site
    P-PEPtidyl TRNA site
    E-exit
  • MISSING SLIDE FILL IN = POLYPLOIDY, ANEURPLOIDY
  • 27 28 105 fa13 transcription and translation skel

    1. 1. Transcription and Translation BIOL 105 Dr. Corl October 25, 2013
    2. 2. The Central Dogma • DNA codes for RNA, which codes for protein.
    3. 3. The Central Dogma • Transcription is a process in which: – The sequence of bases in a particular stretch of DNA (a gene) specifies the sequence of bases in an mRNA molecule. • Translation is a process in which: – A particular mRNA molecule then specifies the exact sequence of amino acids in a protein. • Thus, genes ultimately code for proteins.
    4. 4. Transcription in Bacteria • In transcription: – Instructions stored in DNA are “transcribed” through the synthesis of an mRNA transcript. • Transcription performed by RNA polymerase. • Three phases of transcription: – Initiation, elongation, and termination.
    5. 5. Transcription in Bacteria • mRNA transcript is synthesized by RNA polymerase in the 5’ to 3’ direction. • Template strand: – DNA strand that is read (transcribed) by RNA polymerase. • Non-template strand: – DNA strand that is not read by RNA polymerase.
    6. 6. Transcription in Bacteria • “Activated” complementary ribonucleotide monomers are added on via condensation reactions, resulting in phosphodiester bonds.
    7. 7. Bacterial RNA Polymerase • A holoenzyme: – An enzyme made up of a core enzyme and other required proteins. – Bacterial RNA polymerase is made up of a core enzyme and a regulatory subunit, sigma.
    8. 8. Bacterial RNA Polymerase • Core enzyme: – Contains the active site where mRNA is synthesized. • Sigma: – A regulatory factor required for initiation of transcription. – Tells core enzyme where in a DNA sequence to start transcription.
    9. 9. Transcription: Initiation • Transcription initiation begins at specific sequences of DNA called promoters. • Two important bacterial promoter regions : – Named the -10 box and the -35 box. – Sigma recognizes these promoter regions and brings the RNA polymerase core enzyme to the promoter to initiate transcription.
    10. 10. Transcription: Initiation Core enzyme • 1.) sigma binds to specific promoter regions of DNA (-35 box and -10 box).
    11. 11. Transcription: Initiation • 2.) DNA double helix is opened and the template strand of DNA is threaded through RNA polymerase core enzyme active site. mRNA synthesis begins.
    12. 12. Transcription: Initiation • 3.) Initiation is complete. Sigma dissociates from core enzyme. mRNA synthesis (transcription) continues.
    13. 13. Transcription: Elongation • Core enzymes of RNA polymerase moves along the DNA template and continues to catalyze the addition of complementary ribonucleotides to the growing mRNA transcript.
    14. 14. Transcription: Termination • RNA polymerase encounters a termination signal within the DNA template, which codes for RNA forming a hairpin loop structure. • Hairpin causes RNA polymerase to separate from RNA transcript, ending transcription.
    15. 15. Bacterial Transcription: Summary • Initiation: – Sigma brings RNA polymerase holoenzyme to promoter region of DNA. – DNA helix is opened and transcription begins. – Sigma releases and transcription continues. • Elongation: – Complementary ribonucleotides are added to the growing mRNA transcript as specified by the DNA template strand. • Termination: – RNA polymerase reaches a termination signal in the DNA template. – mRNA forms a hairpin loop. – mRNA dissociates from RNA polymerase.
    16. 16. Transcription in Eukaryotes • Overall similar to bacterial transcription. • Some differences include: – Basal transcription factors: • Proteins that bind DNA promoters independant of RNA polymerase. • RNA polymerase then binds to basal transcription factors and transcription begins. – Greater diversity and complexity of promoters: • Many promoters include a TATA box sequence.
    17. 17. Transcription in Eukaryotes • Three types of RNA polymerase: – RNA polymerase II: • Catalyzes transcription of genes that code for proteins, forming mRNA. – RNA polymerase I and RNA polymerase III: • Catalyze transcription of non-protein coding genes (e.g. genes coding for ribosomal rRNAs and genes coding for transfer tRNAs).
    18. 18. Eukaryotic mRNA Processing Exon 1 Intron 1 Intron 2 Exon 2 Exon 3 • After transcription but before translation, specific regions of the primary RNA transcript are spliced out (cut out) and degraded during RNA processing. • The term intron can be used in two ways: – A stretch of RNA that does get spliced out and will NOT be a part of the final mature spliced RNA transcript. – A stretch of DNA that codes for an RNA intron.
    19. 19. Eukaryotic mRNA Processing Exon 1 Intron 1 Intron 2 Exon 2 Exon 3 • After transcription but before translation, specific regions of the primary RNA transcript are spliced out (cut out) and degraded during RNA processing. • The term exon can be used in two ways: – A stretch of RNA that does NOT get spliced out and will be a part of the final mature spliced RNA transcript. – A stretch of DNA that codes for an RNA exon.
    20. 20. Eukaryotic mRNA Processing -Intron regions of the primary mRNA transcript are spliced out by small nuclear ribonucleoproteins (snRNPs), which assemble to form a spliceosome (a ribozyme).
    21. 21. Eukaryotic mRNA Processing • Intron regions of the primary mRNA transcript are spliced out by snRNPs, which assemble to form a spliceosome. (occurs within nucleus)
    22. 22. Eukaryotic mRNA Processing • Additionally, a 5’ cap is added to the very 5’ end of the mRNA transcript: – 5’ cap serves as recognition signal for the translation machinery of the cell.
    23. 23. Eukaryotic mRNA Processing • Finally, a poly (A) tail is added to the very 3’ end of the mRNA transcript: – Extends the life of the mRNA by protecting it from enzymatic degradation.
    24. 24. Transcription: Summary • Note the similarities and differences between bacterial vs. eukaryotic transcription! • Eukaryotic transcription tends to be more complex.
    25. 25. Translation • In translation: – The sequence of bases in mRNA is converted (“translated”) to an amino acid sequence of a protein. • Ribosomes catalyze the translation of mRNA sequence into protein.
    26. 26. Translation in Bacteria • In bacteria, transcription and translation can occur simultaneously: – Ribosomes begin translating an mRNA even before RNA polymerase has finished synthesizing it!
    27. 27. Translation in Bacteria • In bacteria, transcription and translation can occur simultaneously: – Ribosomes begin translating an mRNA even before RNA polymerase has finished synthesizing it!
    28. 28. Translation in Eukaryotes • Transcription and translation are separated: – Transcription takes place in the nucleus. – Translation takes place in the cytoplasm.
    29. 29. Translation • How do mRNA codons interact with amino acids?
    30. 30. Translation • Adapter molecules: – Small RNAs called transfer RNA (tRNA) – Hold amino acids in place and interact directly and specifically with a codon in mRNA.
    31. 31. Transfer RNA (tRNA) • Different tRNAs covalently link to specific amino acids, forming aminoacyl tRNAs. • Aminoacyl tRNA synthesizes: – Enzymes which catalyze the addition of amino acids to tRNAs.
    32. 32. Loading Amino Acid Onto tRNA Step 1 Step 3 Step 2 Step 4 • Amino acid binds to its specific aminoacyl tRNA sythnetase, which then covelantly attaches the amino acid onto a specific tRNA.
    33. 33. Loading Amino Acid Onto tRNA Step 1 Step 3 Step 2 Step 4 • Is ATP expended in this process? Yes! • Aminoacyl tRNA = “CHARGED” tRNA
    34. 34. Aminoacyl tRNA • Aminoacyl tRNAs then travel to ribosomes and transfer amino acids to growing polypeptides. • Each tRNA carries a specific amino acid that can be transferred to protein.
    35. 35. Transfer RNA Structure • tRNA secondary structure resembles a “cloverleaf.”
    36. 36. tRNA Secondary Structure • 3’ end of tRNA: binding site for amino acids. • Triplet loop at opposite end (anticodon): – Interacts with complementary codon on mRNA.
    37. 37. tRNA Tertiary Structure • tRNA has an “L-shaped” tertiary structure. • Each tRNA has a distinct anticodon and attached amino acid.
    38. 38. Wobble Hypothesis • There are 61 different codons but only 40 different tRNAs. • Explained by “wobble hypothesis”: – The anticodon of certain tRNAs can bind successfully to a codon whose third position requires nonstandard base pairing. – Allows one tRNA to be able to base pair with more than one type of codon.
    39. 39. Ribosomes • Composed of both ribosomal protein and ribosomal RNA (rRNA). • Can be separated into two subunits: – Large subunit and small subunit. • There are three sites within a ribosome that tRNAs can reside: – The A site, the P site, and the E site.
    40. 40. Ribosome: Structure Large subunit E P A Small subunit • Large subunit and small subunit. • Three sites where tRNAs can reside: A, P and E.
    41. 41. Ribosome: Structure and Function • Each tRNA binds at its anticodon to its corresponding mRNA codon and transfers its amino acid to a growing polypeptide chain.
    42. 42. Phases of Protein Synthesis • initiation: – Ribosomal subunits and mRNA assemble. – Translation begins at the AUG start codon. • elongation: – Amino acids are transferred one by one from aminoacyl tRNAs to a growing polypeptide chain. • termination: – Stop codon in mRNA causes protein to be released from ribosome. – Ribosomal subunits separate from mRNA and from one another.
    43. 43. Initiation: Step 1 • mRNA is targeted to the ribosome: – Ribosome binding site on mRNA binds to complementary sequence on small subunit of ribosome, with the help of proteins called initiation factors.
    44. 44. Initiation: Step 2 • Translation begins at the AUG start codon: – Initiator aminoacyl tRNA, bringing in the first amino acid, binds to the mRNA’s start codon, AUG.
    45. 45. Initiation: Step 3 • LARGE subunit of ribosome binds, placing initiator aminoacyl tRNA in the P-site.
    46. 46. Elongation: Step 1 • Incoming aminoacyl tRNA binds to the codon in theA site via complementary base pairing between anticodon and codon.
    47. 47. Elongation: Step 2 • The amino acid in the P site: – Breaks the bond with its tRNA. – Forms a peptide bond with the amino acid in the A site.
    48. 48. Elongation: Step 3 • Translocation: – Ribosome moves down to the next codon on mRNA. – Results in tRNAs being shifted over one spot: • tRNA in P site moves to E site. • tRNA in A site moves to P site.
    49. 49. Another Elongation Cycle • 1.) Incoming aminoacyl tRNA enters into the __ site. • 2.) peptide bond formation. – rRNA acts as a catalyst: ribozyme • 3.) Translocation.
    50. 50. Polyribosome • multiple ribosomes assembled along a single mRNA, synthesizing proteins from the same mRNA at the same time.
    51. 51. Termination: Step 1 • Occurs when a stop codon is revealed in the A site. • A protein called a release factor enters the A site and promotes the hydrolysis of the bond linking the tRNA in the P-site with its polypeptide.
    52. 52. Termination: Steps 2 and 3 • Hydrolysis reaction (cleavage of bond between tRNA and polypeptide) frees the polypeptide • Ribosomal subunits, mRNA, tRNAs, and polypeptide all _________ from one another.
    53. 53. Post-Translational Modifications • Many proteins are modified after translation is complete: – Many proteins fold with the help of molecular chaperones. – Proteins can be modified by attachment of sugars (glycosylation) in the rough ER and/or Golgi. – Proteins can be phosphorylated or dephosphorylated, modifying their activity.
    54. 54. Mutations • Mutation: – Any permanent change in an organism’s DNA. – Changes the genotype (DNA) of the cell. • This may then result in a change in mRNA transcript sequence, resulting in: – Change in sequence of amino acids of the translated protein.
    55. 55. Point Mutations • A single base pair change, often as a result of errors in DNA replication. • Can affect the primary structure of the polypeptide that is ultimately translated.
    56. 56. Point Mutations • missense mutation: – Point mutation in a gene’s DNA sequence that ultimately results in a single amino acid change in the protein encoded by that gene. – Can often be deleterious: • Reduces the individual’s fitness.
    57. 57. Point Mutations • silent mutation: – Does not change amino acid sequence. • nonsense mutation: – Results in an early stop codon: shortened protein. • frameshift mutation: – Addition or deletion of a nucleotide causes entire reading frame to be shifted.
    58. 58. Chromosome-Level Mutations • Chromosome INVERSION and translocation: – Can result in altered patterns of gene expression.
    59. 59. Summary • Transcription: – Bacterial transcription – Eukaryotic transcription • Translation: – Ribosomes – tRNA and aminoacyl tRNA – Ribosomal protein synthesis • Mutations: – Point mutations – Chromosomal mutations
    60. 60. Review Questions on Transcription • Contrast the functions of sigma versus the core enzyme of bacterial RNA polymerase. • What events occur during transcription initiation? Elongation? Termination? • How does eukaryotic transcription differ from bacterial transcription? • What types of post-transcriptional processing occur in eukaryotes? What is splicing?
    61. 61. Review Questions on Translation • What is the function of tRNA? mRNA? rRNA? • What events occur during the three major phases of translation? • What are the A, P, and E sites of a ribosome? • What are the differences between silent, missense, nonsense, and frameshift mutations?

    ×