Transcription and Translation - PowerPoint

							Transcription and Translation

          Chapter 16
                  Objectives
• Understand the process of transcription
• Recognize the role of RNA Polymerase
• Recognize the significance of promoter and terminator
  regions of DNA
• Explain how transcribed RNA is modified prior to
  exiting the nucleus. Understand the significance of
  this process
• Understand the process of translation
• Recognize the role of mRNA, rRNA, and tRNA
• Understand how protein may be modified prior to use
• Be familiar with the molecular basis of mutation
        Transcription in Eukaryotes
• Consists of three stages
   – Initiation: attachment of
     RNA Polymerase to the
     promotor region on DNA
   – Elongation: building of the
     mRNA from the 3’ end of
     the nucleotide polymer
   – Termination: release of
     RNA polymerase and
     mRNA following
     transcription of the
     terminator region of the
     DNA
                          Initiation
• Genes on the DNA begin with a
  promoter region consisting of a
  sequence of A & T (TATA box) and
  the first nucleotide involved in the
  peptide sequence
• Basal Transcription Factors
  (proteins that assist the binding of
  RNA polymerase to the promoter) are
  found in association with the promoter
  region
• Transcription initiation complex:
  transcription factors & RNA
  polymerase bound to the promoter
  region of the DNA
                   Elongation
• Once initiation is
  complete the 2 strands of
  the DNA unwind due to
  the zipper region of the
  enzyme
• RNA polymerase builds a
  mRNA strand
  complimentary to the
  DNA transcription unit
  (60 bases/sec)
• Once the RNA
  Polymerase passes the
  DNA strands reform their
  double helix
Termination
      • When the RNA
        Polymerase
        transcribes the
        terminator region of
        the DNA, the
        Polymerase releases
        the mRNA
      • The transcribed
        termination sequence
        on the mRNA is
        AAUAAA
 Modification of mRNA Transcripts



• Transcribed mRNA (pre-mRNA) must be
  modified before leaving the nucleus
• modifications include:
  – addition of 5’cap
     • Prevents “unraveling”
  – addition of poly A tail
     • Prevents “unraveling”
     • Helps ribosome attach
     • Assists in the export of mRNA from nucleus
   Further Transcript Modifications




• Transcribed RNA is “too long” and is shortened in
  the nucleus through RNA splicing
• Exons are segments of the mRNA that contain
  information that will be reflected in the polypeptide
• Introns are segments of the mRNA that separate
  (intervene) exons
How is this done?
         • Small nuclear
           ribonucleicproteins
           (snRNP) recognize
           intron ends and together
           with proteins form a
           structure called a
           spliceosome
         • Spliceosomes remove
           introns while connecting
           exons together
         • Ribozymes may also
           catylyze this process in
           some organisms (introns
           may act as ribozymes)
     Why bother with introns?
• Introns may regulate gene activity and the
  passage of mRNA into the cytoplasm
• Genes may play roles in multiple proteins,
  introns may enable a gene to be diverse in
  function
• May increase recombination of genetic
  material (easier to cut and paste)
           Process of Translation
• Flow of genetic
  information from mRNA
  to protein
• Change in cellular
  language from nucleotide
  to protein
• tRNA acts as the
  interpreter
• There is a different tRNA
  for “each kind of codon”
  except the stop codons
    How does Transfer RNA work?
• tRNA has a 3-D
  structure that includes
  the formation of three
  loops within the
  molecule
• 3-D structure defines 2
  different working ends    Wobble: the third base in the
   – 3’ end attaches to a   anticodon may be free to bind in a
     specific AA            non-complimentary fashion to the
   – anticodon (2nd loop)   third base of a codon (U to A or G
     that compliments the   & I (inosine) to U,A,C)
     codon on mRNA
 How are AA attached to tRNA?

• Before a peptide can
  be assembled tRNA
  must bind to a specific
  AA
• This process involves
  the enzyme
  aminoacyl-tRNA
  synthetase and ATP
           Ribosome Anatomy




• Ribosomes: structures composed of rRNA (2 subunits)
  and protein
• Aligns tRNA anticodons with mRNA codons
• Stage for synthesis of peptides
              Pieces and Parts
• Each ribosome has:
  – a binding site for
    mRNA
  – three binding sites each
    for a molecule of
    tRNA
     • E: Exit site
     • P: holds tRNA with
       growing peptide chain
     • A: holds tRNA
       containing the next AA
       to be added to the
       polypeptide
                         The Process




• Initiation: Binding of the small and large ribosomal subunits to
  mRNA and the first tRNA
• Initiation factors and energy (GTP) involved in the formation of
  initiation complex
   – Small subunit attaches to mRNA near the 5’cap
   – tRNA carrying the AA Methionine attaches to the start codon (P site)
   – Large subunit attaches
                 Process Continued




Elongation: lengthening of polypeptide
   – Codon recognition: Site A codon forms bonds with the anticodon of
     tRNA; requires elongation factors and energy
   – Peptide bond formation: large subunit rRNA catalyzes the formation of
     a peptide bond between the AA of tRNA’s at P & A sites. AA at P site
     transferred to A site
   – Translocation: rRNA shifts position by one codon with respect to
     mRNA. The H bonded mRNA-tRNA unit is moved from the A site to
     the P site. Any tRNA occupying P site is moved to the E site where it is
     released. Translocation requires energy. mRNA is read from 5’ to 3’.
               Process Continued




• Termination: stoppage of synthesis and peptide release
   – Elongation continues until the a stop codon is read by the ribosome
   – Release factor protein binds to the stop codon
   – Release factor adds H2O to the protein causing it to be released
     from tRNA
   – Translation assembly falls apart
          Oh, One last thing
• Most synthesized proteins must undergo
  further modification before use including
  – attachment of sugar, lipids, functional groups
  – leading end of protein may have some AA
    removed (recall that all protein sequencing
    starts with AA methionine but not all finished
    protein do)
  – Polypeptide chain may be divided up into
    smaller units
  – Protein may require several polypeptide chains
     Mutations:
 Changes in the
 genetic material of
 a cell

• Point mutations:chemical changes in just a single or a
  few base pairs in a gene
   – Base-pair substitutions: replacement of one nucleotide with
     another
      • Silent
      • Missense
      • Nonsense
   – Insertion/Deletion: change in the number of nucleotide pairs
      • Frame shift
     Changes in Gene Sequence*




• Deletion: loss of a chromosomal segment
• Duplication: addition of a chromosomal segment
• Inversion: flipping of information on a
  chromosome                          *discussed in chapter 12