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Protein Synthesis - Transcription and Translation Lecture Powerpoint

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Protein Synthesis - Transcription and Translation Lecture Powerpoint Powered By Docstoc
					From Gene to
   Protein
             The Bread Mold
               Experiment
• Two scientists named Beadle and Tatum
  exposed bread mold to X-rays, creating
  mutants that were unable to survive.
  o The mold was not able to produce certain molecules it
    needed to live.
  o A specific enzyme that produced these molecules was
    missing in the mold’s cells.
• Their hypothesis: “one-gene, one-enzyme”.
  o This means that each gene in the mold’s DNA codes for a
    different enzyme needed for life.
    One Gene One Protein
• However, some proteins aren’t enzymes.
  o Researchers later revised the hypothesis: one
    gene–one protein
• However, many proteins are composed of
  several polypeptides, each of which has its
  own gene
• Therefore, Beadle and Tatum’s hypothesis is
  now restated as one gene–one polypeptide
        Transcription and
           Translation
• Transcription is the production of a molecule
  of RNA based on a segment of DNA in the
  nucleus.
  o RNA is needed because it can leave the nucleus,
    while DNA cannot.
  o This molecule of RNA is called messenger RNA
    (mRNA)
• The overall cellular chain of command is this:




      DNA RNA protein
LE 17-3-1




                            DNA
            TRANSCRIPTION




  Prokaryotic cell
LE 17-3-2




                                       DNA
            TRANSCRIPTION

                                       mRNA
                                     Ribosome
            Prokaryotic cell

                               Polypeptide



  Prokaryotic cell
LE 17-3-3


                                          DNA
                 TRANSCRIPTION

                                          mRNA
                                         Ribosome
                  TRANSLATION

                                 Polypeptide



            Prokaryotic cell



                                        Nuclear
                                        envelope




                                  DNA
                TRANSCRIPTION




            Eukaryotic cell
LE 17-3-4


                                               DNA
                 TRANSCRIPTION

                                               mRNA
                                              Ribosome
                  TRANSLATION

                                      Polypeptide



            Prokaryotic cell



                                             Nuclear
                                             envelope




                                       DNA
                TRANSCRIPTION


                                       Pre-mRNA
                RNA PROCESSING


                               mRNA




            Eukaryotic cell
LE 17-3-5


                                                DNA
                 TRANSCRIPTION

                                                mRNA
                                              Ribosome
                  TRANSLATION

                                      Polypeptide



            Prokaryotic cell



                                             Nuclear
                                             envelope




                                       DNA
                TRANSCRIPTION


                                       Pre-mRNA
                RNA PROCESSING


                               mRNA




                                        Ribosome
             TRANSLATION

                                  Polypeptide




            Eukaryotic cell
        The Genetic Code
• Proteins are made of amino acids.
• There are 20 amino acids, but there are only
  four nucleotide bases in DNA
• This is possible because the four DNA bases (A,
  T, C, G) can be arranged in total of 64
  different ways.
  Codons: Triplets of Bases
• The flow of information from gene to protein is
  based on a triplet code: a series of three-
  nucleotide “words”.
• These triplets are the smallest units of uniform
  length that can code for all the amino acids
  o Example: The triplet A-G-T on a DNA strand
    encodes for the amino acid “serine” to be added
    to the polypeptide chain being formed.
LE 17-4

                                         Gene 2
          DNA
          molecule
                            Gene 1


                                                  Gene 3




          DNA strand   3                                  5
          (template)



          TRANSCRIPTION




          mRNA         5                                  3

                             Codon
           TRANSLATION


          Protein


                            Amino acid
          The DNA Code
• All 64 codons were deciphered by the mid-
  1960s
• Each codon only translates into one type of
  amino acid.
• Each amino acid has more than one codon
  that may produce it.
• Codons must be read in the correct sequence
  in order for the specified polypeptide to be
  produced
                                              LE 17-5
                           Second mRNA base




Third mRNA base (3 end)
  Evolution of the Genetic
           Code
• The existence of DNA is evidence of the
  theory of evolution.
• The genetic code is nearly universal, shared
  by the simplest bacteria to the most complex
  animals.
• Genes can be transcribed and translated
  after being transplanted from one species to
  another.
               Translation
• Translation is the synthesis of a polypeptide,
  which occurs under the direction of mRNA
• Ribosomes are the sites of translation
• A cell translates an mRNA message into
  protein with the help of transfer RNA (tRNA)
• Molecules of tRNA are not identical:
  o Each carries a specific amino acid on one end
  o Each has an anticodon on the other end; the
    anticodon base-pairs with a complementary
    codon on mRNA
LE 17-13




                                                   Amino
                Polypeptide                        acids



                                                  tRNA with
                                                  amino acid
                                                  attached
                                       Ribosome




                                                  tRNA

                                                  Anticodon




           5                 Codons                  3

                mRNA
              Ribosomes
• Ribosomes facilitate specific coupling of tRNA
  anticodons with mRNA codons in protein
  synthesis
• Ribosomes are made of proteins and
  ribosomal RNA (rRNA)
                  Mutations
• Mutations are changes in the genetic material
  of a cell or virus
• Point mutations are chemical changes in just
  one base pair of a gene
• The change of a single nucleotide in a DNA
  template strand leads to production of an
  abnormal protein.
  o Example: “The cat ate the rat” becomes “The cat are the rat.”
• A famous example is Sickle-cell anemia, which
  is caused by a mutation of a single base pair
  within a gene.
     LE 17-23




       Wild-type hemoglobin DNA              Mutant hemoglobin DNA
3                                5   3                             5




      mRNA                                  mRNA


5                                3   5                             3



           Normal hemoglobin                 Sickle-cell hemoglobin
            Substitutions
• A base-pair substitution replaces one
  nucleotide and its partner with another pair of
  nucleotides
• Missense mutations still code for an amino
  acid, but not necessarily the right amino acid
• Nonsense mutations change an amino acid
  codon into a stop codon, nearly always
  leading to a nonfunctional protein
• Missense mutations are more common
LE 17-24
           Wild type


           mRNA
                 5                                             3
           Protein
                                                         Stop
           Amino end                               Carboxyl end
           Base-pair substitution

            No effect on amino acid sequence
                                               U instead of C




                                                         Stop

            Missense
                                      A instead of G




                                                         Stop

            Nonsense
                    U instead of A




                              Stop
   Insertions and Deletions
• Insertions and deletions are additions or losses
  of nucleotide pairs in a gene
• These mutations have a disastrous effect on
  the resulting protein, because the entire
  sequence of amino acids is altered.
• The result is called a frameshift mutation,
  because it shifts the entire way that the
  ribosome translates the mRNA.
Example: “The cat ate the rat.” becomes
         “The cta tet her at.” by deleting an “a” or
         “The caa tat eth era t.” by inserting an “a”
LE 17-25
            Wild type


           mRNA 5                                         3
           Protein
                                                      Stop
            Amino end                            Carboxyl end

            Base-pair insertion or deletion
             Frameshift causing immediate nonsense
                          Extra U




                              Stop
             Frameshift causing
             extensive missense               Missing




             Insertion or deletion of 3 nucleotides:
             no frameshift but extra or missing amino acid

                                Missing




                                                Stop
                  Mutagens
• Spontaneous mutations can occur during
  DNA replication, recombination, or repair.
• Mutagens are physical or chemical agents
  that can increase the likeliness of mutations.
  o Ultraviolet radiation
  o Nuclear radiation

				
DOCUMENT INFO
Description: A Powerpoint lecture covering a unit on protein synthesis. Written for an introduction to biology course.