From Gene to Protein 2

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From Gene to Protein(2)

• In all living cells, DNA molecules are the
  storehouses of information

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Outline

• _______________________________________
• 6. Synthesis of protein
• 7. Mutations and gene
• 8. Key terms
• 9. Conclusions

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The Synthesis of Protein

• The key to translate a genetic message into a
  specific amino acid sequence is that each type of
  tRNA molecule links a particular mRNA codon with
  a particular amino acid.

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The Synthesis of Protein

• 1. tRNA
• 2. Aminoacyl-tRNA synthetases
• 3. Ribosomes
• 4. The synthesis of a polypeptide chain
• initiation, elongation, termination
• 5. Polyribosomes
• 6. From polypeptide to functional protein

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The Synthesis of Protein

• 1. tRNA
• All types of RNA, including tRNA, are
  transcribed from template DNA
• In Eukaryotes, each tRNA can be used repeatedly

tRNA is a single-stranded RNA only about 80
nucleotides. 45 distinct types of tRNA, some
tRNAs recognize two or more mRNA codons
specifying the same AAs. The relaxation of
base-paring rules called Wobble (e.g. U-A ,
U-G/CCI to GGU, GGC, and GGA)
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The Synthesis of Protein

• 2.Aminoacyl-tRNA synthetases
• (enzymes that catalyzes the attachment of an AA
  to its tRNA)
• Each of the 20 AAs has a specific aminoacyl-tRNA
  synthetase
• Attachment of the AA to tRNA
• The aminoacyl-tRNA complex release from the
  enzyme and transfers its AA to a growing
  polypeptide chain

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The Synthesis of Protein

• 3. Ribosomes
• Ribosomes coordinate the pairing of tRNA
  anticodons to mRNA codons
• Two subunits (small and large)
• 60 rRNA and 40 protein
• Both subunits are constructed in the nucleolus
  once in the cytoplasm, are assembled into
  functional ribosomes when attached to an mRNA

Each ribosome has three tRNA binding
sites The P site holds the tRNA carrying the
growing polypeptide chain the A site holds the
next tRNA the E site for exiting
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The Synthesis of Protein

• 4. The synthesis of a polypeptide chain
  (Initiation)
• Initiation brings together mRNA, a tRNA with the
  first AA, and two ribosomal subunits
• a. binding of the small subunit to mRNA and
  initiator tRNA
• b. the large subunit binds to the small one to
  form a functional translation complex
• initiator tRNA fits into the P site
• the vacant A site is ready for next

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The Synthesis of Protein

• 4. The synthesis of a polypeptide chain
  (Elongation)
• Several proteins called elongation factors take
  part in this three-step cycle which adds AAs one
  by one to the initial AA
• a. Codon recognition (form hydrogen bonds)
• b. Peptide bond formation
• An rRNA molecule of the large subunit (as
  ribozyme) catalyze the formation
• The polypeptide separates from its tRNA and is
  trasferred to the new AA in the A site
• c. Traslocation
• The tRNA in the A site is translocated to P
  site, and the one from P to E
• The mRNA is moved through the ribosome only in
  the 5 to 3 direction
• Each elongation cycle takes less than a tenth
  of a second

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The synthesis of a polypeptide chain (Elongation)
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• Ribosomes facilitate the specific coupling of the
  tRNA anticodons with mRNA codons.
• Each ribosome has a large and a small subunit.
• These are composed of proteins and ribosomal RNA
  (rRNA), the most abundant RNA in the cell.

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The Synthesis of Protein

• 4. The synthesis of a polypeptide chain
  (Termination) When a stop codon reaches the A
  site, a protein release factor binds to the codon
  and
• a. Release factor causes the addition of water
  molecule instead of an AA to the polypeptide
  chain
• b. Release factor hydrolyzes the bond between
  the polypeptide and the tRNA in the P site
• c. The polypeptide and tRNA are released
• d. The remainder of the translation complex
  dissociates

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The Synthesis of Protein
5. Polyribosomes A cluster of ribosomes
simultaneously translating an mRNA
molecule Polyribosomes are found in both
prokaryotes and eukaryotes
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The Synthesis of Protein

• 6. From polypeptide to functional protein
• The biological activity of proteins depends on a
  precise folding of the polypeptide chain into 3-D
  conformation
• Some proteins must undergo post-translational
  modification before they become fully functional

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Roles of RNA

• Information carrier mRNA
• Adaptor molecule tRNA
• Catalyst and structural molecule rRNA
• Viral genomes Some viruses use RNA as their
  genetic material

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Mutations and gene

• Point Mutation a mutation limited to about one
  or a few base pairs in a single gene. (two types
  1. Substitutions 2. Insertions or deletion)
• Substitutions
• a. silent substitution
• b. conservative substitution
• c. significant change in protein (rare occasion
  ? enhence success of its descendants)
• d. missense and nonsense
• Insertions and deletions
• Frameshift mutation produces
• nonfunctional protein

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Mutations and gene
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Mutations and gene

• Mutagens
• Mutagenesis the creation of mutations ( may
  caused by natural reason or by exposure to
  mutagens).
• Mutagen physical or chemical agents that
  interact with genetic material to cause mutation
  (radiation is the most common physical mutagen
  many categories of chemical mutagens).
• The Ames test, developed by Bruce Ames, is one of
  the most widely used tests for measuring the
  mutagenic strength of various chemicals.

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Mutations and gene

• What is a gene?
• In molecular terms, a gene is a specific
  sequence of nucleotides at a given location in
  the genome of an organism. Depending on the gene,
  the final gene product may be RNA or a specific
  polypeptide.
• Genome the entire collection of chromosomes
  in each cell of an organism

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Key Terms
Point mutation Base-pair substitution Missense
mutation Nonsense mutation Insertion Deletion Fram
eshift mutation P site A site E
site Triplet Synthetase terminator
Transcription Translation Messenger RNA Ribosomal
RNA Transfer RNA Template strand Codon Anticodon I
ntron Exon Mutation
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In Conclusion

• 1. Cells cannot stay alive without enzymes and
  other proteins
• 2. The amino acid sequence of a polypeptide chain
  corresponds to a gene region in a double-stranded
  DNA molecule
• 3. The path from genes to proteins is
• DNA ---gt RNA ---gt Protein
• 4. In transcription, double-stranded DNA is
  unwound at a gene region

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In Conclusion

• 5. In translation, the three classes of RNAs
  interact in the synthesis of polypeptide chains
• 6. Translation proceeds through three stages
  Initiation, elongation, and termination
• 7. Gene mutations are heritable changes in DNAs
  base sequence