Protein Synthesis

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Protein Synthesis

• Transcription and Translation

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The Central Dogma

• The information encoded with the DNA nucleotide
  sequence of a double helix is transferred to a
  mRNA molecule.
• The mRNA molecule travels out of the nucleus and
  attaches to a ribosome
• Using the RNA nucleotide sequence and the genetic
  code, the ribosome assembles a protein

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The Central Dogma (brief)

• DNA is copied to mRNA
• mRNA is used as blueprint to make protein

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DNA ? Protein in 3 easy steps!

1. Transcription
2. RNA modification
3. Translation

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Genes and DNA

• A gene is a specific sequence of DNA nucleotides
• For each specific protein used by a cell, there
  is a specific DNA sequence (gene) located on a
  chromosome
• 1 gene ? 1 polypeptide

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RNA Structure

• RNA (ribonucleic acid) are nucleotides very
  similar to DNA
• Nitrogenous bases include Cytosine, Guanine,
  Adenine, and Uracil (instead of Thymine)
• Form three basic structures
• mRNA messenger RNA
• rRNA ribosomal RNA
• tRNA transfer RNA

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RNA vs DNA

• RNA has an oxygen on the 2 carbon of the ribose
  sugar

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RNA vs DNA

• RNA is single stranded, DNA is double stranded
• GCAT vs CUGA

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Types of RNA molecules

• mRNA (messenger)
• Relays DNA sequence information to ribosome
• rRNA (ribosomal)
• Combines with proteins to form ribosomes
• tRNA (transfer)
• Acts as bridge between nucleotide sequence and
  growing polypeptide chain

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Transcription

• The process by which the nucleotide base sequence
  of a DNA molecule is copied into a mRNA molecule
• 3 steps
• Initiation
• Elongation
• Termination
• Proteins required
• RNA polymerase
• Transcription factors

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RNA Polymerase

• Creates a mRNA molecule complimentary to template
  strand of DNA
• Works in the 5? 3 direction
• Requires transcription factors to begin its work

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Initiation

• Proteins called transcription factors bind to DNA
  region upstream from gene
• Proteins bind to region called promoter
• RNA polymerase attaches to double helix at
  beginning of gene

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Elongation

• RNA polymerase creates a mRNA molecule with bases
  complimentary to the template strand
• Template strand Anti-sense strand

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Termination

• RNA polymerase reaches end of gene and detaches
  from double helix
• mRNA transcript is released

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Animations

• Transcription showing full complex
• Transcription cool sounds

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Sense or Anti-sense?

• The sense strand of a gene has the same base
  sequence as the mRNA transcript
• The anti-sense strand is used as the template

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Transcript Modification

• Before a mRNA transcript exits the nucleus it is
  modified in 3 three (tres) ways
• Addition of 5 cap
• Addition of poly-A tail
• Removal of introns

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5 cap and poly-A tail

• Protective cap is placed on 5 end
• A long repetitive sequence of adenine nucleotides
  are added to 3 end, also for protection

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mRNA splicing

• Not all of a transcribed DNA sequence will be
  translated
• Genes are composed of introns and exons
• Introns are removed from mRNA transcripts by
  splicosomes

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Transcription Review

1. How is RNA polymerase similar to DNA polymerase
   III? How are they different?
2. Will the mRNA transcript have the same nucleotide
   sequence as the sense or anti-sense strand of
   DNA?
3. How are RNA and DNA different?
4. Name 3 things that happen during mRNA
   modification.

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Translation

• messenger RNA (mRNA) is decoded at a ribosome to
  produce a specific polypeptide according to the
  rules specified by the genetic code.
• 4 steps
• Activation
• Initiation
• Elongation
• Termination
• Requires
• Ribosomes (rRNA proteins), mRNA, tRNA, and
  amino acids

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Activation

• Amino acid is joined with the correct tRNA
• Reaction catalyzed by aminoacyl-tRNA-synthetase
• Occurs continuously

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tRNA - transfer

• Specified amino acids are attached to tRNA
• each anti-codon corresponds to the amino acid
  specified by the genetic code
• Each tRNA has an anti-codon (3 nucleotides)
• Anti-codon region base pairs with mRNA trascript

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Initiation

• Small ribosome subunit recognizes start sequence
  on mRNA and binds to it
• Start codon, AUG, is recognized by tRNA carrying
  a Methionine amino acid
• Large subunit completes the complex

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Elongation

• Ribosome moves down the mRNA in a 5? 3
  direction
• Every three mRNA nucleotides another amino acid
  is added to the growing polypeptide
• 3 steps
• Codon recognition
• Peptide bond formation
• Translocation

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Elongation Codon Recognition

• When the appropriate tRNA anticodon H-bonds to a
  mRNA codon at the ribosomal complex

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Elongation Peptide Bond Formation

• A peptide bond is created between polypeptide
  chain and new amino acid
• polypeptide is transferred to incoming tRNA

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Elongation Translocation

• Ribosome shifts 3 nucleotides (reading frame)
  down mRNA transcript
• tRNA unattached to polypeptide is released

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Elongation
Codon Recognition
Peptide Bond Formation
Translocation
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Termination

• The end of the mRNA coding sequence is reached
• Stop codon is recognized by a release factor
• Ribosome complex dissociates, protein is released

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The Genetic Code

• Each codon corresponds to a specific amino acid
• Degenerate
• 64 possible codons
• only 20 amino acids
• Several codons can code for the same amino acid
• Ex. CCU, CCA, CCG, CCC Proline
• Universal
• The same genetic code is used by all living
  organisms

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The Genetic Code
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Animations

• Translation
• Translation no sound, basic

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Summary

• Genetic information is encoded in the sequence of
  the DNA double helix. To access this information,
  the DNA sequence must be copied, or
  "transcribed", by enzymes known as RNA
  polymerases. The resulting messenger RNA (mRNA)
  molecules carry the genetic information to the
  protein-synthesizing machinery, where it is used
  to define the amino-acid sequence, and therefore
  the structure and function, of proteins.

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