Sections 9.3

1
Sections 9.3 9.5

• Transcription, RNA Processing, Translation

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9.3 RNA Synthesis (Transcription)

• General Info
• Produces molecules of RNA
• One strand of DNA acts as the template for RNA
  synthesis (C ? G A ? U T ? A)
• Happens in the nucleus.
• Main enzyme RNA polymerase, which adds the RNA
  nucelotides to pair with the DNA
• Transcription occurs in 3 stages.

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9.3 RNA Synthesis (Transcription)

• 3 Stages
• 1. Initiation
• - RNA polymerase binds to the DNA at the
  promoter region. It is directed to this region
  by proteins called initiation factors.

4
9.3 RNA Synthesis (Transcription)

• 3 Stages
• 2. Elongation
• - RNA polymerase moves along the coding strand
  of DNA away from the promoter, building the RNA
  strand.
• - RNA polymerase adds the RNA nucleotides to
  their complements on the coding strand (A now
  bonds to U), forming the primary transcript.

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9.3 RNA Synthesis (Transcription)

• 3 Stages
• 3. Termination
• - RNA polymerase reaches the terminator region
  of the DNA is released from the DNA with the
  primary transcript.
• - The primary transcript is considered
  unfinished, and now needs to undergo processing
  before a protein can be made.

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(No Transcript)
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9.4 RNA Processing

• General Info
• All 3 types of RNA (mRNA, tRNA, rRNA) are
  processed in the nucleus before they leave it.
• This can involve adding, chemically changing,
  and/or removing nucleotides.

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9.4 RNA Processing

• All RNA transcripts undergo the following
• Enzymes attach a methyl-guanine (mG) cap to the
  starting end of the RNA molecule.

9
9.4 RNA Processing

• All RNA transcripts undergo the following
• Other enzymes add 100-200 adenine nucleotides to
  the other end (called the poly-A tail).
• These 2 additions help transport RNA out of the
  nucleus protect the RNA from enzymes found in
  the cytoplasm.

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9.4 RNA Processing

• All RNA transcripts undergo the following
• All RNA transcripts also undergo splicing.
• The removal of meaningless segments of RNA,
  called introns, and fusing together the
  remaining, meaningful sections, called exons.
• Requires precise recognition of the site to be
  cut.

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9.4 RNA Processing

• After the 3 major changes, what happens to the
  RNA if it becomes.
• mRNA ? it is completed.
• tRNA ? the molecule is folded into a cloverleaf
  shape.
• rRNA ? proteins are added to it to form the
  ribosome of a cell.

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• Transcription Animation
• Cap Tail Animation

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9.5 Translation (Protein Synthesis)

• General Info
• The sequence of mRNA is translated into the amino
  acid sequence of a protein.
• It happens on ribosomes in the cytoplasm.
• tRNA acts as a carrier molecule that brings
  specific amino acids to the ribosome.
• A tRNA anticodon matches with an mRNA codon.
  (They are complementary)
• ATP provides the energy to form the bonds.
• Divided into same 3 stages as Transcription
  initiation, elongation, termination.

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9.5 Translation (Protein Synthesis)

• Initiation
• Ribosomal subunits bind to the mRNA at the start
  codon (AUG).

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9.5 Translation (Protein Synthesis)

• Elongation
• tRNA molecules pick up their specific amino acids
  bring them to the ribosome.
• Each tRNA molecules anticodon is complementary
  to a codon on the mRNA.
• The amino acid from the previous tRNA is joined
  to the next amino acid via a peptide bond.
• This continues the polypeptide chain grows.

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9.5 Translation (Protein Synthesis)

• Elongation
• (3 sites)
• A site The next site in line for arrival of a
  tRNA.
• P site This is where the newly arriving amino
  acid will join with the previously arrived amino
  acid by a peptide bond.
• E site After a tRNA has dropped off the amino
  acid, it will exit go pick up another amino
  acid.

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9.5 Translation (Protein Synthesis)

• Termination
• Elongation continues until a codon is reached for
  which there is no complementary anticodon (no
  tRNA). No amino acid arrives the polypeptide
  chain ends. This is called a stop codon (UAA,
  UAG, UGA).
• Instead of tRNA, a protein called a release
  factor, knocks the protein factory apart,
  releasing the ribosome, mRNA, the polypeptide.
• The polypeptide is then used to build a
  functional protein for the organism.

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• Translation Animation