From Gene to Protein

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Chapter 17

• From Gene to Protein

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The Bridge Between DNA and Protein

• DNA contains the genes that make us who we are.
• The characteristics we have are the result of the
  proteins our cells produce during the process of
  transcription and translation.

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Main Questions

• Somehow the information content in DNA-- --the
  specific sequence of nucleotides along the
  DNA--strands needs to be turned into protein.
• How does this information determine the
  organisms appearance?
• How is the information in the DNA sequence
  translated by a cell into a specific trait?

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The Bridge Between DNA and Protein

• RNA is the single stranded compound that carries
  the message from the DNA to the ribosome for
  translation into protein.
• Recall, DNA A,T,C,G RNA A,U,C,G
• The order of these bases carries the code for the
  protein which is constructed from any or all of
  the 20 amino acids.

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RNA

• RNA is used because it is a way to protect the
  DNA from possible damage.
• Many copies of RNA can be made from one gene,
  thus, it allows many copies of a protein to be
  made simultaneously.

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mRNA and RNA Polymerase

• mRNA is the messenger or vehicle that carries
  the genetic information from the DNA to the
  protein synthesizing machinery.
• RNA polymerase pries apart the DNA and joins RNA
  nucleotides together in the 5--gt3 direction
  (adding, again, to the free 3 end).
• RNA polymerase is just like DNA polymerase, but
  it doesnt need a primer.

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Transcription and Translation

• The process of going from gene to mRNA is called
  transcription.
• Translation is the process that occurs when the
  mRNA reaches the ribosome and protein synthesis
  occurs.

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Transcription and Translation

• The mRNA produced during transcription is read by
  the ribosome and results in the production of a
  polypeptide.
• The polypeptide is comprised of amino acids.
• The specific sequence of amino acids is
  determined by the genetic code on the DNA.

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Transcription

• The gene determines the sequence of bases along
  the length of the mRNA molecule.
• One of the two regions of the DNA serves as the
  template.
• The DNA is read 3--gt5 so the mRNA can be
  synthesized 5--gt3
• Not all regions of DNA codes for protein.

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Transcription

• There are numerous segments of DNA to which
  transcription factors bind.
• These govern the synthesis of mRNA and regulate
  gene expression.
• Promoter sequence
• Termination sequence
• Enhancers

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Other Functions of Non-Coding DNA

• Other regions of non-coding DNA are involved in
  regulating gene expression, coding for tRNA
  molecules, and ensuring that the DNA maintains
  its length (telomeres).

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tRNA Structure and Function

• tRNA, like mRNA, is made in the nucleus and is
  used over and over again.
• tRNA binds an aa at one end and has an anticodon
  at the other end.
• The anticodon acts to base pair with the
  complementary code on the mRNA molecule, and
  delivers an aa to the ribosome.

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Transcription and Translation

• Additionally, in eukaryotes, once genes get
  transcribed, the RNA that is produced is often
  modified before getting translated.

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Post Transcriptional Modification

• In eukaryotes, once the primary transcript is
  made, it is spliced and modified before getting
  translated into protein.

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mRNA Modification

• The initial transcript (8000 bp) is reduced (to
  1200 on average).
• The large, non-encoding regions of the DNA that
  get transcribed are spliced out.
• Introns--intervening regions are removed.
• Exons--expressed regions are kept.

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mRNA Modification

• Some untranslated regions of the exons are saved
  because they have important functions such as
  ribosome binding.

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Translation

• mRNA triplets are called codons.
• Codons are written 5--gt3
• Codons are read 5--gt3 along the mRNA and the
  appropriate aa is incorporated into the protein
  according to the codon on the mRNA molecule.
• As this is done, the protein begins to take shape.

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Protein Synthesis

• Many copies of protein can be made simultaneously
  within a cell using a single mRNA molecule.
• This is an efficient way for the cell to make
  large amounts of protein in times of need.

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Polyribosome

• Here you can see an mRNA transcript being
  translated into many copies of protein by
  multiple ribosomes in a eukaryote.
• This is a way in which the cell can efficiently
  make numerous copies of protein.

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Polyribosome

• Here it is again in a prokaryote.
• The process essentially the same between
  prokaryotes and eukaryotes.
• The main exception is where it occurs.

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One Main Difference

• Between prokaryotes and eukaryotes, there is one
  main difference between transcription and
  translation. The two processes can occur
  simultaneously in prokaryotes because they lack a
  nucleus.
• In eukaryotes, the two processes occur at
  different times. Transcription occurs in the
  nucleus, translation occurs in the cytoplasm.

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Translation

• So how, exactly, does the cell translate genetic
  code into protein?

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The Genetic Code

• Scientists began wondering how the genetic
  information contained within DNA instructed the
  formation of proteins.
• How could 4 different base pairs code for 20
  different amino acids?
• 11 obviously didnt work a 2 letter code didnt
  work either but a 3 letter code would give you
  more than enough needed.

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The Genetic Code

• Codons are composed of triplets of bases.
• 61 of the 64 codons code for amino acids.
• 3 of the codons code for stop codons and signal
  an end to translation.
• AUG--start codon

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Genetic Code

• The genetic code is said to be redundant.
• More than one triplet codes for the same amino
  acid.
• One triplet only codes for one amino acid.
• The reading frame is important because any error
  in the reading frame codes for gibberish.

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Ribosomes

• rRNA genes are found on chromosomal DNA and are
  transcribed and processed in the nucleolus.
• They are assembled and transferred to the
  cytoplasm as individual subunits.
• The large and small subunits form one large
  subunit when they are attached to the mRNA.

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Ribosomes

• The structure of ribosomes fit their function.
• They have an mRNA binding site, a P-site, an
  A-site and an E-site.
• A-site (aminnoacyl-tRNA) holds the tRNA carrying
  the next aa to be added to the chain.
• P-site (peptidyl-tRNA) holds the tRNA carrying
  the growing peptide chain.
• E-site is the exit site where the tRNAs leave the
  ribosome.
• Each of these are binding sites for the mRNA.

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The 3 Stages of Protein Building

• 1. Initiation
• 2. Elongation
• 3. Termination
• All three stages require factors to help them
  go and GTP to power them.

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1. Initiation

• Initiation brings together mRNA, tRNA and the 2
  ribosomal subunits.
• Initiation factors are required for these things
  to come together.
• GTP is the energy source that brings the
  initiation complex together.

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1. Initiation

• Initiation brings together mRNA, tRNA and the 2
  ribosomal subunits.
• Initiation factors are required for these things
  to come together.
• GTP is the energy source that brings the
  initiation complex together.

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2. Elongation

• The elongation stage is where aas are added one
  by one to the growing polypeptide chain.
• Elongation factors are involved in the addition
  of the aas.
• GTP energy is also spent in this stage.

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3. Termination

• Termination occurs when a stop codon on the mRNA
  reaches the A-site within the ribosome.
• Release factor then binds to the stop codon in
  the A-site causing the addition of water to the
  peptide instead of an aa.
• This signals the end of translation.

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Polypeptide Synthesis

• As the polypeptide is being synthesized, it
  usually folds and takes on its 3D structure.
• Post-translational modifications are often
  required to make the protein function.
• Adding fats, sugars, phosphate groups, etc.
• Removal of certain proteins to make the protein
  functional.
• Separately synthesized polypeptides may need to
  come together to form a functional protein.

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