Chapter 6: Gene Expression: Translation

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Chapter 6 Gene Expression Translation

• Linnea Fletcher Ph.D.
• BIOL 2316

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Ribosomal subunits are made of protein and rRNA
Three-dimensional structure of the 30S ribosomal
subunit.
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• What are the building blocks of proteins, and
  what type of covalent bond holds them together in
  the polypeptide chain?
• How many different types of amino acids are used
  to build proteins?
• What structural elements do all amino acids
  share, and in what way do they differ?
• About how long are polypeptide chains?

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You should be able to draw this picture!
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There are four groups of amino acids based
on chemical and physical Characteristics What
are the characteristics of an acidic amino acid?
Nonpolar and neutral?
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Covalent bond holding amino acids together
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• Describe the 4 levels of protein structure and
  the forces holding these structures together.
• What type of amino acids become buried to the
  interior of proteins, and why?
• Do all proteins have quaternary structure?
• What determines what secondary, tertiary, and
  quaternary structures that proteins assume inside
  the cell?

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What are the bonds that form the primary,
secondary, teritary, And quatenary structures in
proteins?
For example, for secondary structure, it is
primarily the hydrogen bonding between peptide
bonds that form the repeating structure you
see in beta-pleated sheets and alpha-helices
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• Explain why Francis Crick predicted that the
  genetic code was a triplet message, and how he
  and others found experimental support for this
  idea.
• Describe the in vitro translation experiments of
  Nirenberg Khorana that deciphered many of the
  genetic codons.
• What advantages did the ribosome-binding assays
  of Nirenberg Leder have in deciphering the
  genetic code?

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Review Crick and his colleagues experiments with
T4 phage
Figure 6.5 Reversion of a deletion frameshift
mutation by a nearby addition mutation. (a)
Hypothetical segment of normal DNA, mRNA
transcript, and polypeptide in the wild type. (b)
Effect of a deletion mutation on the amino-acid
sequence of a polypeptide. The reading frame is
disrupted. (c) Reversion
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Figure 6.6 Hypothetical example showing how three
nearby mutations restore the reading frame,
giving normal or near-normal function. The
mutations are shown here at the level of the
mRNA.
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• Characteristics of the CODE
• To what extent is the genetic code universal?
• What does it mean that the code is comma free
  and nonoverlapping?
• How many codons are there in the genetic code
  that code for amino acids?
• What does it mean that the code is degenerate?
• What are the start and stop signals?
• Explain the Wobble Hypothesis

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Using this table, you should be able to translate
mRNAs
Figure 6.7 The genetic code. Of the 64 codons, 61
specify one of the 20 amino acids. The other 3
codons are chain-terminating codons and do not
specify any amino acid. AUG, one of the 61 codons
that specify an amino acid, is used in the
initiation of protein synthesis.
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Amino acids that occur more frequently in
proteins have the higher number of codons (e.g.
Thr occurs more frequently than His)
Figure 6.8 Example of base-pairing wobble. Two
different leucine codons (CUC, CUU) can be read
by the same leucine tRNA molecule, contrary to
regular base-pairing rules.
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Describe how aminoacyl-tRNA synthetases produce
charged tRNAs
Figure 6.9 Charging of a tRNA molecule by
aminoacyl-tRNA synthetase to produce an
aminoacyl-tRNA (charged tRNA).
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Figure 6.9 Charging of a tRNA molecule by
aminoacyl-tRNA synthetase to produce an
aminoacyl-tRNA (charged tRNA).
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• Describe the actions of the molecules of
  translation in prokaryotes during these stages of
  translation initiation, elongation,
  termination.
• What types of molecules make up the structure of
  a ribosome?
• What signals initiation and termination?
• What are the steps of ribosome assembly during
  initiation?
• Compare the initiation step for prokaryotes and
  eukaryotes in what ways are they the same and
  in what ways are they different? For example how
  does the 5 CAP and the poly A tail play a role
  in initiation in the eukaryote?
• (I suggest you make a table of the 3 stages that
  indicates the similarities and differences
  between prokaryotes and eukaryotes)

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Figure 6.11 Initiation of protein synthesis in
prokaryotes. A 30S ribosomal subunit, complexed
with initiation factors and GTP, binds to mRNA
and fMet-tRNA to form a 30S initiation complex.
Next, the 50S ribosomal subunit binds, forming a
70S initiation complex. During this event, the
initiation factors are released and GTP hydrolyzed
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• What are the 3 steps of elongation during
  translation in prokaryotes?
• Know detail, such as
• What are the three sites of the large subunit of
  the ribosome positioned at the mRNA? (A, P and
  Eknow this!)
• In what direction do the tRNA and mRNA travel
  along these sites during elongation?
• What happens during the translocation step?
• What becomes of the freed tRNA once peptidyl
  transferase has removed its amino acid?
• What creates a polysome in a prokaryote?

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This is found only in a prokaryote!
Figure 6.12 Sequences involved in the binding of
ribosomes to the mRNA in the initiation of
protein synthesis in prokaryotes. (a) Nucleotide
sequence at the end of E. coli 16S rRNA. (b)
Example of how the end of 16S rRNA can base pair
with the nucleotide sequence upstream from the
AUG initiation codon
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Again, only in a prokaryote
Figure 6.13 Elongation stage of translation in
prokaryotes.
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Prokaryotes
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Enzyme is composed of RNA
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In a prokaryote, translation can start BEFORE
transcription has finished! This cannot occur
with eukaryotic mRNAs that are made in the
nucleus and then transport out to be translated.
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Figure 6.16 Termination of translation. The
ribosome recognizes a chain termination codon
(UAG) with the aid of release factors. A release
factor reads the stop codon, initiating a series
of specific termination events leading to the
release of the completed polypeptide.
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• What signals the termination of translation?
• Describe the structure and action of release
  factors during termination.
• BOTTOM LINE BE ABLE to DIAGRAM TRANSLATION and
  KNOW the difference between prokaryotes and
  eukaryotes!

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• In a eukaryote, what tags a protein for secretion
  or localization to an organelle?

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Problem 6.21 A segment of a polypeptide chain is
Arg-Gly-Ser-Phe-Val-Asp-Arg it is encoded by the
following segment of DNA. Which strand is the
template strand? Label each strand with its
correct polarity (5 and 3)?
Problem 6.29 In the recessive condition in humans
known as sickle-cell anemia, the beta globin
polypeptide of Hb is found to be abnormal.
Explain how the difference in the DNA gives rise
to the difference in the mRNA and the resulting
protein. Compared the abnormal protein to the
normal Hb and explain why this difference causes
the sicklingeffect.