Messenger RNA

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Chapter 7

• Messenger RNA

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7.1 Introduction

• All three types of RNA are central players during
  the process of gene expression.

Figure 7.01 Protein synthesis uses three types
of RNA.
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7.2 mRNA Is Produced by Transcription and Is
Translated

• Within a gene, only one of the two strands of DNA
  is transcribed into RNA.

Figure 7.02 Gene expression transcription
translation.
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7.3 The Secondary Structure of Transfer RNA Is a
Cloverleaf

• A tRNA has a sequence of 74 to 95 bases that
  folds into a cloverleaf secondary structure with
  four constant arms (and an additional arm in the
  longer tRNAs).
• tRNA is charged to form aminoacyl-tRNA by forming
  an ester link from the 2' or 3' OH group of the
  adenylic acid at the end of the acceptor arm to
  the COOH group of the amino acid.

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7.3 The Secondary Structure of Transfer RNA Is a
Cloverleaf
Figure 7.03 tRNA is an adaptor.
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7.3 The Secondary Structure of Transfer RNA Is a
Cloverleaf

• The sequence of the anticodon is solely
  responsible for the specificity of the
  aminoacyl-tRNA during translation.

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7.3 The Secondary Structure of Transfer RNA Is a
Cloverleaf
Figure 7.05 The anticodon determines tRNA
specificity.
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7.4 The Acceptor Stem and Anticodon Are at
Opposite Ends of the tRNA Tertiary Structure

• The cloverleaf forms an L-shaped tertiary
  structure with the acceptor arm at one end and
  the anticodon arm at the other end.

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7.4 The Acceptor Stem and Anticodon Are at
Opposite Ends of the tRNA Tertiary Structure
Figure 7.06 All tRNAs share a tertiary structure.
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7.5 Messenger RNA Is Translated by Ribosomes

• Ribosomes are characterized by their rate of
  sedimentation.
• 70S for bacterial ribosomes and 80S for
  eukaryotic ribosomes.
• A ribosome consists of a
• large subunit (50S or 60S for bacteria and
  eukaryotes)
• small subunit (30S or 40S)
• The ribosome provides the environment in which
  aminoacyl-tRNAs add amino acids to the growing
  polypeptide chain in response to the
  corresponding triplet codons.
• A ribosome moves along an mRNA from 5' to 3'.

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7.5 Messenger RNA Is Translated by Ribosomes
Figure 7.08 Ribosomes dissociate into subunits.
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7.6 Many Ribosomes Can Bind to One mRNA

• An mRNA is simultaneously translated by several
  ribosomes.
• Each ribosome is at a different stage of
  progression along the mRNA.

Figure 7.09 Each ribosome has a
polypeptidyl-tRNA and an aminoacyl-tRNA.
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Figure 7.10 Hemoglobin is synthesized on
pentasomes.
Photo courtesy of Alexander Rich, Massachusetts
Institute of Technology
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Figure 7.11 Ribosomes recycle for translation.
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Figure 7.12 30 of bacterial dry mass is
concerned with gene expression.
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7.7 The Cycle of Bacterial Messenger RNA

• Transcription and translation occur
  simultaneously in bacteria.
• Ribosomes begin translating an mRNA before its
  synthesis has been completed.
• Bacterial mRNA is unstable and has a half-life of
  only a few minutes.

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7.7 The Cycle of Bacterial Messenger RNA
Figure 7.13 Trancription - translation -
degradation.
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7.7 The Cycle of Bacterial Messenger RNA

• A bacterial mRNA may be polycistronic in having
  several coding regions that represent different
  genes.

Figure 7.15 Bacterial mRNA is polycistronic.
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7.8 Eukaryotic mRNA Is Modified During or after
Its Transcription

• A eukaryotic mRNA transcript is modified in the
  nucleus during or shortly after transcription.
• The modifications include the addition of a
  methylated cap at the 5' end and a sequence of
  poly(A) at the 3' end.

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7.8 Eukaryotic mRNA Is Modified During or after
Its Transcription
Figure 7.16 Eukaryotic mRNA is modified at both
ends.
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7.8 Eukaryotic mRNA Is Modified During or after
Its Transcription

• The mRNA is exported from the nucleus to the
  cytoplasm only after all modifications have been
  completed.

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7.8 Eukaryotic mRNA Is Modified During or after
Its Transcription
Figure 7.17 Eukaryotic mRNA is modified and
exported.
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7.9 The 5 End of Eukaryotic mRNA Is Capped

• A 5' cap is formed by adding a G to the terminal
  base of the transcript via a 5'5' link.
• One to three methyl groups are added to the base
  or ribose of the new terminal guanosine.

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7.9 The 5 End of Eukaryotic mRNA Is Capped
Figure 7.18 Eukaryotic mRNA has a methylated 5'
cap.
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7.10 The 3 Terminus of Eukaryotic mRNA Is
Polyadenylated

• A length of poly(A) 200 nucleotides long is
  added to a nuclear transcript after
  transcription.
• The poly(A) is bound by a specific protein
  (PABP).
• The poly(A) stabilizes the mRNA against
  degradation.

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7.11 Bacterial mRNA Degradation Involves Multiple
Enzymes

• The overall direction of degradation of bacterial
  mRNA is 5'3'.
• Degradation results from the combination of
  endonucleolytic cleavages followed by
  exonucleolytic degradation of the fragment from
  3'?5'.

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7.11 Bacterial mRNA Degradation Involves Multiple
Enzymes
Figure 7.19 mRNA is degraded by exo- and endo-
nucleases.
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7.12 Two Pathways Degrade Eukaryotic mRNA

• The modifications at both ends of mRNA protect it
  against degradation by exonucleases.
• Specific sequences within an mRNA may have
  stabilizing or destabilizing effects.
• Destabilization may be triggered by loss of
  poly(A).

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7.12 Two Pathways Degrade Eukaryotic mRNA
Figure 7.20 The structure and sequence of
eukaryotic mRNA determine stability.
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Figure 7.21 An ARE in a 3 nontranslated region
initiates degradation of mRNA.
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7.12 Two Pathways Degrade Eukaryotic mRNA

• Degradation of yeast mRNA requires removal of the
  5' cap and the 3' poly(A).

Figure 7.22 Deadenylation allows decappaing to
occur, which leads
endonucleolytic cleavage from the 5 end.
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7.12 Two Pathways Degrade Eukaryotic mRNA

• One yeast pathway involves exonucleolytic
  degradation from 5'?3'.
• Another yeast pathway uses a complex of several
  exonucleases that work in the 3'?5' direction.
• The deadenylase of animal cells may bind directly
  to the 5' cap.
• Either mutation causes slower degradation of
  mRNA, but loss of both pathways is lethal in
  yeast.

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Figure 7.23 The 3'-5' pathway has three stages.
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7.13 Nonsense Mutations Trigger a Surveillance
System

• Nonsense mutations cause mRNA to be degraded.
• Genes coding for the degradation system have been
  found in yeast and worms.

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7.13 Nonsense Mutations Trigger a Surveillance
System
Figure 7.24 A surveillance system degrades
mutant mRNA.
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Figure 7.25 Splicing junctions are marked by
proteins.
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7.14 Eukaryotic RNAs Are Transported

• RNA is transported through a membrane as a
  ribonucleoprotein particle.
• All eukaryotic RNAs that function in the
  cytoplasm must be exported from the nucleus.
• tRNAs and the RNA component of a ribonuclease are
  imported into mitochondria.
• mRNAs can travel long distances between plant
  cells.

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7.14 Eukaryotic RNAs Are Transported
Figure 7.26 Eukaryotic RNA can be transported
between cell compartments.
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7.15 mRNA Can Be Localized Within A Cell

• Yeast ASH1 mRNA forms a ribonucleoprotein that
  binds to a myosin motor.
• A motor transports it along actin filaments into
  the daughter bud.
• It is anchored and translated in the bud, so that
  the protein is found only in the bud.

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7.15 mRNA Can Be Localized Within A Cell
Figure 7.27 ASH1 mRNA is connected to a motor.