Download Understanding Our Environment

Genes and How They Work
Chapter 15
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Outline
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Cells Use RNA to Make Protein
Genetic Code
Transcription
Translation
Introns and Exons
Bacterial and Eukaryotic Gene Expression
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Cells Use RNA to Make Protein
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During polypeptide synthesis, ribosomal RNA
(rRNA) is the site of polypeptide assembly.
 Transfer RNA (tRNA) transport and
position amino acids.
 Messenger RNA (mRNA) direct which
amino acids are assembled into
polypeptides.
Central Dogma
 DNA  RNA Protein
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Gene Expression
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Transcription
 DNA sequence is transcribed into RNA
sequence.
- Initiated when RNA polymerase binds to
promoter site.
 Moves along DNA strand and adds
corresponding complementary RNA
nucleotide.
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Gene Expression
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Translation
 Nucleotide sequence of mRNA transcript is
translated into an amino acid sequence in
the polypeptide.
- rRNA molecule within ribosome
recognizes and binds to start sequence
on mRNA.
 Moves along three nucleotides at a
time.
 Disengages at stop signal.
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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Genetic Code
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The genetic code consists of a series of
information blocks called codons.
 Reading frame is composed of three
nucleotide blocks (Triplet).
- Each codes for one amino acid.
 Genetic code is universal in almost all
organisms.
 Mitochondria
 Chloroplasts
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Transcription
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RNA Polymerase
 Only one of two DNA strands (template or
antisense strand) is transcribed.
 Non-transcribed strand is termed coding
strand or sense strand.
 In both bacteria and eukaryotes, the
polymerase adds ribonucleotides to the
growing 3’ end of an RNA chain.
- Synthesis proceeds in 5’3’ direction.
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Transcription
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Promoter
 Transcription starts at RNA polymerase
binding sites called promoters on DNA
template strand.
Initiation
 Other eukaryotic factors bind, assembling
a transcription complex.
- RNA polymerase begins to unwind DNA
helix.
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Transcription
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Elongation
 Transcription bubble moves down DNA at
constant rate leaving growing RNA strands
protruding from the bubble.
Termination
 Stop sequences at the end of the gene
cause phosphodiester bond formation to
cease, transcription bubble to dissociate,
and RNA polymerase to release DNA.
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Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Translation
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Begins when initial portion of mRNA
molecule binds to rRNA in a ribosome.
 tRNA molecule with complimentary
anticodon, binds to exposed codon on
mRNA.
- Some tRNA molecules recognize more
than one codon.
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Translation
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Activating Enzymes
 tRNA molecules attach to specific amino
acids through the action of activating
enzymes (aminoactyl-tRNA syntheases).
- Must correspond to specific anticodon
sequences on a tRNA molecule as well
as particular amino acids.
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Translation
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Start and Stop Signals
 Start signal coded by AUG codon.
 Stop signal coded by one of three
nonsense codons: UAA - UAG - UGA.
Initiation
 Polypeptide synthesis begins with the
formation of an initiation complex.
- Initiation Factors
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Formation of Initiation Complex
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Translation
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Elongation
 After initiation complex forms, large
ribosome subunit binds, exposing mRNA
codon adjacent to the initiating codon,
positioning it for interaction with another
amino acid-bearing tRNA molecule.
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Translation
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Translocation
 Ribosome moves nucleotides along mRNA
molecule.
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Translation
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Termination
 Nonsense codons are recognized by
release factors that release the newly
made polypeptide from the ribosome.
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Introns
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DNA sequence specifying a protein is broken
into segments (exons) scattered among longer
noncoding segments (introns).
Initially, a primary RNA transcript is produced
for the entire gene.
 Small nuclear ribonucleicproteins (snRNPs)
associate with proteins to form spliceosome.
- Excise introns and joins exons to form
mature RNA.
 Alternative Splicing
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Introns and Exons
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RNA Splicing
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During RNA processing, intron sequences
are cut out of primary transcript before it is
used in polypeptide synthesis.
 Remaining sequences are not translated.
- Remaining exon sequences are spliced
together to form final processed mRNA.
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Differences Between Bacterial and
Eukaryotic Gene Expression
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Most eukaryotic genes possess introns.
Individual bacterial mRNA molecules often
contain transcripts of several genes.
Eukaryotic mRNA molecules must be
completely formed and must pass across the
nuclear membrane before translation.
In bacteria, translation begins at AUG codon
preceded by special nucleotide sequence.
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Differences Between Bacterial and
Eukaryotic Gene Expression
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Eukaryotic mRNA molecules have introns cut
out and exons joined together before
translation.
Eukaryotic ribosomes are larger than
prokaryotic ribosomes.
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Review
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Cells Use RNA to Make Protein
Genetic Code
Transcription
Translation
Introns and Exons
Bacterial and Eukaryotic Gene Expression
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies
Copyright © McGraw-Hill Companies Permission required for reproduction or display
Raven - Johnson - Biology: 6th Ed. - All Rights Reserved - McGraw Hill Companies