Download a peptide bond forms that adds an amino acid

Transcription
and Translation
Transcription
and Translation
© 2011 Pearson Education, Inc.
RNA Processing in Eukaryotes
• In bacteria, the information in DNA is converted to mRNA
directly. In eukaryotes, however, the product of transcription
is an immature primary transcript, or pre-mRNA. Before
primary transcripts can be translated, they have to be
processed in a complex series of steps.
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Key Concepts
• After RNA polymerase binds DNA with the help of other
proteins, it catalyzes the production of an RNA molecule
whose base sequence is complementary to the base
sequence of the DNA template strand.
• Eukaryotic genes contain regions called exons and regions
called introns; during RNA processing, the regions coded by
introns are removed, and the ends of the RNA receive a cap
and tail.
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Characteristics of RNA Polymerase
• Like the DNA polymerases, an RNA polymerase performs a
template-directed synthesis in the 5′ to 3′ direction. But unlike
DNA polymerases, RNA polymerases do not require a primer
to begin transcription.
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The Discovery of Eukaryotic Genes in Pieces
• The protein-coding regions of eukaryotic genes are
interrupted by noncoding regions.
• To make a functional mRNA, these noncoding regions
must be removed.
• Exons are the coding regions of eukaryotic genes that will be
part of the final mRNA product.
• The intervening noncoding sequences are called introns,
and are not in the final mRNA.
• Eukaryotic genes are much larger than their corresponding
mature mRNA.
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RNA Splicing
• The transcription of eukaryotic genes by RNA polymerase
generates a primary RNA transcript that contains exons
and introns.
• Introns are removed by splicing.
• Small nuclear ribonucleoproteins (snRNPs) form a
complex called a spliceosome. This spliceosome catalyzes
the splicing reaction.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Adding Caps and Tails to RNA Transcripts
• Primary RNA transcripts are also processed by the addition
of a 5′ cap and a poly(A) tail.
• With the addition of cap and tail and completion of splicing,
processing of the primary RNA transcript is complete. The
product is a mature mRNA.
• The 5' cap serves as a recognition signal for the translation
machinery.
• The poly(A) tail extends the life of an mRNA by protecting it
from degradation.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Transcription and Translation in Eukaryotes
• In eukaryotes, transcription and translation are separated.
mRNAs are synthesized and processed in the nucleus and
then transported to the cytoplasm for translation by
ribosomes.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Key Concepts
• Ribosomes translate mRNAs into proteins with the help of
intermediary molecules called transfer RNAs (tRNAs).
• Each transfer RNA carries an amino
acid corresponding to the tRNA’s
three-base-long anticodon.
• In the ribosome, the tRNA anticodon
binds to a three-base-long mRNA
codon, causing the amino acid carried
by the transfer RNA to be added to
the growing protein.
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Introduction
• A cell builds the proteins it needs from
instructions encoded in its genome according
to the central dogma of molecular biology.
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How Does an mRNA Triplet Specify an Amino
Acid?
• There were two hypotheses regarding the specification of
amino acid sequence by a sequence of nucleotide bases:
• mRNA codons and amino acids interact directly.
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How Does an mRNA Triplet Specify an Amino
Acid?
Francis Crick proposed that an adapter molecule holds
amino acids in place while interacting directly and
specifically with a codon in mRNA.
• The adapter molecule was later found to be a small RNA
called transfer RNA (tRNA).
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The Characteristics of Transfer RNA
• ATP is required to attach tRNA to an amino acid.
• Enzymes called aminoacyl tRNA synthetases “charge” the
tRNA by catalyzing the addition of amino acids to tRNAs.
• For each of the 20 amino acids, there is a different aminoacyl
tRNA synthetase and one or more tRNAs.
• A tRNA covalently linked to its corresponding amino acid is
called an aminoacyl tRNA.
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© 2011 Pearson Education, Inc.
What Happens to the Amino Acids Attached to
tRNA?
• Experiments with radioactive amino acids revealed that they
are lost from tRNAs and incorporated into polypeptides
synthesized in ribosomes.
• These results inspired the use of “transfer” in tRNA’s name,
because amino acids are transferred from the RNA to the
growing end of a new polypeptide.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
What Do tRNAs Look Like?
• The CCA sequence at the 3' end of each tRNA is the
binding site for amino acids.
• The triplet on the loop at the opposite end is the anticodon
that base pairs with the mRNA codon.
• The secondary structure of tRNA folds over to produce an Lshaped tertiary structure.
• All of the tRNAs in a cell have the same structure, shaped
like an upside-down L. They vary at the anticodon and
attached amino acid.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
How Many tRNAs Are There?
• There are 61 different codons but only about 40 tRNAs in
most cells.
• To resolve this deficit, Francis Crick proposed the wobble
hypothesis. This hypothesis proposes that the anticodon of
tRNAs can still bind successfully to a codon whose third
position requires a nonstandard base pairing.
• Thus, one tRNA is able to base pair with more than one type
of codon.
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The Structure and Function of Ribosomes
• Ribosomes contain protein and ribosomal RNA (rRNA).
• Ribosomes can be separated into two subunits:
• The small subunit, which holds the mRNA in place during
translation.
• The large subunit, where peptide bonds form.
• During translation, three distinct
tRNAs line up within the ribosome.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Ribosomes and the Mechanism of Translation
• All three tRNAs are bound at their anticodons to the
corresponding mRNA codon.
• The A site of the ribosome is the acceptor site for an
aminoacyl tRNA.
• The P site is where a peptide bond forms that adds an
amino acid to the growing polypeptide chain.
• The E site is where tRNAs no longer bound to an amino
acid exit the ribosome.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Ribosomes and the Mechanism of Translation
• The ribosome is a molecular machine that synthesizes
proteins in a three-step sequence.
1. An aminoacyl tRNA carrying the correct anticodon for
the mRNA codon enters the A site.
2. A peptide bond forms between the amino acid on the
aminoacyl tRNA in the A site and the growing
polypeptide on the tRNA in the P site.
3. The ribosome moves ahead three bases and all three
tRNAs move down one position; the tRNA in the E site
exits.
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The Phases of Translation
• Translation has three phases: initiation, elongation, and
termination.
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Initiation
• The initiation phase of translation begins at the AUG start
codon.
• In bacteria, the start codon is preceded by a ribosome
binding site that is complementary to a section of one rRNA
in the small ribosomal subunit.
• The interaction between the small subunit and the mRNA is
mediated by initiation factors.
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Initiation in Bacteria
• Translation initiation is a three-step process in bacteria:
1. The mRNA binds to a small ribosomal subunit.
2. The initiator aminoacyl tRNA binds to the start codon.
3. The large ribosomal subunit binds, completing the
complex.
• Translation is now ready to begin.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Elongation
• At the start of the elongation phase, the initiator tRNA is in
the P site, and the E and A sites are empty.
• An aminoacyl tRNA binds to the codon in the A site via
complementary base pairing between anticodon and codon.
• Peptide bonds form between amino acids on the tRNAs in
the P and A sites.
• After peptide bond formation, the polypeptide on the
tRNA in the P site is transferred to the tRNA in the A site.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Is the Ribosome an Enzyme or a Ribozyme?
• The active site of the ribosome is entirely ribosomal RNA.
• Thus, ribosomal RNA catalyzes peptide bond formation and
the ribosome is a ribozyme.
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Moving Down the mRNA
• Translocation occurs when elongation factors move the
mRNA down the ribosome three nucleotides at a time, and
the tRNA attached to the growing protein moves into the P
site.
• The A site is now available to accept a new aminoacyl tRNA
for binding to the next codon.
• The tRNA that was in the P site moves to the E site, and if
the E site is occupied, that tRNA is ejected.
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Elongation
• Elongation has three steps:
1. Arrival of the aminoacyl tRNA.
2. Peptide bond formation.
3. Translocation.
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Termination
• The termination phase starts when the A site encounters a
stop codon.
• This causes a protein called a release factor to enter the
site. Release factors resemble tRNAs in size and shape but
do not carry an amino acid.
• These factors catalyze hydrolysis of the bond linking the
tRNA in the P site with the polypeptide chain.
© 2011 Pearson Education, Inc.
© 2011 Pearson Education, Inc.
Post-Translational Modifications
• Most proteins go through an extensive series of processing
steps, collectively called post-translational modification,
before they are ready to go to work in a cell.
• Molecular chaperones speed folding of the protein. Folding
determines a protein's shape and therefore its function.
• Many proteins are altered by enzymes that add or remove a
phosphate group. These changes often switch the protein
from an inactive state to an active state or vice versa.
© 2011 Pearson Education, Inc.