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Samenvatting celbiologie module 2 deel 3 jaar 1 psychobiologie
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SV celbiologie module 2 deel 3.
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Module 2 deel 3: RNA translatie en eiwitvormingHoofdstuk 6
Pag. 333 t/m 346
From RNA to protein
Most genes in a cell produce mRNA molecules that serve as intermediaries on the pathway to proteins.
An mRNA sequence is decoded in sets of three nucleotides
After transcription and processing >> mRNA >> the information present in its nucleotide sequence is used
to synthesize a protein. Transcription = information transfer. The conversion of the information in RNA into
protein >> a translation of the information. The nucleotide sequence of a gene, through the intermediary of
mRNA is translated into the amino acid sequence of a protein by the genetic code.
The sequence of nucleotides is read in groups of three. There are 64 different possible combinations of
three nucleotides, but only 20 amino acids are commonly found in proteins. >> each group of three
nucleotides in RNA = a codon. And each codon specifies either one amino acid or a
stop to the translation process. Mitochondria have their own transcription and protein-
synthesis systems that operate independently from those of the rest of the cell. >>
minor changes to the code.
An RNA sequence can be translated in any one of three different reading frames,
depending on where the decoding process begins. Only one of the three in an mRNA
encodes the required protein.
tRNA molecules match amino acids to codons in mRNA
The codons in an mRNA molecule do not directly recognize the amino acids they
specify: the group of three nucleotides does not, for example, bind directly to the
amino acid. The translation of mRNA into protein depends on adaptor molecules that
can recognize & bind both to the codon and to the amino acid. These adaptors
consist of transfer RNAs (tRNAs). Four short segments of the folded tRNA are
double-helical, producing a molecule that looks like a cloverleaf when drawn
schematically.
Two regions of unpaired nucleotides situated at either end of the L-shaped molecule are
crucial to the function of tRNA in protein synthesis.
The anticodon, a set of three consecutive nucleotides that pairs with the complementary codon in an
mRNA molecule.
a short single- stranded region at the 3ʹ end of the molecule; this is the site where the amino acid that
matches the codon is attached to the tRNA.
Some amino acids have more than one tRNA and some tRNAs are constructed so that
they require accurate base-pairing only at the first two positions of the codon and can
tolerate a mismatch (or wobble) at the third position.
tRNAs are covalently modified before they exit from the nucleus
Both bacterial and eukaryotic tRNAs are typically synthesized as larger precursor
tRNAs, which are then trimmed to produce the mature tRNA. & some tRNA precursors
contain introns that must be spliced out. >> differs from pre-mRNA splicing, tRNA splicing
uses a cut-and-paste mechanism that is catalyzed by proteins. Trimming and splicing both require the
precursor tRNA to be correctly folded in its cloverleaf configuration >> misfolded tRNA precursors will not
be processed properly, the trimming and splicing reactions serve as quality-control steps in the generation
of tRNAs.
>> some of the modified nucleotides affect the conformation and base-pairing of the anticodon and thereby
facilitate the recognition of the appropriate mRNA codon by the tRNA molecule. Others affect the accuracy
with which the tRNA is attached to the correct amino acid.
, Specific enzymes couple each amino acid to its appropriate tRNA molecule
Recognition and attachment of the correct amino acid depends on enzymes >> aminoacyl-
tRNA synthetases >> covalently couple each amino acid to its appropriate set of tRNA
molecules. Most cells have a different synthetase enzyme for each amino acid. Many bacteria
have fewer than 20 >> a sinlge synthetase places the identical amino acid on two different
types of tRNAs, only one of which has an anticodon that matches. A second enzyme then
chemically modifies each ‘incorrectly’ attached amino acid >> so that it corresponds to the
anticodon displayed by its covalently linked tRNA.
The synthetase-catalyzed reaction >> coupled to hydrolysis of ATP.
The aminoacyl-tRNA synthetase enzymes and
the tRNAs are equally important in the
decoding process. Cells have several quality
control mechanisms to avoid a type of mishap,
>> the genetic code is translated by two sets
of adaptors that act sequentially. Each
matches one molecular surface to another
with great specificity, and it is their combined
action that associates each sequence of three
nucleotides in the mRNA molecule with its particular amino acid.
Editing by tRNA synthetases ensures accuracy
Several mechanisms working together ensure that an aminoacyl-tRNA synthetase links the correct amino
acid to each tRNA. Most synthetase enzymes select the correct amino acid by a 2-step mechanism:
1. The correct amino acid has the highest affinity for the active-site
pocket of its synthetase and is therefore favored over the other 19.
However, accurate discrimination between two similar amino acids
is very difficult to achieve in a single step.
2. Occurs after the amino acid has been covalently linked to AMP:
when tRNA binds, the synthetase tries to force the adenylated
amino acid into a second editing pocket in the enzyme. The precise
dimensions of this pocket exclude the correct amino acid, while
allowing access by closely related amino acids. In the editing
pocket, an amino acid is removed from the AMP by hydrolysis. This
hydrolytic editing increases the overall accuracy of tRNA charging.
The tRNA synthetase must also recognize the correct set of tRNAs,
and extensive structural and chemical complementarity between the synthetase and the tRNA allows the
synthetase to probe various features of the tRNA. Most tRNA synthetases directly recognize the matching
tRNA anticodon; these synthetases contain three adjacent nucleotide-binding pockets, each of which is
complementary in shape and charge to a nucleotide in the anticodon. For other synthetases, the nucleotide
sequence of the amino acid-accepting arm (acceptor stem) is the key recognition determinant. In most
cases, however, the synthetase “reads” the nucleotides at several different positions on the tRNA.
Amino acids are added to the C-terminal end of a growing polypeptide chain
Each amino acid is first coupled to specific tRNA molecules. Fundamental reaction of protein synthesis >>
the formation of a peptide bond between the carboxyl group at the end of a growing polypeptide chain and
a free amino group on an incoming amino
acid. >> a protein is synthesized stepwise
from its N-terminal end to its C-terminal end.
The growing carboxyl end of the polypeptide
chain remains activated by its covalent
attachment to a tRNA molecule.
>> each amino acid added carries with it the
activation energy for the addition of the next amino acid rather than the energy for its own addition.
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