CLINICAL CASES
55
Anticodon: The three-base sequence on a tRNA that will base pair with the
three-base sequence on the mRNA. The anticodon is specific for an
amino acid; it translates the DNA sequence into an amino acid sequence
in the protein produced.
Codon: The three-base sequence of an mRNA that code for a particular
amino acid.
Elongation factors: Proteins required for bringing the aminoacyl-tRNA to
the A-site, codon recognition, and translocation of the newly elongated
peptidyl-tRNA from the A-site to the P-site.
Initiation factors: Proteins required for assembly of the ribosomal com-
plex with mRNA and Met-tRNA so that protein synthesis can proceed.
Nucleolar organizing region (NOR): Area of the nucleolus where a great
deal of rRNA transcription and synthesis occurs.
P-site: The peptidyl site on the ribosome to which Met-tRNA is brought to
base pair with the mRNA sequence AUG. It is also the site to which the
peptidyl RNA is moved in a process known as translocation following
the formation of a new peptide bond.
Posttranscription modification of tRNA: The synthesis of tRNA involves
modification of some uridine nucleotides to unusual nucleotides, such as
pseudouridine, ribothymidine, and dihydrouridine.
Ribosomes: Complexes of proteins and rRNA on which protein synthesis
occurs. There are two major subunits to all ribosomes, a larger subunit
(50S for prokaryotes, 60S for eukaryotes) and a smaller subunit (30S for
prokaryotes, 40S for eukaryotes).
D ISC U SSIO N
The synthesis of proteins involves converting the nucleotide sequence of spe-
cific regions of DNA into mRNA
(transcription)
followed by the formation of
peptide bonds in a complex set of reactions that occur on ribosomes
(transla-
tion).
The amino acids incorporated into the protein are first activated by being
attached to a family of tRNA molecules, each of which recognizes, by com-
plementary base-pairing interactions, particular sets of three nucleotides
(codons) in the mRNA. Proteins are synthesized on ribosomes by linking amino
acids together in the specific linear order stipulated by the sequence of codons
in an mRNA. Ribosomes are compact ribonucleoprotein particles found in
the cytosol of all cells. All ribosomes are composed of a small and a large
subunit. The two subunits contain rRNAs of different lengths, as well as a dif-
ferent set of proteins. All ribosomes consist of two major rRNA molecules
(23S and 16S rRNA in bacteria, 28S and 18S rRNA in eukaryotes) and one or
two small RNAs. Small and large subunits are brought together by an mRNA
molecule and protein synthesis starts immediately. After the protein is synthe-
sized, the ribosomal subunits separate and are reused.
Protein synthesis is divided into three stages: initiation, elongation, and
termination.
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