Biomedical Engineering Reference
In-Depth Information
 end, a chain of adenylic acid residues is added. The most important step in RNA
processing is RNA splicing to remove the introns and ligate back the broken coding
sequences. The fact that eukaryotic genes exhibit several introns allows for coding
of several related proteins by alternative splicing and is the reason for the presence of
isoforms of several proteins [1-] .
1.4 Translation
Transcription is usually followed by translation, the process by which the blueprint
for protein production now copied into mRNA is used to form a functional protein
by the addition of amino acids in the specified sequence. Protein synthesis is carried
out in the cytoplasm in conjunction with all three types of RNA. The mRNA carries
the information in a codon, the triplet-base language wherein each triplet of bases, or
codon, can code for an amino acid or act as a stop codon. Of the existing 6 codons
from combinations of four bases, 61 code for amino acids and are stop codons. In
general, with the exception of methionine and tryptophan, all other amino acids are
coded by multiple codons.
The initiation of synthesis of the polypeptide chain in both prokaryotes and
eukaryotes begins with the amino acid methionine. The start codon specifying it is
AUG, and the sequence of codons from start to stop codon is called a reading frame.
An mRNA molecule can be read in different reading frames but is generally read in a
single reading frame, where a functional protein is formed before termination.
The conversion of the four-base language of nucleic acids into the 20-amino
acid language of proteins requires an adaptor, and tRNA fits the role of an adap-
tor molecule aptly. To fulfill this requirement, it is necessary that an amino acid
link to tRNA to form aminoacyl tRNA, and tRNA must have an anticodon to pair
with mRNA. tRNAs are 0-0 nucleotides long and have a well-defined clover
leaf-type three-dimensional structure. The four stems are stabilized by base pairing.
The  end of the unlooped stem is the acceptor stem. The base pairing rule
demands the presence of 61 tRNAs, each specific for one codon coding for an
amino acid, although cells may contain a lesser number of tRNA. This is because
the first position of the anticodon on tRNA is the wobble position. It allows base
pairing with more than one type of bases and accounts for the fewer number of
tRNA molecules.
The attachment of tRNA with the appropriate amino acid is catalyzed by spe-
cific aminoacyl tRNA synthetase. The 20 different enzymes recognize one of the 20
amino acids and link them to their appropriate tRNA at the  terminus. This reaction
requires energy, and the aminoacyl tRNA is said to be activated. The fidelity of the
attachment is ensured by the proofreading activity of the synthetase enzymes.
The next step in protein synthesis involves the ribosomes. Ribosomes are rich
in RNA, containing about 60% RNA. They are composed of three to four diffe-
rent rRNA molecules and around 0 proteins, and form a large subunit and a small
subunit. The ribosomal subunits and rRNA are differentiated by their sedimentation
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