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CASE FILES: BIOCHEMISTRY
polymerase and several transcription factors.
The DNA strand that directs
RNA synthesis via
complementary base pairing
is called the
template. RNA
synthesis is always unidirectional from 5' (phosphate) to 3' (hydroxyl).
One
can organize transcription in a stepwise fashion: (1)
binding of RNA poly-
merase
to the DNA; (2) formation of the
transcription bubble
(separation of
the DNA strands); (3)
addition of the first ribonucleic acid residue;
and
(4) addition of the
second residue,
formation of a
phosphodiester bond
between
the ribonucleic acid residues, and
release of pyrophosphate.
The addition
process continues until the
termination signal
is encountered. Once the
nascent
RNA chain emerges, it is capped with methylguanylate at the 5' end and
polyadenylated in the 3' end.
This primary RNA transcript is processed further
by splicing machinery to yield the mature functional mRNA.
Heterogenous
nuclear RNA (hnRNA) contains exons and introns.
The introns are excised
and
mature RNA enters the cytoplasm.
The
mRNA
is then processed in the cytoplasm by a process called
trans-
lation.
Translation involves the
ribosomes
along with a battery of
initiating
(IF), elongation (EF), and release (RF) factors.
An important rule to remem-
ber in translation is that each
triplet nucleotide codon codes for a specific
amino acid (the three-letter genetic code).
Maintaining the order of amino
acids is important to obtain a functional protein. mRNA is translated into pro-
tein with the help of
transfer RNA (tRNA),
which
brings an amino acid
along with it (aminoacyl tRNA) and ribosomes.
Ribosomes are in turn com-
posed of numerous proteins and several rRNA molecules. The stepwise
process of
translation
is as follows: (1) The ribosomal unit along with the
ini-
tiation factors and formyl-methionine (fMet)-tRNA bind close to the ini-
tiating codon AUG in the 5' region
of the mRNA. (2) The
ribosomal
complex consists of three sites, namely, A, P, and E.
The
aminoacyl tRNA
binds the triplet codon at the A site.
The
formyl-methionyl group
forms a
peptide bond with the amino group of the aminoacyl tRNA in
the P site,
releasing the now
uncharged tRNA
fMet, which exits the ribosome assembly
through the
E site.
(3)
Elongation
continues until the ribosome encounters
one of the
stop codons UAA, UAG, and UGA.
(4)
Termination of protein
synthesis
occurs with the help of release factors which cleave the peptide
chain from the tRNA and dissolve the ribosomal complex.
In
retroviruses such as HIV,
where the genetic material is
RNA
rather
than DNA,
reverse transcription
occurs. The life cycle of HIV begins when
the virion binds to cell surface receptors (CD4 receptors) on helper T lym-
phocytes. This results in a conformational change that enables the viral coat to
fuse with the lymphocyte membrane, thus releasing the viral RNA and viral
proteins into the cytosol, including reverse transcriptase and integrase. The
RNA genome (Figure 5-2) is reverse transcribed into a double-stranded
DNA
molecule by utilizing the
reverse transcriptase enzyme.
This enzyme
is an unusual
DNA polymerase, because it uses both DNA and RNA as
template.
First, it makes DNA from the RNA and uses this in turn to make the
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