CASE FILES: BIOCHEMISTRY
polymerase and several transcription factors.
The DNA strand that directs
RNA synthesis via
complementary base pairing
is called the
synthesis is always unidirectional from 5' (phosphate) to 3' (hydroxyl).
can organize transcription in a stepwise fashion: (1)
binding of RNA poly-
to the DNA; (2) formation of the
the DNA strands); (3)
addition of the first ribonucleic acid residue;
(4) addition of the
formation of a
the ribonucleic acid residues, and
release of pyrophosphate.
process continues until the
is encountered. Once the
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.
nuclear RNA (hnRNA) contains exons and introns.
The introns are excised
mature RNA enters the cytoplasm.
is then processed in the cytoplasm by a process called
Translation involves the
along with a battery of
(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),
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
is as follows: (1) The ribosomal unit along with the
tiation factors and formyl-methionine (fMet)-tRNA bind close to the ini-
tiating codon AUG in the 5' region
of the mRNA. (2) The
complex consists of three sites, namely, A, P, and E.
binds the triplet codon at the A site.
peptide bond with the amino group of the aminoacyl tRNA in
the P site,
releasing the now
fMet, which exits the ribosome assembly
continues until the ribosome encounters
one of the
stop codons UAA, UAG, and UGA.
Termination of protein
occurs with the help of release factors which cleave the peptide
chain from the tRNA and dissolve the ribosomal complex.
retroviruses such as HIV,
where the genetic material is
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
molecule by utilizing the
reverse transcriptase enzyme.
is an unusual
DNA polymerase, because it uses both DNA and RNA as
First, it makes DNA from the RNA and uses this in turn to make the