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chapter 25
RNA and Protein Synthesis
4. Other modifications of the primary transcript are
possible such as
alternative splicing
which produces
mRNAs with different sets of exons and
RNA editing
in which bases are modified or changed in the original
transcript.
5. The functional mRNA is transported to the cytoplasm
where translation occurs on ribosomes bound to the
endoplasmic reticulum of the cell.
25.3 Enzymatic Synthesis of RNA
The basic chemical features of the synthesis of RNA are
the following (Figure 25-3):
1. The precursors of RNA synthesis are the four
ribonucleoside 5'-triphosphates (rNTPs): ATP,
GTP, CTP, and UTP. The ribose portion of each
NTP has an -OH group on both the
2!
and the
3' carbon atoms.
2. In the polymerization reaction, a 3'-OH group of one
nucleotide reacts with the 5r-triphosphate of a second
nucleotide; a pyrophosphate is removed and a
phosphodiester bond is formed. This same reaction
occurs in the polymerization of DNA.
3. The sequence of bases in an RNA molecule is
determined by the base sequence of the DNA
template strand. Each base added to the growing end
of an RNA chain is chosen by base pairing with the
appropriate base in the template strand; thus, the
bases C, T, G, and A in a DNA strand cause
incorporation of G, A, C, and U, respectively, in the
newly synthesized RNA molecule. The RNA is
complementary to the template DNA strand, which is
called the
coding (sense) strand
or template strand.
4. The RNA chain grows in the 5' -» 3' direction, which
is the same as the direction of chain growth in DNA
synthesis. The RNA strand and the DNA template
strand are also antiparallel.
5. RNA polymerases, in contrast with DNA
polymerases, can initiate RNA synthesis, i.e., no
primer is needed.
6
. Only ribonucleoside 5'-triphosphates participate in
RNA synthesis, and the first base to be laid down in
the initiation event is a triphosphate. Its 3'-OH group
is the point of attachment of the subsequent
nucleotide. Thus, the 5' end of a growing RNA
molecule terminates with a triphosphate. In tRNAs
and rRNAs, and in eukaryotic mRNAs, the
triphosphate group is removed.
E. coli
RNA polymerase consists of five subunits—two
identical
a
subunits and one each of /3, /3', and
o
—having
a total molecular weight of 465,000; it is one the largest
enzymes known.
The
a
subunit is easily dissociated from the enzyme and,
in fact, does so shortly after polymerization is initiated.
The term
core enzyme
describes the
a
-free unit («
2
/
3
/
3
');
the complete enzyme is called the
holoenzyme
(o^/^'cr).
In this chapter, the name RNA polymerase is used when the
holoenzyme is meant. Several different RNA polymerases
exist in eukaryotes and are described below.
An
E. coli
cell contains 3000-6000 RNA polymerase
molecules; the number is greater when cells are growing
rapidly. In eukaryotes, the number of RNA polymerase
molecules varies significantly with cell type and is greatest
in cells that actively make large quantities of protein, e.g.,
secretory cells.
25.4 Prokaryotic Transcription
The first step in prokaryotic transcription is the binding
of RNA polymerase to DNA at a particular region called
FIGURE 25-3
Model of RNA synthesis by RNA polymerase from the sense strand of DNA. See text for details.
