Cellular Structure, Proteins, and Metabolism
57
of a sentence—they indicate the end of a genetic message has
been reached.
The genetic code is a universal language used by all living
cells. For example, the triplets specifying the amino acid tryp-
tophan are the same in the DNA of a bacterium, an amoeba, a
plant, and a human being. Although the same triplets are used
by all living cells, the messages they spell out—the sequences
of triplets that code for a specifi
c protein—vary from gene to
gene in each organism. The universal nature of the genetic
code supports the concept that all forms of life on earth
evolved from a common ancestor.
Before we turn to the specifi c mechanisms by which
the DNA code operates in protein synthesis, an important
qualifi cation is required. Although the information coded in
genes is always fi rst transcribed into RNA, there are several
classes of RNA—including messenger RNA, ribosomal RNA,
and transfer RNA. Only messenger RNA
directly
codes for
the amino acid sequences of proteins, even though the other
RNA classes participate in the overall process of protein syn-
thesis. For this reason, the customary defi
nition of a gene as
the sequence of DNA nucleotides that specifi es the amino acid
sequence of a protein is true only for the vast majority of genes
that are transcribed into messenger RNA.
Protein Synthesis
To repeat, the fi rst step in using the genetic information in
DNA to synthesize a protein is called transcription, and it
involves the synthesis of an RNA molecule containing coded
information that corresponds to the information in a single
gene. The class of RNA molecules that specifi es the amino
acid sequence of a protein and carries this message from DNA
to the site of protein synthesis in the cytoplasm is known as
messenger RNA (mRNA).
Transcription: mRNA Synthesis
Recall from Chapter 2 that ribonucleic acids are single-chain
polynucleotides whose nucleotides differ from DNA because they
contain the sugar ribose (rather than deoxyribose) and the base
uracil (rather than thymine). The other three bases—adenine,
guanine, and cytosine—occur in both DNA and RNA. The pool
of subunits used to synthesize mRNA are free (uncombined)
ribonucleotide triphosphates: ATP, GTP, CTP, and UTP.
Recall also that the two polynucleotide chains in DNA
are linked together by hydrogen bonds between specifi c pairs
of bases: A–T and C–G. To initiate RNA synthesis, the two
strands of the DNA double helix must separate so that the
bases in the exposed DNA can pair with the bases in free ribo-
nucleotide triphosphates (
Figure 3–18
). Free ribonucleotides
containing U bases pair with the exposed A bases in DNA,
and likewise, free ribonucleotides containing G, C, or A bases
pair with the exposed DNA bases C, G, and T, respectively.
Note that uracil, which is present in RNA but not DNA, pairs
with the base adenine in DNA. In this way, the nucleotide
sequence in one strand of DNA acts as a template that deter-
mines the sequence of nucleotides in mRNA.
The aligned ribonucleotides are joined together by the
enzyme
RNA polymerase,
which hydrolyses the nucleotide
triphosphates, releasing two of the terminal phosphate groups
and joining the remaining phosphate in covalent linkage to
the ribose of the adjacent nucleotide.
Because DNA consists of
two
strands of polynucleotides,
both of which are exposed during transcription, it should the-
oretically be possible to form two individual RNA molecules,
one from each strand. However, only one of the two potential
RNAs is ever formed. Which of the two DNA strands is used
as the
template strand
for RNA synthesis from a particular
gene is determined by a specifi c sequence of DNA nucleotides
called the
promoter,
which is located near the beginning of
A
A
G
U
U
U
C
A
U
G
C
A
G
A
T
A
A
G
T
A
C
T
T
A
A
G
T
T
T
C
A
T
G
A
C
G
A
U
A
G
A
C
T
T
C
Codon 1
Codon 2
Codon 3
Codon n
Template strand
of DNA
Nontemplate strand
of DNA
DNA
Primary RNA transcript
Promoter base sequence
for binding RNA polymerase
and transcription factors
Figure 3–18
Transcription of a gene from the template strand of DNA to a primary mRNA transcript.
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