131
CHAPTER FOUR
Cellular Metabolism
Those amino acids, in the proper order, are now represented
by a series of three base sequences, called
codons,
(ko
donz)
in mRNA. To complete protein synthesis, mRNA must leave
the nucleus and associate with a ribosome. There, the series
of codons of the mRNA is translated from the “language” of
nucleic acids to the “language” of amino acids. This process
is F ttingly called
translation
(see F g. 4.23).
Table 4.1
com-
pares DNA and RNA molecules.
Protein Synthesis
Synthesizing a protein molecule requires that the speciF ed
amino acid building blocks in the cytoplasm align in the proper
sequence along an mRNA. A second type of RNA molecule,
transcribed in the nucleus and called
transfer RNA
(tRNA),
aligns amino acids in a way that enables them to bond to each
other. A tRNA molecule consists of seventy to eighty nucle-
otides and has a complex three-dimensional shape, some-
what like a cloverleaf. The two ends of the tRNA molecule are
important for the “connector” function (see F g. 4.23).
At one end, each tRNA molecule is a binding site for a
particular amino acid. At least one type of tRNA speciF
es
each of the twenty amino acids. An amino acid must be
activated for a tRNA to pick it up. Special enzymes catalyze
this step. ATP provides the energy for an amino acid and its
tRNA to bond
(f
g. 4.24)
.
The other end of each transfer RNA molecule includes
a speciF c three nucleotide sequence, called the
anticodon,
unique to that type of tRNA. An anticodon bonds only to the
complementary mRNA codon. In this way, the appropriate
tRNA carries its amino acid to the correct place in the mRNA
sequence (F
g. 4.24).
In mRNA synthesis, RNA polymerase binds to a pro-
moter, a DNA base sequence that begins a gene. Then a sec-
tion of the double-stranded DNA unwinds and pulls apart,
exposing a portion of the gene. RNA polymerase moves
along the strand, exposing other portions of the gene. At
the same time, a molecule of mRNA forms as RNA nucle-
otides complementary to those along the DNA strand are
strung together. ±or example, if the sequence of DNA bases
is TACCCGAGG, the complementary bases in the developing
mRNA molecule will be AUGGGCUCC, as F
gure 4.23 shows.
±or different genes, different strands of the DNA molecule
may be used to manufacture RNA.
RNA polymerase continues to move along the DNA
strand, exposing portions of the gene, until it reaches a special
DNA base sequence (termination signal) that signals the end
of the gene. At this point, the RNA polymerase releases the
newly formed mRNA molecule and leaves the DNA. The DNA
then rewinds and assumes its previous double helix structure.
Each amino acid in the protein to be synthesized was
originally represented by a series of three bases in DNA.
S
C
S
S
S
S
G
G
G
U
A
U
U
P
P
P
P
S
S
S
S
A
P
P
U
S
P
P
P
P
DNA
RNA
S
G
S
C
S
S
S
S
C
G
T
A
S
S
S
S
G
C
A
U
Direction of “reading” code
P
P
P
P
P
P
P
P
P
P
FIGURE 4.21
±RNA±dif
ers From DNA in that it is single-stranded,
contains ribose rather than deoxyribose, and has uracil (U) rather than
thymine (T) as one oF its Four bases.
FIGURE 4.22
Transcription oF RNA From DNA. When an RNA
molecule is synthesized beside a strand oF DNA, complementary
nucleotides bond as in a double-stranded DNA molecule, with one
exception: RNA contains uracil nucleotides (U) in place oF thymine
nucleotides (T).
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