104
UNIT 1
Organization of the Body
3
littered with introns. Before the newly formed RNA can be used
as a messenger, it must be processed, or edited—that is, sections
corresponding to introns must be removed. Large RNA-protein
complexes called
spliceosomes
snip out the introns and splice to-
gether the remaining exon-coded sections in the order in which
they occurred in the DNA, producing functional mRNA.
Although many introns degrade naturally, some contain ac-
tive segments (such as microRNAs) that can function to control,
interfere with, or silence other genes. Additionally, a number of
specific RNA-binding proteins, called
mRNA complex proteins
,
must become associated with the edited mRNA. Tese mRNA
complex proteins guide the export of mRNA from the nucleus,
determine its localization, translation, and stability, and check it
for premature termination codons.
Translation
A translator takes a message in one language and restates it in
another. In the
translation
step of protein synthesis, the lan-
guage of nucleic acids (base sequence) is translated into the lan-
guage of proteins (amino acid sequence).
Genetic Code
Te rules by which the base sequence of a gene
is translated into an amino acid sequence are called the
genetic
code
. For each triplet, or three-base sequence on DNA, the cor-
responding three-base sequence on mRNA is called a
codon
.
Since there are four kinds of RNA (or DNA) nucleotides, there
are 4
3
, or 64, possible codons. Tree of these 64 codons are “stop
signs” that call for termination of polypeptide synthesis. All the
rest code for amino acids.
Because there are only about 20 amino acids, some are speci-
fied by more than one codon. Tis redundancy in the genetic
code helps protect against problems due to transcription (and
translation) errors.
Figure 3.36
shows the genetic code and a
complete codon list.
Role of tRNA
±ranslation involves the mRNAs, tRNAs, and
rRNAs mentioned above. Before we get into the actual details of
the translation process, let’s look at how the tRNAs are so well
suited for their roles in translation.
Shaped like a handheld drill, tRNA is well suited to its dual
function of binding to both an amino acid and an mRNA co-
don. Te amino acid (picked up from the cytoplasmic pool) is
bound to one end of tRNA, at a region called the stem. At the
other end, the head, is its
anticodon
(an
0
ti-ko
9
don), a three-
base sequence complementary to the mRNA codon calling for
the amino acid carried by that particular tRNA. Because an-
ticodons form hydrogen bonds with complementary codons,
tRNA is the link between the language of nucleic acids and the
language of proteins. For example, if the mRNA codon is AUA,
which specifies isoleucine, the tRNAs carrying isoleucine will
have the anticodon UAU, which can bind to the AUA codons.
Tere are approximately 45 types of tRNA, each capable of
binding with a specific amino acid. Te attachment process is
controlled by an aminoacyl-tRNA synthetase enzyme and is ac-
tivated by A±P. Once its amino acid is loaded, the tRNA (now
called an
aminoacyl-tRNA
because of its amino acid cargo)
migrates to the ribosome, where its amino acid is maneuvered
into the proper position, as specified by the mRNA codons and
described below. Te ribosome is more than just a passive at-
tachment site for mRNA and tRNA. Like a vise, the ribosome
holds the tRNA and mRNA close together to coordinate the
coupling of codons and anticodons, and positions the next (in-
coming) amino acid for addition to the growing polypeptide
chain. ±o do its job, the ribosome has a binding site for mRNA
and three binding sites for tRNA: an A (aminoacyl) site for an
incoming aminoacyl-tRNA, a P (peptidyl) site for the tRNA
holding the growing polypeptide chain, and an E (exit) site for
SECOND BASE
UUG
UUA
UUC
UUU
Phe
Leu
FIRST BASE
THIRD BASE
CUG
CUA
CUC
CUU
Leu
AUA
AUC
AUU
Ile
GUG
GUA
GUC
GUU
Val
UCG
UCA
UCC
UCU
Ser
CCG
CCA
CCC
CCU
Pro
ACG
ACA
ACC
ACU
Thr
GCG
GCA
GCC
GCU
Ala
UAC
UAU
Tyr
CAG
CAA
CAC
CAU
His
Gln
AAG
AAA
AAC
AAU
Asn
Lys
GAG
GAA
GAC
GAU
Asp
Glu
UGC
UGU
Cys
Trp
CGG
CGA
CGC
CGU
Arg
AGG
AGA
AGC
AGU
Ser
Arg
GGG
GGA
GGC
GGU
Gly
UAA
Stop
UGA
Stop
AUG
Met or
Start
UAG
Stop
UGG
U
C
A
G
G
A
C
U
G
A
C
U
G
A
C
U
G
A
C
U
U
C
A
G
Figure 3.36
The genetic code.
The three bases in an mRNA
codon are designated as the first, second, and third. Each set of
three specifies a particular amino acid, represented here by an
abbreviation (see list below). The codon AUG (which specifies
the amino acid methionine) is the usual start signal for protein
synthesis. The word
stop
indicates the codons that serve as signals
to terminate protein synthesis.
Abb.*
Amino acid
Abb.*
Amino acid
Ala
alanine
Leu
leucine
Arg
arginine
Lys
lysine
Asn
asparagine
Met
methionine
Asp
aspartic acid
Phe
phenylalanine
Cys
cysteine
Pro
proline
Glu
glutamic acid
Ser
serine
Gln
glutamine
Thr
threonine
Gly
glycine
Trp
tryptophan
His
histidine
Tyr
tyrosine
Ile
isoleucine
Val
valine
*Abbreviation for the amino acid
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