C h a p t e r 5
Genetic Control of Cell Function and Inheritance
93
G
G G
G
G
G
G
C
C C
C
C
C C
C C
A
A
G A A
AU
UUU
U
CU
U
U U U
C
C
U
U
A
U
U
G
A
A
Peptide bond
Transfer RNA
"head" bearing anticodon
Ribosome
Direction of
messenger RNA
advance
Codon
Messenger RNA
Forming
protein
Amino acid
Translation.
The process of
translation involves taking the
instructions transcribed from
DNA to mRNA and transferring
them to the rRNA of ribosomes
located in the cytoplasm. It is the
recognition of the mRNA codon
by the tRNA anticodon that
ensures the proper sequence of
amino acids in a synthesized pro-
tein. In order to be functional, the
newly synthesized protein must
be folded into its functional form,
modified further, and then routed
to its final position in the cell.
2
Synthesis
Translation
Proteins are made from a standard set of amino acids,
which are joined end to end to form the long polypep-
tide chains of protein molecules. Each polypeptide chain
may have as few as 100 to more than 300 amino acids in
it. The process of protein synthesis is called
translation
because the genetic code is translated into the language
of this polypeptide assembly.
Translation requires the coordinated actions of all three
forms of RNA. It begins when mRNA contacts and passes
through the ribosome, where it bonds to rRNA. As the
RNA complex passes through the ribosome, tRNA trans-
lates its codons into complementary
anticodons
, which
determine which amino acids it then delivers to the rRNA
of ribosomes for attachment to the growing polypeptide
chain. The long mRNA strand usually travels through
and directs protein synthesis in more than one ribosome
Transcription
mRNA
U
C A
U A A G U A
U
C
C
G
AT G
AT
T TC
DNA
A
B
Transcription
factors
Promoter
region
Transcription unit
TATA
box
RNA
polymerase
Transcription
initiation site
Termination site
with a stop codon
FIGURE 5-4.
Transcription of
messenger RNA (mRNA) from DNA
double helix.
(A)
Transcription of mRNA
involves attachment of RNA polymerase
along with transcription factors to a
specific nucleotide sequence, theTATA
(thymine-adenosine-thymine-adenosine)
region on the promoter region of the
DNA.Transcription moves along the
transcription unit and terminates at the
stop codon.
(B)
Transcription creates
a complementary copy mRNA from
one of the DNA strands in the double
helix. DNA, deoxyribonucleic acid; RNA,
ribonucleic acid.
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