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Transcfption Versus neplication
Like DNA replication,
transcription uses DNA as a template for making a newmolecule.
ln transcription, a new molecule of RNA is made from the DNA.
However, in DNA replication, a new molecule ofDNA is made from
the DNA. Also, in DNA replication, both strands ofDNA se e as
templates.In contrasl, du ngtansc ption only part of one ofthe
two stands ofDNA (a gene) serves as a template for the new RNA.
scl,*xs.
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Topie Genetic Code
Code:HX80648
The Genetic Gode: Three-Letter "Words"
A gene can be thought ofas a "sentence" of "words" that is first
transcribed and then tmnslated into a functional protein. Once a
section ofa gene is tmnscribed into mRNA, the words can be caffied
ftom thenucleus todbosomes in the cfoplasm. There, thewords are
translated to make proteins.
Codoos of mRI{A Each ofthewords in mRNA is made up of
codon
three adjacent nucleotide bases. Each three-nucleotide sequence
is called a codon. Each codon is matched to I of20 amino acids or
acts as a start or stop si8nal for the ffanslation stage. Flgure 13 shows
this matching system for each ofthe possible 64 mRNA codons. For
example, the codon GCU specifres the amino acid alanine. Notice
that each codon specifies only one amino acid but thatseveral amino
acids have more than one codon. This system of matching codons
sequence that encode! an am no acrd or si{nilies
a
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fi[st
Ftnd the
base of the nRNA
codon ln this column
sta(signalor
FiElure
13
a stopsrEnal
The amino acid coded
for by a speciiic mRNA codon can be
determined byfollowlngthethree steps
and amino acids is called the genetic code.>fhe genetic code is based
on codons that each represent a specific amino acid.
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in DNA and mRNA. a thrce-nucleotide
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beloiv.
What amino acid does the
codon GM code fo/?
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matches the second
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AGU
ACC -,
AUG-Stan
ACG
AAG lLYsine
AGG ]Arginine
GUU I
GUC I.
GCirl
GAU-'JAspartic
GGI]
U
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GAC
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c
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AUA
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SECTIoN
3
RNA and Gene
Expression 307