T ranscription • Figure 3.18
During transcription, an enzyme called RNA polymerase binds to the DNA
and makes a copy (mRNA) of the information encoded in DNA.
During transcription, RNA polymerase
binds to a promoter region on the DNA.
Step 2: Translation
Q RNA nucleotides pair
DNA nucleotides to
form a growing
When the enzyme
reaches a terminator
region it falls off. The
mRNA strand leaves
the nucleus through
the nuclear pores.
The DNA that encodes (puts into code) for the protein
(plans) is located in the nucleus (architectural firm). The
sequence of nucleotides in the DNA specifies the order
of amino acids in the protein. It takes one sequence of 3
nucleotides (codon) to code for one specific amino acid.
There are at least 20 codons to account for the various
amino acids, plus some codons for start and stop signals.
A copy of the DNA is made in the nucleus. This copy
is messenger RNA (mRNA)
(the blueprint). The
copying process is called transcription (Figure 3.18):
polymerase binds to the DNA at a sequence called
• RNA polymerase moves along the DNA strand, making
a copy of the DNA as it goes. The nucleotides of mRNA
pair up with the complementary nucleotides of DNA.
• When the enzyme reaches a sequence of DNA called
a terminator, it stops copying, falls off the DNA,
and releases the mRNA. The newly made mRNA has
a nucleotide sequence complementary to that of the
DNA so it can specify the amino acid order in the
Cells Carry Out Many Processes 65
p r o c e ss