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.
DNA
Promoter
Gene
Terminator
Newly synthesized
pre-mRNA
RNA nucleotides
RNA
rvSr*
Step 2: Translation
Nucleus
DNA
RNA
Nuclear pore
Plasma
membrane
Cytoplasm
RNA
Ribosome
Protein
Key:
A
Q RNA nucleotides pair
DNA nucleotides to
form a growing
mRNA strand.
DNA strand
being
transcribed
When the enzyme
reaches a terminator
region it falls off. The
mRNA strand leaves
the nucleus through
the nuclear pores.
w
C
E
Adenine
Guanine
Thymine
Cytosine
Uracil
mRNA
envelope
Cytoplasm
1.
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.
2.
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):
• During
transcription,
an
enzyme
called
RNA
polymerase binds to the DNA at a sequence called
a promoter.
• 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
protein.
Cells Carry Out Many Processes 65
p r o c e ss
diagram
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