When the RNA nucleotides are added during transcription, they
are linked together with covalent bonds. As RNA polymerase
moves down the strand, a single strand of RNA grows. Behind
RNA polymerase, the two strands of DNA close up by forming
hydrogen bonds between complementary bases, re-forming the
DNA double helix.
Like DNA replication, transcription uses DNA nucleotides as a
template for making a new molecule. However, in DNA replication,
the new molecule made is DNA. In transcription, the new molecule
made is RNA. In addition, in DNA replication, both strands of DNA
serve as templates, whereas in transcription, only part of one of
the two strands of DNA (a gene) serves as a template.
Transcription in prokaryotic cells occurs in the cytoplasm
(because prokaryotic cells have no nucleus); transcription in
eukaryotic cells occurs in the nucleus, where the DNA is located.
During transcription, many identical RNA molecules are made
simultaneously from a single gene, as shown in Figure 3.The RNA
being made fans out from the gene to give a “feathery” appear-
ance. The long line along the length of the “feather” is the DNA
being transcribed. The circles along the length are the RNA poly-
merase molecules. The “hairs” on the feather are the RNA chains
being made.210 CHAPTER 10How Proteins Are Made
Figure 3 Multiple copies
of RNA.In eukaryotes, RNA
polymerase adds about 60
nucleotides per second.
There are typically about 100
RNA polymerase molecules
per gene.
Modeling Transcription
You can use paper and pens to model the process
of transcription.
Materials
paper, scissors, pens or pencils (two colors), tapeProcedure- Cut a sheet of paper
into 36 squares, each
about 2.5 2.5 cm
(1 1 in.) in size.
2.To make one side of your
DNA model, line up
12 squares in a column.
Using one color, randomly
label each square with one of
the following letters: A, C, G,
or T. Each square represents
a DNA nucleotide. Use
tape to keep the squares
in a column.
3.To make the second side
of your DNA model, line up
12 squares next to the first
column. Use the same color
you used in step 2 to label
each square with the comple-
mentary DNA nucleotide.
Tape the squares together in
a column.
4.Separate the two columns.
The remaining 12 squares
represent RNA nucleotides.
Use a different color to
“transcribe” one of the
DNA strands.Analysis- Propose a reason for using
different colors for the DNA
and RNA “nucleotides.” - Predict how a change in the
sequence of nucleotides in a
DNA molecule would affect
the mRNA transcribed from
the DNA molecule.
3.Critical Thinking
Applying Information Use
your model to test your pre-
diction. Describe your results.
3E 6B