Intervening DNA
in Eukaryotic Genes
While it is tempting to think of a gene as an unbroken stretch of
nucleotides that code for a protein, this simple arrangement is usually
found only in prokaryotes. In eukaryotes, many genes are interrupted
by (IN trahnz)—long segments of nucleotides that have no
coding information. (EK sahnz) are the portions of a gene that
are translated (expressed) into proteins. After a eukaryotic gene is
transcribed, the introns in the resulting mRNA are cut out by complex
assemblies of RNA and protein called spliceosomes.The exons that
remain are “stitched” back together by the spliceosome to form a
smaller mRNA molecule that is then translated.
Many biologists think this organization of genes adds evolution-
ary flexibility. Each exon encodes a different part of a protein. By
having introns and exons, cells can occasionally shuffle exons
between genes and make new genes. The thousands of proteins that
occur in human cells appear to have arisen as combinations of only
a few thousand exons. Some genes in your cells exist in multiple
copies, in clusters of as few as three or as many as several hundred.
For example, your cells each contain 12 different hemoglobin genes,
all of which arose as duplicates of one ancestral hemoglobin gene.
Exons
introns
218 CHAPTER 10How Proteins Are Made
The “int” in the word intron
comes from the “int” in the
word intervening.The “ex”
in the word exoncomes
from the “ex” in the word
expressed.
Modeling Introns and Exons
You can use masking tape to represent introns
and exons.
Materials
masking tape, pens or pencils (two colors), metric
ruler, scissors
Procedure
1.Place a 15–20 cm strip of
masking tape on your desk.
The tape represents a gene.
2.Use two colors to write the
words appropriately joined on
the tape exactly as shown in
the diagram below. Space the
letters so that they take up
the entire length of the strip
of tape. The segments in one
color represent introns; those
in the other color represent
exons.
- Lift the tape. Working
from left to right, cut
apart the groups of letters
written in the same color.
Stick the pieces of tape to
your desk as you cut them,
making two strips according
to color and joining the pieces
in their original order.
Analysis
- Determine from the result-
ing two strips which strip is
made of “introns” and which
is made of “exons.”
2.Critical Thinking
Predicting Outcomes
Predict what might happen to
a protein if an intron were not
removed.
Transcription
Introns
removed
mRNA leaves
nucleus
Translation
Exon Intron
mRNA
(exons spliced together)
mRNA
3E 6A
