130
UNIT ONE
4.1
FROM SCIENCE TO TECHNOLOGY
DNA Prof
ling Frees A Prisoner
T
he human genome sequence dif
ers From
person to person because it includes 3.2
billion bits oF inFormation. Techniques
called DNA pro±
ling (or ±
ngerprinting) compare
the most variable parts oF the genome among
individuals For several purposes—to identiFy
remains at crime scenes or aFter natural disas-
ters; to con±
rm or rule out “blood” relationships;
and, increasingly, to establish innocence when
other types oF evidence are questionable. The
Innocence Project is a national litigation and pub-
lic policy organization that provides DNA testing
to people who claim that they have been wrong-
Fully convicted. So Far the Innocence Project has
exonerated more than 200 people. Among them
is Josiah Sutton.
Sutton had served Four and a halF years oF
a twenty-±
ve-year sentence For rape when DNA
proFiling established his innocence. He and a
Friend had become suspects aFter a woman in
Houston identiFied them as the men who had
raped and threatened her with a gun, leaving her
in a ±
eld. The two young men supplied saliva and
blood samples, From which DNA proFiles were
done and compared to DNA pro±
les From semen
Found in the victim and in her car. At the trial, an
employee oF the crime lab doing the DNA analy-
sis testi±
ed that the probability that Sutton’s DNA
matched that oF the evidence by chance was 1
in 694,000—a number so compelling that it led
jurors to convict him, even though Sutton did not
±
t the victim’s description oF her assailant.
A DNA pro±
le analyzes only 13 parts oF the
genome, known to vary in most populations.
Usually this is suFFicient inFormation to rule out
a suspect. Using these criteria, Sutton’s DNA at
±
rst seemed to match the evidence. The problem,
though, wasn’t in the DNA, but in the population
to which it was compared. Although Sutton’s pat-
tern may indeed have been very rare in the large
population to which it was compared, among
black men, it wasn’t rare at all—1 in 16 black men
have the exact same pattern!
Proclaiming his innocence all along, Sutton
had asked right away For an independent DNA
test, but was told he couldn’t aFFord one. So
while he was in prison, he read voraciously about
DNA pro±
ling and again, in a handwritten note,
requested retesting. Then he got lucky. Two jour-
nalists began investigating the Houston crime
laboratory. They sent inFormation on a Few cases
to a proFessor oF criminology, who immediately
saw the errors made in Sutton’s DNA analysis,
claiming that the test wasn’t even oF the qual-
ity oF a middle school science project. Retesting
Sutton’s DNA, and comparing it to a relevant pop-
ulation, proved his innocence.
Genetic Code
Genetic information speci±
es the correct sequence of amino
acids in a polypeptide chain. Each of the twenty different
types of amino acids is represented in a DNA molecule by
a triplet code, consisting of sequences of three nucleotides.
That is, the sequence C, G, T in a DNA strand represents one
type of amino acid; the sequence G, C, A represents another
type. Other sequences encode instructions for beginning or
ending the synthesis of a protein molecule, and for determin-
ing which genes are accessed for their information.
The genetic code is said to be universal because all species use the
same DNA base triplets to speciFy the same amino acids. Researchers
deciphered the code in the 1960s. When the media mentions an indi-
vidual’s genetic code, they really are reFerring to the sequence oF DNA
bases comprising a certain gene or genome—not the genetic code
(the correspondence between DNA triplet and amino acid).
DNA molecules are in the nucleus and protein synthe-
sis occurs in the cytoplasm. Because the cell must keep a
permanent copy of the genetic instructions, genetic infor-
mation must get from the nucleus into the cytoplasm for
the cell to use it. RNA molecules accomplish this transfer of
information.
RNA Molecules
RNA (ribonucleic acid)
molecules differ from DNA mole-
cules in several ways. RNA molecules are single-stranded,
and their nucleotides have ribose rather than deoxyribose
sugar. Like DNA, RNA nucleotides each have one of four
nitrogenous bases, but whereas adenine, cytosine, and gua-
nine nucleotides are part of both DNA and RNA, thymine
nucleotides are only in DNA. In place of thymine nucle-
otides, RNA molecules have uracil (U) nucleotides (
f g. 4.21
and
Appendix D, p. 949
). In RNA U pairs with A
(f g. 4.22)
.
Different types of RNA have different size ranges and func-
tions. The process of copying DNA information into an RNA
sequence is called
transcription
(trans-krip
-shun).
The ± rst step in delivering information from the nucleus
to the cytoplasm is the synthesis of
messenger RNA
(mRNA).
RNA nucleotides form complementary base pairs
with one of the two strands of DNA that encodes a particular
protein. However, just as the words in a sentence must be
read in the correct order to make sense, the base sequence
of a strand of DNA must be “read” in the correct direction
and from the correct starting point. Furthermore, only one
of the two antiparallel strands of DNA contains the genetic
message. An enzyme called RNA polymerase recognizes
the correct DNA strand and the right direction for RNA syn-
thesis. The “sentence” always begins with the mRNA base
sequence AUG
(f
g. 4.23)
.
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