933
CHAPTER TWENTY-FOUR
Genetics and Genomics
a postage stamp, to which DNA molecules of known, short
sequences are afF xed at known points, creating a grid pattern.
A DNA microarray might include selected genes expected to be
present in a particular medical condition, or the entire human
genome. ±or example, a DNA microarray used to study cardio-
vascular disease includes thousands of genes whose protein
products control blood pressure; blood clotting; and synthe-
sis, transport, and metabolism of cholesterol and other lipids.
Using a whole genome “chip,” however, can detect genes not
expected to be active in a particular condition.
To track gene expression, a cell type of interest is
sampled and separated from its tissue. Its messenger RNA
molecules are collected and copied using a special enzyme
(reverse transcriptase) into DNA, and during the copying
process, a chemical “tag” is included that makes the DNA
fluoresce under a laser scanner. Sometimes two samples
of cells to be compared are each labeled with a different
color. The DNA copies are then added to the DNA microar-
ray. Genetic material from the sample binds complementary
sequences embedded in the microarray.
The resulting pattern of fluorescent spots seen with
a laser scanner reveals which genes are expressed in the
sampled cells. ±luorescence intensity refl ects degree of gene
expression. Software analyzes the patterns and identifies
which genes are turned on or off in a sample. The mRNAs in
a differentiated cell type refl ect instructions for “housekeep-
ing” proteins essential for all cells, as well as the proteins
that provide the cells’ particular characteristics, such as con-
tractile proteins in muscle cells or signaling proteins in cells
of the nervous or endocrine systems.
RECONNECT
To Chapter 3, Stem and Progenitor Cells, pages 104–105.
RECONNECT
To Chapter 4, Nucleic Acids and Protein Synthesis,
pages 131–133.
DNA microarrays that proF
le gene expression have var-
ied uses in clinical medicine and in basic research. In cancer
management, for example, microarray-based gene expres-
sion proF
les on tissue samples can identify cancer cells very
early, when treatment is more likely to work; estimate if
because it samples only maternal blood, yet it provides the
high accuracy of these tests. It is also more accurate than
measuring maternal serum markers.
Table 24.2
and F
gure
24.13 summarize the tests used to visualize fetal chromo-
somes as a window onto health.
PRACTICE
19
Why do deviations from the normal chromosome number of 46
aF
ect health?
20
Distinguish between polyploidy and aneuploidy.
21
How do extra sets of chromosomes or extra individual
chromosomes arise?
22
How are fetal chromosomes examined?
24.7
GENE EXPRESSION EXPLAINS
ASPECTS OF ANATOMY
AND PHYSIOLOGY
This book opened with a look at the deep roots of anatomy
and physiology. Today much more recent F
elds—genetics
and the even newer genomics—are adding to what we
know about the structure and function of the human body.
SpeciF
cally, identifying which genes are active and inac-
tive in particular cell types, under particular conditions,
can add to our understanding of physiology. This approach
goes well beyond the rare, single-gene disorders on which
the F
eld of genetics focused for many years. Gene expres-
sion monitors the proteins that a cell produces, providing
snapshots of physiology in action.
The technology that provides these glimpses of gene
function is termed
gene expression profiling,
and identi-
fying the sets of proteins in a cell is
proteomics.
Clinical
Applications 3.1 and 3.2 (pp. 82 and 87) describe single gene
disorders. In contrast, gene expression proF
ling considers
sets of genes whose functioning underlies cell survival and
specialization as well as how cells interact as they respond
to the environment and form tissues.
Devices called DNA microarrays (or “DNA chips”) are
used to reveal the subset of genes expressed in a particular cell
type. A microarray is a square of glass or nylon, smaller than
TABLE
24.2
|
Prenatal Tests
Procedure
Time (Weeks)
Source
Information Provided
Maternal serum markers
15–16
Maternal blood
Small liver may indicate increased risk of trisomy
Amniocentesis
14–16
±etal skin, urinary bladder, digestive system cells
in amniotic ²
uid
Karyotype of cell from fetus
CVS
10–12
Chorionic villi
Karyotype of cell from chorionic villus
±etal cell sorting
Not yet established
Maternal blood
Karyotype of cell from fetus
Ultrasound
Any time
Applied externally or through vagina
Growth rate, head size, size and location of organs
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