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.
To Chapter 3, Stem and Progenitor Cells, pages 104–105.
To Chapter 4, Nucleic Acids and Protein Synthesis,
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-
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.
24.13 summarize the tests used to visualize fetal chromo-
somes as a window onto health.
Why do deviations from the normal chromosome number of 46
Distinguish between polyploidy and aneuploidy.
How do extra sets of chromosomes or extra individual
How are fetal chromosomes examined?
GENE EXPRESSION EXPLAINS
ASPECTS OF ANATOMY
This book opened with a look at the deep roots of anatomy
and physiology. Today much more recent F
and the even newer genomics—are adding to what we
know about the structure and function of the human body.
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
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,
fying the sets of proteins in a cell is
Applications 3.1 and 3.2 (pp. 82 and 87) describe single gene
disorders. In contrast, gene expression proF
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
Maternal serum markers
Small liver may indicate increased risk of trisomy
±etal skin, urinary bladder, digestive system cells
in amniotic ²
Karyotype of cell from fetus
Karyotype of cell from chorionic villus
±etal cell sorting
Not yet established
Karyotype of cell from fetus
Applied externally or through vagina
Growth rate, head size, size and location of organs