108 ■ CHAPTER 06 Cell DivisionCELLS
a wide variety of consumer products and that
we are probably all exposed to—can cause an
increased risk of miscarriage and babies with
birth defects.”Shuffling the DNA
As a glance at a pair of parents and their
biological children will tell you, the offspring
resulting from sexual reproduction are similar
to their parents, but—unlike the clones result-
ing from asexual reproduction—they are not
identical. Because each half of a sexually repro-
ducing organism’s DNA comes from a different
parent, meiosis and fertilization maintain the
constant chromosome number of a species
while allowing for genetic diversity within the
population.
Meiosis generates genetic diversity in two
ways: crossing-over between the paternal and
maternal members of each homologous pair of
chromosomes, as well as independent assort-
ment of these paired-up chromosomes duringmeiosis I. Crossing-over is the physical exchange
of identical chromosomal segments during
meiosis I between the nonsister chromatids in
each duplicated homologous pair. These nonsis-
ter chromatids make physical contact at random
sites along their length, and each exchange the
exact same segments of DNA (Figure 6.9). The
chromatids are said to be recombined, and the
exchange of DNA segments is known as genetic
recombination. Without crossing-over, every
chromosome inherited by a gamete would be
just the way it was in the parent cell.
The independent assortment of chromo-
somes—the random distribution of the homol-
ogous chromosomes into daughter cells during
meiosis I—also contributes to the genetic vari-
ety of the gametes produced. Each homolo-
gous chromosome pair in a given meiotic cell
orients itself independently when it lines up at
the imaginary equatorial plane known as the
metaphase plate during meiosis I, leading to
many possible combinations of maternal and
paternal chromosomes in the daughter cells
(Figure 6.10).
As with crossing-over, the independent
assortment of chromosomes creates gametes
that are likely to be genetically different from
the parent, and also from each other. Then,
during fertilization, the fusion of two gametes
adds a tremendous amount of genetic variation
because it combines a one-in-a-million egg with
a one-in-a-million sperm. These three processes
together—crossing-over, independent assort-
ment, and fertilization—give each of us our
genetic uniqueness.Ten Years Later
Hunt and her team published their results in
- “We got a firestorm,” she recalls with a
grimace. The press reported the findings exten-
sively, and many people and companies were
upset over the allegations that BPA was toxic.
Members of the plastics industry who did not
agree with the paper’s conclusions criticized
Hunt’s work. But there was more supporting
research to come. Soto and Sonnenschein had
also turned their attention to BPA because in
everyday products it is a far more common
chemical than nonylphenol and is therefore of
greater concern.
T
here are things you can do to reduce your own risk of exposure to
BPA. Today, the U.S. Food and Drug Administration recommends
that individuals not put hot or boiling liquid intended for consumption
in plastic containers made with BPA. (Some, but not all, plastics that
are marked with the recycling code 3 or 7 may be made with BPA.) The
organization also recommends discarding all bottles with scratches,
which may harbor bacteria and, if the plastic contains BPA, may lead to
greater release of the chemical.
“Get educated about your world,” says Heather Patisaul, a BPA
researcher at North Carolina State University. “You can either
become completely reliant on the information you get from media and
government, or you can educate yourself, which is vastly more useful.”
If you are concerned about BPA exposure, it is possible to cut it down
by making lifestyle changes like not eating canned food, not drinking
bottled water, and not putting plastic in the microwave. “You can be
empowered,” says Patisaul. “Those types of things can effect great
change.”
Still, BPA and similar chemicals are ubiquitous in modern life, says
Ana Soto. Ultimately, the best way to avoid them will be for government
regulators to take a stand and outlaw the use of these chemicals in
consumer products, she notes. She encourages individuals to contact
their representatives and ask them to push legislation limiting the use
of BPA in manufacturing.