10 regulation by sirnas and mirnas which is extensive

10. Regulation by siRNAs and miRNAs, which is extensive in
eukaryotes, is absent from bacterial cells.
Model Genetic Organism
The Plant Arabidopsis thaliana
Much of the early work in genetics was carried out
on plants, including Mendel’s seminal discoveries
in pea plants as well as discoveries in important
aspects of heredity, gene mapping, chromosome genetics,
and quantitative inheritance in corn, wheat, beans, and
other plants. However, by the mid-twentieth century, many
geneticists had turned to bacteria, viruses, yeast, Drosophila,
and mouse genetic models. Because a good genetic plant
model did not exist, plants were relatively neglected, partic-
ularly for the study of molecular genetic processes.
This neglect of plants changed in the last part of the
twentieth century with the widespread introduction of a
new genetic model organism, the plant Arabidopsis thaliana
( Figure 12.21 ). Arabidopsis thaliana was identified in the
sixteenth century, and the first mutant was reported in 1873;
but this species was not commonly studied until the first
detailed genetic maps appeared in the early 1980s. Today,
Arabidopsis figures prominently in the study of genome
structure, gene regulation, development, and evolution in
plants, and it provides important basic information about
plant genetics that is applied to other economically impor-
tant plant species.
Advantages of Arabidopsis as a model genetic
organism
The thale cress Arabidopsis thaliana is a
member of the mustard (Brassicaceae) family and grows
as a weed in many parts of the world. Except in its role as
a model genetic organism, Arabidopsis has no economic
importance, but it has a number of characteristics that make
it suitable for the study of genetics. As an angiosperm, it has
features in common with other flowering plants, some of
which play critical roles in the ecosystem or are important
sources of food, fiber, building materials, and pharma-
ceuticals. Arabidopsis ’s chief advantages are its small size
(maximum height of 10 to 20 cm), prolific reproduction,
and small genome (see Figure 12.21).
Arabidopsis thaliana completes development—from
seed germination to seed production—in about 6 weeks. Its
small size and ability to grow under low illumination make
it ideal for laboratory culture. Each plant is capable of pro-
ducing from 10,000 to 40,000 seeds, and the seeds typically
have a high rate of germination; so large numbers of prog-
eny can be obtained from single genetic crosses.
Another key advantage for molecular studies is Arabi-
dopsis ’s small genome, which consists of only 125 million
base pairs of DNA on five pairs of chromosomes, compared
with 2.5 billion base pairs of DNA in the maize genome and
16 billion base pairs in the wheat genome. The genome of
A. thaliana was completely sequenced in 2000, providing
detailed information about gene structure and organization
in this species. A number of variants of A. thaliana —called
ecotypes—that vary in shape, size, physiological characteris-
tics, and DNA sequence are available for study.