Principles of Plant Genetics and Breeding

38 CHAPTER 3
Table 3.2 Number of chromosomes per cell possessed by
a variety of plant species.
Scientific Chromosome
Species name number (2n)
Broad bean Vicia faba 24
Potato Solanum tuberosum 48
Maize Zea mays 20
Bean Phaseolus vulgaris 22
Cucumber Cucumis sativus 28
Wheat Triticum aestivum 42
Rice Oryza sativa 24
Tobacco Nicotiana tabacum 48
Soybean Gycine max 40
Peanut Arachis hypogeae 40
Cotton Gossypium hirsitum 52
Alfalfa Medicago sativa 32
Sugar beet Beta vulgaris 18
Sunflower Helianthus annus 34
Bermudagrass Cynodon dactylon 18, 36
(called alleles). At various stages in the plant life cycle,
a cell nucleus may divide according to one of two
processes – mitosis and meiosis.
Mitosis
Mitosis occurs only in somatic cells and is characterized
by a division of the nucleus (karyokinesis) into two so
that each daughter nucleus contains the same number
of chromosomes as the mother cell (Figure 3.2). The
cytoplasm divides (cytokinesis) so that the mitotic products
are genetically identical (equational division). This
conservative process produces new cells for growth and
maintenance of the plant. Cells in tissue culture divide
mitotically. Through the application of appropriate
chemicals and other suitable environmental conditions,
plant cells can be made to proliferate into an amorphous
mass called callus. Callus is an undifferentiated mass of
cells (cells with no assigned functions). It is a material
used in genetic engineering to receive and incorporate
foreign DNA into cells.
The nuclear division process may be disrupted (e.g.,
using a chemical called colchicine) on purpose by scientists,
by interfering with the spindle fibers (the structures
that pull the chromosomes to opposite poles of the
cell). The consequence of this action is that the chromosomes
fail to separate properly into the daughter cells.
Instead, a mitotic product may contain a duplication of
Interphase
Chromatin is diffuse
Early prophase
Chromosome visible as long
threads as chromatin condenses
Late prophase
Each chromosome duplicates
into two sister chromatids.
Nuclear membrane breaks
down at the end of prophase
Metaphase
Chromosomes align at
equitorial plate attached to
mitotic spindle
Anaphase
Centromere divides; sister
chromatids separate and move
towards corresponding poles
Telephase
Daughter chromosomes arrive
at poles; microtubules disappear;
chromatin expands; nuclear
membrane reappears; cytoplasm
divides; ultimately producing two
daughter cells (cytokinesis)
Figure 3.2 Diagrammatic presentation of mitosis in a cell
with a diploid number of 4. The male and female
chromosomes are presented in black and white. Mitosis
produces genetically identical daughter cells.
all or some of the original set of chromosomes (ploidy
modification; see Chapter 13).
Meiosis
Meiosis occurs only in specialized tissues in flowers of
plants and produces daughter cells that contain the haploid
number of chromosomes (Figure 3.3). This nuclear
division is responsible for producing gametes or spores.
A meiotic event called crossing over occurs in the
diplonema stage, resulting in genetic exchange between
non-sister chromatids. This event is a major source of
genetic variability in flowering plants. It is responsible
for the formation of new combinations of genetic material
(recombinants) for use by plant breeders. Closely linked
genes may also undergo recombination to separate
them. Hence, plant breeders sometimes take advantage
of this phenomenon of recombination to attempt to
break undesirable genetic linkages through repeated
crossing, and more importantly to forge desirable linkage
blocks. Meiosis is also critical in the life cycle of
flowering species as it pertains to the maintenance of