chromosome number, and this number often defines a species as being different from
another. The number of chromosomes within a nucleus is defined as theploidyof the
cell. For example, the model plantArabidopis thalianahas a total of 10 chromosomes (5
pairs), while the crop plant soybean (Glycine max) has 40 chromosomes (20 pairs).
Some plants have tremendously large genomes. For example, some lilies have hundreds
of chromosomes. Chromosomes vary in length (i.e., in the number of nucleotides that
make up the DNA molecule) and therefore vary in size when visualized under the micro-
scope. Each chromosome has hundreds to thousands of genes contained within the
sequence of DNA, along with sequences between the genes. This connecting DNA has
been historically called “junk DNA,” but current research is discovering that intergenic
DNA sequences may play several critical roles such as regulating how genes and chromo-
somes interact at higher levels.
To understand biotechnology and genetics, it is essential to define and understand how
chromosomes exist within the nucleus. Chromosomes are organized in two different
basic physical structures during the life of the cell. During most of the cells’ adult
life, the chromosome exists in a relaxed state, where the DNA is loosely wrapped
around chromosomal proteins (Fig. 2.2a). This physical state allows the DNA to be
read (transcribed and translated) so that the appropriate proteins are produced. As the
chromosomes prepare for cell division, they become tightly wound around chromosomal
proteins and are described as being in the condensed state (Fig. 2.2a). Chromosomes can
be visualized under the light microscope only when they are condensed. During different
points in the cell cycle, chromosomes may be in different conformations. Initially after
cell division, a chromosome exists as a single molecule of double-stranded DNA with
a single centromere, called achromatid(Fig. 2.2b). After the DNA synthesis phase of
the cell cycle, the chromosome exists as two molecules of identical double-stranded
DNA connected at the single centromere. The two DNA molecules within a chromosome
are calledsister chromatids, and they stay connected until they are separated by one of
the types of cell division. DNA synthesis does not represent a change in the total
chromosome number, as chromosome numbers remain the same during the lifetime of
the plant. A single chromosome then may exist in either a prereplicated (one chromatid)
or replicated state (two sister chromatids). The different states of chromosomal arrange-
ments within the life of a cell will be important as we describe cell division and sexual
reproduction.
Most cells in a plant have two copies of each chromosome, which are calledhomologous
chromosomesor achromosome pair(Fig. 2.2c). Generally speaking, one of the individual
chromosomes in a pair is derived from the maternal parent and one from the paternal parent.
Gender identity and parenting is sometimes confusing to think about in plants that have the
ability toself-fertilize(when the same plant’s pollen fertilizes the ovum), but one of
the homologous chromosomes comes from the pollen and one from the ovum even if all
the chromosomes come from the same plant. Hermaphrodites (organisms with both male
and female organs) and selfing are considered to be anomalies in the animal kingdom
but are frequent among plants. As we will discuss later in this chapter, plants have a
wide array of reproductive strategies to achieve the pairing of the chromosomes.
Most adult plant cells have two copies of all chromosomes, and the ploidy level is
defined as thediploidstate (2N). In order to sexually reproduce, the total chromosome
number is divided in half, and this reduced chromosome number in the sexual gametes
is defined as thehaploid state (N). During most of an angiosperm plant’s life, the
diploid sporophyte stage dominates and produces diploid cells during cell division.
24 MENDELIAN GENETICS AND PLANT REPRODUCTION