broken at the centromere and the two daughter cells each acquire a complete copy of the
cell’s genome.
2.3.2 Meiosis
Meiosis is the type of cell division used to make sex cells or gametes. The goal of meiosis is
to generate haploid cells, which have half the number of chromosomes as the original cell.
Meiosis is a two-step process, where the original cell undergoes two divisions in order to
make haploid cells. In the first division (I), homologous chromosomes line up together
and separate from one another to generate haploid cells. In the second meiotic division
(II), sister chromatids of each chromosome divide in a process identical to mitosis.
It can be said that meiosis simply adds a reductive division to separate the homologous
chromosomes, and then goes through a mitotic division of remaining chromosomes.
The two meiotic divisions proceed in stepwise fashion similar to that described above for
mitosis, with the condensation of the chromosomes, alignment in the center of the cell,
pulled to opposite poles, followed by cell division. The differences occur in how the homo-
logous chromosomes interact with one another (Fig. 2.8). In the first meiotic division, the
homologous chromosomes find one another and form a structure called thetetrad. During
prophase I, the homologous chromosomes interact with one another, which allows for the
transfer of genetic material between the homologous chromosomes in a process known as
crossing over(orcrossover)orrecombination. Recombination in this fashion generates
diversity when the homologous chromosomes swap DNA. Metaphase I is also different
in meiosis, as the homologous chromosomes in the tetrad straddle the metaphase plate,
with each chromosome on one side. During anaphase I, complete homologous chromo-
somes, each with their two sister chromatids, are pulled to the opposite poles of the cell.
The centromere remains completely intact as each separate homologous chromosome is
pulled to the opposite end of the cell. After cell division, each daughter cell has only
one of each homologous chromosome and therefore only half of the genetic material.
The first meiotic division results in a reduction of genetic material by half.
The second meiotic division is exactly like mitosis, but with half the genetic material per
cell, with the chromosomes lining up at the metaphase platewith the sisterchromatids on each
side of the cell. The centromeres are then broken, and the sister chromatids are pulled to oppo-
site ends of the cell. This division results in two cells with identical genetic material, which is
exactly the same process as mitosis, except with a haploid number of chromosomes. Meiosis
and mitosis are similar processes but differ in how the chromosomes are pulled apart. In
mitosis, the complete genome is retained in the daughter cells, while meiosis reduces the
genome size in half by separating the homologous chromosomes. Therefore, growth is
achieved by mitosis as numerous exact copies of the diploid cells are made, allowing for
each cell to function in the adult plant. Meiosis prepares for sexual reproduction by generating
haploid cells, which will be combined by the process of fertilization with other haploid cellsto
reconstitute the normal number of two homologous chromosomes.
2.3.3 Recombination
Recombination, or the crossing over of DNA between chromosomes during meiosis, a
process first documented in Drosophila (Bridges 1916), is a critically important process
that generates genetic diversity in plant species. If recombination did not occur, each
chromosome would be essentially static and “immortal,” with the same alleles always
32 MENDELIAN GENETICS AND PLANT REPRODUCTION