subject is not needed to make an important contribution in science. To continue our
discussion of plant reproduction, we must describe the two types of cell division that
separate chromosomes from one another during the life of the cell.
2.3 Mitosis and Meiosis
Mendel’s observations and subsequent research prompted cell biologists to study the move-
ment of chromosomes during the process of cell division. Plant growth and sex cell production
are the result of two different types of cell division: chromosome copying (mitosis) and
chromosome reducing (meiosis). Most cells in a plant and any other complex organism
undergo an exact copying process in which the original chromosome number remains the
same. This process that allows simple plant growth is calledmitosis, in which a cell divides
into two exact copies of the original (Fig. 2.7). In mitosis, the chromosome number is main-
tained in each daughter cell as a result of the division of sister chromatids at the centromere. In
order to proceed through sexual reproduction, cells must undergo the process ofmeiosis,a
form of cell division where the resultant cells have half (haploid) the total number of chromo-
somes (Fig. 2.8). If the chromosome number were not reduced in sex cells (gametes), the
number of chromosomes would double after each generation of sexual reproduction. This,
of course, is not the case, as each plant species generally retains its chromosome number
over generations. Meiosis allows for two haploid cells to join during fertilization to reconstitute
the two copies of each chromosome in the progeny. Mitosis and meiosis are the two processes
by which a cell may divide, and each process has a different goal according to the total number
of chromosomes required in the daughter cells.
Figure 2.7.The stages of mitosis based on arrangement of the chromosomes.30 MENDELIAN GENETICS AND PLANT REPRODUCTION