Multiplying and Dividing
All living organisms begin as a single cell. In organisms that arise via sexual reproduction, this very special single cell is known as a zygote. How this zygote is formed and what happens afterwards is the essence of plant growth and development. The heart of understanding how a plant grows and develops lies in knowing about the means whereby one cell becomes two. This process is known as cell division, and can occur in two different ways. Which of the two ways takes place determines whether the cells produced will contribute to the formation of the plant body, or will form into sex cells, known as gametes. The main difference in the two ways is in the difference between multiplying and dividing.
Each of the two ways has a special biological term. When one cell divides to form two cells identical to the original cell, the original cell has, essentially, duplicated itself. This first type of cell division occurs as a plant grows in size, producing more leaves, roots, and stems. Cell division like this is known as mitosis, or cell multiplication. Now, if the original cell divides, but the two resulting cells have only half the number of chromosomes that the original cell had, that cell has, essentially, reduced itself to two "half cells" or haploid cells. This second type of cell division is known as meiosis and is also referred to by the phrase, "reduction division". In meiosis, cells actually "divide in half". Cell division of this type is how sex cells, or gametes, are produced in the flowers or reproductive structures of a plant.
Pulling apart or pulling together
Achieving an understanding of exactly how these processes work will show you the reason why cloning and sexual reproduction are not the same and do not produce the same results. You see, in cloning the process involved is mitosis, while in sexual reproduction the process is meiosis.
In mitosis, the cell's complement of chromosomes must first double, or duplicate itself. For example, if a cell normally has 4 chromosomes, the first thing that must happen is these must duplicate themselves. The cell then ends up with two sets of 4, or a total of 8 chromosomes. Next, these chromosomes line up, "back to back", as it were, and then together in their separate groups of 4, move to opposite ends of the cell (see thumbnail picture above, right). A new wall forms to separate the two and you now have two cells, each with the normal set of 4 chromosomes! This happens whether you are taking a cutting that will root to form a new plant, or whether you are culturing plant tissue cells (called somatic cells) in a test tube to form little plantlets for propagation. The resulting cells (and plants) are clones of each other, with identical sets of chromosomes (barring unexpected mutations in the process!).
In meiosis, though, the results are much different, although the division process has some similarities to that of mitosis. To begin understanding meiosis, first realize that a body or somatic cell received half of its chromosomes from one parent and half from the other. This means that each chromosome has an "opposite number", as it were, with which it is similar in terms of the types of genes it has. In the diagram at left, the red chromosomes represent the contribution of one parent and the light blue, the contribution of the other. Let's say that each of a pair of chromosomes has only one gene, and that this gene specifies plant height. In this example, one of the pair will code for tallness and the other for shortness. Whichever is dominant will be how the plant looks when it is grown. Real chromosomes have multiple genes on them, but each gene will have its counterpart in the "opposite number" chromosome. A pair of chromosomes that are similar in the way that I have just described are said to be homologous. The important thing to remember is that they are not identical. Each chromosome specifies a particular expression of a set of characteristics in the plant.
Next, starting with a cell containing 4 single chromosomes, the chromosomes duplicate themselves but do not separate. Then, the doubled chromosomes pair off such that each pair is homologous. That means that each doubled chromosome from the one parent pairs off with the doubled "opposite number" from the other parent that is homologous to it. In a cell with 4 doubled chromosomes, you will end up with 2 homologous pairs of doubled chromosomes. Notice that the two groups of doubled chromosomes add up to 4, not 8 as in mitosis. These two groups arrange themselves "back to back", as it were and move to opposite ends of the cell so 2 are at one end and 2 at the other end. A wall forms between them and you end up with two cells, each with 2 doubled chromosomes within it. This first division is called meiosis I. You see that these are not identical cells because none of the doubled chromosomes separated after duplicating. Now, unlike in mitosis, a second division occurs, called meiosis II, but this time the previously doubled chromosomes do separate. Each of the two cells from meiosis I then divide into two cells with 2 single chromosomes each. The final result is four cells with 2 single chromosomes each. This is half the number of a somatic cell and so these are called haploid cells. They are the type of cell that becomes a sex cell or gamete.
Note another crucial difference here; see how some of the chromosomes are shown part red and part blue? That is to illustrate that, during the interphase, or initial duplication process, an event called crossing over took place, whereby homologous chromosomes swapped pieces to mix things up a bit. Crossing over is what helps diversify plants during sexual reproduction.
Putting them back together
The final step in sexual reproduction is when a male gamete and a female gamete come together to form a zygote, which is where we started at the beginning of the article. The two haploid cells, one male and one female, fuse and, continuing my example, 2 plus 2 yields a zygote with 4 chromosomes, the beginning of a new plant produced by sexual means. New plants form this way via many subsequent mitotic divisions (divisions via mitosis) and we enjoy them in our homes and gardens.
Image credit: Florida Center for Instructional Technology Clipart and Public Domain image