2. Meiosis and its Phases

Meiosis Introduction

Mitosis, as just reviewed above, is a process by which a cell can reproduce itself and the number of chromosomes and the nature of the DNA will be identical to the original parent cell. Very few species will grow or live indefinitely, so there must be some way to ensure the continuity of the species. Reproduction is the only way a species can be perpetuated, without perpetuation the species will become extinct. Reproduction can occur in several ways as vegetative propagation, such as in the development of runners in strawberry plants, or by special cells called vegetative spores which are products of mitosis. In these processes, the ‘offspring’ have identical cells and identical chromosomes to the parent cells and thus the processes are called asexual reproduction—a means without, so without sexual reproduction. Most plants, however, will undergo sexual reproduction which involves the production and recombining of sex cells called gametes. In flowering and cone-bearing plants, this involves the production of seeds. The gametes produced are male and female, and are called sperm cells and egg cells, correspondingly. When the gametes combine together, the cells fuse and form a single cell called a zygote. It is the zygote that will go on to become the plant embryo and eventually a mature, adult plant.

However, in thinking about this process, what would happen if both gametes had the same number of chromosomes as the rest of the cells in the organism? When they fused to become a zygote, they would have two times the number of chromosomes as the rest of the cells in the organism. The number of chromosomes would increase exponentially through the generations if this occurred. This is where meiosis comes in to play. Meiosis is the process by which gametes, sex cells, are formed. It is unique because gametes have exactly half of the total number of chromosomes as the rest of the cells in the parent organism. When two gametes, each with half the number of chromosomes, get together they are able to restore the chromosome number to the same as the rest of the cells in the parent organism. When the zygote develops into a plant embryo and eventually a mature plant, it will have the exact number of chromosome-specific to the species. Note that the processes and steps in meiosis are very similar to mitosis, so make certain you have a good understanding of mitosis so that you will be able to compare the two processes.

Before we get into the nitty-gritty of meiosis, keep in mind that all living cells have two sets of chromosomes—one from a male and one set from a female parent. The genes in the chromosomes may control the same characteristics but in contrasting ways—for example: genes for plant height, genes for plant color, genes for fruit color, etc—the female gamete might code for short plants, while the male gamete might code for tall plants. That is more of a genetics topic though. But you should know that the chromosomes that code for the same characteristics are called homologous chromosomes.

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