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DNA is the nucleic acid that is responsible for "programming" many of our traits. As the material that composes our genes, DNA is a fundamental molecule. In this unit, we will learn how DNA, our genetic material, is passed on from generation to generation. We will also look at how genetic diversity is created through this process and different frameworks of genetics and inheritance. We will also explore phenotypes, the observable characteristics of an organism, and how they are influenced by both genetics and the environment.
In the previous unit, we learned about mitosis. Mitosis is the process that results in two daughter cells having identical copies of the chromosomes that were in the parent cell. This type of cell division is found in normal tissue growth which consists of somatic cells. In this section, we introduce meiosis. Meiosis is marked by two nuclear divisions, as opposed to the one division found in mitosis (Figure 5.1). The end result is four cells that each have half the number of chromosomes than that of the original cell. These cells are called gametes. The difference between mitosis and meiosis can also be thought of as the difference between sexual and asexual reproduction. Many lower-order cells create entirely new organisms with each round of mitosis (asexual reproduction).
In higher organisms, each cell usually contains two similar copies of each chromosome. One of these copies is from one parent and the other is from the other parent. Together, these are called a homologous pair and each alone is called a homologue. The haploid number of a cell refers to the total number of chromosomes in a gamete. This number varies from species to species; in humans, it is 23. The diploid number of a cell refers to the total number of chromosomes in a somatic cell and is equal to two times the haploid number. If the haploid number is thought of as N the diploid number would be 2N, where N equals the number of chromosomes. In humans, the diploid number is 46. The haploid nature of gametes is vital to the process of sexual reproduction.
There are two different types of gametes in most eukaryotes: sperm and eggs. Testicles produce sperm and ovaries produce eggs. In order to create a new individual via sexual reproduction, a sperm cell needs to fertilize, or combine with an egg. When these two haploid cells unite, a diploid cell results. This specialized cell, called a zygote, can then develop into a new individual. The sexual reproduction process just described ensures that the resulting offspring will have an equal maternal and paternal genetic contribution.
Figure 5.01: Events of meiosis.
As was mentioned earlier, meiosis is marked by two nuclear divisions. The phases of these two nuclear divisions (meiosis I and meiosis II) bear the same name as those for mitosis, with a catch. The phases of meiosis I are called prophase I, metaphase I, anaphase I, telophase I, and cytokinesis I. The phases of meiosis II are called prophase II, metaphase II, anaphase II, telophase II, and cytokinesis II. In the following sections, we will review DNA replication and then go through the steps of meiosis, noting how they differ from mitosis.
