Meiosis

Some Important Vocabulary

Before we begin discussing meiosis, there are a few vocabulary words to learn. Here they are:

Gamete - a gamete is a special cell that contains only half the normal number of chromosomes. Gametes are also known as sex cells. Male gametes are called sperm and female gametes are called eggs, or ova. The singular form of ova is ovum. A gamete is a haploid cell/

Zygote - A zygote is the cell that is created when a male gamete (sperm) fertilizes a female gamete (egg). A zygote is a diploid cell.

Haploid - Haploid refers to the number of chromosomes. A cell that is haploid contains only half the normal number of chromosomes.

Diploid - Diploid refers to the number of chromosomes. A cell that is diploid contains the normal number of chromosomes.

Why Does Meiosis Happen?

As you know, organisms that result from sexual reproduction receive half of their chromosomes from the male parent and the other half from the female parent. These chromosomes are then arranged as pairs in every cell in the organism. Obviously, offspring could not have received all of the father's chromosomes and all of the mother's chromosomes, because then the offspring would have twice as many chromosomes as it should have.

Sexual reproduction involves the joining of a male and female gamete. You will remember from the short vocabulary lesson above that gametes are haploid cells, meaning that they have only half the normal number of chromosomes. When the chromosomes in the male gamete (sperm) join with the chromosomes in the female gamete (egg), the result is a diploid cell (a zygote). This zygote grows and eventually becomes a new organism.

Since eggs and sperm contain only half the usual number of chromosomes, you might guess that they are not produced through the process of mitosis. You would guess correctly. The process that produces eggs and sperm is called meiosis.

Why is Meiosis So Confusing?

Many students are confused by meiosis. First, the name sounds very similar to mitosis. Second the stages of meiosis have the same names as the stages of mitosis. The names of the stages may be the same, but don't let that fool you. The things that are going on during each stage of meiosis are very different than the things going on in the stage of mitosis with the same name.

Here is the order of the stages of meiosis:

Interphase

Meiosis I

Prophase I

Metaphase I

Anaphase I

Telophase I

Cytokinesis

Meiosis II

Prophase II

Metaphase II

Anaphase II

Telophase II

Cytokinesis

Let's take a closer look at each of these stages.

Interphase

Interphase, you will remember from the last chapter in the Online Textbook, is the stage in the Cell Cycle when the cell is growing, copying organelles, and making a duplicate copy of its DNA.

Meoisis I

Meiosis is separated into two halves, called Meiosis I and Meiosis II. Meiosis I and Meiosis II are each separated into 4 stages. Unfortunately, those stages have the same names as the stages of mitosis even though they are not really the same. Let's take a look at what happens in each stage and see how it is different from what happens in mitosis.

Prophase I

In Prophase I, the chromosomes condense into chromatids.