Module 5 Lesson 1 - 4

Cell Division

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As you may recall from earlier science courses, life does not occur spontaneously. Instead, life comes from existing life and is organized around small units called cells. The first part of the cell theory was proposed by two German biologists, Mathias Schleiden and Theodore Schwann. Based on their observations, they concluded that all plants and animals were made of cells. This conclusion has been extended to include all living things and, since their discoveries, no exceptions have been found.

The continuity of life from one cell to another is based on the reproduction of cells through cell division and occurs as part of the cell cycle. Cell division is of two major types: mitosis and meiosis.

Mitosis is a simple cell replication that results in two identical daughter cells. Mitotic division in multi-cellular organisms is responsible for growth, development, and repair.

Meiosis is a more complex process. After a meiotic division, the resulting cells have half the genetic information. These cells are called gametes. The gametes unite in a process known as fertilization. Although meiosis takes longer and requires more energy to perform, it results in genetic variation. Genetic variation increases the chances for survival in a species. The next few lessons examine these processes in more detail.

Cell Cycle

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Our bodies are made of an amazing array of specialized cells that keep us healthy and able to maintain an internal balance in an environment that seems bent on changing. During our daily combat with the environment, cells must be replaced as they wear out. Exactly how fast this occurs depends mostly on the role those cells perform. Skin, which is constantly being scratched, rubbed, or cut, replaces itself very quickly. However, muscle or nerve cells may remain healthy for most of our lives and, therefore, may not need to divide to replace themselves. Some cells, such as red blood cells, which lack a nucleus and genetic material, or gametes, which have only one of each chromosome, will never go through a cell cycle.

The cell cycle refers to the moment a cell is created until it divides into two daughter cells. A cell cycle involves two phases: interphase (growth) and mitotic phase (division).

Interphase is the much longer phase in which the cell performs its intended function in the body. Before a cell divides by mitosis, it undergoes replication of its chromosomes during interphase. This is necessary because each new daughter cell requires a full set of chromosomes to perform the work of the cell properly. From origin through G1, S, and G2 of interphase, the cell grows, replicates its DNA, then gets ready for cell division.

The mitotic phase consists of mitosis and cytokinesis. Mitosis is the division of the genetic material and cytokinesis is the division of the cytoplasm into two identical daughter cells. If you need to refresh your understanding of this process, review the following animation. You will consider the cell cycle in detail in the next lesson.

Sister Chromatids

Before mitosis and meiosis, a cell goes through the S phase of interphase in which the cell replicates its DNA.  Therefore, after S phase, a cell has two copies of each DNA double strand. The exact copies of a single chromosome are referred to as sister chromatids and they are bound to each other in a pinched region called the centromere.

In other words, after DNA replication in S-phase, each chromosome consists of 2 sister chromatids. Once the sister chromatids separate, each one is considered to be a daughter chromosome.  This means that after anaphase, there are no chromatids (since the structures are no longer called chromatids but rather chromosomes). 

So even though the amount of DNA doubles after S phase, human somatic cells remain diploid with 46 chromosomes throughout the cell cycle. Mitosis is a process to separate the genetic material, so think about what is happening at each pole after the chromosomes separate in anaphase because these will eventually become the nuclei of two new cells. 

Do not confuse the chromatids with homologous chromosomes. The homologous chromosomes are two chromosomes of similar shape and size: one chromosome is inherited from the maternal parent and the other is from the paternal parent. They have the same genes at the same location, but they can carry different alleles. A pair of sister chromatids are exact copies of each other and are bound to each other at the centromere. 

The sister chromatids separate during mitosis and meiosis II.  Homologous chromosomes separate during meiosis I. 

Explore

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Have some practice with dealing with karyotypes before working with them in a lab. Play the game called Chromosome Mapping Game 

a process pioneered in Canada by Dr. Irene Uchida.

Self-Check

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Complete the following review questions on cell division.

1. In what three general ways is cell division important to your body?
   
2. Give the main advantage of meiosis and state the reason this is important to populations.
   
3. Differentiate among chromosome, chromatid, and chromatin.
   
4. What characteristics are common in homologous chromosomes?
   
5.  What can scientists learn from producing a karyotype of a developing fetus?