CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

9. Cell Division—Proliferation and Reproduction

9.2. The Cell Cycle and Mitosis

The cell cycle consists of all the stages of growth and division for a eukaryotic cell (figure 9.2). All eukaryotic cells go through the same basic life cycle, but different cells vary in the amount of time they spend in the various stages. The cell’s life cycle is a continuous process without a beginning or an end. As cells complete one cycle, they begin the next.

FIGURE 9.2. The Cell Cycle

Cells spend most of their time in interphase. Interphase has three stages—G₁, S and G₂, During G₁ of interphase, the cell produces tRNA, mRNA, ribosomes, and enzymes for everyday processes. During the S phase of interphase, the cell synthesizes DNA to prepare for division. During G₂ of interphase, the cell produces the proteins required for the spindles. After interphase, the cell can enter mitosis. Mitosis has 4 stages—prophase, metaphase, anaphase, and telophase. The nucleus is replicated in mitosis and two cells are formed by cytokinesis. Once some organs (i.e., the brain) have completed development, some cells (i.e., nerve cells) enter theG_q stage and stop dividing.

Interphase is a stage of the cell cycle during which the cell engages in normal metabolic activities and prepares for the next cell division. Most cells spend the greater part of their life in the interphase stage. After the required preparatory steps the cell proceeds into the stages of mitosis. Mitosis is the portion of the cell cycle in which the cell divides its genetic information. Scientists split interphase and mitosis into smaller steps in order to describe how the cell divides in more detail. Interphase contains three distinct phases of cell activity—G₁, S, and G₂. During each of these parts of interphase, the cell is engaged in specific activities needed to prepare for cell division.

The G₁ Stage of Interphase

During the G₁ stage of interphase, the cell gathers nutrients and other resources from its environment. These activities allow the cell to perform its normal functions. Gathering nutrients allows the cell both to grow in volume and to carry out its usual metabolic roles, such as producing tRNA, mRNA, ribosomes, enzymes, and other cell components. In multicellular organisms, the normal metabolic functions may be producing proteins for muscle contraction, photosynthesis, or glandular-cell secretion.

Often, a cell stays in G₁ for an extended period. This is a normal process. For cells that remain in the G₁ stage for a long time, the stage is often renamed the G₀ stage, because the cell is not moving forward through the cell cycle. In the G₀ stage, cells may become differentiated, or specialized in their function, such as becoming nerve cells or muscle cells. The length of time cells stay in G₀ varies. Some cells entering the G₀ stage remain there more or less permanently (e.g., nerve cells), while others can move back into the cell cycle and continue toward mitosis (e.g., cells for bone repair, wound repair). Still others divide more or less continuously (e.g., skin-, blood-forming cells).

If a cell is going to divide, it commits to undergoing cell division during G₁ and moves to the S stage.

The S Stage of Interphase

A eukaryotic cell’s genetic information, DNA, is found as a component of chromosomes. During the S stage of interphase, DNA synthesis (replication) occurs. With two copies of the genetic information, the cell can distribute copies to the daughter cells in the chromosomes. By following the cell’s chromosomes, you can follow the cell’s genetic information while mitosis creates two genetically identical cells.

The structure of a chromosome consists of DNA wrapped around histone proteins to form chromatin. The individual chromatin strands are too thin and tangled to be seen with a compound microscope. As a cell gets ready to divide, the chromatin coils and becomes visible as a chromosome. As chromosomes become more visible at the beginning of mitosis, you can see two threadlike parts lying side by side. Each parallel thread is called a chromatid (figure 9.3). A chromatid is one of two parallel parts of a chromosome. Each chromatid contains one DNA molecule. After DNA synthesis, the chromosome contains two DNA molecules, one in each chromatid. Sister chromatids are the 2 chromatids of a chromosome that were produced by replication and that contain the identical DNA. The centromere is the sequence of bases at the site where the sister chromatids are attached.

FIGURE 9.3. Chromosomes

During interphase, when chromosome replication occurs, the two strands of the DNA molecule unzip and two identical double-stranded DNA molecules are formed, which remain attached at the centromere. Each chromatid contains one of these DNA molecules. The two identical chromatids of the chromosome are sometimes termed a dyad, to reflect that there are two double-stranded DNA molecules, one in each chromatid. The DNA contains the genetic data. Different genes are shown here as different shapes along the DNA molecule.

The G2 Stage of Interphase

The final stage of interphase is G₂. During the G₂ stage, final preparations are made for mitosis. The cell makes the cellular components it will need to divide successfully, such as the proteins it will use to move the chromosomes. At this point in the cell cycle, the nuclear membrane is intact. The chromatin has replicated, but it has not coiled and so the individual chromosomes are not yet visible (figure 9.4). The nucleolus, the site of ribosome manufacture, is also still visible during the G₂ stage.

FIGURE 9.4. Interphase

Growth and the production of necessary organic compounds occur during this phase. If the cell is going to divide, DNA replication also occurs during interphase. The individual chromosomes are not visible, but a distinct nuclear membrane and nucleolus are present. (Some cells have more than one nucleolus.)

9.2. CONCEPT REVIEW

3. What is the cell cycle?

4. What happens to chromosomes during interphase?