There are three stages in the cell cycle. Interphase is the stage in which cells carry out the functions necessary for survival and cells that divide prepare for cell division.
Mitosis divides the duplicated contents of the cell’s nucleus into two equal parts.
Cytokinesis separates the two nuclei and cell contents into two daughter cells.
Proteins monitor the activities of the cell at checkpoints in the cell cycle. Cancer may result when errors occur in the cell cycle.
The Cell Cycle
The life of a cell is divided into three stages known as the cell cycle
(Figure 5.4). The stages of the cell cycle are interphase, mitosis, and cytokinesis. • Interphase is the stage in which cells carry out the functions necessary for survival and cells that divide prepare for reproduction.
• Mitosis divides the duplicated contents of the cell’s nucleus into two equal parts.
• Cytokinesis separates the two nuclei and cell contents into two daughter cells.
Figure 5.4 shows that interphase is the longest stage in the cell cycle.
This is a time when a cell carries out its various functions within the organism. For example, a cell in your stomach lining might be making and releasing enzyme molecules that aid in digesting the food you eat.
During interphase, the cell roughly doubles everything in its cytoplasm.
Growth and preparation
During the first phase of interphase, a cell increases in size and makes the proteins and molecules necessary for the cell to function. Some organelles begin to duplicate.
Cytokinesis growth and preparation
DNA replication continued growth and preparation
In the next phase, DNA copies or makes a “replica” of itself in a process called replication. During replication, the cell copies the 3 billion base pairs of DNA information in the nucleus of the cell. Then the cell temporarily has two complete sets of DNA. Enzymes control this process.
To replicate itself, the DNA molecule unwinds and the steps of the
DNA ladder break apart as shown in Figure 5.5. Each side then becomes a pattern or a template on which a new side forms. In section 4.1, you saw that base A will pair with base T, and base G will pair with base C.
The process of replication results in two new DNA molecules that have the same sequence of bases as the original DNA molecule.
Continued growth and preparation
After the DNA replicates, the cell continues to grow and is active making materials such as proteins for the new cells that will be formed after cytokinesis. Early biologists referred to these cells as “daughter cells,” and scientists continue to use this term today. The chromatin, which contains the replicated DNA, is in its loosely coiled form (Figure 5.6A and Figure 5.6B).
In its loosely coiled form, the DNA can be copied into
RNA so that proteins can be made in preparation for cell division. In addition, during this final phase before cell division, organelles such as mitochondria and chloroplasts will be duplicated. Chromatin, which contains the DNA, is located in the nucleus.
For plants that cannot grow roots from cuttings, growers use a method called grafting to produce new plants. In grafting, stems called scions are attached to the rooted stock (or “rootstock”) of a similar plant species (Figure 5.28A). This technique is often used to reproduce