A cell’s cycle of growing and dividing is called the cell cycle. The cell cycle has three main stages. They are described in the table on the next page. During interphase, the cell grows, carries out cellular functions, and copies its DNA. Interphase is followed by mitosis, the period when the nucleus divides. Cytokinesis follows mitosis and is the stage when the cytoplasm divides and two cells are created.
The time it takes a cell to complete the cell cycle varies depending on the type of cell. A typical animal cell takes 12–24 hours to complete the cell cycle. Some cells might complete the cycle in eight minutes. Other cells might take as long as a year to complete one cycle.
What happens during interphase?
The stages of the cell cycle are shown in the figure below. Most of the cell cycle is taken up by interphase. During interphase the cell grows, performs normal cell functions, and copies its DNA in preparation for cell division. Interphase is divided into three stages: G1, S, and G2, also called Gap 1, Synthesis, and Gap 2. After Gap 2, the cell begins mitosis. As soon as a cell is finished dividing, it enters the G1 stage of interphase. During this stage, a cell grows, performs normal cell functions, and prepares to copy its DNA. Some cells, such as muscle and nerve cells, exit the cycle at this stage and do not divide again. The second stage of interphase is the synthesis, or S stage. During this stage, the cell copies its DNA forming 2 identical copies that exist as sister chromatids and are connected by a centromere. The G2 stage is after the S stage. This is the period when the cell prepares for the division of its nucleus. A protein that makes microtubules for cell division is synthesized at this time. The cell also makes sure that it is ready to move forward in the cell cycle. When preparations are complete, the cell enters mitosis.
What happens during mitosis?
Recall that during interphase, the cell copies its DNA in preparation for cell division. Mitosis follows interphase. During mitosis the two identical copies of DNA separate. Each copy will become part of a new cell, called a daughter cell. Daughter cells are genetically identical because they each have the same DNA. Mitosis increases the number of cells as a young organism grows to its adult size. Mitosis also replaces damaged cells, such as skin cells that are damaged when you get a cut.
What are the stages of mitosis?
Like interphase, mitosis is divided into stages. These four stages are prophase, metaphase, anaphase, and telophase. The figure on the next page shows the four stages of mitosis. Follow the diagram as you read about each stage.
What happens during prophase?
The first and longest stage of mitosis is called prophase. Before prophase, DNA is in a relaxed, or unwound, form known as chromatin. During prophase, chromatin becomes tightly wound, or condenses, into chromosomes. During prophase, each chromosome is X-shaped. The left half of the X is one chromosome copy. The right half is an identical copy. Each half of the X is a sister chromatid containing identical copies of DNA. The sister chromatids are attached near the center of the chromosome by a structure called the centromere. The centromere ensures that a complete copy of the DNA becomes part of each daughter cell at the end of the cell cycle.
What happens at the end of prophase?
As prophase continues, the nucleolus seems to disappear. Structures called spindle fibers form in the cytoplasm. In animal and protist cells, centrioles migrate to opposite ends, or poles, of the cell. Star-shaped aster fibers come out of the centrioles. Spindle fibers, centrioles, and aster fibers are all made of microtubules. These structures form the spindle apparatus which helps move and organize the chromosomes before cell division. Centrioles are not present in plant cells. As prophase ends, the nuclear envelope disappears. The spindle fibers attach to the sister chromatids of each chromosome on both sides of the centromere and attach to opposite poles of the cell. One spindle fiber connects the centromere to one pole of the cell. The other spindle fiber connects the centromere to the opposite pole of the cell. This ensures that each new cell gets one copy of the DNA.
What happens during metaphase and anaphase?
During the second stage of mitosis, metaphase, the chromatids are pulled by motor proteins along the spindle apparatus toward the center of the cell. The chromatids line up in the middle, or the equator, of the cell. If metaphase is completed successfully, each daughter cell will have a copy of each chromosome. During anaphase, the sister chromatids are pulled apart. The microtubules of the spindle apparatus shorten and pull at each centromere to separate into two identical chromosomes. At the end of anaphase, the microtubules move each identical chromosome toward the poles of the cell.
What happens during telophase?
Telophase is the final stage of mitosis. During telophase, the chromosomes arrive at the poles of the cell and begin to change back into chromatin. Two new nuclear membranes form around each set of chromosomes, the nucleoli reappear, and the spindle apparatus is taken apart, as shown below.
What happens during cytokinesis?
Near the end of mitosis, cytokinesis begins. During cytokinesis, the cytoplasm divides. The result of cytokinesis is two daughter cells, each with an identical nucleus. In animal cells, cytokinesis is accomplished by using microtubules to constrict, or pinch, the cytoplasm of the cell in half. The area where constriction occurs is called the furrow. The cell splits into two daughter cells.
How is cytokinesis different in plant cells?
Plant cells complete cell division a different way. Recall that plant cells are surrounded by a rigid cell wall. During cytokinesis, plant cells form a new structure, called the cell plate, between the two daughter nuclei. New cell walls then form on either side of the cell plate, dividing the cell into two identical daughter cells.