Cell Division and the Cell Cycle
Cell division is involved in growth, development and repair.
Multicellular organisms vary in size and complexity. Through cell division, a single cell becomes two cells. Those cells divide into four, and the four divide into eight, and so on. A multicellular organism grows because cell division increases the number of cells in it. As the organism develops and its cells divide, many of the cells become specialized, and most of them continue to divide.
If cell division were the only process occurring in cells, all multicellular organisms would end up as spheres of identical cells. But during development, cells become specialized to perform particular functions. These cells may take on shapes or structures that help to perform their functions. Some cells might become layered skin cells, while others might become long, thin nerve cells. These cells still have the same set of genetic material as all the other cells in an organism’s body, but as the organism develops they specialize.
Even when growth and development appear to stop, cell division is still occurring. When an organism ages or is injured, the worn-out or damaged cells need to be replaced by new cells formed when healthy cell divide. For example, the cells that make up the lining of your throat have a short life span – two to three days. Living throat cells are constantly dividing and replacing the cells that have died.
The genetic material of eukaryotic cells is organized in chromosomes.
The genetic material of a cell contains information needed for the cell’s growth and other activities. When a cell divides into two new cells, each new cell receives a full set of genetic material. Most of the genetic material in cells is contained in a molecule called DNA.
DNA
The genetic material in cells is DNA – deoxyribonucleic acid. DNA is a chemical that contains information for an organism’s growth, and functions. DNA is made of two strands of molecules joined together in a structure that resembles a twisted ladder or a double helix.
Chromosomes
In eukaryotic cells, most of the cell’s DNA is in the nucleus. During most of a cell’s life cycle, DNA exists as a mass of loose strands. While the DNA is spread throughout the nucleus, the cell performs the functions needed for survival. During this time, the DNA is duplicated, or copied.
Before a cell divides, the DNA becomes wrapped around proteins like thread around a spool. The DNA is then further compacted into structures called chromosomes. A duplicated chromosome consists of two identical structures called sister chromatids that are held together near the center by a structure called a centromere.
The cell cycle includes interphase and cell division.
All living things live, grow, and reproduce, and die in a process called a life cycle. Cells have a life cycle, too, called the cell cycle. The cell cycle is the normal sequence of development and division of a cell. The cell cycle consists of two main phases: one in which the cell carries out its functions, called interphase, and one in which the cell divides, which included mitosis. All cells divide, but only eukaryotes undergo mitosis. Each phase in the cell cycle requires a certain period of time – from hours to days to years, depending on the type of cell.
Interphase
Interphase is the part of the cell cycle during which a cell is not dividing. Much activity takes place in this phase of the cell’s life. During interphase, the cell grows to about twice the size it was when it was first produced. The cell also engages in normal life activities, such as transporting materials in and transporting wastes out. Also, cellular respiration occurs, which provides the cell with energy.
Changes that occur during interphase prepare a cell for division. Before a cell can divide, it duplicates its DNA exactly. Correct copying of the DNA is very important. It ensures that, after cell division, each new cell gets a complete set of DNA. During most of interphase, the DNA is in the unwound, loose form.
Mitosis
Mitosis is the part of the cell cycle during which the nucleus divides. Prokaryotes do not undergo mitosis because they have only one chromosome and no nucleus. In most cells, mitosis is the shortest period of the cell cycle. The function of mitosis is to move the DNA and other material in the parent cell into position for cell division. When the cell divides, each new cell gets a full set of DNA and other cell structures. The final stage of mitosis, cytokinesis, is the division of the parent cell’s cytoplasm.
As a result of mitosis, the original – or parent – cell splits into two genetically identical daughter cells. In this case, the term daughter cell does not imply gender. It is a term scientists use to refer to these new cells. Each daughter cell comes from half of the parent cell. Each daughter cell also receives a complete set of DNA from the parent cell.
Cell division produces daughter cells that are genetically identical to each other, as well as to their parent cell, which no longer exists. Being genetically identical to their parent cell helps the new cells function properly. The process of mitosis is essential in evenly dividing the genetic material between the daughter cells. Although mitosis is a continuous process, scientists divide the events of mitosis into five phases:
1. Prophase (Chromosomes form) – The DNA in the nucleus of the cell condenses into chromosomes. Each chromosome consists of two identical chromatids held together by a centromere. The membrane around the nucleus disappears.
2. Metaphase (Chromosomes line up) – The chromosomes line up down the middle of the cell.
3. Anaphase (Chromosomes separate) – The chromatids of each chromosome separate, forming two chromosomes. The chromosomes are pulled to opposite sides of the cell.
4. Telophase (Nuclei form) – A new nuclear membrane forms around each group of chromosomes. The chromosomes unravel.
5. Cytokinesis (Cytoplasm separates; Membrane pinches together) – In an animal cell, a fiber ring forms in the center dividing the cell. The fiber ring contracts, pulling the cell membrane inward. Eventually, the cell is pinched into two daughter cells.