The part of information is from the book "Memmler's The Human Body In Health And Disease" by Barbara Janson Cohen and Jason J. Taylor
For growth, repair, and reproduction, cells must multiply to increase their numbers. The cells that form the sex cells (egg and sperm) divide by the process of meiosis, which cuts the chromosome number in half to prepare for union of the egg and sperm in fertilization. If not for this preliminary reduction, the number of chromosomes in the offspring would constantly double. All other body cells, known as somatic cells, divide by the process of mitosis. In this process, described below, each original parent cell becomes two identical daughter cells. Before mitosis can occur, the genetic information (DNA) in the parent cell must be doubled, so that each of the two new daughter cells will receive a complete set of chromosomes. For example, a cell that divides by mitosis in the human body must produce two cells with 46 chromosomes each, the same number of chromosomes that was present in the original parent cell. DNA duplicates during interphase, the stage in the life of a cell between one mitosis and the next. During this phase, DNA uncoils from its double-stranded form, and each strand takes on a matching strand of nucleotides according to the pattern of A-T, G-C pairing. There are now two strands, each identical to the original double helix. The strands are held together at a region called the centromere until they separate during mitosis. A typical cell lives in interphase for most of its cycle and spends only a relatively short period in mitosis. For example, a cell reproducing every 20 hours spends only about 1 hour in mitosis and the rest of the time in interphase.
Stages of Mitosis
Although mitosis is a continuous process, distinct changes can be seen in the dividing cell at four stages (Fig. 9).
* In prophase, the doubled strands of DNA return to their tightly wound spiral organization and become visible under the microscope as dark, threadlike chromosomes. The nucleolus and the nuclear membrane begin to disappear. In the cytoplasm, the two centrioles move toward opposite ends of the cell and a spindle-shaped structure made of thin fibers begin to form between them.
* In metaphase, the chromosomes line up across the center (equator) of the cell attached to the spindle fibers.
* In anaphase, the centromere splits and the duplicated chromosomes separate and begin to move toward opposite ends of the cell.
* As mitosis continues into telophase, a membrane appears around each group of separated chromosomes, forming two new nuclei.
Figure 9 The stages of mitosis. When it is not dividing, the cell is in interphase. The cell shown is for illustration only. It is not a human cell, which has 46 chromosomes.
Also during telophase, the plasma membrane pinches off to divide the cell. The midsection between the two areas becomes progressively smaller until, finally, the cell splits in two. There are now two new cells, or daughter cells, each with exactly the same kind and amount of DNA as was present in the parent cell. In just a few types of cells, skeletal muscle cells for example, the cell itself does not divide following nuclear division. The result, after multiple mitoses, is a giant single cell with multiple nuclei. This pattern is extremely rare in the human body. During mitosis, all the organelles, except those needed for the division process, temporarily disappear. After the cell splits, these organelles reappear in each daughter cell. Also at this time, the centrioles usually duplicate in preparation for the next cell division. Body cells differ in the rate at which they reproduce. Some, such as nerve cells and muscle cells, stop dividing at some point in development and are not replaced if they die. They remain in interphase. Others, such as blood cells, sperm cells, and skin cells, multiply rapidly to replace cells destroyed by injury, disease, or natural wearand-tear. Cells that multiply slowly may be triggered to divide when tissue is injured, as in repair of a bone fracture. Immature cells that retain the ability to divide and mature when necessary are known as stem cells (see Box 1-1). All blood cells, for example, are produced from stem cells in the red bone marrow. Research has been done on stimulating stem cells to divide into various cell types in the laboratory, but these studies have been controversial. Although it may be possible some day to use such cells to replace cells injured by disease, some people consider these studies to be unethical.
|Box 1-1 Stem Cells: So Much Potential
At least 200 different types of cells are found in the human body, each with its own unique structure and function. All originate from unspecialized precursors called stem cells, which exhibit two important characteristics: they can divide repeatedly and have the potential to become specialized cells. Stem cells come in two types. Embryonic stem cells, found in early embryos, are the source of all body cells and potentially can differentiate into any type of cell. Adult stem cells, found in babies and children as well as adults, are stem cells that remain in the body after birth and can differentiate into only a few cell types. They assist with tissue growth and repair. For example, in red bone marrow, these cells differentiate into blood cells, whereas in the skin, they differentiate into new skin cells after a cut or scrape. The potential healthcare applications of stem cell research are numerous. In the near future, stem cell transplants may be used to repair damaged tissues in treating illnesses such as diabetes, cancer, heart disease, Parkinson disease, and spinal cord injury. This research may also help explain how cells develop and why some cells develop abnormally, causing birth defects and cancer. Stem cells may also be used to test drugs before trying them on animals and humans. But stem cell research is controversial. Some argue that it is unethical to use embryonic stem cells because they are obtained from aborted fetuses or fertilized eggs left over from in vitro fertilization. Others argue that these cells would be discarded anyway and have the potential to improve lives. A possible solution is the use of adult stem cells. However, adult stem cells are less abundant and lack embryonic stem cells’ potential to differentiate, so more research is needed to make this a viable option.