Cells can divide, and in unicellular organisms, this makes more organisms. In multicellular organisms, cell division is used for growth, development, and repair of the organism. Cell division is controlled by DNA, but exact copies of the DNA must be given to the daughter cells (note use of mother and daughter). Bacteria reproduce by a simple process called binary fission. They have one chromosome which is attached to the cell membrane. This chromosome replicates, then the two copies are pulled apart as the cell grows. Eventually the cell pinches in two to make two cells. Eukaryotes do mitosis. In mitosis, each daughter cell gets about half of the cytoplasm from the mother cell and one set or copy of the DNA.
Before cell division occurs, the cell first has to replicate the chromosomes so each daughter cell can have a set. When the chromosomes are replicated and getting ready to divide, they consist of two, identical halves called sister chromatids which are joined by a central region, the centromere. Each chromosome is one long molecule of DNA and special proteins. DNA makes up the genes, and we say that genes are on chromosomes, or chromosomes contain or are made of genes. Some of the proteins in the chromosomes turn off the genes that are not needed in that cell. For example, while every cell in your body contains exactly the same genes, you dont need your eye-color gene operational in cells in your big toe, nor toenail-shape genes active in cells in your stomach.
Two basic types of cells occur in the bodies of eukaryotes. Somatic cells are general body cells. These have the same number of chromosomes as each other within the body of an organism. The number of chromosomes in somatic cells is consistent among organisms of the same species, but varies from species to species. These chromosomes come in pairs, where one chromosome in each pair is from the mother and one is from the father. Actually, since most organisms have more than one pair of chromosomes, it would also be correct to say that the organism received one set of chromosomes from its mother and one matching set from its father, and that these sets match in pairs. The other type of cells found in eukaryotes is gametes or sex cells, consisting of eggs in females and sperm in males. These special reproductive cells have only one set (half as many) of chromosomes consisting of one chromosome from each pair. In humans ONLY, the somatic cells have 46 chromosomes arranged in 23 pairs (= two sets of 23 each), while gametes have 23 individual chromosomes (= one set). In fruit flies, somatic cells have 8 chromosomes (= 4 pairs or 2 sets) and gametes have 4 chromosomes (= 1 set). Geneticists use the term “-ploid” to refer to one set of chromosomes in an organism, and that term is typically combined with another wordstem that describes the number of sets of chromosomes present. For example, a cell with one set of chromosomes is called haploid, a cell with two sets of chromosomes is diploid, and a cell with four sets of chromosomes (not usually a “normal” condition, but sometimes possible) is tetraploid.
Technically, mitosis is specifically the process of division of the chromosomes, while cytokinesis is officially the process of division of the cytoplasm to form two cells. In most cells, cytokinesis follows or occurs along with the last part of mitosis.
Remember centrioles? They consist of nine sets of three microtubules, occur in animal cells only, and are involved in division of the chromosomes. Each animal cell has a pair of centrioles located just outside the nucleus. The two centrioles in the pair are oriented at right angles to each other. Just before mitosis, the centrioles replicate, so the cell now has four (two sets of two) as it starts mitosis.
The stages in mitosis include (interphase), prophase, metaphase, anaphase, and telophase. Remembering IPMAT or Intelligent People Meet At Three (or is that Twelve?) can help you remember the stages in order. Strictly speaking, interphase is the stage in which a cell spends most of its life and is not part of the process of mitosis, per se, but is usually discussed along with the other stages.
Interphase may appears to be a resting stage, but cell growth, replication of the chromosomes, and many other activities are taking place during this time. Near the end of interphase just before the cell starts into the other stages of mitosis, if the cell is an animal cell, the centrioles replicate so there are two pairs. At this time, the strands of DNA that make up the chromsosomes are unwound within the nucleus and do not appear as distinct chromosomes. Thus, at this stage, the genetic material is often referred to as chromatin. From here, the cell goes through all other stages of mitosis.
In prophase, the chromosomes start to coil, shorten, and become distinct. In animals, the centrioles begin to migrate to the poles of the cell. The mitotic spindle or polar fibers begin to form from the poles of the cell towards the equator. In animals, this starts as asters around the centrioles. Eventually, the spindle mechanism finishes growing toward the equator and interacts with the centromeres to line up and, later, move the chromosomes. Also at this time, the nuclear envelope starts to disintegrate.
Metaphase is characterized by the lining up of the chromosomes along the equator of the cell or what is called the metaphase plate. The nuclear envelope has totally disintegrated and the polar fibers have reached the centromeres of the chromosomes and have begun interacting with them.
In anaphase, the sister chromatids separate at the centromeres, thus can now be called chromosomes. These are pulled to the poles of the cell by the mitotic spindle.
onion root tip
In telophase, the new daughter nuclei and nuclear envelopes start to reform and the chromosomes uncoil. Telophase frequently includes the start of cytokinesis. In animal cells, cytokinesis starts with a cleavage furrow or indentation around the middle that eventually pinches in, dividing the cell in two. In plants, cytokinesis begins with a series of vesicles that form at the equator of the cell, which subsequently join until the cell is divided in two.
|Animal Cytokinesis||Plant Cytokinesis|
One interesting offshoot of the study of mitosis is tissue culture. In tissue culture, the cells to be studied are removed from the organisms body and grown on a sterile, artificial medium. When grown in this manner, typically normal cells grow one layer thick on the surface of the sterile medium and will undergo only 20 to 50 mitotic divisions then cease to be able to reproduce. Also, typically, when all cells are touching neighbors all around, they stop dividing. This phenomenon is known as contact inhibition. In sharp contrast, cancer cells will not stop growing with one layer on the surface of the medium, but grow multiple layers and fill the dish. They do not exhibit contact inhibition: they dont stop growing when touching on all sides. Also cancer cells appear to have no limit to the number of generations they can produce. Back in the mid-1950s, a biopsy of cervical cancer was removed from a woman named Henrietta Lacks and grown in tissue culture. While Ms. Lacks died long ago, HeLa cells are a widely-cultured research organism available through a number of biological supply companies. Within the past few years, an interesting issue has arisen regarding these cells: are they still human? While HeLa cells currently being grown in tissue culture are descendents of the original human cancer cells, by now they have mutated so much that its questionable whether they can still be considered human tissue, especially since they were abnormal, cancer cells to begin with.
Tissue culture is now a widely-used means of more effectively and quickly finding the right drugs to treat cancer. Typically, in the past, people with cancer were subjected to one toxic drug after another in hopes of finding one that would be effective against that particular cancer. Unfortunately, by the time the right drug was found, it frequently was too late to do any good. Now, when a person is diagnosed with cancer, a biopsy can be taken and a number of cultures of cells can be grown. Each of these cultures can be subjected to a different drug, thus enabling doctors to find the right drug sooner, while it may still be of help, and without needlessly subjecting the person to many kinds of toxic chemicals.
Within our bodies, different cells do mitosis at different rates. Skin cells continuously do mitosis and divide, thus our skin is constantly renewed and repaired. In sharp contrast, most nerve cells stop doing mitosis soon after birth (Caution: overconsumption of alcohol can kill nerve/brain cells, and they can never be replaced, they will never “grow back.”). Liver cells are somewhere in between. In a healthy adult, liver cells normally do not divide, but can divide to repair minor damage. Major liver damage or a disease like cirrhosis is too much damage to be repaired through mitosis. In contrast, it is possible to use one adult liver to do liver transplants for four babies, and if all goes well, these pieces can eventually regenerate whole livers.
Note this comparison between mitosis and meiosis.
Copyright © 1996 by J. Stein Carter. All rights reserved.
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