Cell Division - All new cells come from previously existing cells. New cells are formed by the process of cell division which involves replication of DNA and division of the cell’s nucleus (karyokinesis) and division of the cytoplasm (cytokinesis). There are two types of nuclear division: mitosis and meiosis. Mitosis typically results in new somatic (body or non-sex) cells. Formation of an adult organism from a fertilized egg, asexual reproduction, regeneration, and maintenance or repair of body parts (e.g. new skin after a bad sunburn) are accomplished through mitotic cell division. (yeast cell division, double click to play)
Where does one find cells undergoing mitosis? Plants and animals differ in this respect. In higher plants the process of forming new cells is restricted to special growth regions called meristems. These regions usually occur at the tips of stems or roots. In animals, cell division occurs anywhere new cells are formed or as new cells replace old ones. However, some tissues in both plants and animals rarely divide once the organism or organ is mature.
To study the stages of mitosis, you need to look for tissues where there are many cells in the process of mitosis. This restricts your search to the tips of growing plants such as the onion root tip or, in the case of animals, to developing embryos.
Plant Cell division - Roots consist of different regions. The root cap functions in protection. The apical meristem is the region that contains the highest percentage of cells undergoing mitosis. The region of elongation is the area in which growth occurs. The region of maturation is where root hairs develop and where cells differentiate to become xylem, phloem, and other tissues.
Cell Division and Chromosomes
In almost all organisms, chromosomes come in pairs called homologues. You may have noticed on the Karyotype on the left, that there are two chromosomes labeled #1, two chromosomes labeled #2, and so on.
These Homologous chromosomes have a special relationship. They carry genes for the same proteins. This means that you have two copies for any given gene on a non-sex chromosome (not X or Y). Organisms with two copies of each gene (and/or chromosome) are called diploid. We should note that, there are organisms that have one, three, four, or even six copies of each chromosome in their genome.
These two copies of every gene are present in each somatic cell of a diploid organism. However, even though the the two copies of the gene have information for making the same protein, they need not be identical. They are sometimes found as alleles (different forms of a gene). For instance you might have a normal copy of the gene for Alcohol Dehydrogenase on one chromosome and a mutant (perhaps nonfunctional) copy on the other.
Thus, the alleles an individual carries for any one type of gene can be identical or different.
Alleles can be normal, i.e. exhibiting no abnormal phenotypic effect, or they can be mutant, i.e. variants having an observable phenotype that is different from normal.
Use the animation above to practice your understanding of homologous chromosomes and alleles. To better understand the animation, pretend that mutant alleles are labeled RED and normal alleles labeled BLACK. A gene whose name is shown both red and black has two different forms (alleles) of the same gene.