A clone is a group of identical cells that share a common ancestry, meaning they are derived from the same mother cell.

Clonality implies the state of a cell or a substance being derived from one source or the other. Thus there are terms like polyclonal--derived from many clones; oligoclonal--derived from a few clones; and, monoclonal--derived from one clone. These terms are most commonly used in context of antibodies or immunocytes.

Contexts

This concept of clone assumes importance as all the cells that form a clone share common ancestry, which has a very significant consequence: shared genotype

Genotype

The genotype is the genetic makeup of a cell, an organism, or an individual usually with reference to a specific character under consideration...

.

1. One of the most prominent usage is in describing a clone of B cell
   B cell
   B cells are lymphocytes that play a large role in the humoral immune response . The principal functions of B cells are to make antibodies against antigens, perform the role of antigen-presenting cells and eventually develop into memory B cells after activation by antigen interaction...
   
   s. The B cells in the body have two important phenotypes (functional forms) -- the antibody secreting, terminally differentiated (that is, they cannot divide further) plasma cells, and the memory and the naive cells -- both of which retain their proliferative potential.
2. Another important area where one can talk of "clones" of cells is neoplasms. Many of the tumors derive from one (sufficiently) mutated cell, so they are technically a single clone of cells. However, during course of cell division, one of the cells can get mutated further and acquire new characteristics to diverge as a new clone. This is how, in fact, drug resistance could be acquired in many cases.
3. All the granulosa cells in a Graafian follicle
   Ovarian follicle
   Ovarian follicles are the basic units of female reproductive biology, each of which is composed of roughly spherical aggregations of cells found in the ovary. They contain a single oocyte . These structures are periodically initiated to grow and develop, culminating in ovulation of usually a single...
   
    are in fact a clone.
4. Paroxysmal nocturnal hemoglobinuria
   Paroxysmal nocturnal hemoglobinuria
   Paroxysmal nocturnal hemoglobinuria , sometimes referred to as Marchiafava-Micheli syndrome, is a rare, acquired, potentially life-threatening disease of the blood characterised by complement-induced intravascular hemolytic anemia , red urine and thrombosis...
   
    is a disorder of bone marrow cells resulting in shortened life of red blood cells, which is also a result of clonal expansion, i.e., all the altered cells are originally derived from a single cell, which also somewhat compromises the functioning of other "normal" bone marrow cells.

Basis of clonal proliferation

Most other cells cannot divide indefinitely as after a few cycles of cell division the cells stop expressing an enzyme telomerase

Telomerase

Telomerase is an enzyme that adds DNA sequence repeats to the 3' end of DNA strands in the telomere regions, which are found at the ends of eukaryotic chromosomes. This region of repeated nucleotide called telomeres contains non-coding DNA material and prevents constant loss of important DNA from...

. The genetic material, in the form of deoxyribonucleic acid (DNA), continues to shorten with each cell division, and cells eventually stop dividing when they sense that their DNA is critically shortened. However, this enzyme in "youthful" cells replaces these lost bits (nucleotides) of DNA, thus making almost unlimited cycles of cell division possible. It is believed that the above mentioned tissues have a constitutional elevated expression of telomerase. When ultimately many cells are produced by a single cell, clonal expansion is said to have taken place.

Concept of clonal colony

A somewhat similar concept is that of clonal colony

Clonal colony

A clonal colony or genet is a group of genetically identical individuals that have grown in a given location, all originating vegetatively from a single ancestor. In plants, an individual in such a population is referred to as a ramet...

(also called a genet), wherein the cells (usually unicellular) also share a common ancestry, but which also requires the products of clonal expansion to reside at "one place", or in close proximity. A clonal colony would be well exemplified by a bacterial culture colony, or the bacterial films that are more likely to be found in vivo (e.g., in infected multicellular hosts). Whereas, the cells of clones dealt with here are specialized cells of a multicellular organism (usually vertebrates), and reside at quite distant places. For instance, two plasma cells belonging to the same clone could be derived from different memory cells (in turn with shared clonality) and could be residing in quite distant locations, such as the cervical (in the neck) and inguinal (in the groin) lymph node

Lymph node

A lymph node is a small ball or an oval-shaped organ of the immune system, distributed widely throughout the body including the armpit and stomach/gut and linked by lymphatic vessels. Lymph nodes are garrisons of B, T, and other immune cells. Lymph nodes are found all through the body, and act as...

s.