Immortalised cell line
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An immortalised cell line is a population of cells from a multicellular organism which would normally not proliferate indefinitely but, due to mutation, have evaded normal cellular senescence and instead can keep undergoing division. The cells can therefore be grown for prolonged periods in vitro. The mutations required for immortality can occur naturally or be intentionally induced for experimental purposes. Immortal cell lines are a very important tool for research into the biochemistry and cell biology of multicellular organisms. Immortalised cell lines have also found uses in biotechnology.
An immortalised cell line should not be confused with stem cells which can also divide indefinitely but form a normal part of development of multicellular organism.
Relation to natural biology and pathology 
There are various immortal cell lines. Some of them are normal cell lines - e.g. derived from stem cells. Other immortalised cell lines are the in vitro equivalent of cancerous cells. Cancer occurs when a somatic cell which normally cannot divide undergoes mutations which cause de-regulation of the normal cell cycle controls leading to uncontrolled proliferation. Immortalised cell lines have undergone similar mutations allowing a cell type which would normally not be able to divide to be proliferated in vitro. The origins of some immortal cell lines, for example HeLa human cells, are from naturally occurring cancers.
Role and uses 
Immortalised cell lines are widely used as a simple model for more complex biological systems, for example for the analysis of the biochemistry and cell biology of mammalian (including human) cells. The main advantage of using an immortal cell line for research is its immortality; the cells can be grown indefinitely in culture. This simplifies analysis of the biology of cells which may otherwise have a limited lifetime.
Immortalised cell lines can also be cloned giving rise to a clonal population which can, in turn, be propagated indefinitely. This allows an analysis to be repeated many times on genetically identical cells which is desirable for repeatable scientific experiments. The alternative, performing an analysis on primary cells from multiple tissue donors, does not have this advantage.
Immortalised cell lines find use in biotechnology where they are a cost effective way of growing cells similar to those found in a multicellular organism in vitro. The cells are used for a wide variety of purposes, from testing toxicity of compounds or drugs to production of eukaryotic proteins.
While immortalised cell lines often originate from a well-known tissue type they have undergone significant mutations to become immortal. This can alter the biology of the cell and must be taken into consideration in any analysis.
Methods for generating immortalised cell lines 
There are several methods for generating obtaining immortalised cell lines:
- Isolation from a naturally occurring cancer. This is the original method for generating an immortalised cell line. Major examples include human HeLa cells, obtained from a cervical cancer and mouse Raw 264.7 cells, obtained from a murine leukaemia.
- Spontaneous or induced random mutagenesis and selection for cells which are able to undergo division.
- Introduction of a viral gene that partially deregulates the cell cycle (e.g., the adenovirus E1 gene was used to immortalize the HEK 293 cell line).
- Artificial expression of key proteins required for immortality, for example telomerase which prevents degradation of chromosome ends during DNA replication in eukaryotes
- Hybridoma technology, specifically used for the generation of immortalised antibody-producing B cell lines, where an antibody-producing B cell is fused with a myeloma (B cell cancer) cell.
Examples of immortalised cell lines 
There are several examples of immortalised cell lines, each with different properties. Most immortalised cell lines are classified by the cell type they originated from or are most similar to biologically.