What we means by carcinogenesis a tumour or neoplasm is an overgrowth of tissue formed by a clone of cells bearing cumulative genetic injuries confers an additional growth advantage to the clone that possesses it these mutations can be 1- congenital :already present in the genome heritable cancers 2- acquired mutations brought about by exposure to a carcinogen (sporadic cancers)what are the stage process of carcinogenesis
Carcinogenesis:is a generic term for the acquisition of a series of genetic mutations that  lead up to the expression of full malignant cells undergo carcinogenesis and become neoplastic they become transformed some criteria of process of tumrogenesis include the following
• Neoplasms are monoclonal they arise from a single cell)
Neoplasms arise due to cumulative genetic injury Neoplasms may develop more aggressive sub-clones as genetic injuries accumulate
• Genetic injuries confer growth advantages
o Increased proliferation (failure of control of division
o Immortalisation (failure of cell senescence
o Loss of apoptotic control
• Genetic injuries may include
o Point mutations o Amplifications
o Deletions
o Changes in control regions(eg .gene promoters, enhancer sequences
o Translocations of chromosomal material
Carcinogens can be divided into three types
• Infectious (oncogenic viruses, bacteria, protozoa

Chemical carcinogens

Chemical carcinogens may act directly to damage DNA (eg alkylating agents)whereas the  majority require metabolic conversion from a pro-carcinogen state to become activated (eg polycyclic hydrocarbons (smoke), aromatic amines, amides, and azo dyes nature plant products, and nitrosamines).The carcinogen is often activated by metabolism hepatic P450 mixed function oxidase system of the liver.
chemical carcinogens can be either mutagens (irreversibly directly damage DNA) or non mutagens (reversibley promote cell division. Some heavy metals depolymerise DNA
the process of initiation is exposure to a carcinogen that causes irreversible DNA damage not directly lead to a change in phenotype. This is followed by the process of promotion  which allows initiated cells to grow into tumours by promoting cell division eg hormonal influences on tumour growth chemicals
are tested for mutagenicity by a variety of in-vitro and in-vivo procedures p
reduction of mutations in bacteria colonies (eg the Ames test), yeast colonies, and in  cultured mammalian cells ,charting unexpected DNA synthesis in cultured mammalian cells

• use of higher plants to look at chromosome damage

physical carcinogens

 These consist of a wide range of agents
• Electromagnetic radiation (UV light, ionising radiation) Extremes of temperature
  • mechanical trauma
  • foreign bodies and implants

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