A minimal level of intra-tumour heterogeneity is a simple consequence of the imperfection of DNA replication: whenever a cell (normal or cancerous) divides, a few mutations are acquired - leading to a diverse population of cancer cells.
There are two main sources of vitamin A, animal sources and plant sources. Known as retinol, animal sources of this essential vitamin contain preformed vitamin A. Vegetable sources contain carotenoids, such as beta-carotene, which is converted to retinol by the body.lant sources of vitamin A include many fruits and vegetables that have rich, deep, intense color. The more intense the color of the vegetable or fruit, the more beta-carotene it contains. Plant sources of carotenes include: 1  Sweet potatoes. 2  Carrots. 3  Pumpkin. 4  Winter squash. 5  Cantaloupe. 6  Pink Grapefruit.
The Roles of Vitamin A. Vitamin A is much more than just a vitamin that helps you see at night. It is a fat-soluble vitamin that plays a role in immunity, the body's development and maintenance of major organs. Available in two sources, vitamin A is derived from animals and from plants. Vitamin A from animal sources is preformed vitamin A and is active immediately. From plant sources, it is provitamin A â including a group of carotenoids, such as beta-carotene. Beta-carotene is the most common form of provitamin A and in plant foods it must be converted in the body to the active form of vitamin A.