in early development of animal and plant embryos, cells are unspecialised (stem cells), and can become any type of cell needed. by time baby is born, most of its cells are specialised to carry out particular job such as nerve cells, skin cells, muscle cells- they have differentiated and some of their genes have been switched on or off so different types of specialised cells have different sub-cellular structures to carry out specific functions
most specialised cells can divide by mitosis but can only form same sort of cell- muscle cells divide to produce more muscle cells. some differentiated cells such as red blood cells and skin cells cannot divide at all and so adult stem cells replace dead or damaged cells. nerve cells do not divide once differentiated so when damaged not replaced. in mature animal, little or no growth takes place and cell division almost entirely restricted to repair and replacement of damaged cells, each differentiated cell type divides only to make more of same cells
in contrast to animal cells, most plant cells can differentiate all through their lives- undifferentiated cells are formed at active regions of stems and roots (meristems). in these areas mitosis takes place almost continuously and cells then elongate and grow before finally differentiate
plants keep growing throughout their lives at these growing points and plant cells produced do not differentiate until in final position in plant but it is not permanent as moving a plant cell to another part of plant it can redifferentiate and become different type of cell- cannot do this with animal cells
producing identical offspring is cloning. huge numbers of identical plant clones can be produced from tiny piece of leaf tissue- in right conditions plant cells produced will become unspecialised and undergo mitosis many times- each of these undifferentiated cells will produce more cells by mitosis. given different conditions these will then differentiate to form tissues such as xylem, phloem, photosynthetic cells and root hair cells needed to form tiny new plant identical to original parent
difficult to clone animals because most animal cells differentiate permanently early in embryo development so cannot change back so artificial animal clones can only be made by cloning embryos in same way, but adult cells can be used to make an embryo
