B2

• Stem cells are unspecialised cells in early development of animal and plant embryos
• most animal cells differentiate early in development
• cell division is mainly for repair and replacement
• some differentiated cells cannot divide so they are replaced by adult stem cells
• plant cells can differentiate throughout life as they have actively dividing plant tissues (meristems) found at growing points of plants
• the cells produced by mitosis in meristem then elongate and differentiate
• stem cells replace cells that die or are lost
• stem cells allow body to renew and repair its tissue
• stem cells both renew themselves and creates new cells of whatever tissue they belong to
• embryonic stem cells are found when egg and sperm fuse to form zygote. the cell then divides many times to form ball of cells in embryo and inner layers of embryo
• adult stem cells in bone marrow usually change into blood cells
• stem cells can be used to treat conditions like paralysis by differentiating into new nerves cells, macular degeneration in eye to restore lost vision, diabetes by producing cells sensitive to blood sugar and can produce insulin
• cloning is producing genetically identical offspring
• plants are easy to clone because cells can become unspecialised, divide by mitosis and differentiate into various types of plant cells
• its difficult to clone animals because once cells are differentiated they cannot become unspecialised again
• STAGE 1 OF CELL CYCLE: longest stage where cells grow, increase in mass and carry out normal cell activities. at the end of stage 1, the cells replicate their DNA to make 2 copies of every chromosome and sub-cellular structures
• STAGE 2 OF CELL CYCLE: mitosis occurs in which one set of chromosomes are pulled to each end of the cell and nucleus divides
• STAGE 3 OF CELL CYCLE: cytoplasm and cell membrane divide to form two identical cells