B2 - Cell Division
B2.1 - Cell Division
chromosomes carry genes that contain instructions for making both new cells and tissues and organs - made up of hundreds or thousands of genes
genes contain info that controls a characteristic or part of one, it is a section of DNA
most of your characteristics are a result of many different genes rather than just one
you have 46 chromosomes in the nucleus of your body cells - arranged in 23 pairs
body cell division cycle = cell cycle
involves a process called mitosis
mitosis produces cells needed for
replacement of worn out or damaged cells
growth and development of in multicellular organisms
in asexual reproduction the cells of the offspring are produced by mitosis from parent cells - identical
cell cycle
time taken depends on cells involved and stage of life of organism
cell cycle is short in developing baby/fetus where new cells are being made all the time
remains quite quick in childhood - slows down when puberty is over and body is adult
in adults there are regions where there is continued growth or regular replacement e.g hair follicles, skin, blood and digestive system lining
STAGES
ONE
longest stage
cells grow bigger, increase mass and carry out normal cell activities
replicate DNA to form 2 copies of each chromosome ready for cell division
also increase numbers of sub cellular structures e.g mitchondria, ribosomes and chloroplasts ready for cell to divide
TWO
mitosis
one set of chromosomes is pulled to each end of dividing cell
nucleus divides
THREE
cytoplasm and cell membranes divide to form 2 identical daughter cells
B2.2 - Growth and Differentiation
growth = permanent increase in size as a result of cell division or cell enlargement
as cells divide, grow and develop they begin to differentiate
animal cells
in animals many types of cells become specialised very early in life
in animal and plant embryos the cells are unspecialised (stem cells) and can become any type of cell that is needed
by the time a human baby is born most of its cells are specialised to carry out a specific job e.g nerve, skin or muscle cells - they have been differentiated
most specialised cells can divide by mitosis but can only form the same type of cell e.g muscle cells
however some cant e.g blood and skin cells so adult stem cells replace dead or damaged cells
nerve cells can't divide or be replaced
plant cells
most cells are able to differentiate all through their lives
stem cells are formed at active regions of stems and roots known as meristems
in these areas mitosis takes place almost continuously - cells elongate and grow before they finally differentiate
plant cells produced dont differentiate until theyre in their final position in the plant and even then the differentiation is not permanent
you can move a plant cell from one part to another and it can redifferentiate and become a completely new type of cell
cloning plants
producing identical offspring is known as cloning
large numbers of identical plant clones can be produced from a small piece of leaf tissue because in the right conditions the cell will become unspecialised and undergo mitosis many times
these will then differentiate to form tissues e.g xylem, phloem, photosynthetic cells and root hair cells to form a new plant
new plant is identical to original parent
only plants can clone because their cells can redifferentiate - artificial animal clones can be made with an embryo
B2.3 - Stem Cells
animals
that cell divides and becomes a hollow ball of cells - embryo
inner cells are called embryonic stem cells - differentiate to form the specialised cells in your body
bone marrow + also tiny amount in blood, brain, muscle and liver - adult stem cells
differentiated cells can divide to replace themselves but some tissues can't do this so stem cells stay in these tissues for years - only needed if cells are injured or affected by disease
can be used to treat paralysed people, people going blind from macular degeneration and diabetic people
egg and sperm cell fuse to form a zygote - a single new cell
plants
stem cells from meristems can be used to make clones of mature parent quickly and economically
gives us a way of producing large numbers of rare plants reliably and safely saving them from extinction
also allows us to produce large populations of identical plants for research - scientists can change variables and observe effects on genetically identical plants
also produces large numbers of plants for sale
used in agriculture to produce large numbers of crops with special features such as disease resistance
B2.4 - Stem Cell Dilemmas
embryonic stem cells come from aborted embryos or spare embryos from fertility treatment donated because they will otherwise not be used
embryo cant give permission - violation of human rights
religious people - interference with process of reproduction
progress has been slow, difficult, expensive and hard to control
embryonic stem cells might cause cancer if they are used to treat humans - has sometimes been a problem when used to treat mice
adult stem cells might be infected with viruses
patients body may reject adult stem cells
the future
scientists have found stem cells in umbilical cord blood of newborn babies and amniotic fluid that surrounds the fetus as it grows - using these can overcome ethical issues
scientists are finding ways of growing adult stem cells - avoids controversy
therapeutic cloning - cloning of adult stem cells to produce cloned early embryo of themselves - prevents rejection as it has been made with bodys own cells
stem cells found in tubes that connect liver and pancreas to small intestine - turned into insulin producing cells in pancreas important for controlling blood sugar - these are the cells that are missing or destroyed in people with type 1 diabetes