B2

• Cell division is necessary for the growth and development of an organism, and for the repair of damaged tissues.
• The cell divides in a series of stages called The cell cycle. The length of the cycle can vary. In embryos it's very short as cells divide rapidly to form a new organism. In adults the process slows down but in some cells such as hair follicles, skin, blood and the lining of the digestive system, continue to divide rapidly.
• Mitosis is one stage in the cell cycle, which results in two identical cells being produced from the original cell.

There are three stages in the cell cycle:

• The longest stage. The cells grow, increase in mass, and carry out normal cell activities. At the end of stage 1 the cells replicate their DNA to make two copies of every chromosome. They also make more sub-cellular structures such as mitochondria,ribosomes and chloroplasts.
• Mitosis: in this process one set of chromosomes is pulled to each end of the dividing cell and the nucleus divides.
• The cytoplasm and cell membrane divide to form two new identical cells.

• The chromosomes contain the genes, which must be passed on to each new cell.
• Human body cells have 23 pairs of chromosomes. One of each pair is from your mother, the other from your father. Egg and sperm (gametes) have 23 chromosomes each, and combine to form the 46 chromosomes in body cells.
• When offspring are produced by asexual reproduction, their cells are produced by mitosis from the parent cell. They contain exactly the same genes as the parent.

Mitosis is a type of cell division. Mitosis occurs wherever more cells are needed. It produces two new cells that are identical to each other, and to the parent cell. The process of growth and division is called the cell cycle.

The cycle starts as the number of organelles - the different parts of the cell - increases. This is to ensure that each of the two new cells receives copies of all the organelles.

Meiosis is a different kind of cell division. It is used to produce male and female gametes. A human body cell contains 46 chromosomes arranged in 23 pairs. The gametes are sperm or eggs, and only contain half as many chromosomes (23). This is why meiosis is sometimes called reduction division.

At fertilisation, the nuclei of the sperm and an egg join to form the zygote. The zygote contains 23 pairs of chromosomes - 23 single chromosomes from the sperm, and 23 single chromosomes from the egg, thereby creating the correct number of 46 chromosomes for all body cells. It also means the zygote contains a complete set of chromosomes from each parent.

Organisms start as one cell, which divides by mitosis. The growth of an organism is a result of cell enlargement and cell division. An adult human contains trillions of cells.

• In early developmen tof animal and plant embryos the cells are unspecialised and are called stem cells.
• Most animal cells differentiate early in development and cell devision is mainly for repair and replacement.
• Some differentiated cells cannot divide so they are replaced by adult stem cells such as those found in the bone marrow.
• Plant cells can differentiate throughout the life of the plant as it continues to grow. Actively dividing plant tissues are called meristems, found at the growing points of plants. The cells produced by mitosis in the meristem then elongate and differentiate.
• Producing genetically identical offspring is known as cloning. Plants are easy to clone because the cells can become unspecialised, divide by mitosis, and then differentiate into the various types of plant cell.
• It is difficult to clone animals because once the cells are differentiated they cannot become unspeciallised again.

• Stem cells are unspecialised cells that are found in the human embryo and the adult bone marrow, they have the ability to turn into any specialised cell.
• An egg and sperm fuse to form a zygote. This cell divides many times to form a ball of cells - the embryo. The inner layers of the ball are embryonic stem cells.
• Layers of cells in the embryo differentiate into all the cells the body needs.
• When stem cells change into all the different types of body cell, such as nerve cells or muscle cells, we say the cells differentiate.
• Adult stem cells in the bone marrow can change into other types of cell, such as blood cells.
• Researchers hope that human stem cells can be made to differentiate into many types of cell. The cells formed could then be used to treat conditions such as:
  • Paralysis, by differentiating into new new nerve cells 
  • Macular degeneration in the eye, to restore lost vision
  • Diabetes, by producing cells that are sensitibe to blood sugar and can produce insulin

The stem cells from plant meristems can be used to make clones of the mature parent plant very wuickly and economically therefore:

• Rare plants can be saved from extinction
• Large populations of genetically identical plants can be used for scientific research

Problems with stem cell research:

There are many potential benefits when using stem cells in human medicine, but there are also risks, as well as social and ethical issues.

Embryonic stem cells come from aborted embryos or spare embryos from fertility treatment.

• Some people believe this is a violation of the embryo's rights as it cannot give permission and has the potential to create a new human life - ethical objection
• People believe that we should not interfere with the natural processes of reproduction - religious objection
• Some people think that a lot of money, which could be used to treat other medical conditions, is wasted on stem cell research - ethical and social problem
• Scientists are finding it difficult to 'persuade' stem cells to differentiate - development of the process is proving slow and expensive
• Embryonic stem cells divide rapidly so there is a concern they could cause cancer
• Embryonic stem cells are preferred to adult stem cells. it is possible that adult stem cells could transmit viruses to another person of trigger an immune response.

• It is hoped that human stem cells can be made to differentiate into many types of cell. The cells formed could then be used to treat conditions such as paralysis. This are of research called therapeutic cloning. It has already been successful in producing new organs such as tracheas.
• Therapeutic cloning involves the development of an embryo cloned from the adult who needs treatment. The cells would be genetically identical to that adult so the risk of rejection is reduced.
• Scientists have discovered stem cells in previously unknown areas of the body. such as the tubes connecting the liver and pancreas to the small intestine. They hope to develop insulin-producing cells from these stem cells to treat type 1 diabetes.