Cell Division
Cells divide to replace old and damaged cells and to enable organisms to grow. All cells originate as stem cells and differentiate into more specialised cells. Stems cells have medical applications, but their use is controversial.
Chromosomes
In eukaryotic cells, including animal cells and plant cells, DNA is wrapped around proteins to form structures called chromosomes. Chromosomes are found in the nucleus of the cell.
A gene is stretch of DNA that codes for a protein. Genes provide the instructions to produce all of our characteristics. Each chromosome carries hundreds, or sometimes thousands, of genes.
Chromosomes are arranged in pairs – one that has been inherited from your father and the other from your mother. Humans have 23 pairs of chromosomes, so a total of 46 individual chromosomes.
Mitosis and the cell cycle
Mitosis is a type of cell division where cells produce identical copies of themselves and is used for growth, repair and asexual reproduction. It differs from meiosis, which is the type of cell division used to produce gametes (sex cells).
Mitosis occurs as part of the cell cycle which consists of four distinct phases. First, interphase takes place which is made up of three growth phases (called G1 phase, S phase and G2 phase), followed by mitosis:
Gap Phase 1 (G1) - cell grows bigger and replicates its organelles. The number of ribosomes increases (for building proteins) as well as the number of mitochondria (for releasing energy for cell division).
Synthesis Phase (S) - the cell replicates its DNA to form two copies of each chromosome.
Gap Phase 2 (G2) - the cell keeps growing until all of the organelles have duplicated.
Mitosis – one set of chromosomes is pulled to each end of the cell and the two new nuclei are formed. Then the cytoplasm and cell membranes divide to form two identical cells.
Stem cells
All cells start out as unspecialised cells, called stem cells. Stem cells can differentiate (specialise) into any type of cell. A developing embryo contains a lot of stem cells, which will differentiate to become all the cells of the human body, including heart cells, brain cells and muscle cells. Animal cells tend to differentiate at a very early stage in development. By the time our bodies are fully formed, cell differentiation is mostly finished. We still have some stem cells present in our bone marrow, but cell division is mainly restricted to repair and replacement. In contrast, plant cells retain the ability to differentiate into specialised cells throughout their life.
Stem cells play different roles depending on whether they are found in an embryo, an adult or in the meristem of plants:
Embryonic stem cells – these can differentiate into any type of cell. They form when an egg and sperm cell fuse to form a zygote. Researchers can culture these cells and turn them into almost any type of specialised cell.
Adult stem cells – these have now lost the ability to divide into any kind of cell and are found in limited body parts (bone marrow and blood). These cell types divide into related types only — bone marrow cells can only differentiate into the cells of the immune system and blood cells.
Stem cells in plant meristems – the meristem is found at roots tips and shoot tips and is where cells are actively dividing. Cells in the meristem are able to differentiate into any type of plant cell. Meristem cells could be used to make plant clones — this is useful if the parent plant has desirable characteristics, such as drought tolerance.
Stem cells are being used in medicine as a potential treatment for a variety of diseases. Scientists can take stem cells and differentiate them into another type of cell in the lab with the potential to form whole organs and tissues. For example, a scientist could take a stem cell and differentiate it into a pancreatic cell. The pancreatic cells can be grown on a support to form a whole pancreas to be transplanted into a diabetic person. Scientists are also in the process of using cells to grow spinal cord tissue to be transplanted into people with paralysis. This is still an area of research undergoing development, but much progress has been made in recent years.
Therapeutic cloning is a technique which produces an embryo with the same genes as the patient (it’s basically an embryonic clone of patient). It is done by removing the nucleus taken from one of the patient’s cells (e.g. a skin cell) and placing it into an egg cell which has had its nucleus removed. The cell is stimulated to divide and will grow into an embryo. After 4-5 days stem cells are removed from the embryo and can be used to treat the patient. The benefit of this technique it that stem cells from the embryo are not rejected by the patient’s body.
Issues with stem cells
There are various concerns with the use of stem cells in medical research:
There is a risk of viral infection when the stem cells are placed into a patient’s body
Many people consider the creation and destruction of an embryo morally wrong and contradictory to religious beliefs
The embryo is unable to consent to being used in medicine – at what point does the embryo become a human being and has the right to decide whether to be used in medical treatments?
It is difficult to find suitable stem cell donors
Stem cells which have been growing for a number of generations have a higher rate of mutations and have the potential to become cancerous
Stem cell therapies is still a relatively new area of research and it is unclear how effective it is
Whereas the technology around human stem cells proceed with caution, stem cells from plants have been used for a while to produce clones of plants quickly and cheaply. Cloning plants can protect rare species from extinction and can provide large numbers of desirable plants for farmers.
Plant cloning
Plant cloning is carried out using a technique called micropropagation. During micropropagation, small pieces of a plant (explants) are grown in vitro. They are grown in a petri dish using sterile agar jelly containing plant hormones and nutrients. This is an important way to preserve a rare plant species. It is also used commercially to produce large numbers of genetically identical plants with desired characteristics and can be done at any time of the year.