1.2 - Cell Division

4.1.2.1 Chromosomes

Keyword: Chromosome
FSL: GCSE Biology Revision "Cell division by Mitosis"

Chromosome: A thread-like structure composed of DNA and proteins found in the nucleus of eukaryotic cells. It carries genetic information in the form of genes. Humans have 46 (23 pairs). One of each pair is inherited from each parent.

Chromosomes are made from highly coiled strands of relatively long DNA.

Before a cell divides, its genetic material needs to be doubled, and that is why we get X-shaped chromosomes (each side is a replica of each other, so they can split).

4.1.2.2 Mitosis

Keywords: Haploid cell, Diploid cell, Interphase, Cytokinesis, Mitosis
FSL: GCSE Biology Revision "Cell division by Mitosis" (again)

Haploid cell: A cell with half the number of chromosomes (e.g. egg/sperm cell).
Diploid cell: A cell with a full set of chromosomes (e.g. zygote, which is what the egg is called once it merges with the sperm).
Interphase/growth phase: When the cell replicates subcellular structures (e.g. ribosomes, mitochondria). The cell grows bigger into an elliptical shape.
Mitosis: Chromosomes are replicated in the nucleus. Chromosomes line up in the centre of the cell, and one copy of each chromosome are pulled to opposite ends of the cell. One cell ends up with the original chromosomes, the other ends up with the duplicates.
Cytokinesis: The cytoplasm and membrane divide. This forms two genetically identical cells.

Cells divide in a series of stages called the cell cycle.
During the cell cycle the genetic material is doubled and then divided into two identical cells.
Before a cell can divide it needs to grow and increase the number of subcellular structures such as ribosomes and mitochondria.
The DNA replicates to form two copies of each chromosome. In mitosis one set of chromosomes is pulled to each end of the cell and the nucleus divides.
Finally, the cytoplasm and cell membranes divide to form two identical daughter cells.
(taken from the spec)

The role of Mitosis

Mitosis is used for growth and repair.
As reproduction starts with just one cell, that cell (and its daughter cells later) has to replicate over and over to produce new cells for growth.
Cell division is used to repair damaged tissue and wounds.

4.1.2.3 Stem Cells

Keyword: Stem Cells
FSL: GCSE Biology Revision "Stem Cells"

Stem Cells: Stem cells are unspecialised cells, they have not undergone (much) differentiation. They can differentiate into other cells.

Specialised Cells

Specialised cells in animals cannot change their function (differentiate), and when they undergo mitosis the daughter cells will have the same function. As some cells cannot undergo mitosis (e.g. red blood cell, skin cell) stem cells are needed to replace them.

Embryonic Stem Cells

Embryonic stem cells are cells which are found in embryos. They are completely undifferentiated, and can differentiate into all types of cells found in the human body.

Adult Stem Cells

Adult stem cells are cells which can be found in multiple places in humans, and are most commonly found in bone marrow and skin. They were set aside in the womb while most cells were undergoing differentiation, however they may have already gone through a bit of differentiation and therefore cannot differentiate into everything.

Stem cells from bone marrow are most commonly used for red blood cells and cells of the immune system.

Stem cells from the skin are most commonly used for different types of cells in the skin and hair follicles.

The role of adult stem cells is primarily growth during childhood, and repair during adulthood.

Meristem Cells

Meristem cells are stem cells which are found in plants. They are found in the tips of the roots and shoots (the growing regions of plants), and are fully undifferentiated. They have the ability to make a whole new plant through their own division, and are primarily used in the regions of the plant that are growing.

Stem Cells in Plants

Meristem cells can be easily extracted from plants and used to make clones of plants cheaply and speedily. This is especially useful for preserving rare or endangered plants, and for making more resistant crop plants or new plants with special features.

This is how you do it:

  1. Take a small length of steam from a healthy plant.
  2. Dip the base in rooting powder.
  3. Once the cutting has started to grow roots, it can be planted.
  4. The new plant will be genetically identical to the parent.

Stem Cells as Treatment

Treatment with stem cells may be able to help conditions such as diabetes and paralysis. They can be extracted from human embryos grown in the lab, or as adult stem cells from alive humans (although these can be made into less new types of cell).

In practice, some people argue that harvesting stem cells from embryos is unethical, but some diseases cannot be cured with adult stem cells (due to the fact that they cannot differentiate into some types) and often doesn't work anyway, since the body will reject (and the immune system will fight) cells with different antigens.

There is also a third option, umbilical cord stem cells, which are rich in blood cells and can be used to treat a variety of conditions. The problem with this is that they cannot be used on most people due to their age.

Therapeutic Cloning

In therapeutic cloning an embryo is produced with the same genes as the patient, by extracting the patient's DNA to make new cells. Stem cells from the embryo are not rejected by the patient's body so they may be used for medical treatment.

Therapueutic cloning is done like this:

  1. Nucelus from the patient's skin cell is removed.
  2. The nucleus from a patient's cell is put into an empty egg cell (the egg cell's nucleus is discarded).
  3. The egg cell is stimulated to divide and develops into a new embryo.
  4. Stem cells are taken from the embryo after 4-5 days.
  5. The stem cells are grown in a contianer of warm nutrients.
  6. Stem cells are treated to develop into required cell types for a patient.

This has the risks of passing on diseases or viruses, some stem cells can form cancer like tumours, the cost is very high, and has ethical issues.

Despite the risks, therapueutic cloning could be used to create new organs, especially since anti-rejection drugs are not required.

Cloning entire animals (like Dolly the Sheep) is often illegal, and the animals have many many developmental problems.