(a) Somatic and Germline cells

• A somatic cell is any cell in the body other than cells involved in reproduction.
• Somatic stem cells divide by mitosis to form more somatic cells.
• During cell division the nucleus of a somatic cell divides by mitosis to maintain the diploid chromosome number.
• Diploid cells have 23 pairs of homologous chromosomes.

• Germline cells are gametes (sperm and ova) and the stem cells that divide to form gametes.
• Germline stem cells divide by mitosis to produce more germline cells or by meiosis to produce haploid gametes.

Meiosis

• The nucleus of a germline stem cell can divide by meiosis.
• It undergoes two divisions, firstly separating homologous chromosomes and secondly separating chromatids.
• Haploid gametes contain 23 single chromosomes.

(b) Cellular differentiation

• Cellular differentiation is the process by which a cell expresses certain genes to produce proteins characteristic for that type of cell.
• This allows a cell to carry out specialised functions
• Stem cells are unspecialised somatic cells that can divide to make copies of themselves (self-renew) and/or differentiate into specialised cells.
• Tissue (adult) stem cells are involved in the growth, repair and renewal of the cells found in that tissue.
• They are multipotent.
• Tissue stem cells are multipotent as they can make all of the cell types found in a particular tissue type.
• For example, blood stem cells located in bone marrow can give rise to red blood cells, platelets, phagocytes and lymphocytes.
• Cells in the very early embryo can differentiate into all the cell types that make up the individual and so are pluripotent.
• All the genes in embryonic stem cells can be switched on so these cells can differentiate into any type of cell.

(c) Therapeutic and research uses of stem cells

• Therapeutic uses involve the repair of damaged or diseased organs or tissues.
• The therapeutic uses of stem cells should be exemplified by how they are used in corneal repair and the regeneration of damaged skin.
• Stem cells from the embryo can self-renew, under the right conditions, in the lab.
• Research uses involve stem cells being used as model cells to study how diseases develop or being used for drug testing.
• Stem cell research provides information on how cell processes such as cell growth, differentiation and gene regulation
• There are many ethical issues.
• Use of embryonic stem cells can offer effective treatments for disease and injury; however, it involves destruction of embryos.

(d) Cancer cells

• Cancer cells divide excessively because they do not respond to regulatory signals.
• This results in a mass of abnormal cells called a tumour.
• Cells within the tumour may fail to attach to each other, spreading through the body where they may form secondary tumours.