Division and differentiation in human cells

• Somatic Cells - Any cell in the body other than cells involved in reproduction.
• Germline Cells - Gametes and the stem cells that divide to produce gametes.
• Somatic cells form several different types of

  body tissue.

  
  

  Epithelial, Connective, Muscle, Nerve and

  Blood.
• Division by mitosis produces more germline stem cells.

  
  

  The nucleus of germline cells can divide by mitosis to maintain the diploid chromosome number. Diploid cells have 23 pairs of homologous chromosomes.
• Division by meiosis produces haploid gametes.

  
  

  The nuclei of germline cells can divide by meiosis.

  
  

  It undergoes two divisions, firstly separating

  homologous chromosomes and secondly separating chromatids. Haploid gametes contain 23 single chromosomes.
• Every cell contains exactly the same genetic information.

  
  

  Differentiation involves the selective use of the genetic code.

  
  

  Different genes are active in different types of cells.

  
  

  Some genes are ‘switched on’ and others are ‘switched off’.

  
  

  We say that the genes which are switched on are ‘expressed’ by that particular cell.
• Stem cells are unspecialised cells that have the ability to reproduce and differentiate into a diverse range of specialised cells.
• Embryonic stem cells are derived from an embryo about 4-5 days old (blastocyst).
• Embryonic stem cells have the ability to differentiate into all of the cell types that make up the individual and so are pluripotent.
• All genes in embryonic stem cells can be switched on so these can differentiate into any cell type.
• Tissue stem cells are involved in the growth, repair and renewal of the cells found in that tissue. They are multipotent.
• Tissue stem cells can differentiate into all the types of cell 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.
• Induced pluripotent stem cells are tissue cells that have been genetically reprogrammed to an embryonic stem cell-like state.
• Stem cells can be used in corneal repair through transplant and the regeneration of damaged skin which can then be grafted onto the patient.
• Stem cell research provides information on :

  
  

  • How cell processes such as growth, differentiation and gene regulation work

  
  

  • Used as models to study diseases in their development

  
  

  • Drug testing

  
  

  • Therapeutic uses in the treatment of diseases, such as leukemia (bone marrow transplant), Hunter's disease and heart disease
• 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 a tumour may fail to attach to each other, spreading throughout the body where they may form secondary tumours.
• Somatic stem cells divide by mitosis to produce more somatic stem cells which can differentiate to form different body tissues.
• Meiosis makes distinct gametes (diploid -> haploid), while mitosis clones body cells (diploid -> diploid).
• In a developing embryo, tissues such as muscle and nerve are produced by somatic cells dividing by mitosis.
• This pathway describes the production of haploid gametes from diploid germline cells:

  Diploid germline cell - mitosis -> Diploid germline cell - meiosis -> Haploid gametes