01 Replication and Division of Nuclei and Cells

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  • 1. CELL AND NUCLEAR DIVISIONReplication and division of nuclei and cells ALBIO9700/2006JK
  • 2. Haploid and diploid cells• n – number of chromosomes in one set of chromosomes• Diploid – cells that contain 2 sets of chromosomes (2n)• Haploid – cells that contain 1 set of chromosomes (n)• Humans: a 2n body cell has 46 chromosomes and a gamete has 23 ALBIO9700/2006JK
  • 3. Two types of nuclear division• Growth – when diploid zygote grows into a multicellular diploid adult the daughter cells must keep the same number of chromosomes as the parent cell: Mitosis• Sexual reproduction – there must be a point when the number of chromosomes is halved. If there was no point in the life cycle when the number of chromosomes halved then it would double every generation: Meiosis ALBIO9700/2006JK
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  • 5. Biological importance of mitosis• Mitosis – nuclear division that produces 2 genetically identical daughter nuclei, each containing the same number of chromosomes as the parent nucleus• Allows growth of multicellular organisms from unicellular zygotes. Growth may occur over the entire body (animals) or be confined to certain regions (meristems of plants)• Replacement of cells and repair of tissues. Dead cells replaced by identical ones (skin, lining of the gut and even whole parts of the body)• Basis of asexual reproduction (production of new individuals of a species by one parent organism). The ability to generate whole organisms from single cells is becoming important in biotechnology and genetic engineering ALBIO9700/2006JK
  • 6. Chromosomes• Chromo means ‘coloured’; somes means ‘bodies’• Chromosomes are matched in pairs (homologous pairs). In the original zygote, one of each pair came from the mother and one from the father.• Accurate and precise nuclear division during growth results in all cells of the body containing the two sets of chromosomes.• The pairs of chromosomes can be distinguished because each pair has a distinctive banding pattern when stained.• 2 chromosomes are displayed to one side (sex chromosomes) which determine the sex. All other chromosomes are called autosomes.• Each chromosome has a characteristic set of genes which code for different features ALBIO9700/2006JK
  • 7. Banding pattern of human chromosomes ALBIO9700/2006JK
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  • 9. Structure of chromosomes• A double structure made of 2 identical structures (chromatids)• 2 chromatids are held together by a narrow region (centromere)• Centromere position is characteristic for a particular chromosome• Each chromatid contains one DNA molecule• The fact that the 2 DNA molecules in sister chromatids, and hence their genes, are identical is the key to precise nuclear division• The gene for a particular characteristic is always found on the same position (locus/loci) on a chromosome ALBIO9700/2006JK
  • 10. • Homologous chromosomes – a pair of chromosomes in a diploid cell that have the same structure as each other, with the same genes (but not necessarily the same alleles of those genes) at the same loci• Each member of a pair possesses genes for the same characteristics but differing in how they code for those characteristics• Alleles – a particular variety of a gene ALBIO9700/2006JK
  • 11. Cancer• Cancer shows the importance of controlling cell division precisely – result of uncontrolled mitosis• Cancerous cells divide repeatedly, out of control, and a tumour develops which is an irregular mass of cells. The cells usually show abnormal changes in shape. ALBIO9700/2006JK
  • 12. Carcinogens• Starts when changes occur in the genes that control cell division• Oncogene – mutated gene that causes cancer• Mutation – a change in the gene• Cancerous cells manage to escape early death and being destroyed by the body’s immune system. The mutation is passed on to all that cell’s descendents.• Mutagen – factor which brings about any mutation• Carcinogen – agent that causes cancer ALBIO9700/2006JK
  • 13. • Ionising radiation – X-rays, gamma rays, UV light and particles from the decay of radioactive elements. Cause the formation of damaging ions inside cells which can break DNA strands• Chemicals – Tar of tobacco smoke, certain dyes (aniline dyes), Etidium bromide (EtBr)• Virus infection – Burkitt’s lymphoma, leukaemia, papilloma viruses (cervical cancer) – Viruses that cause cancer usually carry oncogenes or regulatory genes that can become oncogenes• Hereditary predisposition – Susceptibility to the factors that cause the disease is inherited or inheritance of a single faulty gene – Retinoblastoma (error on chromosome 13) ALBIO9700/2006JK
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  • 15. • Primary growth – a small group of tumour cells• Benign tumours – do not spread from their site of origin, but can compress and displace surrounding tissues (warts, ovarian cysts and some brain tumours)• Malignant (cancerous) tumours – spread throughout body, invade other tissues and eventually destroy them• Cells can break off and spread through blood and lymphatic system to other parts of the body (secondary growths) – metastasis ALBIO9700/2006JK
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