Cell division

• When a cell is not dividing the DNA exists as chromatin within the nucleus.
• Chromatin


  A complex of DNA and histone proteins found in the nucleus of eukaryotic cells.
• When cell division begins

  Chromatin condenses to form chromosomes.
• Chromosomes


  Consist of DNA, protein and a little RNA. The DNA is made up of many small sections called genes.
• Homologous chromosomes


  Chromosomes found in matching pairs.
• At the start of cell division

  The DNA replicates (during interphase) and the chromosomes are visible as two identical sister chromatids joined at the centromere.
• Humans have 23 pairs of homologous chromosomes, but other species have different numbers of chromosomes.
• One chromosome in each homologous pair comes from the mother and the other from the father.
• Diploid number


  The total number of chromosomes (i.e. 2 of each type of chromosome) in a somatic (body) cell.
• Gametes


  Sex cells that have half the diploid number, (i.e. 1 of each type of chromosome) and are haploid (n).
• What is the diploid number of chromosomes for humans? 2n=46
• What is the haploid number of chromosomes for humans? n=23
• All cells undergo a cell cycle.
• Interphase


  The period when a newly formed cell increases in size and carries out a number of activities.
• Cellular activity during interphase

  • DNA replicates
  • Organelles are produced
  • Protein synthesis occurs
  • ATP synthesis occurs (aerobic respiration)
• Mitosis phases


  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Prophase


  1. Chromatin condenses and becomes visible as two sister chromatids joined at the centromere.

  2. Centrioles move to opposite poles. Protein microtubules form from each centriole and the spindle develops.

  3. The nuclear envelope disintegrates, the nucleolus disappears and chromosomes move towards the equator of the cell.

  4. Prophase is the longest stage of mitosis.
• Metaphase


  The chromosomes arrange themselves along the equator of the cell and become attached to spindle fibres at the centromere.
• Anaphase


  The spindle fibres contract and shorten, the centromere splits and sister chromatids are pulled apart. The chromatids are then drawn towards opposite poles with the centromere leading.
• Telophase


  1. The chromosomes uncoil and lengthen.

  2. The spindle fibres break down.

  3. The nucleoli and nuclear membranes reform.

  4. Cytokinesis (splitting of the cytoplasm)
• Cytokinesis in animal cells

  Occurs by the constriction of the parent cell from the outside inwards, forming a cleavage furrow.
• Cytokinesis in plant cells

  A cell plate forms across the equator of the parent cell from the inside outwards, and a new cellulose cell wall is laid down.
• Cell cycle stages


  • Interphase
  • Prophase
  • Metaphase
  • Anaphase
  • Telophase
• Details of cell cycle stages

  • DNA replication
  • ATP and protein synthesis
  • New organelles produced
  • Chromosomes condense
  • Nucleolus disappears, nuclear membrane disintegrates
  • Centrioles synthesise spindle
  • Chromosomes line up along the equator
  • Spindle attaches to centromere
  • Spindle fibres contract and shorten
  • Chromatids pulled to opposite poles
  • Nucleolus and nuclear envelope reform
  • Chromosomes uncoil
  • Spindle disintegrates
• Differences between mitosis in plant and animal cells

  • Animal cells become rounded before mitosis, plant cells have no shape change
  • Animal cells have centrioles, plant cells are absent
  • Animal cells have cleavage furrow developing from outside inwards, plant cells have cell plate developing from centre outwards
  • Animal cells have spindle degenerating at telophase, plant cells have spindle remaining throughout new cell wall formation
• Significance of mitosis


  Mitosis produces two daughter cells with the same number of chromosomes as the parent cell. Each chromosome is an exact replica of one of the original, so mitosis produces cells that are genetically identical to the parent cell. This gives genetic stability.
• Mitosis leads to


  Growth of an organism and also allows for repair of damaged tissues and replacement of dead cells.
• Mitosis in plants


  • Root tips
  • Growing points of shoots
• Mitosis in animals


  • Human skin
• Advantages of asexual reproduction

  • Offspring are genetically identical to parent, so they are able to survive successfully if environmental conditions are stable
  • Less time and energy is wasted as organisms do not need to find a reproductive partner
  • The population can rapidly increase in number, providing the environmental conditions are stable
• Disadvantages of asexual reproduction

  • Lack of genetic variation would mean offspring are unable to adapt to a change in environmental conditions
• Asexual reproduction produces offspring that are genetically identical to the parent as a result of mitosis.
• Most animals and plants also produce haploid gametes which are not identical to the parent cell and so are able to reproduce sexually.
• Types of tumour

  • Benign (non-cancerous and won't invade nearby tissues)
  • Malignant (cancerous and can spread to other tissues)
• Carcinogens


  Radiation, certain chemicals and some viruses can mutate DNA and affect the genes which control the cell cycle.
• Proto-oncogenes


  Control the cell cycle, they tell the cell when to divide. If mutated, uncontrollable cell division can occur to form a tumour.
• Oncogenes


  Mutated proto-oncogenes.
• Meiosis


  Used to produce gametes in sexually reproducing organisms.
• Meiosis


  Produces four genetically different daughter cells.
• Stages of meiosis


  • Meiosis I - chromosome number is halved
  • Meiosis II - the new haploid cells divide again to produce 4 genetically different daughter cells (meiosis II is identical to mitosis)