Cell Division

• In body cells the chromosomes are normally found in pairs. Body cells divide by mitosis.
• When a body cell divides by mitosis copies of the genetic material are made then the cell divides once to form two genetically identical body cells.
• Mitosis occurs during growth or to produce replacement cells.
• The Cell Cycle


  1. Interphase

  2. Nuclear Division (Mitosis or Meiosis)

  3. Cell Division (Cytokinesis)

  
• Interphase


  Period between divisions
• G1 phase


  Cell increases in size/volume and new biomass is made (proteins)
• S phase


  DNA replicates by semi-conservative DNA replication
• G2 phase


  Cell prepares for division, synthesis & stores of ATP and new organelles synthesised

  
• Not all cells go through all parts of cell cycle, particularly in multicellular organisms like humans and plants!

  
• Stem cells (e.g. in bone marrow) divide and differentiate to produce specialised cells, (e.g. red blood cells).
• Specialised cells cannot undergo cell division to produce more identical cells of that type (e.g. red blood cells cannot go through cell cycle and divide to produce more red blood cells, they must be made by the bone marrow stem cells).
• The cell cycle and mitosis are not the same – mitosis is part of the cell cycle.

  
• Gene


  Section of DNA that codes for one specific polypeptide (protein)
• Chromosome


  Independent DNA molecule which has been supercoiled into a condensed form. Chromosomes contain many different genes.

  
• Semi conservative DNA replication

  1. 2 genetically identical copies of the DNA molecule are made

  2. They remain attached at a central point called the centromere

  3. Each of the DNA copies now supercoils (using histone proteins to help them coil in eukaryotes)

  4. While they are still joined, the two condensed DNA molecules are called identical sister chromatids
• Somatic cells


  Body cells that have their chromosomes in homologous pairs (one from each parent)
• Diploid cells


  Somatic cells that have their chromosomes in homologous pairs (2n)

  
• Gametes


  Sex cells that have only one copy of each chromosome (haploid, n)
• Organism


  • Fruit fly
  • Pig
  • Human
  • Potato
  • Chicken
  • Rattlesnake fern
• Organism


  • Total number of chromosomes
  • Somatic cell 2n=
  • Haploid cell n=
• Homologous chromosomes


  Have the same genes in the same gene loci (positions) but may have different alleles (versions) of the genes. One chromosome is Maternal and the other is Paternal.

  
• Mitosis


  • Increasing cell numbers and growth of an organism
  • Repair of damaged tissues (not cells)
  • Replacement of worn out / dead cells
• In mitosis, the semi conservative replication of DNA must take place first.

  
• The 2 daughter cells are genetically identical. No variation occurs. They have the same number of chromosomes as the parent cell. With the same alleles of the same genes in the same gene loci. They are genetically identical clones of each other.
• Mitosis is also used by some organisms for asexual reproduction. All the offspring will be identical. This tends to occur in favourable, stable environments (where there is no environmental change).
• Prophase


  1. The nuclear membrane starts to break down

  2. The centrioles start to move to the poles of the cell and make spindle fibres

  3. The chromosomes supercoil and condense / shorten / thicken and become visible

  4. Each chromosome appears as 2 identical sister chromatids joined at the centromere

  
• Metaphase


  1. The centrioles complete the production of spindle fibres (contractile protein fibres)

  2. The chromosomes are attached to the spindle fibres by their centromere

  3. The chromosomes align down the equator of the cell

  
• Anaphase


  1. The spindle fibres contract/shorten

  2. The centromere splits

  3. The identical sister chromatids are pulled to opposite poles

  4. Making a "V" shape
• Telophase


  1. A nuclear membrane starts to reform around each set of chromosomes

  2. The chromatids / chromosomes unwind / uncoil / become longer / thinner and become invisible

  
• Allele


  Different version of the same gene
• Gene


  Section of DNA on a chromosome coding for one or more polypeptide

  
• Chromosome


  Independent DNA molecule in a CONDENSED form which contains many genes (using HISTONES)

  
• Sister chromatid


  One of the two strands of a replicated chromosome that are joined together by a single centromere prior to cell division

  
• Haploid


  Cell that contains only a single copy of each chromosome (i.e gametes)

  
• Diploid


  Cell in which the nucleus contains two sets of homologous chromosomes
• Somatic cell


  A normal body cell
• Homologous chromosomes


  Same genes are in the same location
• Unregulated Mitosis and Cancer


  Cells that undergo rapid uncontrolled mitosis
• In multicellular organisms, the zygote develops into the adult by undergoing repeated cell division by mitosis.
• As cells are produced they differentiate (become specialised) and produce the various tissues, organs, organ systems and the whole organism.