3.7 Mitosis

3.7 Mitosis

1. Daughter cells are genetically identical to parent cells, except in the rare event of a mutation.

2. Always preceded by a period during which the cell is not dividing , called INTERPHASE

   • Period of considerable cellular activity that includes a very important event, the replication of DNA

3. Continuous process, however is can be divided into four stages

4. PROPHASE

   • Chromosomes first become visible as long thin threads, which then shorten and condense
   • Animal cells contain two cylindrical organelles called centrioles, each of which moves to opposite ends (poles) of the cell.
   • From each centriole, spindle fibres develop which span the cell from pole to pole. Collectively, these fibres are called spindle apparatus.
   • The nucleolus dissapears
   • Nucelar Envelope breaks down
     • leaves chromosomes free in the cytoplasm of the cell.
   • Chromosomes are drawn towards the equator of the cell by the spindle fibres attached to the centromere

5. Produces two daughter cells

6. METAPHASE

   • Chromosomes are seen to be made up of two chromatids
   • Each chromatid is an identical copy of DNA form the parent cell
   • Chromatids are joined by the centromere
   • It is to this centromere that some microtubules from the poles are attached and the chromosomes are pulled along the spindle apparatus and arrange themselves across the equator of the cell

7. ANAPHASE

   • Centromeres divide into two and the spindle fibres pull the individual chromatids making up the chromosome apart
   • The chromatids move rapidly to their respective , opposite poles of the cell and we now refer to them as chromosomes
   • The energy for the process is provided by mitochondria which gather around the spindle fibres

8. TELOPHASE AND CYTOKINESIS

   • Chromosomes reach respective poles
   • Chromosomes become longer and thinner, finally disappearing altogether, leaving only widely spread chromatin
   • The spindle fibres disintegrate and the nuclear envelope and nucleolus re-form.
   • Finally, the cytoplasm divides in a process called Cytokinesis

9. Cell Division in Prokaryotic Cells

   • 1) The circular DNA molecule replicates and both copies attach to the cell membrane
   • 2) The plasmids also replicate
   • 3) The cell membrane begins to grow between the two DNA molecules and begins to pinch inward, dividing the cytoplasm into two
   • 4) A new cell wall forms between the two molecules of DNA, dividing the original cell into two identical daughter cells, each with a single copy of the circular DNA and a variable number of copies of the plasmids

10. Replication of Viruses

    • Viruses are non-living meaning they can't undergo cell division
      • Instead they replicate bt attaching to their host cell with the attachment proteins on their surface
    • They inject their nucleic acid into the host cell
    • The genetic information on the injected viral nucleic acid then provides the 'instructions' for the host cells metabolic processes to start producing the viral components which are then assembled into viruses
      • nucleic acid
      • enzymes
      • structural proteins

11. Mitosis is important in organisms as it produces genetically identical daughter cells. Why is it essential to make exact copies of existing cells?

    • Growth: When two haploid cells (e.g sperm and an ovum) fuse together to form a diploid cell, it has all the genetic information needed to form the new organism. If the new organism is to resemble its parents, all the cells that grow from this original cell must be genetically identical. Mitosis ensures this happens.
    • Repair: If cells are damaged or die it is important that the new cells produced have an identical structure and function to the ones that have been lost
    • Reproduction: Single - celled organisms divide by mitosis to give two new organisms. Each new organism is genetically identical to the parent organism