Cells and Control

• Chromosomes are coiled up lengths of DNA molecules
• When a cell divides by mitosis it makes two cells identical to the original cell - the nucleus of each new cell contains the same number of chromosomes.
• Body cells in multicellular organisms divide to produce new cells during a process called the cell cycle.
• Multicellular organisms use mitosis to grow or to replace cells that have been damaged.
• In Interphase the DNA is spread out in long strings before it divides. The cell has to grow and increase the amount of subcellular structures such as mitochondria and ribosomes. It then duplicates its DNA so there is one copy for each new cell. The DNA is copied and forms X-shaped chromosomes.
• Prophase

  The chromosomes condense, getting shorter and fatter. The membrane around the nucleus breaks down and the chromosomes lie free in the cytoplasm.
• Metaphase


  The chromosomes line up at the centre of the cell
• Anaphase


  Spindle fibres pull the chromosomes apart. The the chromatids are pulled to opposite ends of the cell.
• Before telophase ends, the cytoplasm and cell membrane divide to form two seperate cells - this process is called cytokenesis
• Cell differentiation 


  the process by which a cell changes to become specialised for its job. Having specialised cells allows multicellular organisms to work more efficiently.
• Cell division


  By mitosis. Plants also grow by cell elongation. This is where a plant cell expands, making the cell bigger and so making the plant grow.
• All growth in animals happens by cell division.
• When you are young cells divide at a fast rate but once you're an adult, most cell division is for repair - the cells divide to replace old or damaged cells.
• In plants, growth in height is mainly due to cell elongation - cell division usually just happens in the tips of the roots and shoots in meristems.
• The rate that cells divide by mitosis is controlled by the genes in an organisms DNA
• If there is a change in one of the genes that controls cell division, the cell might start dividing uncontrollably. This can result in a mass of abnormal cells called a tumour
• If the tumour invades and destroys surrounding tissue it is called cancer.
• Undifferentiated cells are called stem cells.
• Stem cells are found in early human embryos. These embryonic stem cells have the potential to divide and produce any kind of cell.
• Adults have stem cells but they are only found in certain places, like bone marrow. These aren't as versatile as embryonic stem cells.
• Meristem tissue is found in the areas of a plant that are growing.
• Meristems produce unspecialised cells that are able to divide and form any cell type in the plant - they act like embryonic stem cells:
• The unspecialised cells go on to form specialised tissues like xylem and phloem.
• Doctors use adult stem cells to cure some diseases. E.g sickle cell anaemia can sometimes be cured with a bone marrow transplant.
• It might be possible to use stem cells to create specialised cells to replace those which have been damaged by disease or injury.
• Tumour development 


  Stem cells divide very quickly. If scientists are unable to control the rate, a tumour may develop
• Disease transmission


  Viruses live inside cells. If donor stem cells are infected with a virus and this isn't picked up, the virus could be passed to the recipient and so make them sicker.
• Rejection 


  if the transplanted cells aren't grown using the patient's own stem cells l, the patient's body may recognise the cells as foreign and trigger an immune response.
• The body has a lot of sensory receptors - groups of cells that can detect a change in your environment.
• When a stimulus is detected by receptors, the information is converted to a nervous (electrical) impulse and sent along sensory neurones to the CNS.
• The CNS coordinates the response. Impulses travel through the CNS along relay neurones.
• The CNS send information to an effector along a motor neurone.
• The time it takes you to respond is called your reaction time.
• The cell body has extensions that connect to other neurones - dendrites and dendrons carry nerve impulses towards the cell body, and axons carry nerve impulses away from the cell body.
• Some axons are surrounded by a myelin sheath. This acts as an electrical insulator. Speeding up the electrical impulse.
• Neurones can be very long which also speeds up the impulse.
• Sensory Neurone

  • One long dendron carries nerve impulses from receptors to the cell body, which is located in the middle of the neurone.
  • One short axon carries nerve impulses from the cell body to the CNS
• Motor Neurone

  • Many short dendrites carry nerve impulses from the CNS to the cell body
  • One long axon carries nerve impulses from the cell body to effector cells.
• Relay Neurone

  • Many short dendrites carry never impulses from sensory neurones to the cell body
  • An axon carries nerve impulses from the cell body to motor neurones.
• The connection between two neurones is called a synapse.