Biology section 2

• Mitosis


  The process by which a cell divides to produce two genetically identical daughter cells
• Chromosomes


  • Contain genetic information
  • Most cells have two copies (diploid)
  • One copy comes from each parent
• Cell cycle


  1. Interphase

  2. Mitosis and cytokinesis
• Interphase


  • DNA is duplicated
  • Cell grows and increases subcellular structures
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• At the end of mitosis, the cell has produced two new daughter cells that are genetically identical to the parent cell
• The formula to calculate the number of cells after multiple divisions by mitosis is: number of cells = 2^n, where n is the number of divisions
• Cell differentiation


  The process by which a cell changes to become specialised for its job
• Cell elongation


  Where a plant cell expands, making the cell bigger and so making the plant grow
• In animals, most cell division is for repair - to replace old or damaged cells
• In plants, growth in height is mainly due to cell elongation, while cell division happens in the tips of roots and shoots (meristems)
• Cancer


  Uncontrolled cell division due to a change in a gene that controls cell division
• Percentile charts


  Used to monitor a child's growth over time and identify any problems
• Stem cells


  Undifferentiated cells that can divide and produce specialised cell types
• Embryonic stem cells


  • Found in early human embryos
  • Can produce any cell type
• Adult stem cells


  • Found in certain places like bone marrow
  • Can only produce certain cell types
• Meristems


  Plant tissues containing stem cells that can divide and differentiate to generate any cell type
• Doctors use adult stem cells to cure some diseases, and scientists are experimenting with using embryonic stem cells to create specialised cells to replace damaged ones
• Stem cells


  Unspecialised cells that are able to divide and form any cell type in the plant
• Unlike human stem cells, plant stem cells can divide and differentiate to generate any type of cell for as long as the plant lives
• Stem cells forming specialised tissues

  Unspecialised cells go on to form specialised tissues like xylem and phloem
• Doctors already use adult stem cells to cure some diseases, e.g. sickle cell anaemia can sometimes be cured with a bone marrow transplant
• Scientists have experimented with extracting stem cells from very early human embryos and growing them
• It might be possible to use stem cells to create specialised cells to replace those which have been damaged by disease or injury, e.g. new cardiac muscle cells could be transplanted into someone with heart disease
• Before using stem cells for cures, a lot of research needs to be done as there are many potential risks which scientists need to learn more about
• Potential risks of using stem cells

  • Tumour development
  • Disease transmission
  • Rejection
• Research using embryonic stem cells raises ethical issues, e.g. some people argue that human embryos shouldn't be used for experiments because each one is a potential human life
• The nervous system is what lets you react to what goes on around you, so you'd find life tough without it
• Sensory receptors


  Groups of cells that can detect a change in your environment (a stimulus)
• How the nervous system coordinates a response

  1. Receptors detect stimulus

  2. Converts to nervous impulse

  3. Sent to CNS

  4. CNS coordinates response

  5. Sends impulses to effector
• Reaction time


  The time it takes you to respond to a stimulus
• Neurones


  • Have a cell body with a nucleus, dendrites carry impulses towards the cell body, axons carry impulses away from the cell body
  • Some axons have a myelin sheath which acts as an electrical insulator, speeding up the impulse
  • Neurones can be very long, which also speeds up the impulse
• Types of neurones


  • Sensory neurone
  • Motor neurone
  • Relay neurone
• Synapse


  The connection between two neurones
• Neurotransmitters


  Chemicals that diffuse across the synapse to transfer the nerve signal
• Reflex


  Automatic rapid responses to stimuli that can reduce the chances of being injured
• How a reflex arc works

  1. Receptor detects stimulus

  2. Impulses sent to relay neurone in CNS

  3. Neurotransmitters released to send impulses to motor neurone

  4. Impulses travel to effector (muscle) which contracts
• A reflex to protect the eye involves light receptors detecting bright light, sending a message to the brain, which then contracts the iris muscles to make the pupil smaller