biology 2.6

• what is a stem cell?


  an unspecialised cell

  they can undergo cell division multiple time in order to reproduce cells for growth, development and tissue repair
• what is potency?


  a stem cells ability to differentiate into different cell types
• what is a totipotent stem cell?


  can differentiate into any type of cell
• what is a pluripotent stem cell?


  can form all tissue types but not whole organisms
• what is a multipotent stem cell?

  only form a range of cells within a certain type of tissue
• what is differentiation?


  where a cell becomes specialised to carry out a particular function and is no longer able to divide
• what are mammalian erythrocytes?

  red blood cells

  essential for the transport of oxygen - adapted as they have fewer organelles allowing for more haemoglobin

  only last 120 days so need to be replaced constantly
• what are embryonic stem cells?


  totipotent

  present at a very early age of embryo development

  after 7 days, a mass of cells has formed - now pluripotent - and remain this state in fetus until birth
• what are adult stem cells?


  present throughout life from birth

  found in specific areas of the body eg: bone marrow

  multipotent
• where are stem cells found in plants?


  present in meristematic tissue (meristems)

  this tissue is found where growth occurs (roots/shoots)

  pluripotent
• what are the phases of the cell cycle?


  Interphase, Prophase, Metaphase, Anaphase, Telophase, Cytokinesis
• in eukaryotic cells, what are the two main phases of the cell cycle?


  interphase and mitotic (division) phase
• what happens during interphase?


  • DNA is replicated and checked for errors in the nucleus
  • protein synthesis occurs in the cytoplasm
  • mitochondria grow and divide, increasing the number in the cytoplasm
  • the normal metabolic processes of cells occur (including cell respiration)
• what are the 3 stages of interphase?

  • G1 - the 1st growth phase - proteins from which organelles are synthesised are produced and organelles replicate. the cell increases in size
  • S - synthesis phase - DNA is replicated in the nucleus
  • G2 - the 2nd growth phase - cell continues to increase In size, energy stores increased and the duplicated DNA is checked for errors
• how many stages are there in interphase?


  3 - G1, S and G2
• what is the mitotic phase?


  the period of cell division
• what are the 2 stages of cell division?


  mitosis - nucleus divides

  cytokinesis - the cytoplasm divides and 2 cells are produced
• what is the phase G0?


  the phase where the cell leaves the cycle either temporarily or permanently.

  there are a number of reasons for this including differentiation, the DNA being damaged (can't divide) or diseases and health issues like cancer
• what is differentiation?


  a cell that becomes specialised to carry out a particular function and is no longer able to divide.
• how is the cell cycle checked and controlled?


  checkpoints are the control mechanisms of the cell cycle, they monitor and verify wether the processes at each phase of the cell cycle have been accurately completed before moving onto the next stage
• at what points do checkpoints occur in the cell cycle?

  G1

  G2

  spindle assembly checkpoint
• what is checked for at the G1 checkpoint?


  cell size

  nutrients

  growth factors

  DNA damage

  if it meets requirements it get triggered to begin DNA replication, if not it goes into a resting state
• what is checked for at the G2 checkpoint?


  cell size

  DNA replication without error

  DNA damage

  if it meets requirements, the cell initiates the molecular processes that signal the beginning of mitosis, if not it goes into a resting state
• what is checked for at the spindle assembly checkpoint?

  chromosome attachment to the spindle and aligned correctly

  mitosis can't proceed if until this checkpoint is passed

  also called metaphase checkpoint
• what is mitosis important?

  ensures that both daughter cells produced when a parent cell divides are genetically identical

  each new cell will have an exact copy of the DNA present in the parent cell and the same number of chromosomes
• what is the purpose of mitosis?

  when all daughter cells have to be identical, often during growth replacement and repair of tissues in multicellular organisms

  also necessary for asexual reproduction
• what are chromatids?

  before mitosis occurs, the DNA is replicated during interphase

  each DNA molecule is converted into 2 identical DNA molecules called chromatids
• why do chromatids need to be kept together during mitosis?

  two chromatids are joined together at a region called centromere.

  they must be kept together so they can be precisely maneuvered and segregated equally, one into each of the new daughter cells
• what are the four stages of mitosis?


  prophase, metaphase, anaphase, telophase
• you can view and identify these stages using...?

  a light microscope
• where can dividing cells be easily obtained from in plants?


  growing root tips

  the root tips can be treated with a chemical to allow the cells to be separated - they can then be quashed to form a single layer of cells on a slide, then they are stained to make the chromosomes more visable
• what happens during prophase?


  1. chromatin fibres begin to coil and condense to form chromosomes
  2. the nucleolus that is responsible for RNA synthesis disappears, the nuclear membrane begins to break down
  3. protein microtubules form spindle-shaped structures linking the poles of the cell
  4. two centrioles migrate to opposite poles of the cell
  5. spindle fibres attache to specific areas on the centromeres and start to move the chromosomes to the centre of the cell
  6. by the end of the prophase, the nuclear envelope has dissapeared
• what are centrioles for in prophase?


  cyclindrical bundles of proteins that help in the formation of the spindle
• what are the fibres forming the spindle needed for during prophase?


  necessary to move the chromosomes into the correct positions before division
• what happens during metaphase?

  chromosomes are moved by the spindle fibres to form a plane in the centre of the cell, called the metaphase plate and then held in position
• what happens during anaphase?


  • the centromeres holding together the pairs of chromatids in each chromosome divide during anaphase.
  • the chromatids are separated - pulled to opposite poles of the cell by the shortening spindle fibers
  • the characteristic 'v' shape of the chromatids moving towards the poles is a result of them being dragged by their centromeres through the liquid cytosol
• what happens during the telophase?


  • in telophase, the chromatids have reached the poles and are now called chromosomes
  • the new sets of chromosomes assemble at each pole and the nuclear envelope reforms around them
  • the chromosomes start to uncoil and the nucleolus is formed
  • cytokinesis begins
• what is cytokinesis?


  the actual division of the cell into two separate cell begins during telophase
• how does cytokinesis happen in animal cells?


  in animal cells a cleavage furrow forms around the middle of the cell

  the cell-surface membrane is pulled inwards by the cytoskeleton until it is close enough to fuse around the middle, forming two cells
• how does cytokinesis happen in plant cells?

  • plant cells have cell walls so it isn't possible for a cleavage furrow to be formed
  • vesicles from the golgi apparatus begin to assemble in the same place as where the metaphase plate was formed
  • the vesicles fuse with each other and the cell surface membrane, dividing the cell into 2
  • new sections of cell wall then form along the new sections of membrane