cell division

• interphase- cells carry out major functions (produce enzymes for cell division)

  mitotic phase- period of cell division
• mitosis- nucleus divides

  cytokinesis- cytoplasm divides
• G1

  • organelles replicate
  • first growth phase
  • checkpoint for size, replication, damage
• s

  • DNA replicated
  • (synthesis)
• G2

  • energy stores increase
  • second growth phase
  • checkpoint for size, nutrients, damage
• G0

  • differenciation
  • damage
• stem cells

  -cells with the potential to differenciate into a specialised cell
• potency- a stem cells ability to differenciate into different cell types
• totipotent

  • ability to differenciate into any type of cell and organism
  • zygotes
• pluripotent

  • can form all tissue types (not all organisms)
  • embryo
• multipotent

  • only a range of cells within certain tissue can be formed
  • bone marrow
• embryonic stem cells

  • pluripotent
  • found in early stages of embryo development
• meristem stem cells

  • totipotent
  • found between phloem and xylem
• adult stem cells

  • multipotent
  • found in bone marrow
• mitosis- prophase

  1. chromatids coil and condense to form chromosomes
  2. the nuclear membrane begins to break down
  3. microtubules form spindle structures which link the poles
  4. two centrioles migrate to opposite poles of the cell
  5. spindle fibers attach to the centromeres and start to move the chromosomes to the center of the cell
  6. the nuclear envelope has disappeared
• mitosis- metaphase

  1. chromosomes are moved by spindle fibers to form a plate
• mitosis- anaphase

  1. the chromatids are separated
  2. they are pulled to opposites pole by the shortening spindle fibers
• mitosis- telophase

  1. the chromatids have reached the poles and are now called chromosomes
  2. nuclear envelope reforms around them
  3. the chromosomes start to uncoil and the nucleolus is formed
• mitosis- cytokinesis

  1. a cleavage furrow forms around the middle of the cell
  2. the cell membrane fuses around the middle forming two cells

  cytokinesis in plant cells

  1. plant cell walls dont allow a cleavage furrow to form
  2. vesicles assemble where the metaphase plate was formed
  3. they fuse together dividing the cell in two
• meiosis 1- pairs of homologus pairs chromosomes are separated into two cells

  meiosis 2- pairs of chromatids are separated into two more cells
• homologus pair- matching pair of chromosomes, one inherited from each parent
• prophase 1

  1. the nuclear envelope disintergrates and spindle formation begins
  2. homologus chromosomes pair up, chromatids entangle as they moves through the cytoplasm (crossing over)
• metaphase 1

  1. homologus pairs of chromosomes assemble along the metaphase plate
  2. maternal and paternal chromosomes can end up facing either pole (independent assortment) this creates genetic variation
• anaphase 1

  1. homologus chromosomes are pulled to opposite poles (chromosomes stay joined together)
  2. recombinant chromosomes allow genes to be exchanged between chromatids.
  3. this causes genetic variation
  4. the point where chromatids break and rejoin is called chiasmata
• telophase 1

  1. chromosomes assemble at each pole and the nuclear membrane reforms
  2. the cell undergoes cytokinesis, dividing into two cells
  3. chromosome number reduces from diploid to haploid
• haploid- one chromosome of each type (half normal chromosome number)

  diploid- two chromosomes of each type (one inherited from each parent)
• prophase 2

  1. chromosomes condense again
  2. the nuclear envelope breaks down
  3. spindle formation begins
• metaphase 2

  1. chromosomes assemble on the metaphase plate
  2. chromatids are no longer identical due to crossing over
  3. independent assortment occurs creating further genetic variation
• anaphase 2

  1. chromatids are parallel to opposite poles after the division of the centromeres
• telophase 2

  1. chromatids assemble at the poles
  2. chromosomes uncoil and form chromatin again
  3. the nuclear envelope reforms and nucleus visible
  4. cytokinesis results in the further division of 2 cells (forming 4 genetically different daughter cells)
• mitosis features

  • one cell division
  • produces two daughter cells
  • produces diploid cells
  • genetically identical cells
  • produces new body cells
  • no crossing over
• meiosis features

  • two cell divisions
  • produces four daughter cells
  • produces haploid cells
  • non identical cells
  • produces gametes
  • crossing over