The Cell Cycle

• Make new molecules (e.g. phospholipids), membranes and organelles, and increase in size, not indefinitely as diffusion distances would be too great for O_2 to reach the mitochondria and the SA wouldn’t be large enough relative to cell size
• Before any problems arise, cells divide into two daughter cells, if they can divide, e.g. stem cells (animals) and meristematic cells (plants), as any lose the ability as they develop into specialised cells, e.g. neurones, blood cells, etc
• The cell cycle shows the different stages of a cell's life (G1 → S → G2 → M → C); which is controlled by a series of checkpoints that occur at intervals to ensure the previous stage is complete before the cell moves to the next stage
• G_1 Checkpoint: determines whether the cell goes through the cell cycle or not; checks for mutations in the DNA, any type of damage, and the age of the cell
• G_2 Checkpoint: determines whether the cell is ready to enter mitosis; final check that everything went as planned
• Metaphase Checkpoint: determines whether all chromosomes are attached to the spindles and ready for anaphase
• Cell cycle duration varies between organisms and between cells in the same organism: Bacteria (20 minutes), Liver (2 hours), Skin Cell (12 hours), and Mature Muscle Cell (never)
• Cell Cycle

  A) G1

  B) organelle

  C) Protein

  D) storing energy

  E) s

  F) DNA replication

  G) G2

  H) mitochondria

  I) chloroplasts

  J) Protein synthesis

  K) G0

  L) job

  M) always

  N) divide

  O) damaged

  P) old

  Q) erythrocytes