The Cell Cycle

• Asexual reproduction


  The production of genetically identical offspring from a single organism
• Sexual reproduction


  The production of offspring that are genetically different from the parent organisms (because of the fusing of the two gametes)
• What are chromosomes made of?

  A mass of coiled DNA threads and proteins
• What happens to the chromosomes during eukaryotic cell division?


  . Chromosomes replicate

  . In a cell that is not actively dividing, the chromosomes cannot be seen

  . When the cell divides, the chromosomes condense and take up stains readily - this enables us to see them

  . DNA winds around histones to form dense clusters known as nucleosomes
• Number of chromosomes in humans

  46 (in 23 pairs)
• The cell cycle


  A regulated, three-stage process in which cells divide into two genetically identical daughter cells
• Stages of the cell cycle

  • Interphase
  • (including G1, S and G2)
  • Mitosis
  • (including prophase, metaphase, anaphase and telophase)
  • Cytokinesis
• Interphase


  Period between active cell divisions where cells increase in size and mass, replicate their DNA and carry out metabolic activities
• S stage of interphase

  
  

  • Chromosomes replicate and become double-stranded chromatids ready for next division
  • DNA will double (through DNA replication)
• G2 stage of interphase


  • Organelles and other materials are synthesised - cell needs double of everything so that it can divide
  • Centrioles (source of spindle fibres) are made
• Control of the cell cycle

  • The cell cycle will repeat frequently during development but may slow/stop once organism is mature
  • Controlled by cyclins
  • small proteins
  • Cyclins attach to cyclin-dependent kinases (CDKs)
  • phosphorylate other proteins
• Phosphorylation


  Addition of a phosphate group
• Examples of CDK phosphorylation


  • Phosphorylation of chromatin in nucleus - results in chromosomes becoming denser
  • Phosphorylation of proteins in nuclear membrane - results in the breakdown of structure during cell division
• Histones


  Positively charged proteins involved in DNA coiling to form dense chromosomes
• Nucleosomes


  Dense clusters of DNA wound around histones
• Chromatid


  One strand of the replicated chromosome pair that is joined to the other chromatid at the centromere
• Cyclins


  Small proteins built up during interphase and involved in the control of the cell cycle
• Cyclin-dependent kinases (CDKs)


  Enzymes involved in the control of the cell cycle by phosphorylating other proteins, activated by attachment to cyclins