cell cycle

• Cell cycle


  The pattern of events involved in the formation of new cells, including the production of additional cell contents before a cell can divide
• Cell cycle


  1. Interphase (G1 and G2 phases)

  2. Synthesis (S) phase

  3. Mitosis (nuclear division)

  4. Cytokinesis (cell division)
• Interphase


  • Two gap (growth) phases (G1 and G2) separated by a synthesis (S) phase
• Mitosis


  • Nuclear division during which the chromosomal material is partitioned into daughter nuclei
• Cytokinesis


  • The cell divides into two daughter cells
• The terms cell cycle, mitosis and cytokinesis are quite distinct and should not be confused
• Mitosis


  One form of nuclear division
• Meiosis


  The other form of nuclear division, the process by which the nucleus divides
• During interphase, the cell synthesises new proteins and organelles
• During S phase, DNA is replicated and the amount of chromatin is doubled
• Cell division follows nuclear division, and the cell cycle represents the entire life of the cell during which it grows, and includes nuclear division and cytokinesis
• S phase


  1. DNA synthesis occurs

  2. Histones (proteins that bind and support DNA) are produced

  3. DNA and chromatids formed are identical and remain attached until separated during mitosis
• G2 phase


  1. Proteins such as tubulin are synthesised (tubulin forms the microtubules of the spindle fibres)

  2. Energy stores are increased

  3. The cell continues to increase in size
• Mitosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase

  5. Cytokinesis
• Prophase


  • Chromatin condenses to form chromosomes
  • Centrioles move towards opposite poles
  • Spindle begins to form
  • Chromosomes continue to condense, with two chromatids joined at the centromere
• Metaphase


  • Nuclear envelope breaks down
  • Spindle formation is completed
  • Microtubules of the spindle attach to the centromere of each chromosome
  • Chromosomes (chromatid pairs) are moved by the microtubules onto the equator of the spindle
• Anaphase


  • Centromeres divide
  • Spindle fibres pull the centromeres of sister chromatids apart
  • Sister chromatids move towards opposite poles
• Telophase


  • Each chromatid is now a separate chromosome
  • Two groups of chromosomes reach opposite poles of the cell
  • New nuclear envelope forms around each group
• Cytokinesis


  • In animal cells, a cleavage furrow forms and the cell is cleaved into two
  • In plant cells, Golgi bodies produce vesicles that fuse to form an equatorial cell plate, with new cell walls laid down on each side
• Checkpoints in the cell cycle


  • G1 checkpoint: Assess nutrient/growth factor supply, DNA damage, cell size
  • G2 checkpoint: Check DNA replication, DNA damage
  • Metaphase checkpoint: Ensure chromosomes are correctly attached to spindle fibres
• If a mutation occurs in a gene encoding a checkpoint protein, the cell undergoes repeated, uncontrolled division, leading to a tumour
• Benign tumour


  Stops growing, encapsulated in a fibrous sheath, does not travel to other locations
• Malignant tumour


  Continues to grow unchecked and uncontrolled, cells are shed and carried around the body, producing secondary tumours (metastases)
• Anti-cancer drugs


  • Inhibit DNA unzipping
  • Inhibit nucleotide synthesis
  • Inhibit mitotic spindle formation
• Genetic locus


  Specific length of DNA along a chromosome that represents a gene
• Homologous chromosomes


  Chromosomes that are similar in size and shape, and possess alleles of the same genes (one from each parent)
• Homozygous


  Alleles on homologous chromosomes are the same
• Heterozygous


  Alleles on homologous chromosomes are different
• Diploid


  Cell contains two sets of chromosomes
• Haploid


  Cell contains only one set of chromosomes
• Gametes are haploid, and diploidy is restored at fertilisation
• Karyotype and karyogram can be used to study chromosome number and ploidy
• Meiosis


  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Meiosis prophase


  • Homologous chromosomes pair up to form bivalents
  • Crossing over occurs between non-sister chromatids of the bivalent
• If a cell has 46 chromosomes, it is diploid
• Haploid


  A cell that contains only one of each type of chromosome
• In mammals the gametes are haploid, and diploidy is restored at fertilisation
• Karyotype


  A photomicrograph of a cell during nuclear division (at metaphase) showing the chromosomes
• Karyogram


  Arranging the chromosomes into homologous pairs, recognised by their length and centromere position
• A human karyogram contains 23 pairs including a pair of sex chromosomes