cell cycle

• stages of cell cycle

  1. interphase
  2. G1
  3. S-phase
  4. G2

  2. nuclear division (meiosis/mitosis)

  3. cell division (cytokinesis - cytoplasmic division)
• interphase

  G1: cell grows by producing proteins and cell organelles (mitochondria, endoplasmic reticulum)

  synthesis: 

  • growth continues + DNA replication (S-phase)
  • protein molecules (histones) synthesised and attached to DNA = chromosomes
  • each chromosome becomes 2 sister chromatids

  G2:

  • growth continues by protein and cell organelle synthesis
  • mitochondria and chloroplasts divide
  • spindle forms
• nuclear division

  mitosis

  type of cell division giving rise to genetically identical cells

  chromosome number is maintained

  two cells formed containing 2 sets of chromosomes - diploid cells (2n)

  • importance:
  • asexual reproduction
  • growth in multicellular organisms
  • repair of damaged tissues
• meiosis

  • parent cell contains 2 pairs homologous chromosomes
  • results in 4 genetically dissimilar daughter cell
  • reduction division
  • type of cell division that gives rise to genetically dissimilar cells in which chromosome number is halved due to separation of homologous chromosomes

  haploid cell: one set of chromosomes

  diploid cell: two sets of chromosomes, one from each parent

  importance

  • produces haploid gametes
  • produces gametes that are genetically dissimilar
• nucleus

  1. chromosomes
  2. long thread-like structure found within nucleus
  3. made up of DNA
  4. condense into thick, rod-shaped structures when cell is dividing
  5. nuclear envelope
  6. separates contents of nucleus from rest of cytoplasm
  7. role
  8. control cell activities
  9. information in nucleus copied from cell to cell when cells divide
  10. passed into new individuals when sex cells fuse together
• Chromatin

  Long thin uncoiled threads of DNA coiled around histone proteins in a non-dividing cell
• Chromosome

  Compact, coiled structure of DNA when a cell is preparing to divide (condensed state)
• Chromosomes

  • Hold genes
  • Particular gene always occurs on same chromosome at same position
  • Shape
  • Fixed length with a centromere (always in the same position)
  • Number of chromosomes per species is fixed
  • Occur in pairs called homologous pairs
  • Copied before nuclear division occurs, forming sister chromatids attached at the centromere until separation to become chromosomes again
• Mitosis

  1. Prophase

  2. Metaphase

  3. Anaphase

  4. Telophase
• Prophase

  • Chromatin condenses by supercoiling to become 2 sister chromatids held together at centromere
  • Nucleolus gradually disappears
  • Nuclear envelope breaks down
  • Centrosome (absent in plants) divides and 2 centrioles replicate to form 2 centrosomes
• Metaphase

  • 2 centrosomes move to opposite ends of the cell
  • Microtubules of the cytoplasm start to form into a spindle, out from centrioles
  • Each pair of sister chromatids is attached to a microtubule of the spindle, arranged at the equator
• Anaphase

  • Centromeres separate
  • Spindle fibres shorten
  • Sister chromatids are pulled by centromeres to opposite poles
  • Once separated, they become chromosomes again
• Telophase

  • Nuclear envelope reforms around both groups of chromosomes
  • Chromosomes 'decondense' by uncoiling becoming chromatin again
  • Nucleolus reforms in each nucleus
• Meiosis

  The reductive division
• Prophase I

  1. Synapsis

  2. Pairing of homologous chromosomes

  3. Crossing over between non-sister chromatids resulting in new combinations of alleles on chromosomes

  4. Chiasma - point of join between non-sister chromatids
• Meiosis


  • A type of cell division that gives rise to genetically dissimilar cells
  • The chromosome number is halved due to separation of homologous chromosomes
• Sexual reproduction

  1. Two haploid sex cells fuse

  2. To form a diploid zygote
• Meiosis

  • Produces haploid gametes
  • Keeps chromosome number from doubling each time sexual reproduction occurs
• Meiosis

  Involves 2 divisions of the nucleus
• Homologous pairs of chromosomes

  • Both are duplicated chromosomes
  • Matching pairs of chromosomes that can possess different versions of the same genes/alleles
  • One member of the pair comes from the male and the other from the female parent
• Homologous pairs of chromosomes are maintained by exact replication which takes place prior to each mitotic division
• Prophase II

  1. Nuclear envelope breaks down

  2. Chromosomes shorten and re-thicken by coiling

  3. Centrioles (animal cells only) duplicate and move to opposite poles of the cell

  4. Spindle apparatus reformed at right angles to original spindle
• Metaphase I

  1. Spindle forms

  2. Homologous pairs become attached to individual microtubules of the spindle by their centromeres

  3. Homologous pairs are arranged at the equator of the spindle
• Genetic variation

  1. independent assortment of maternal and paternal homologous chromosomes
  2. the way the chromosomes of each homologous pair line up at the equator of the spindle in meiosis I is entirely random
  3. crossing over of segments of individual maternal and paternal homologous chromosomes
  4. results in new combinations of alleles on chromosomes of haploid cells produced
  5. random fusion of male and female gametes in sexual reproduction
• Anaphase I

  Separation of homologous pairs of chromosomes but not the sister chromatids
• Metaphase II

  Chromosomes line up at the equator of spindle, attached by centromeres
• Cytokinesis

  Animal cells

  • in-tucking of the cell surface membrane at equator of spindle, 'pinching' the cytoplasm in half
  • first sign of cleavage is appearance of cleavage furrow
  • cleavage furrow deepens until parent cell is pinched into 2, producing 2 completely separated cells
• Plant cells

  • golgi apparatus forms vesicles of new cell wall materials, collecting along the lines of equator of spindles
  • vesicles merge, forming new cell surface membrane and cellulose cells walls between the 2 cells
• Telophase I

  1. Homologous chromosomes arrived at opposite poles

  2. Chromosomes uncoil

  3. Nuclear envelope reform around both nuclei

  4. Spindle breaks down; continues into meiosis II directly
• Anaphase II

  1. Centromeres divide

  2. Sister chromatids move to opposite poles of the spindle, centromeres first
• Telophase II

  1. Nuclear envelopes form around the four groups of sister chromatids so four nuclei are formed, each with half the chromosome number of the original parent cell

  2. Chromosomes uncoil and become dispersed as chromatin

  3. Nucleoli reform
• 2 haploid cells formed (chromosomes are doubled)