Cell division

• Mitosis

  • nuclear division that produces genetically identical daughter cells
  • DNA replicates in the parent cell and is distributed so each daughters cell has an identical copy of DNA
  • only source of variation is via mutations
• Mitosis is essential in the growth and repair of tissues and in asexual reproduction
• During mitosis, the chromatin becomes organised into structures called chromosomes
• In somatic cells the chromosomes homologous pairs line up separately from each other
• The diploid number (2n) represents the total number of chromosomes in a normal body cell
• The haploid number (n) represents a single set of chromosomes i.e. one from each homologous pair
• Mitosis produces cells with the same number of chromosomes as the parent cell so that a diploid parent cell will divide to produce two genetically identical diploids cells
• Interphase

  • represents the non-dividing cell including stages G1, S and G2
  • the cell carries out normal cell functions by at the end prepares for cell division

  1. DNA content is doubled via DNA replication
  2. other organelles are replicated e.g. mitochondria, ribosomes
  3. RNA and protein synthesis
  4. ATP content is increased
• Prophase

  1. previously indistinct chromatin is now organised into chromosomes
  2. each chromosome then shortens and thickens by condensation achieved by spiralisation
  3. condensation of chromosomes prevents tangling during movements in the cell
  4. centrioles move to opposite poles
  5. nucleoli and nuclear membrane break down
• Metaphase

  1. centrioles at the poles form a spindle across the cell which consist of protein microtubules
  2. each chromosome moves to the equator of the spindle and attached to it via its centromere with its sister chromatids orientated toward opposite poles of the cell
  3. there is no association between members of each homologous pair on the spindle
• Anaphase

  1. centromere splits and sister chromatids separate
  2. sister chromatids are pulled to opposite poles of the cell by contraction of the spindle microtubules
  3. numerous mitochondria supply the energy for this movement
• Telophase

  1. chromatids are at opposite poles of the cell and the nucleoli reappear and nuclear membranes reform around each group of chromosomes
  2. sister chromatids represent the chromosomes of each new cell which are identical in genetic content to each other and to the original parent cell
  3. 2 distinct cells are formed as cytoplasmic cleavage occurs and a cell membrane forms between the cells
• Cytokinesis

  • last stage before G1 to produce 2 daughter cells

  1. organelles move towards the poles of the cell
  2. the cell membrane constricts at the equator of the cell

  • in plant cells a cell plate forms where new cell wall material is organised to form a new primary cell wall that eventually fuses with the old cell walls separating the 2 cells
• gametes produced are haploid containing one chromosome from each homologous pair
• Importance of meiosis

  1. ensures haploid gametes are produced from diploid cells
  2. when haploid gametes fuse at fertilisation to form a zygote the diploid number is restored
  3. the ensure each generation possess a constant number of chromosomes
  4. the process of meiosis produces genetic variation in gametes
• Meiosis

  • a single cell divides twice but DNA replication only occurs once
  • the number of chromosomes is reduced from the diploid number to the haploid number
  • a single cell produces four cells which are genetically varied
  • cells produced usually function as gametes i.e. reproductive cells
• Important features of meiosis

  1. independent segregation of homologous chromosomes
  2. genetic recombination by crossing over
• Independent segregation of homologous chromosomes

  • homologous chromosomes pair together and then separate so that one member from each pair enters the gamete
  • the pairing and subsequent separation of the 2 members of a pair is completely independent of the separation of another pair
  • the chromosomes randomly associate within a gamete
  • gametes produces will possess one member from each original homologous pair due to independent segregation of homologous chromosomes
  • the calculate the number of variations 2 to the power n is used where n is the number of homologous pairs
• Crossing over

  • only occurs in meiosis during prophase of the first meiotic division
  • 2 members of each homologous pair lies side by side forming a bivalent

  1. chromatids of homologous chromosomes intertwine
  2. chromatids break and equivalent portions of the chromatids are exchanges to produce recombinants
  3. chromosomes then separate
• Gametes

  • sexual reproduction involves gamete formation and fertilisation
  • DNA from one generation is passed to the next generation by gametes
  • gametes are sexual reproductive cells and are haploid
• Female gametes

  • large with more food reserves and cytoplasm than male gametes
  • less produced than male gametes
• Male gametes

  • produced in large numbers
  • mobile and swim to the female gamete
  • contain large numbers of mitochondria to provide energy for swimming to the ovum
• Haploid adult life cycle

  1. adult haploid (n)
  2. mitosis
  3. gametes (n)
  4. fertilisation
  5. zygote diploid (2n)
  6. meiosis
  7. adult haploid (n)
• Diploid adult life cycle

  1. adult diploid (2n)
  2. meiosis
  3. gametes (n)
  4. fertilisation
  5. zygote (2n)
  6. mitosis
  7. adult diploid (2n)
• Life cycle of a moss

  1. gametophyte adult (n)
  2. mitosis
  3. gametes (n)
  4. fertilisation
  5. zygote (2n)
  6. mitosis
  7. sporophyte adult (2n)
  8. meiosis
  9. spores (n)
  10. mitosis
  11. gametophyte adult (n)