cell cycle, mitosis and meiosis

• somatic cell = any cell of a multicellular organism that is not a gamete

  the life cycle - from formation of cell -> eventual division into 2 daughter cells

  life cycle contains 3 stages:

  • interphase: growth and synthesis has 3 phases: G1, S, G2
  • mitosis: nuclear division has 4 phases: prophase, metaphase, anaphase and telophase
  • cytokinesis: cell division
• interphase

  longest phase, has 3 distinct stages: G1, S and G2

  • G1 = first growth phase: organelles duplicate, intake of nutrients, growth
  • S = synthesis: DNA replication, listone synthesis
  • G2 = second growth phase: organells duplicate, production of proteins requires for mitosis and cytokinesis

  the first stage G1, can enter G0: cellular state outside replicative cell cycle
• G0 phase:

  during G1 phase cell may go into G0 phase for a number of reasons:

  1. resting phase - a period of inactivity or dormancy before going back to G1 called quiescent cells, cells may re-enter G1 due to external stimuli
  2. cell differentiona nd specialisation - when cells become specialised they lose ability to divide and remain in G0 for the rest of their lifetime
  3. senescence - ageing of cells
  4. appoptosis - programmed cell death
• mitosis = nuclear division

  • appearance of chromosomes and their districbution to 2 daughter cells
  • mitosis is a continuous process and is divided into 4 dynamic stages
• there are 3 checkpoints that occur during different phases of cell cycle

  metaphase checkpoint:

  • to ensure correct attachment of chromosomes to spindle fibres

  restriction checkpoint:

  • commits cells to dividing if all requirements are met: cell size, nutrients, DNA damage

  final checkpoint to ensure any incorrectly replicated DNA is repaired
• definition of a chromosome:

  • a molecule of DNA (present as chromatin in interphase) chromosomes condense and appear in mitosis and meiosis
  • human cells have 23 pairs of chromosomes

  homologous chromosomes

  • a pair of chromosomes, one from each parent which have:
  • a similar length
  • same genes at same loci (positions)
  • NB - each gene may have a dif. allele (allele = alternative form of a gene)
  • centromere in same position

  after DNA replication, each chromosome is made up of 2 identical sister chromatids
• mitosis

  • the seperation of identical sister chromatids (into 2 daughter cells)
  • centrioles: attachment point for spindle fibres
  • centrosome = organelle containing the centrioles enables formation of spindle fibres during prophase
• interphase -

  • intact nuclear envelope
  • DNA in form of chromatin, replicating in S phase
  • nucleolus present
  • pair of centrioles. duplication of centrioles happens in S phase

  prophase -

  • spindle start to emerge from centrosome at each pole
  • centrioles have migrated to opposite poles and formed centrosomes
  • nuclear envelope starst to degrade
  • chrosomes appear and each has a pair identical chromatids
  • centromere = attachment point for sister chromatids
• metaphase -

  • spindle fibres (microtubules) fully formed
  • chromosomes line up at equator
  • centromeres attach sister chromatids to spindle fibres

  anaphase -

  • (identical sister) chromatids are seperated by being pulled apart to opposite poles by shortening spindle fibres

  telophase -

  • centrioles (duplicated in interphase S that follows)
  • chromosomes disperse and chromatin reforms
  • nuclear envelope reforms

  • cytokinesis in animals involves equator region invaginating
  • cytokinesis in plant cells involved formation of a cell wall at equator
• mitosis, beheviour of chromosomes and other events:

  interphase:

  • DNA replicates (chromosomes duplicate) in S phase
  • centrioles duplicate

  prophase:

  • chromosomes appearing/condensing
  • centrioles migrate to opp. poles
  • spindle forming
  • nuclear envelope degrading

  metaphase:

  • chromosomes line up at equator
  • centromeres attach to spindle fibres

  anaphase:

  • chromatids seperating

  telophase:

  • chromosomes disappearing/ decondensing
  • spindle shortening/disassemblding

  cytokinesis

  • nuclear envelope reforming
  • cells seperating
• G1 (gap 1) phase - growth phase

  • cell growth
  • seize increases
  • new organelles synthesises
  • vol. of cytoplasm increases
  • protein synthesis - cell produces proteins and enzymes needed for DNA replication, histone proteins synthesised
  • organelle duplication
  • metabolic activity
  • checkpoint control: cell size, nutrient availability, DNA integrity
• G0 (gap 0) phase:

  • cell cycle xit
  • reduced metabolic activity
  • maintenance of cell function
• S (synthesis) phase:

  • DNA replication - chromosomes duplicated
  • histone protein synthesis
  • centrosome duplication
  • checkpoint control: monitors accuracy of DNA replication, can repair DNA before G2 phase
• G2 (gap 2) phase:

  • cell growth
  • protein synthesis
  • DNA damage check and repair
  • chromosome condensation preperation
  • duplication of centrosomes complete
  • checkpoint: complete DNA replication, DNA integrity , sufficient resources for mitosis
• M phase:

  • prophase: chromosomes condense, nuclear envelope begins breakdown, spindle apparatus begins to form as centrosomes move to poles
  • metaphase: chromosomes line up along equator, spindle assembly checkpoint ensures that all chromosomes are properly attached to spindle fibres
  • anaphase: sister chromatids pulled apart by shortening spindle fibres, chromatids now an individual chromosome, moves to opposite poles of cell
  • telophase: chromosomes reach poles and decondense, nuclear envelope reforms, two nuclei formed, spindle apparatus disassembles
  • cytokinesis
• cytokinesis in animal cells: cleavage of an animal cell to form a new membrane

  cytokinesis in plant cells: cell plate formation in a plant cell to form new plant cell walls and membrane
• significance of mitosis:

  • mitotic division of chromosomes ensures: equal quantity of DNA to both daughter cells, the DNA is genetically identical for both daughter cells (genetic variation only occurs if mutations occur)
  • growth of multi-cellular organisms by cell division
  • tissue repair and replacement
  • increase in numbers of immune cells
  • asexual reproduction: vegetative propagation in plants - cloned plants grow from meristematic tissues of parent plants, also occurs in some insects e.g. aphids
• Binary fission (asexual reproduction in bacteria) is different to mitosis: there is no appearance of chromosomes or spindle fibres or degredation of the nuclear envelope
• the mitotic index = the number of cells in mitosis/ total number of cells