cell division

• eukaryotic cells have 2 main phases in its cycle: the interphase and the miotic division phase.
• interphase: cells DO NOT divide continuously, long periods of growth and normal working separate divisions. the cell spends most of its time in this.
• during interphase:

  • DNA is replicated and checked for errors in nucleus
  • protein synthesis occurs in the cytoplasm
  • mitochondria grow and divide, increasing in the number in the cytoplasm, multiplying by dividing
  • chloroplasts grow and divide in plant and agal cell cytoplasm, increasing in number in the cytoplasm, multiplying by dividing
  • the normal metabolic processes of cells occur (some, including cell respiration, also occur throughout cell division)
• 3 stages of interphase:

  1. G1 which is the first growth phase, proteins are produced, organelles replicate and cell increases in size.
  2. S which is the synthesis phase, DNA is replicated in the nucleus.
  3. G2 the second growth phase where the cell continues to increase in size, energy stores are increased and the duplicated DNA is checked for errors.
• G1 checkpoint checks for:

  • cell size
  • nutrients
  • growth factors
  • dna damage

  if it doesn’t fit into requirements then it enters G0
• G0 is the cell cycle arrest (cell leaving cycle temporarily or permanently) reasons for leaving cell cycle:

  • differentiation: cell becomes specialised to carry out its function and is no longer able to divide.
  • the dna of a cell may be damaged
  • natural aging process
• checkpoints in cell cycle: controls mechanisms of cell cycle. monitor and verify whether the processes at each phase of the cell have been accurately completed before the cell is able to move into the next phase.
• G2 checkpoint checks for:

  • cell size
  • dna replication
  • dna damage
• metaphase checkpoint (spindle assembly checkpoint) checks for:

  • chromosome attached to spindle formed on the centrioles.

  mitosis cannot proceed until this checkpoint is passed.
• the passing of a cell cycle checkpoint is brought about by kinases, the class of enzyme that catalyse the addition of a phosphate group to a protein (phosphorylation). this changes the tertiary structure of checkpoint proteins, activating them at certain points of the cycle.
• kinases involved in cell cycle regulation are activated by binding to a variety of checkpoint proteins called cyclins. the binding of the correct cyclin to the appropriate kinase forms a cyclin- dependent kinase (CDK) complex. these are activated by enzymes.
• CDK complexes catalyse the activation pf key cell cycle proteins by phosphorylation, which insures a cell progresses through the cycle at appropriate times. different enzymes break down cyclins when they are not needed (signalling cell to move to next phase).
• cancer is caused by uncontrolled division of cells

  -abnormal mass of cells = tumour which can be benign (do not travel and stop growing) or malignant (grows and is uncontrolled)
• tumours are often a result of damaged or spontaneous mutations of genes that encode the proteins involved in regulating the cell cycle, including checkpoints. eg: overexpression of cyclin results from mutation, more cyclin produced, disrupts regulation of the cell cycle.
• CDK can be used as a possible target for chemical inhibitors in the treatment of cancer.
• mitosis occurs in eukaryotic cells. it ensures both daughter cells produced (when a parent cell divides) are genetically identical (except when mutations occur)
• mitosis is necessary when:

  • growth
  • replacement
  • repair

  of tissues (in plants, animals and fungi)
• prokaryotic (bacteria) = no nucleus so reproduce asexually by binary fission.
• before mitosis occurs, all the DNA in the nucleus is replicated during the interphase.

  each DNA molecule (chromosome) is converted into 2 identical DNA molecules (chromatids)
• chromatids are joined together at a region called the centromere which is necessary to keep chromatids together.
• mitosis: prophase:

  • chromosomes condense and supercoil.
  • centrioles divide and move to cell poles, forming a spindle protein network (animal cells)
  • nuclear envelope breaks down.
• mitosis: metaphase:

  • chromosomes line up along equator.
  • spindle attaches to a centromere of each chromosome.
• mitosis: anaphase:

  • centrioles divide, separating the chromosomes into sister chromatids.
  • spindles contract, pulling chromatids to poles of cell.
• mitosis: telophase:

  • chromatids reach opposite poles and uncoil.
  • nuclear envelope forms around both groups, forming nuclei.
• mitosis: cytokinesis:

  two diploid daughter cells produced.
• mitosis (tow=2):

  • dell division results in 2 genetically identical daughter cells- number of chromosomes (23 pairs) maintained across the diploid cells (a cell that has 2 copies of each chromosome)
  • it is necessary for: asexual reproduction, growth and tissue repair.

  meiosis:

  • cell division that results in the formation of 4 genetically distinct daughter cells containing half the number of chromosomes as the parent cells (haploid)
  • it produces gametes (sex cells) which are essential for sexual reproduction.
• centriole: barrel- shaped organelles located in the cytoplasm (animal cells) near nuclear envelope - organise microtubules.
• sister chromatids- one of the two identical halves of a chromosome that has been replicated for cell division.
• chiasmata- non sister chromatids wrap around each other and join at certain points = chiasmata
• alleles: alternative forms of a gene
• homologous chromosomes: two chromosomes in a pair, in one cell
• centromere: a structure on a chromosome that holds sister chromatids together
• genetic diversity produced in meiosis:

  • crossing over creates different allele combinations on sister chromatids
  • dividing the chromosomes and then sister chromatids in anaphase’s ensures each of the four daughter cells has a different combination of alleles.
  • each daughter cell is haploid, it has to combine its dna with that if another cell = fertilisation
  • mutations
• stem cells: sources:

  • embryonic stem cells= formed when a zygote begins to divide
  • umbilical cord blood
  • certain tissues (skin, bone marrow and brain)
  • can be created by “reprogramming” differentiated cells , called induced pluripotent stem cells (iPS cells)
• the different cells in a multicellular organism are specialised for different functions. the process of a cell becoming specialised is called differentiation.
• despite being differentiated in structure and function, all body cells in an organism have the same dna
• differentiation involves the expression of some gene but not others in the cells genome
• all cells in plants and animals begin as undifferentiated cells, they are not adapted to any particular function. = unspecialised. they have the potential to differentiate to become any one of the range of specialised cell types in the organism.
• undifferentiated cells = stem cells
• stem cells are able to undergo cell division a lot and are the source of new cells, needed for growth, development and tissue repair.