Cell Division

• the cell cycle - a highly ordered sequence of events that takes place in a cell resulting in division of the cell, and the formation of daughter cells
• Interphase:

  • the cell spends the majority of its time in interphase
  • this is when the cell grows and goes about its normal working e.g. synthesising proteins
  • sometimes referred to as the resting phase as no division occurs
  • DNA is replicated and checked for errors
  • mitochondria grow and divide
  • chloroplasts grow and divide
  • normal metabolic processes occur
• G1 - 1st growth phase

  • proteins from which organelles are synthesised are produced
  • organelles replicate
  • the cell increases in size
• S- synthesis phase

  • DNA is replicated in the nucleus
• G2 - the second growth phase

  • the cell continues to increase in size
  • energy stores within the cell increase
  • copied DNA is checked for errors
• Mitotic Phase:

  • period of cell division, involving both mitosis and cytokinesis
  • mitosis: the nucleus divides
  • cytokinesis: the cytoplasm divides and two cells are produced
• G0:

  • when the cell leaves the cycle temporarily or permanently
  • certain cells that enter G0 can be stimulated to reenter the cell cycle and start dividing again e.g. lymphocytes
• Reasons for G0:

  • Differentiation:
  • a cell becomes specialised to carry out a function, making it unable to divide
  • once differentiated, it will carry out its function indefinitely and not reenter the cell cycle
  • Damaged DNA:
  • a damaged cell may no longer be able to divide and enters a period of permanent arrest
  • the majority of cells will only divide a limited number of times and will then become senescent
  • the number of senescent cells increases over time
• Control of the cell cycle:

  • it is vital that the cell only divides once
  • checkpoints occur at various stages of the cycle
• G1 checkpoint:

  • the cell must be large enough, have enough nutrients, have enough proteins to stimulate tissue growth, no damaged DNA. If passed, cell can begin DNA replication, if failed, cell enters G0
• G2 checkpoint

  • occurs before the start of the mitotic phase
  • cell must have undamaged, completely copied DNA
  • if passed, cell begins mitosis
  • if damaged, cell will stop at G2 for repairs. If the damage is irreparable, the cell may undergo apoptosis
• Metaphase checkpoint (spindle assembly checkpoint)

  • metaphase checkpoint is the point in mitosis when all the chromosomes should be attached to spindles and be aligned in a plane along the middle of the cell
  • mitosis cannot proceed until this checkpoint is passed
  • the separation of sister chromatids during anaphase is irreversible so the cycle will not proceed until all the chromosomes are firmly attached to at least two spindle fibres from opposite poles of the cell
• Mitosis:

  • only the nuclear division stage of cell division
  • produces genetically identical daughter cells (unless a mutation occurs)
  • needed for growth, repair, asexual reproduction
  • bacteria do not produce asexually by mitosis as they do not have a nucleus, they undergo binary fission
• Chromosomes:

  • before mitosis can occur, the DNA must be replicated by semiconservative replication during interphase
  • each DNA molecule is converted into two identical DNA molecules called chromatids
  • chromatids are joined at the centromere. they need to be kept together so that they can be moved and then segregated precisely
• mitosis has 4 stages which flow from one to the other. they can be seen by doing a root tip squash
• Prophase:

  • chromatin fibres coil and condense to form chromosomes
  • the nucleolus disappears
  • the nuclear membrane begins to break down
  • protein microtubules form the spindle fibres needed to move the chromosomes in the correct place in the cell
  • centrioles migrate to the opposite poles of the cell
  • spindle fibres attach to the centromere at the kinetochore
  • nuclear envelope completely disappears
• Metaphase:

  • chromosomes are moved by spindle fibres to form a plane in the centre of the cell, this is called the metaphase plate
• Anaphase:

  • centromeres holding the chromatids together divide
  • chromatids are separated and are pulled to the opposite poles of the cell
  • the spindle fibres shorten
  • the chromatids are pulled through the cytosol
• Telophase:

  • chromatids have reached the poles - at this point they are called chromosomes
  • the chromosomes assemble at the poles and the nuclear envelope reforms around them
  • the chromosomes start to uncoil and the nucleolus is formed
  • at this point cytokinesis begins
• Cytokinesis:

  in animal cells:

  • a cleavage furrow forms around the middle of the cell
  • the cell surface membrane is pulled inwards by the cytoskeleton
  • when it is close enough it fuses in the middle and two cells are formed

  in plant cells:

  • With a cell wall, a cleavage furrow cannot be formed
  • vesicles from the Golgi assemble where the metaphase plate was formed
  • vesicles fuse with each other and fuse with the cell membrane around the cell to divide the cell in two
  • new sections of cell wall form along the new sections of membrane. Holes remain in the cell wall, known as plasmodesmata
• Meiosis:

  • nucleus divides twice producing 4 daughter cells - haploid gametes
  • meiosis is known as the reduction division, as it produces haploid cells with half the genetic information or a parent cell
• homologous chromosomes:

  • each characteristic within an organism is coded for by two copies of each gene
  • each nucleus contains two full sets of genes, meaning that there are matching sets of homologous chromosomes
  • each chromosome in a homologous pair have the same genes at each loci
  • homologous chromosomes have the genes in the same positions, so they are the same size and length
• Alleles:

  • in homologous chromosomes the gene at each loci might code for different varieties of the same characteristic e.g. eye colour
  • the different versions of the same gene are called alleles
  • different alleles of the same gene have the same locus
• The stages of Meiosis:

  Meiosis I:

  • the first division is the reduction division
  • the pairs of homologous chromosomes are separated into two cells
  • each intermediate cell only contains one full set of genes instead of two

  Meiosis II:

  • the second division
  • the pairs of chromatids in each daughter cell are separated and form two more cells
  • this produces 4 genetically different haploid cells
• Prophase I:

  • chromosomes condense
  • nuclear envelope disintegrates
  • nucleolus disappears
  • spindle formation begins
  • homologous chromosomes pair up forming bivalents
  • as this occurs the chromatids entangle and cross over
• Metaphase I:

  • pairs of chromosomes assemble along the metaphase plate
  • orientation of the homologous pairs on the metaphase plate is random
  • this is known as independent assortment
  • independent assortment results in many different combinations of alleles facing the poles
  • I.A causes genetic variation
• Anaphase I:

  • homologous chromosomes are pulled to the opposite poles and the chromatids stay joined together
  • sections of DNA on sister chromatids which are entangled due to crossing over break off and rejoin
  • this can result in an exchange of DNA
  • the point where chromatids break and rejoin are called Chiasmata
  • the exchange of DNA results in the formation of recombinant chromatids
  • this can result in chromatids with a different combination of alleles from the original chromatid, leading to genetic variation
• Telophase I:

  • chromosomes assemble at each pole
  • nucleus reforms
  • chromosomes uncoil
  • cell undergoes cytokinesis
• Prophase II:

  • chromosomes (two chromatids) condense and become visible
  • nuclear envelope breaks down
  • spindle formation begins
• Metaphase II:

  • individual chromosomes assemble on the metaphase plate
  • due to crossing over, chromatids are no longer identical
  • there is more independent assortment causing more genetic variation
• Anaphase II:

  • chromatids of the individual chromosomes are pulled to opposite poles after division of their centromeres
• Telophase II:

  • chromatids assemble at the poles
  • chromosomes uncoil and form chromatin
  • the nuclear envelope reforms and the nucleolus becomes visible
  • cytokinesis occurs
  • 4 genetically different haploid cells are formed
• Differentiation: the selective expression of genes in a cell's genome causing a cell to become specialised
• Stem cells:

  • all plant and animal cells begin as undifferentiated cells
  • these cells are unspecialised and cannot perform a particular function
  • the cells can differentiate to become any type of cell
  • stem cells can divide many times - they are the source of new cells for growth and tissue repair
  • once stem cells have become specialised, they lose the ability to divide and enter G0
  • if they do not divide fast enough, tissues are not effectively replaced but if they divide too fast, tumours are formed
• Stem cell potency: a stem cell's ability to differentiate into different cell types
• Totipotent:

  • can differentiate into any type of cell
  • found in zygotes and first 8-16 cells
  • can produce a whole organism
• Pluripotent:

  • can produce all tissue types but not a whole organism
  • present in early embryos
  • the origin of different types of tissues within the body
• Multipotent:

  • stem cells that can only produce a range of cells within a certain type of tissue
  • e.g. Haematopoetic stem cells in the bone marrow
• Erythrocytes:

  • reduced no. of organelles and no nucleus; short lifespan (120 days)
  • need constant replacement
  • stem cell colonies produce around 3 billion erythrocytes per kg per day to keep up with demand
• Neutrophils:

  • essential role in immune system (phagocytosis)
  • live for only 6 hours
  • stem cells produce 1.6 billion per kg per hour
  • this figure increases during an infection