cell division

• Stages of the cell cycle:

  S - Each of the chromosomes is duplicated

  G2 - The cell 'double checks' the duplicated chromosomes for errors, making any needed repairs. Further increase in size and build up of energy store.

  Mitosis - Division of the nucleus (Prophase, Metaphase, Anaphase, Telophase)

  Cytokinesis - division of the cytoplasm

  (G0 - cell division stopped. Can undergo differentiation and specialisation)

  G1 - Cellular contents excluding chromosomes are duplicated. Organelles duplicate, more cytoplasm, more membrane, energy storage
• the three stages of interphase:

  G1 - growth of cell, duplication of organelles etc.

  S - chromosomes duplicate

  G2 - Increase in size, Energy store, centrosomes replicate
• the significance of G0 is that cells no longer divide to reproduce but can differentiate to become specialised for a specific function.
• Checkpoints insure that each division is successful in producing two genetically identical daughter cells. They monitor and verify each stage is completed before the cell can move on to the next stage.
• the 3 examples of checkpoints in the cell cycle are:

  G1 checkpoint: cell size, nutrients, growth factors, DNA damage (at end of G1)

  G2 checkpoint: cell size, DNA replication, DNA damage (at end of G2)

  Spindle assembly check point: chromosome attachment to spindles (at end of metaphase)
• Mitosis - nuclear division stage of the mitotic phase of the cell cycle. Results in two daughter cells each having the same number and kind of chromosomes as the parent nucleus
• Chromosome - Structures of condensed and coiled DNA in the form of chromatin. Chromosomes become visible under a light microscope when the cell is preparing to divide
• Chromatid - each of the two thread-like strands into which a chromosome divides longitudinally during cell division
• Sister chromatids - two identical copies of DNA (a chromosome) joined at a centromere
• Centromere - a region at which two chromatids are held together
• Centrioles - component of the cytoskeleton of most eukaryotic cells composed of microtubules
• Spindle fibres - a network of filaments that collectively form a mitotic spindle (in mitosis) and meiotic spindle (in meiosis) and are responsible in moving and segregating the chromosomes during nuclear division
• Homologous pairs - matching pair of chromosomes, one inherited from each parent
• how DNA is packaged into a chromosome:

  double-stranded DNA loops around histones, forming the nucleosome. DNA can be further packaged by forming coils of nucleosomes, called chromatin fibres. These fibres are condensed into chromosomes during mitosis, or the process of cell division.
• the stages of mitosis (in order) are:

  • prophase
  • metaphase
  • anaphase
  • telophase
• Prophase - Duplicated chromosomes condense, Spindle fibres extending/being formed towards centromeres, Nuclear envelope fragmenting
• Metaphase - Chromosomes line up in centre of cell (equator of cell/mitotic plate) Spindle fibres attach to centromere of each chromosome
• Anaphase - Each chromatid is separated into its sister chromatids. Spindle fibres shorten, pulling chromatids to each side of cell
• Telophase - Nuclear envelope reforming, chromosomes less condensed, unravelling.
• the roles of centrioles and spindle fibres in mitosis:

  Spindle fibres form from the centrioles. They attach to centromeres to separate the chromatids and draw them to each side of the cell to from the two new nuclei.
• cytokinesis in animal cells:

  Cleavage furrow forms around the centre of the cell. Cytoskeleton draws membrane in until it is close enough to fuse to form two separate cells
• cytokinesis in plant cells:

  Vesicles from golgi apparatus line up along centre of cell and fuse together forming the new cell membranes. New sections of wall then form along the new surface membranes.
• Diploid - A cell with 2n chromosomes (two copies of each chromosome, one from each parent)
• Haploid - A cell with n chromosomes (one copy of each chromosome)
• Gamete - A haploid germ cell - sperm of egg cell in animals. They fuse at fertilisation to produce a zygote.
• Zygote - The cell produced from the fertilisation of a haploid egg cell by a haploid sperm cell
• Meiosis - The form of nuclear division that results in the production of haploid nuclei from a diploid nucleus
• Reduction division - Any form of nuclear division in which the chromosome number is reduced
• the role of meiosis:

  Meiosis is needed for sexual reproduction. It halves the number of chromosomes so that the diploid number of chromosomes is restored at fertilisation. It produces genetic variation in offspring.
• the two ways in which meiosis produces variation is independent assortment and crossing over
• Genetic variation is important in populations. Makes them less vulnerable to disease or change in conditions as some will be more suited to new environment.
• Homologous chromosomes - matching pair of chromosomes, one inherited from each parent
• Bivalent - The name for two homologous chromosomes that have paired up in prophase I of meiosis (The pair of chromosomes join by synapsis and an alternative name for the structure is a tetrad)
• Crossing over - The exchange of part of a chromosome between chromatids of homologous pairs. This occurs at chisamata.
• Chiasmata - Sections of DNA which became entangled during crossing over, break and rejoin during anaphase I of meiosis, sometimes resulting in an exchange of DNA between bivalent chromosomes, forming recombinant chromosomes and producing genetic variation
• Recombinant chromatid - chromatids with a combination of DNA from both homologous chromosomes, formed by crossing over and chiasmata in meiosis
• Random independent assortment - The fact that which daughter cell a chromosome ends up in after Meiosis I or a chromatid ends up in after Meisosis II is random and independent of the fate of chromosomes from other homologous pairs.
• Meiosis I - Chromosomes paired up - bivalents line up along centre of cellIt is the whole chromosomes that are pulled to each side of the cell still in duplicated form (2n to n)
• Meiosis II - Same mechanism as mitosis but with fewer chromosomes - chromatids are separated into each side of the cell.
• the stages of meiosis are: prophase I, metaphase I, anaphase I, telophase I, cytokinesis I, prophase II, metaphase II, anaphase II, telophase II, cytokinesis II