Nucleotides and nucleic acids

• Multipotent


  Stem cells that can only form into a limited range of cells determined by the type of tissue, e.g. haematopoietic stem cells give rise to various types of blood cells
• Pluripotent


  Stem cells that can divide into any type of cell but cannot form a full organism
• Totipotent


  A stem cell that can divide into any type of cell and can form a whole organism, e.g. the first 8/16 cells of an embryo
• Stem cell potencies

  totipotent, pluripotent, multipotent
• Stem cells


  undifferentiated cells that are able to renew themselves for long periods of time by cell division
• Categories of tissues in plants

  - Epidermis tissue, adapted to cover plant surfaces

  - Vascular tissue, adapted for water and nutrient transport
• Categories of tissues in animals

  • nervous tissue, adapted to support the transmission of electrical impulses

  • epithelial tissue, adapted to cover body surfaces, internal and external

  • muscle tissue, adapted to contract

  • connective tissue, adapted either to hold other tissues together or as a transport medium.
• Examples of Organ Systems

  digestive system, circulatory system, gas exchange system
• Layers of organisation

  Specialised cells -> Tissues -> Organs -> Organ Systems -> Whole Organism
• Crossing over


  Process in which homologous chromosomes exchange portions of their chromatids during meiosis.
• Meiosis II


  Similar to mitosis, the intermediate cells divide again to form a total of 4 gametes
• Meiosis I


  The first division in meiosis when the pairs of homologous chromosomes are separated into two cells. Each intermediate cell will only contain one full set of genes, so the cells are haploid
• Stages of Meiosis

  Meiosis I: Prophase I, Metaphase I, Anaphase I, Telophase I

  Meiosis II: Prophase II, Metaphase II, Anaphase II, Telophase II
• Locus


  Position of a gene on a chromosone
• Alleles


  Different versions of a gene with the same locus
• Homologous chromosones

  Each pair of chromosomes has the same genes and loci
• Meiosis


  Cell division that produces 4 haploid gametes in sexually reproducing organisms
• Differences in mitosis between plant and animal cells

  Animal Cells:

  - A cleavage furrow forms around the cell centre, the cell-surface membrane is pulled inwards until it is close enough to fuse around the middle and then does so, forming two cells

  Plant cells:

  - No cleavage furrow is formed due to the cell wall

  - Vesicles from the Golgi apparatus begin to assemble in the cell centre, the vesicles fuse with each other and the cell surface membrane, dividing the cell into two

  - New sections of cell wall then form along the new sections of membrane to prevent osmotic lysis
• Cytokinesis

  Division of the cell into two separate cells, beginning during telophase
• Telophase


  The fourth and final phase of cell division in which the chromatids or chromosomes at the poles of the cell and two nuclei are formed as nuclear envelopes and necleoli form.
• Anaphase


  The third phase of mitosis, during which the chromosome pairs separate after the centromeres divide and move toward opposite poles, moving in a v shape due to being dragged through Cytosol
• Metaphase


  Second phase of mitosis, during which the chromosomes line up across the centre of the cell, forming the metaphase plate
• Prophase


  first and longest phase of mitosis, during which the chromosomes become visible and the centrioles separate, the nucleolus and nuclear envelope disappear and take up positions on the opposite sides of the nucleus
• Process of Prophase

  1) Chromatin fibres begin to coil and condense to form chromosomes. The nucleolus disappears and the nuclear membrane begins to break down.

  2) Protein microtubules form spindle structures linking the poles of the cell

  3) The centrioles migrate to opposite poles of the cell

  4) The spindle fibres attach to specific areas on the centromeres and start to move the chromosomes to the centre of the cell

  5) By the end of prophase, the nuclear envelope has dissapeared
• Chromosome

  Contains genetic information
• Chromatids


  two identical chromosomes that split and contain the same genetic material
• Centromere


  Area where the chromatids of a chromosome are attached
• Phases of Mitosis

  prophase, metaphase, anaphase, telophase
• Mitosis

  Cell division into two identical daughter cells
• Spindle assembly/metaphase checkpoint


  checkpoint for chromosome attachment to spindles, and chromosome alignment.
• G2 checkpoint

  The cell checks to make sure the DNA is copied correctly
• G1 checkpoint

  checks to see if cell size is adequate, chromosomes replication is successfully completed and checks for DNA errors
• Checkpoints


  control mechanisms of the cell cycle.
• Senescent cells

  Cells that can no longer divide
• Reasons for G0 phase

  - Differentiation: The cell becomes specialised and is no longer able to divide, causing it to carry out its function indefinitely and not to enter the cell cycle again

  - DNA damage renders the cell unviable: A damaged cell can no longer divide, causing it to enter a period of permanent cell arrest
• Mitosis (Mitotic phase)


  The nucleus divides
• G0


  The phase in which the cell leaves the cycle, whether permanently or temporarily
• Cytokinesis (mitotic phase)


  The cytoplasm divides, producing two cells
• Mitotic phase


  The phase in which the cell divides into two new cells
• Stages of Interphase

  G1 - Growth phase: Proteins used for organelle synthesis are produced and organelles replicate, causing the cell to grow in size

  S - Synthesis Phase: DNA is replicated in the Nucleus

  G2 - Second Growth Phase: The Cell continues to grow in size while energy stores are increased and duplicated DNA is checked for errors