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How Organisms Develop

  • Cells are grouped into tissues e.g. muscle cells ---->Muscular tissue
  • Different tissues are grouped together and work together in organs
  • Organsims begin life as a zygote (fertilised egg) whcih divides by mitosis to form an embryo
  • In humans, cells are identical upto the eight cell stage. These are embryonic stem cells which produce any cell type in the body
  • After eight cell stage, cells become specialised (differentiation), and different tissues form.
  • In specialised cells, only the genes are needed to enable the cell to function, as that type of cells is switched on.
  • Specialised plant cells form tissues such as xylem (transports water) and phloem (transports glucose)
  • Meristems are unspecialised
  • When meristem cells divide into two, new cell can differentiate into different cell types
  • In plants, the only cells that divide are in meristems
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Plant Development

  • New plants can be grown by placing the cut end of a shoot in water or soil
  • Pieces of plants that have meristems are used to produce clones are called cuttings:
  • They can be used to produce new plants wiht the same desirable features as the parent
  • Or produce clones that are genetically identical to the parent plant.
  • Root growth in cuttings is promoted by plant hormones
  • Tissue Culture - small piece of tissue placed on agar jelly containing nutrients and plant hormones. Each grows into a small plant
  • Plant hormones called auxins are included in agar for tissue culture and hormone rooting powder
  • Auxins increase cell division and enlargement, promoting growth of the plant tissue
  • Plants grow towards light (positive phototropism), roots grown away from light (negative phototropism) - Increases chance of survival
  • Auxins produced at the tip and diffuses down and produces growth below the tip
  • When a shoot tip is exposed to light, auxins distributed on shaded side
  • Auxin causes the cells to elongate faster on shaded side so shoots grow towards light 
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Cell Division - Mitosis

  • Type of cell division that takes place when an organism grows and cells divide to repair tissues
  • Mitosis results in production of two daughter cells that are genetically identical
  • Before mitosis, the DNA in each chromosome is copied. 
  • Each double chromosome separates so that 2 nuclei and two cells are produced.
  • Cell cycle consists of: 
  • Cell growth: cell increases in size; number of organells increase; DNA in each chromosome is copied
  • Mitosis: two daughter cells, each identical to parent cell and containing identical set of chromosomes, are produced as the strands of each double chromosome separate and two nuclei are formed.
  • 1) Each chromosome makes an identical copy of itself
  • 2) Chromosomes line up at the centre of the cell and cell fibres pull them apart
  • 3) Membranes form around each of the sets of the chromosomes, becoming nuclei of two new cells 
  • 4) Cytoplasm divides to form two new cells
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Cell Division - Meiosis

  • Used to produce gametes (sex cells-egg and sperm in animals; eggs and pollen grains in flowering plants)
  • Gametes contain half the number of chromosoms (23) as body cells (46)
  • At fertillisation, gametes join to form a zygote with 46 chromosome
  • Zygote contains a set of chromoses from each parent
  • 1) Before cell divides, it duplicates DNA
  • 2) In first division, chromosome pairs line up in centre of cell
  • 3) Pairs are then pulled apart so each new cell has one copy of each chromosome
  • 4) In second division, chromosomes line up again in centre of cell. Arms of the chromosomes are pulled apart.
  • 5) Produces four gametes with half the number of chromosomes
  • Mitosis produces two daughter cells with an identical number of chromosomes as the parent cell
  • Meiosis producues four daughter cells with half the number of chromosomes
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Chromosomes, Genes, DNA and Proteins

  • DNA molecule is a double helix which are made of bases - A,T,C and G
  • Order of bases in a gene makes up genetic code which gives instructions for the assembly of a protein (amino acids and the order in which they're orranged)
  • Each set of three bases (triplet) codes for one amino acid
  • Protein Synthesis occurs in the cytoplasm
  • Genes do not leave the nucleus so mRNA is produced in the nuclues, using DNA as a template 
  • 1) The two DNA strands unzip. mRNA is made using one strand of DNA as a template
  • 2) mRNA moves out of the nucleus and joins with a ribosome in the cytoplasm
  • 3)Ribosome sticks amino acids together in a chain to make a protein, following the order of bases in the mRNA.
  • Number and sequence of amino acids determines the type of protein
  • Sequence of amino acids is determined by the genetic code 
  • Bases work in threes (triplets) to code for an amino acid
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Cell Specialisation

  • A cell only produces the proteins it needs to carry out its function
  • The genes to make these proteins are switched on whilst others are switched off
  • Up to the 8 cell stage, the embryonic stem cells are identical
  • Specialised cells begin to make specific proteins
  • In embryonic stem cells, any gene can be switched on so they can produce any cell type.
  • Embryonic stem cells have the potentical to replace cells need to replace damaged tissues
  • Adult stem cells are found in the bone marrow and can be used to produce a limited number of cell types
  • Embryonic stem cells raises ethical issues:
  • Embryo is destroyed
  • Embryos have a right to life from when they're conceived
  • Creation of embryos with intent to destroy would be more controversial
  • Therapeutic cloning overcomes some ethical issues:
  • Involves replacing the nucleus of an egg by the nucleus of a body cell
  • Involves stimulating the egg cell to divide to produce an embryo
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