OCR 21st Century B5

Growth and Development
1. Protein Synthesis
  1. Adenine-Thymine
    1. Guanine-Cytosine
    2. Uracil-A in MRNA
  2. A protein is a chain of amino acids. The sequence of amino acids produces different proteins.There are many thousands of different sequences
    1. Firstly the gene unzips and mRNA moves in to the nuclear membrane (through the pores as it is able to fit). The mRNA bases pair with the DNA bases to form a strand of mRNA
      1. The RNA moves out of the nucleus and into the cytoplasm. There it finds a ribosome
        The ribosome attaches to one end of the mRNA. As it moves along the mRNA it reads the bases in groups of three (triplet code/codon). Each codon codes for a single amino acid.
        Different combinations of the four bases produces 64 triplet codes. So there is more than one code for each amino acid.
        The ribosome reads the genetic code so that it can join the amino acids together in the correct order
        When it has finished, the ribosome releases the protein into the cytoplasm and starts to make another one
  3. Gene Switching
    1. Although all body cells in an organism contain the same genes, many genes in a particular cell are not active (switched off) because the cell only produces the specific proteins it needs
      1. Understand that in specialised cells only the genes needed for the cell can be switched on, but in embryonic stem cells any gene can be switched on to produce any type of specialised cell.
      2. In carefully controlled conditions of mammalian cloning, it is possible to switch on inactive genes in the nucleus of a body cell to form cells of all tissue types.
        ethical decisions need to be taken when using embryonic stem cells
        1.nucleus taken out of a human egg cell 2. nucleus from a patient's cell put into the egg cell 3. egg cell stimulated to develop into an embryo 4. stem cells taken from the embryo 5. stem cells grown in a container of warm nutrients 6. stem cells treated to develop into required cell types
Cell Specialisation
1. Multicellular organisms
  1. each kind of cell is specialised to perform particular jobs
    1. groups of specialised cells are called tissues, and groups of tissues form organs
      1. a fertilised egg cell (zygote) divides by mitosis to form an embryo (a ball of about 100 cells)
        In a human embryo up to (and including) the eight cell stage, all the cells are identical (embryonic stem cells) and could produce any type of cell required by the organism
        After this stage, most of the embryonic stem cells are specialised
  2. Adult stem cells remain unspecialised and can become specialised at a later stage but cannot become any type of cell mainly blood cells
    1. both these and embryonic stem cells have the potential to produce cells needed to replace damaged tissues
2. Plants
  1. Only meristem cells are mitotically active
    1. Unspecialised cells
      1. Cells specialise to form different types of tissue (including xylem and phloem) within organs (including flowers, leaves, stems and roots
        Found in, shoot tips, root tips, and in rings (stem/roots) to increase girth
  2. The presence of meristems allows the production of clones of a plant from cuttings. Cuttings are taken because they can be produced quickly and cheaply. Due to them being clones, a farmer can produce a plant with desired features.
    1. The cuttings are placed in rooting powder which contains the plant hormone-auxin. This stimulates the meristems to divide thus producing a root system, allowing the cutting to be planted.
3. Phototropism
  1. The bending of growing plant shoots towards the light
    1. By doing this a plant increases its chance of survival. This is because it is able to absorb more light which it uses for photosynthesis to produce glucose (food).
      1. Auxins are hormone that grow near tips of shoots and roots. They diffuse backwards to stimulate cell elongation
        When a shoot tip is exposed to light, more auxins accumulate on the shady side (via diffusion). They stimulate meristems to divide and cause growth at an increased rate on the shaded side so that the plant bends towards the light
Cell Cycle
1. B. Mitosis
  1. A type of cell division which Produces two new cells that are genetically identical to each other and to the parent cell.
    1. Growth and repair is dependent on mitosis
      1. Asexual reproduction takes place via mitosis
        During Mitosis copies of chromosomes separate and the whole cell divides
    2. copies of the chromosomes separate & the nucleus divides
2. A. Cell Growth
  1. Weak Hydrogen bonds between the bases split, unzipping the DNA from one end to form two strands
    1. Immediately, new strands start to form from free bases in the cell
      1. As A-T and C-G, the two new chains are identical
        This process shows how the chromosomes copy themselves. Secondly, before a cell can divide, it must also grow and make copies of organelles (ribosomes and mitochondria)
3. Meiosis
  1. a type of cell division that produces gametes
    1. produces four genetically different haploid cells
      1. a zygote contains a set of chromosomes from each parent
      2. 23 chromosomes
      3. Genetic variation is as a result of crossing over before meiosis.

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OCR 21st Century B5

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	Creado por Pritesh Patel hace alrededor de 8 años