B5 Growth and Development

B5 Growth and Development

1. The Development of Organisms

   • Development of New Organisms
     • Up to the 8 cell stage, all human cells are unspecialised and they can have any gene switched on to form any kind of specialised cell.
       • After the 8 cell stage the cells in an embryo become specialised and form differernt types of tissue
         • The cells contain the same genes, but many genes are not active (switched off) because the cell only produces the proteins it needs to do its job.
   • Living organisms are made up of cells. In multicellular organisms such as humans and plants.
     • Similar cells from a tissue, groups of tissues form an organ, groups of organs make up systems within the whole organism.

2. Mitosis

   • This is the division of body cells to produce new cells.
     • Each new cell has;
       • Identical sets of chromosomes as the parent cell.
       • The same number of chromosomes as the parent cell
       • The same genes as the parent cell
   • For growth, for repair, to replace old tissues
   • To enable mitosis to take place, cells go through a cycle of growth and then division. The cycle repeats itself until the cell can no linger divide.
     • When a cell enters the growth phase.
       • The number of organelles increase, the chromosomes are copied- the two strands of each DNA molecule seperate and new strands form alongside them.
     • When a cell enters the division phase of the cycle.
       • The copies of the chromosomes separate, the cell divides

3. Meiosis

   • Meiosis only takes place in the testes and ovaries and is a special type of cell division that produces gametes (sex cells,e.g egg and sperm).

4. Fertilisation

   • During fertilisation a male gamete (sperm) and a female gamete (egg) fuse together to produce a single body, cell called a zygote
     • The zygote then divides by mitosis to produce a cluster of cells called an embryo.
       • The embryo continues to divide by mitosis, after which the cells become specialised, until birth as a fully developed babyb

5. Variation

   • Meiosis and sexual reproduction produce variation between offspring and parents
     • When the gametes fuse, genetic information from two individuals is combined.
     • For each gene, just one of each parent's alleles is passed on.
     • Each offspring have a different combination of alleles from either parent.
     • The offspring can have different characteristics from each other.

6. Genes

   • Genes are present in the chromosomes in each cell nucleus.
     • The control
       • Growth and development in organisms
       • The development of characteristics, e.g. eye colour.
   • Genetic code
     • Genes control characteristics by providing intructions for the production of proteins
       • The instructions are in the form of a code, made up of four bases that hold the two strands of the double helix of the DNA molecule together.
         • The four bases always pair up in the same way.
           • Adenine (A) pairs with Thymine (T)
           • Cytosine (C) pairs with Guanine (G)

7. Controlling Growth and Development

   • DNA is too large to leave the nucleus. The genes therefore stay inside the nucleus but the production of proteins takes place outside the nucleus.
     • Information stored in the genes has to be transferred into the cytoplasm.The transfer is done in the following way.
       • 1). The relevant section of DNA is unzipped.
         • 2).Instructions are copied onto smaller molecules(messenger RNA or mRNA)
           • 3). These molecules leave the nucleus and carry the instructions to the ribosomes
             • 4). The ribosomes follow the instructions to make a relevant protein.
   • The sequence of bases in a gene determines the order in which amino acids are joined together to make a particular protein.
     • A group of three base pairs codes for one amino acid in a protein chain, called a triplet code. There are are 20 different amino acids are made.
       • The structure of the protein depends on the amino acids that make it up, the process is as follows:
         • 1).DNA unravels at the correct gene.
           • 2). A copy of the coding strand is made to produce mRNA
             • 3). The mRNA copy moves from the nucleus into the cytoplasm.
               • 4). The triplet code is decoded by the ribosomes.
                 • 5).Amino acids are joined together to form a polypeptide (protein)

8. Stem cells

   • stem cells could be used to.
     • help treat diseases and disorders.
     • Repair damage to various tissues
   • There are three sources
     • Embryos
       • Only embryonic stem cells are completely unspecialised and can be used to form any cell type
         • Ethical decisions need to be taken when using embryonic stem cells and this work is subject to government regulation.
     • Blood from the umbilical cord
     • Adult stem cells from bone marrow

9. Differentiation in Plants

   • Plant cells divide by the process of mitosis, New cells in plants specialise into the cells of: roots, leaves, flowers

10. Meristems

    • plant growth only occurs in areas called meristems, which are sites where unspecialised cells are dividing by mitosid
      • The two types of meristem: Lateral, which leads to increased girth and Apical which leads to increased height and longer roots
        • some plant cells remain unspecialised and can develop into any kind of plant cell. these cells allow clones of plants with desireble features

11. Xylem and Phloem

    • Xylem tubes are used by the plant to: transport water and soluble mineral salts from the roots to the stem and leaves.
    • Phloem tubes are used by the plant to transport dissolved food to the whole plant for respiration or storage.

12. Cuttings

    • Produced in the following way.
      • 1). Cuttings are taken from a plant.
        • 2). the cuttings are put in a rooting hormone.
          • 3).roots start to form and the new plants develop.

13. Photopism

    • Plants respond to light by changing the direction in which they grow. This is called phototropism.They grow towards a light source.
      • Auxin is produced at the shoot tip, it moves down the shoot, causing cells further down the shoot to grow
        • When light shines on a shoot, auxin near the light source is slowly destroyed, so there's more auxin on the far side away from the light.
          • Making the plant bend towards the light.