B5

B5

1. DNA

   • A double helix of paired bases
   • Each strand is made up of little nucleotides
   • Each nucleotide contains a base: A, T, C, G
     • A-T and C-G

2. Proteins

   • Genes are sections of DNA that contain instructions for particular proteins
   • Cells make proteins by joining amino acids in a particular order
   • The order of the bases in a gene tell the cell in what order to put the amino acids together
   • They are made in the cytoplasm by ribosomes
   • DNA cannot move out of the nucleus because it is too big, so a copy of the DNA is made using mRNA
     • mRNA is shorter and only has one strand
     • 1, The two DNA strands unzip and a molecule of mRNA is made using one strand of DNA as a template
     • 2. The mRNA moves out of the nucleus and joins the ribosome In the cytoplasm
     • 3. The ribosome sticks the amino acids together in a chain to make a protein, following the order of the bases in the mRNA

3. Cell Division

   • Mitosis
     • 1. The cell has two copies of its DNA all spread out in long strings
     • 2. Before the cell divides, the DNA forms X-shaped chromosomes. The two arms are duplicates of each other
     • 3. The chromosomes line up at the centre of the cell and their fibres pull them apart; the two arms of each chromosome go to opposite ends of the cell
     • 4. Membranes form around each sets of the chromosomes; these become the nucluei of the two new cells
     • 5. Finally, the cytoplasm divides
     • You now have two new cells containg exactly the same DNA - they're genetically identical to each other and the parent cell
   • Meiosis
     • 1. The cell duplicates its DNA - one arm of each chromosome is an exact copy of the other
     • 2. The chromosome pairs line up in the centre of the cell
     • 3. The pairs are pulled apart so each new cell only has one copy of each chromosome
     • 4. The chromosomes line up again in the centre of the cell and the arms of the chromosomes are pulled apart
     • 5. You get four gametes with only a single set of chromosomes in it
     • After two gametes join at fertilisation, the zygote grows by repeatedly dividing by mitosis

4. Animal Development

   • Embryonic stem cells can be divided to produce ANY type of specialised cell
     • Used to replace faulty cells in sick people
   • Adult stem cells can be divided into SOME but NOT all types of cells
     • Used to cure sickle-cell anaemia (blood disease) by bone marrow transplants as it contains stem cells that can turn into new blood cells to replace the faulty ones
   • Genes can be switched on if they're needed during development
   • Cloning
     • 1. Remove a nucleus from an egg cell
     • 2. A nucleus from a body cell of the adult you're cloning is inserted into 'empty' egg cell
     • 3. Under correct conditions, inactive genes in the nucleus of the body cell can be reactivated (switched on) so that an embryo forms
     • 4. Embryonic stem cells can then be extracted from the embryo - these stem cells could then be controlled to form any type of specialised cell

5. Meristem Cells

   • The only cells that are mitotically active are found here
   • Found in areas of a plant that are growing - roots and shoots
   • Produce unspecialised cells that are able to divide and form any cell type
   • Unspecialised cells become specialised to form tissues like xylem and phloem (the water and food transport tissues)
   • These tissues can group together to form organs like leaves, roots, stems and flowers

6. Phototropism

   • A certain direction in which a plant responds to light
   • Positive Phototropism - plants grow towards sunlight for photosynthesis
   • Negative Phototropism - plants grow away from light so the roots travel down into the soil where they can absorb water

7. Auxins

   • Chemicals that control growth near the tips of shoots and roots
   • Produced in the tips and diffuse backwards to stimulate the cell elongation process
   • If the tip of a shoot is removed, no auxins are available and the shoot may stop growing
   • When a shoot tip is exposed to the light, more auxins accumulate on the side that's in the shade than the side that's in the light
   • This makes the cell grow (elongate) faster on the shaded side, so the shoot grows towards the light