B5-GROWTH + DEVELOPMENT

B5-GROWTH + DEVELOPMENT

1. DNA + PROTEIN SYNTHEIS

   • DNA- is a double helix of paired basis. Its made of two strands made from nucleotides, each nucleotide contains a base. DNA has 4 bases A, T, C, G. The two strands are held together by the base pairings A+T, C+G
   • DNA contains genes, each gene contains the instruction for one particular protein. Cells make proteins by doing amino acids together in a particular order. The order of the bases tell the cell the order of the amino acids. Each set of 3 bases (triplet) codes for an amino acid
   • Protein synthesis- proteins are made by ribosomes in the cytoplasm. But DNA which is found in the nucleus is too big to leave, so to get the genetic info to the ribosomes the DNA molecule unzips and an MRNA molecule is made using the strand as a template, copying its genetic info. The MRNA leaves the nucleus + joins a ribosome which attaches amino acids in an ordered chain according to the order of bases in MRNA

2. CELL DIVISION-MITOSIS

   • mitosis is part of the cell cycle as cells grow + repeatedly divide + when cells need to be repaired/replaced
   • 3) mitosis produces 2 identical daughter cells
   • 1) the molecule of DNA splits + free floating bases from other nucleotides pair up + form cross links so strands are joined together= 2 identical molecules
   • 2) The DNA forms X shaped chromosomes in an exact duplicate, the chromosomes line up in the centre of the cell + cell fibres pull them apart so the two arms go to opposite ends of the cell. The cell membrane forms + he cytoplasm divides

3. CELL DIVISION-MEIOSIS

   • produces gametes- only 23 chromosomes
   • 1) meiosis involves two divisions. First the cell duplicates DNA into two armed chromosomes (X)
   • 2) first division- pairs line up in centre of cell, arms pulled apart so new cells only have 1 copy of each chromosome
   • 3) Second division- the chromosomes line up + arms pulled apart--> 4 gametes with single set of chromosomes created

4. ANIMAL DEVELOPMENT

   • embryonic stem cells are undifferentiated, so have all genes switched on, up to the 8 cell stage
   • stem cells becoming specialised is called differentiation. After the 8 cell stage this occurs + tissues and organs begin to form. Adults only have stem cells in certain places like the bone marrow and the stem cells can only differentiate into a few cells not all as not all the genes are switched on.
   • adult stem cells are already used to cure disease. e.g. people with blood diseases (sickle cell anaemia) can be treated by bone marrow transplants as bone marrow contains new blood cells to replace old ones
   • embryonic stem cells + their use for curing diseases is still in research stage. Although they have the potential to help cure any disease- create nerve cells, heart muscle cells etc.However, much objection to research is ethically based as embryos used to provide stem cells are destroyed. so research is regulated by the Gov.
   • cloning- can also be used to make stem cells. The DNA of an egg cell is replaced with the DNA of an adult body cell and under the right conditions inactive genes can be switched on.

5. PLANT DEVELOPMENT

   • in plants the only cells that are mitotically active (divide by mitosis) are meristem cells which are plant tissues found in areas of the plant which are growing e.g. roots+shoots
   • meristems produce unspecialised cells that are able to divide+ form any cell type in the plant for the entirety of the plants life span. The unspecialised cells can become tissues like xylem or phloem or organs like buds and flowers.
   • clones of plants can be produced from taking cuttings from an area of the plant that is growing so contains meristem cells. So an exact new replica can grow. People clone plants tor create desirable features.
   • rooting powder (or agar jelly which contains rooting powder0 helps cuttings to grow into complete plants because it contains the plant hormone auxin that helps the plant to grow faster.

6. PHOTOTROPHISM + AUXINS

   • plant shoots are positively phototrophic so they grow towards the light. Plants need sunlight for photosynthesis to produce glucose and to grow.
   • plant roots are negatively phototrophic + grow way from the light. Plants need nutrients +water from the soil to grow so they need to grow into the soil
   • auxins are chemicals that grow at the tips of plants in the shoots and roots. They diffuse backwards to stimulate cell elongation behind the tips of the plant.When a shoot tip is exposed to light more auxins go to the shady side, so the shady sides elongates at a faster rate and the plant grows towards the light.