B3 - Molecules of Life section

Cell Structure
1. Ribosomes
  1. Some cell structures are too small to be seen with the light microscope. Ribosomes are like this.
    1. They can only be seen using an electron microscope.
  2. They are found in the cytoplasm and are the site of protein synthesis.
2. Mitochondria
  1. Mitochondria are tiny structures found in cells (one is called a mitochondrion).
  2. Respiration, the chemical reaction that releases energy from glucose, happens in mitochondria.
    1. This provides energy for life process such as movement and growth.
  3. Tissues and organs that need a lot of energy have large numbers of mitochondria in their cells. For example, liver cells and muscle cells contain a lot of mitochondria.
DNA and genetic code
1. Chromonsomes
  1. The cell's nucleus contains chromosomes.
  2. These are long, coiled molecules of DNA, divided up into regions called genes.
  3. The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.
2. DNA
  1. The information contained in genes is in the form of coded instructions called the genetic code.
    1. The genetic code controls cell activity, which means that it also controls some characteristics of the organism.
  2. DNA controls the production of different proteins.
  3. These are needed for the growth and repair of cells.
Structure of DNA
1. Discovery
  1. Two scientists, James Watson and Francis Crick, worked out the structure of DNA.
  2. By using data from other scientists they were able to build a model of DNA.
    1. The data they used showed that bases occurred in pairs. Further x-ray data showed that there were two chains wound into a double helix.
2. A Double Helix
  1. Each strand of DNA is made of chemicals called bases.
    1. There are four different types of bases, shown as A, T, C and G in the diagram. In DNA, two strands coil together to form a double helix.
    2. There are chemical cross-links between the two strands, formed by pairs of bases.
3. Genes and Proteins
  1. Each gene in a molecule of DNA contains:
    1. A different sequence of bases
    2. Codes for a particular protein
      1. Proteins are made in the cytoplasm of a cell, not in the nucleus.
        Genes cannot leave the nucleus, so a copy of the gene is needed.
        This copy is able to leave the nucleus to go into the cytoplasm so that proteins can be made by the cell.
Making Proteins
1. Complementary base pairs
  1. The four bases of DNA are A, T, C and G.
    1. They always pair up in a particular way, called complementary base pairing: A – T and G – C
2. The DNA base code
  1. Protein structure is determined by the DNA base code.
  2. Proteins are made from lots of amino acids joined together.
    1. Each amino acid is coded by the sequence (order) of three bases. For example, GGT codes are found in glycine but TCA codes are found in serine, a different amino acid.
  3. The sequence of bases determines the sequence of amino acids in a protein molecule.
  4. DNA controls the functions of a cell by controlling its production of proteins. Some of these proteins are enzymes.
3. Messenger RNA (mRNA)
  1. Ribosomes are the site of protein synthesis.
  2. They are found in the cytoplasm but DNA is found in the nucleus.
  3. The genetic code needed to make a particular protein is carried from the DNA to the ribosomes by a molecule called mRNA.
  4. Making:
    1. mRNA from DNA is called transcription
    2. Proteins from mRNA is called translation

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B3 - Molecules of Life section

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