B3 - Molecules of Life section

B3 - Molecules of Life section

1. Cell Structure

   • Ribosomes
     • Some cell structures are too small to be seen with the light microscope. Ribosomes are like this.
       • They can only be seen using an electron microscope.
     • They are found in the cytoplasm and are the site of protein synthesis.
   • Mitochondria
     • Mitochondria are tiny structures found in cells (one is called a mitochondrion).
     • Respiration, the chemical reaction that releases energy from glucose, happens in mitochondria.
       • This provides energy for life process such as movement and growth.
     • 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.

2. DNA and genetic code

   • Chromonsomes
     • The cell's nucleus contains chromosomes.
     • These are long, coiled molecules of DNA, divided up into regions called genes.
     • The diagram shows the relationship between the cell, its nucleus, chromosomes in the nucleus, and genes.
   • DNA
     • The information contained in genes is in the form of coded instructions called the genetic code.
       • The genetic code controls cell activity, which means that it also controls some characteristics of the organism.
     • DNA controls the production of different proteins.
     • These are needed for the growth and repair of cells.

3. Structure of DNA

   • Discovery
     • Two scientists, James Watson and Francis Crick, worked out the structure of DNA.
     • By using data from other scientists they were able to build a model of DNA.
       • 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.
   • A Double Helix
     • Each strand of DNA is made of chemicals called bases.
       • 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.
       • There are chemical cross-links between the two strands, formed by pairs of bases.
   • Genes and Proteins
     • Each gene in a molecule of DNA contains:
       • A different sequence of bases
       • Codes for a particular protein
         • 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.

4. Making Proteins

   • Complementary base pairs
     • The four bases of DNA are A, T, C and G.
       • They always pair up in a particular way, called complementary base pairing: A – T and G – C
   • The DNA base code
     • Protein structure is determined by the DNA base code.
     • Proteins are made from lots of amino acids joined together.
       • 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.
     • The sequence of bases determines the sequence of amino acids in a protein molecule.
     • DNA controls the functions of a cell by controlling its production of proteins. Some of these proteins are enzymes.
   • Messenger RNA (mRNA)
     • Ribosomes are the site of protein synthesis.
     • They are found in the cytoplasm but DNA is found in the nucleus.
     • The genetic code needed to make a particular protein is carried from the DNA to the ribosomes by a molecule called mRNA.
     • Making:
       • mRNA from DNA is called transcription
       • Proteins from mRNA is called translation

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