Genes and Protein Synthesis

• DNA contains genes which are instructions for making proteins.
• A gene is a sequence of DNA nucleotides that codes for a polypeptide - the sequence of amino acids in a polypeptide forms the primary structure of a protein.
• Different proteins have a different number and order of amino acids. It's the order of nucleotide bases in a gene that determines the order of amino acids in a particular protein.
• EAch amino acids is coded for by a sequence of three bases (called a triplet) in a gene.
• Different sequences of bases code for different amino acids. So the sequence of bases in a section of DNA is a template that's used to make proteins during protein synthesis.
• DNA molecules are found in the nucleus of the cell, but the organelles that make proteins (ribosomes) are found in the cytoplasn
• DNA is too large to more out of the nucleus, so a section is copied into mRNA. This process is called transcription.
• The mRNA leaves the nucleus and joins with a ribosome in the cytoplasm, where it can be used to synthesise a protein. This process is called translation.
• Messenger RNA

  • Made in the nucleus.
  • Three adjacent bases are called a codon.
  • It carries the genetic code from the DNA in the nucleus to the cytoplasm, where it's used to make a protein during translation.
• Transfer RNA

  • Found in the cytoplasm.
  • It has an amino acid binding site at one end and a sequence of three bases at the other end called an anticodon.
  • It carries the amino acids that are used to make proteins to the ribosomes during translation.
• Ribosomal RNA

  • Forms the two subunits in a ribosome (along with proteins)
  • The ribosome moves along the mRNA strand during protein synthesis. The rRNA in the ribosome helps to catalyse the formation of peptide bonds between the amino acids.
• The genetic code is the sequence of base triplets in DNA or mRNA, which codes for specific amino acids.
• In the genetic code, each base triplet is read in sequence, seperate from the triplet before it and after it. Base triplets don't share their bases - the code is non-overlapping.
• The genetic code is also degenerate - there are more possible combinations of triplets than there are amino acids. This means that some amino acids are coded for by more than one base triplet.
• Some triplets are used to tell the cell when to start and stop production of the protein - these are called start and stop signals. They're found at the beginning and end of the gene.
• The genetic code is universal - the same specific base triplets code for the same amino acids in all living things.