genes and protein synthesis

• DNA codes 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
• the order of nucleotide bases in a gene determines the order of amino acids in a particular protein
• each amino acid is coded for by a sequence of 3 bases (triplet) in a gene
• the sequence of bases is the template used to make proteins during protein synthesis
• 1. DNA molecules found in nucleus but ribosomes are in the cytoplasm
  2. DNA is too large to move out the nucleus so a section is copied into mRNA this is called transcription
  3. the mRNA leaves the nucleus and joins with a ribosome in the cytoplasm where it can be used to synthesise a protein - translation
• 3 main types of RNA:

  Messenger RNA

  Transfer RNA

  Ribosomal RNA
• Messenger RNA -

  • single stranded
  • used to transfer DNA code from nucleus to cytoplasm
  • complementary to the DNA code
  • once in cytoplasm it associates with ribosomes
  • determines the sequence of amino acids during protein synthesis
  • easily broken down only exists when needed
  • sequence of 3 nucleotides in mRNA is called a codon
• Transfer RNA -

  • small molecule and single stranded, clover leaf shape with one side slightly longer
  • longer section attached to an amino acid
  • each tRNA carries a different amino acid
  • 3 bases at opposite end of the tRNA are called anticodon
  • each amino acid has different anticodon
  • anticodon pairs with complementary codon on mRNA
  • carries amino acids used to make protein to ribosomes during translation
• tRNA structure -

  A) 3 bases forming anticodon

  B) unpaired bases

  C) paired bases

  D) point of attachment of amino acid

  
• Ribosomal RNA -

  • forms 2 subunits in the ribosome along with proteins
  • ribosome moves along the mRNA strand during protein synthesis
  • the rRNA helps to catalyse the formation of peptide bonds between amino acids
• the genetic code is non-overlapping, degenerate, and universal
• the genetic code is the sequence of base triplets (codons) in DNA, mRNA, which codes for specific amino acids
• non overlapping - each base is read only once to create a codon
• universal - the same specific base triplets code for the same amino acids in all living things
• stop/start signals - found at the end and start of a gene and are used to tell the cell when to stop of start production of a protein
• degenerate - more possible combinations of triplets then there are amino acids so some amino acids are coded for by more than one base triplet