DNA, genes and protein synthesis

• Prokaryotic cell DNA


  • Short
  • Circular
  • Not associated with proteins
• Eukaryotic DNA


  • Very long
  • Linear
  • Associated with proteins (histones)
  • Forms chromosomes
• Mitochondrial and chloroplast DNA in eukaryotic cells

  • Short
  • Circular
  • Not associated with proteins
• Gene


  A base sequence of DNA that codes for the amino acid sequence of a polypeptide or a functional RNA
• Locus


  The fixed position of a gene on a DNA molecule
• Triplet


  A sequence of three DNA bases that codes for a specific amino acid
• Genetic code characteristics 


  Universal, non-overlapping, and degenerate
• The genetic code has the following features: 

  • Universal - Each DNA triplet codes for the same amino acid in all organisms (with a few minor exceptions). 
  • Non-overlapping - Each base in the DNA sequence is only read once (e.g. CGTATC is read as CGT and ATC).
  • Degenerate Most amino acids are coded for by more than one triplet (e.g. ACA and ACG both code for cysteine).
• Exon - The sections of DNA between introns which contain the coding sequences for proteins.
• mRNA - Messenger RNA carries information from the nucleus to ribosomes where it directs the assembly of amino acids into proteins.
• characteristic's of mRNA- longer than TRNA, linear, no hydrogen bonding, no amino acid binding site
• tRNA - Transfer RNA transfers an amino acid to the growing polypeptide chain at the ribosome.
• Characteristics of tRNA- short, folded structure, anticodon, amino acid binding site
• Protein synthesis is the process of making proteins from amino acids.
• steps involved in protein synthesis

  • transcription
  • translation
• Transcription

  1. Hydrogen bonds between DNA bases break
  2. one DNA strand acts as a template
  3. Free RNA nucleotides align by complementary base pairing; For ‘align by complementary base pairing
  4. In RNA Uracil base pairs with adenine on DNA OR In RNA Uracil is used in place of thymine
  5. RNA polymerase joins adjacent RNA nucleotides by phosphodiester bonds between adjacent nucleotides
  6. pre mRNA is spliced to remove introns and forms mRNA
  7. mRNA leaves via nuclear pores
• Translation

  1. mRNA attaches to ribosomes
  2. tRNA anticodons bind to complementary mRNA codons
  3. tRNA brings a specific amino acid
  4. Amino acids join by peptide bonds
  5. Amino acids join together with the use of ATP
  6. tRNA released after amino acid joined to polypeptide
  7. The ribosome moves along the mRNA to form the polypeptide
• the role of a ribosome in the production of a polypeptide

  • mRNA binds to ribosome
  • Allows tRNA with anticodons to bind
  • formation of peptide bond between amino acids
• How is DNA adapted to perform its function

  • stable due to strong hydrogen bonding- therefore rarely mutates
  • hydrogen bonds can separate between complementary bases to allow DNA replication
  • extremely large cannot leave cell
  • phosphodiester backbone to protect from corruption