Protein Synthesis

• Proteins:

  • Made by joining amino acids together in a specific order
  • Job of small cell organelles called ribosomes
• Gene:

  • A section of DNA that codes for the production of a protein
• How does a ribosome ‘ know’ which amino acids to join together?

  • The sequence of bases on the DNA acts as a code
  • This ‘genetic code’ is based on non-overlapping groups of 3 bases called triplets
  • Each triplet translates to a different amino acid
• A-T

  C-G
• T- thymine
• A - Adenine
• G - guanine
• C - cytosine
• If the DNA is too big to leave the nucleus, how is the triplet code ’communicated’?


  RNA
• Triplet of bases on mRNA: CODON
• Triplet of bases on tRNA: ANTICODON
• tRNA pair up with the mRNA codons
• Ribosomes attaches to the messenger RNA
• Ribosomes has enzymes in it which allows the peptide bond to form
• Changes in DNA bases happen all the time - mutating

  • Can have negative consequences ie. uncontrolled division can become cancerous
  • Xrays, ultra-violet radiation etc increase mutation rate
  • There are some base changes which don’t change the sequence therefore it won’t cause a problem

  We can change genes by changing the DNA
• Proteins can be broken down into their constituent amino acids using proteases.
• When all the amino acids have been joined together, the polypeptide chain folds into its correct shape (protein)
• Gene therapy is where we replace faulty genes with good ones
• The folding process is called protein synthesis
• Translation occurs at ribosomes.
• The process of protein synthesis is called translation because the genetic code is translated from nucleotides to amino acids.
• Different amino acid sequences produce different proteins
• Transcription - the DNA bases are used to make a strand of RNA (ribonucleic acid)
• Translation -

  • mRNA strands travel out of the nucleus
  • mRNA attach to ribosomes in cytoplasm
  • A ribosome moves along an mRNA strand three bases at a time - triplet base called a codon
  • At each codon a molecule of transfer RNA (tRNA) with complementary bases lines up
  • tRNA molecule carries a specific amino acid
  • As the ribosome moves along it joins the amino acids from the tRNA molecules together, forming a polypeptide
• Mutation - change in the DNA of a cell

  • Can change the sequence of the amino acid: called frame shift mutation
  • Some mutations had no effect
• Mutations in non-coding DNA:

  • The first stage of protein synthesis is attachment of RNA polymerase to a non-coding region of DNA in front of the gene
  • If this non-coding region mutates RNA polymerase cannot attach and less protein is produced
  • In some instances the mutation in the non-coding region makes it easier for RNA polymerase to attach and more protein is produced
• Mutation rates can be increased by:

  • Ionising radiation (UV, Xrays)/radioactive sources
  • Certain chemicals (cigarette smoke, mustard gas, nitrous oxide)
  • Rapidly dividing cells will mutate more rapidly

  Effects:

  • Mostly harmful
  • In reproductive cells, the mutation can pass to the next generation (genetic disease)
  • In body cells mutation is lost when an organism dies