3.4.2 DNA and protein synthesis

• Genome- the complete set of genes in a cell/ organism, including those in the mitochondria or chloroplasts
• 'Complete' proteome- the complete set of proteins produced by the genome. The cell's proteome is the proteins produced by a given cell under certain conditions
• Messenger RNA (mRNA):

  • Short polynucleotide (same length as the gene it came from)
  • Contains uracil, adenine, cytosine and guanine
  • Single stranded
  • Contains a ribose sugar
  • Has a codon
• Transfer RNA (tRNA):

  • Clover shaped due to intramolecular bonding (hydrogen bonding of base pairs)
  • Contains uracil, adenine, cytosine and guanine
  • Has an amino acid binding site
  • Has an anti-codon which is complementary to the mRNA codon
• The mRNA which is made in the nucleus, has been created from loose nucleotides which attach to the DNA molecule using the enzyme RNA polymerase. Every three nucleotides is called a codon and will bind to a tRNA molecule's anti-codon in the cytoplasm.
• The process of transcription:

  1. At the locus of the gene, hydrogen bonds are broken exposing the template for the mRNA.
  2. Individual, free-floating nucleotides complementary base pair to the template.
  3. RNA polymerase moves down the template, forming phosphodiester bonds between the adjacent nucleotides.
  4. RNA polymerase moves down the entire gene and reaches the Stop triple code, then releases itself and the new mRNA is also released.
  5. In a eukaryote, the mRNA exits the nucleus, via the nuclear pores, into the cytoplasm. In the prokaryotes, all of protein synthesis occurs in the cytoplasm.
• Activation of amino acids: tRNA molecules bind to specific amino acids to activate them, ready for protein synthesis. This forms amino acid complexes, which is a process which requires ATP (which is converted to ADP).
• Stages of translation:

  1. tRNA's anti-codon (with amino acid binding site) complementary base pairs to the mRNA/ ribosome in the first codon.
  2. A second tRNA molecule with amino acid bonds to the next position codon.
  3. A peptide bond then forms between amino acids, and the first tRNA molecule is released and leaves.
  4. A third tRNA molecule arrives and binds to the mRNA.
  5. Another peptide bond forms, continuing the polypeptide chain, up until the STOP codon.
• What happens next (to a protein):

  • Ribosome- produces a polypeptide chain from amino acids
  • Rough endoplasmic reticulum- transports protein and puts it into a vesicle
  • Golgi body- modifies and packages the protein into a vesicle
  • Vesicle- exocytosis, fuses with the outer membrane, releasing the protein. OR, protein stays inside the vesicle, as a lysosome
• Processes involved in protein synthesis:

  1. Transcription
  2. Activation
  3. Translation
• Post-transcription- only occurs in eukaryotes because prokaryotes do not have introns

  A) exons

  B) introns

  C) exons

  D) template

  E) transcription

  F) enzyme

  G) enzyme

  H) pre

  I) splicing

  J) mature