DNA, RNA, and Protein Synthesis

• universal - the same triplets code for the same amino acid in all organisms
• degenerate - there are more combinations of triplets (64) than amino acids (20) - so there are more than 1 triplet for some amino acids
• non-overlapping - the triplets are read discretely, in order with each base being part of only one triplet
• gene - a base sequence of DNA coding for: the amino acid sequence of a polypeptide or a functional RNA (including rRNA and tRNAs)
• locus - the position of an allele on a chromosome
• Transcription:

  • in the nucleus
  • DNA helicase at the start of the gene breaks the hydrogen bonds and DNA unwinds
  • A single strand is transcribed
  • Free floating complementary RNA nucleotides are attached to the DNA template (complementary base pairing)
  • RNA polymerase joins adjacent RNA nucleotides together creating phosphodiester bonds
  • Hydrogen bonds reform between the DNA bases and the strands coil back up
  • Pre-mRNA strand is spliced to remove non coding regions, i.e. introns and repeats
  • mRNA strand leaves via nuclear pore
• Mutations are the change in the DNA base sequence
• Mutations occur spontaneously - especially during transcription and DNA replication
• The rate of mutation increases on exposure to mutagenic agents. These include: ionizing radiation, phenolic compounds and viruses.
• Substitution:

  This is where one base is replaced by another. It doesn't always result in a change to the amino acid due to degeneracy.
• Deletion:

  This causes a 'frameshift downstream' of the mutation - i.e. after the mutation, all of the triplets will code for a different primary structure/ tertiary structure.
• Addition:

  This also causes a 'frameshift downstream'
• tRNA:

  • clover leaf shape
  • single stranded
  • the anticodon is specific to the amino acid
• Translation (Part 1):

  • the mRNA attaches to a ribosome in the cytoplasm; it attaches via 2 codons
  • a tRNA molecule attaches to the ribosome and to the first codon via codon/anticodon interactions (complementary base pairing)
  • a second tRNA molecule attaches to the second codon and a peptide bond forms between it and the previous amino acid. This is a condensation reaction and requires energy from the hydrolysis of ATP.
• Translation (Part 2):

  • the first tRNA then leaves and finds another amino acid to bind to. This binding also requires energy from the hydrolysis of ATP.
  • the ribosome moves down the mRNA by one codon and another tRNA delivers another amino acids which binds with the previous one.
  • this process repeats - ribosome moving down the mRNA until the primary structure of the polynucleotide has been completed.