Functions of Nucleic Acids

• Two groups of nitrogenous bases, pyrimidines (single ring) and purines (double ring)
• Pyrimidines: Thymine, cytosine, and uracil

  Purines: Adenine, guanine
• Pentose sugar in DNA is Deoxyribose
• DNA strands are arranged antiparallel in a double helix structure
• Each strand goes from the 5' end to the 3' end

  
• DNA Replication:

  1. DNA unwinds
  2. Hydrogen bonds holding complementary base pairs break, catalysed by DNA Helicase, and the two strands separate
  3. Each DNA strand acts as a template strand
  4. Free nucleotides align opposite their complementary bases
  5. DNA Polymerase catalyses the condensation reaction between 2 DNA nucleotides. This occurs from the 5' to the 3' end of the chain
  6. Each new DNA molecule is made from the original template strand and one newly synthesised strand. This is semi-conservative replication
• Conservative replication - Original double helix remains intact, completely conserved, with a fully new DNA molecule being synthesised.
• Semi-Conservative replication - Each strand in the parental double helix acts as a template to synthesise a new polynucleotide strand. Each new DNA molecuce contains one template strand and one newly replicated strand.
• Semi-conservative replication was proven by the Meselson-Stahl experiment.

  1. E.coli was grown in a medium containing amino acids with the heavy isotope 15N. All the bacterial DNA contained this isotope
  2. The bacteria were washed and transferred to a medium containing amino acids with a lighter isotope 14N. The bacteria was allowed to divide once
  3. They were then allowed to divide once again.

  
• Transcription - occurs in the nucleus. The genetic code for a specific protein is copied. a complementary mRNA strand is formed, which then leaves by a nuclear pore.
• Translation - occurs at the ribosome. Amino acids that correspond to the codons on mRNA are brought to the ribosome by tRNA. Amino acids are joined together at the ribosome to form the polypeptide chain
• Transcription:

  1. RNA Polymerase binds to DNA at the start of the gene
  2. RNA polymerase then acts as DNA helicase, breaks the hydrogen bonds between base pairs and the strands unwind
  3. One strand acts as the template strand
  4. free RNA nucleotides align against exposed complementary bases on the template strand
  5. RNA polymerase moves along the template strand, catalysing the addition of RNA nucleotides to each other until a stop codon is reached.
  6. This forms a strand of mRNA
  7. The DNA strands rewind
  8. mRNA leaves via a nuclear pore
• Translation:

  1. Ribosome attaches to a start codon on the mRNA
  2. First tRNA binds to the first attachment site. Anti-codon on tRNA joins to the complementary codon on mRNA by hydrogen bonds
  3. A second tRNA forms a codon-anticodon complex on the second attachment site
  4. Ribosomal enzyme catalyses formation of a peptide bond between the two amino acids
  5. First tRNA leaves site 1
  6. Ribosome moves down one codon, second tRNA now on site 1
  7. New tRNA binds to site 2
  8. sequence repeats until a stop codon is reached
  9. Ribosome-mRNA-polypeptide complex separates
• After translation, the polypeptide can be transported to the Golgi body where it is further modified:

  • addition of carbohydrate to make glycoproteins
  • addition of lipid to make lipoproteins
  • addition of phosphate to make phospho-proteins
• Bonds in tertiary and quaternary structure:

  • Di-sulphide bridge
  • Ionic bonds
  • Hydrogen bonds
  • Hydrophilic interactions
• Genetic code is universal - The same triplet codes for the same amino acid in all living organisms

  Genetic code is non overlapping - each base in the sequence occurs in only triplet
• Introns - non-coding nucleotide sequences in DNA that are removed after transcription by endonuclease

  Exons - coding sequences which are left behind and are spliced together by ligase to form mature mRNA
• pre-mRNA is made of exons and introns, after the introns are removed and exons are spliced together, mature mRNA is formed, ready for translation