3 - Nucleotides and nucleic acids

• Nucleotides are made of a deoxyribose sugar, a phosphate group, and a nitrogenous base.
• The two types of bases found in DNA are purines (adenine and guanine) and pyrimidines (cytosine and thymine).
• DNA bases are joined by hydrogen bonds. A and T form 2 bonds while C and G form 3.
• In RNA, uracil replaces thymine as the pyrimidine base.
• The phosphate group is joined to the nucleotide by a condensation reaction. It attaches to carbon 5 of the pentose sugar and forms a phosphoester bond.
• Phosphodiester bonds form between nucleotides in a condensation reaction.
• DNA strands are anti-parallel (meaning they face opposite directions)
• Each gene codes for a specific polypeptide.
• The genetic code is universal - the same in all organisms.
• The triplet code in DNA is degenerate, meaning each amino acid is coded for by more than one triplet.
• The first stage of protein synthesis is transcription, where the triplet code of an amino acid is converted into a molecule of mRNA.
• mRNA is different to DNA in that:

  • It is single stranded
  • It contains ribose, not deoxyribose
  • It contains uracil not thymine
• The DNA base triplets are converted into mRNA codons.
• The genetic code is non-overlapping.
• Once a molecule of mRNA has been transcribed, it leaves the nucleus via a nuclear pore into the cytoplasm.
• The mRNA combines with a ribosome to build a polypeptide chain through translation.