Nucleic Acids and Nitrogenous Bases

Nucleic Acids

• Nucleic acids are unbranched polymers of nucleotides.
• Nucleotides consist of a pentose sugar (ribose or 2-deoxyribose), a phosphate group, and a nitrogenous base.
• Two main types of nucleic acids are DNA and RNA.
  • DNA stores and transfers genetic information.
  • RNA is primarily involved in protein synthesis.

Nitrogenous Bases

• Five nitrogen-containing heterocyclic bases exist.

  • Three are pyrimidines (monocyclic): thymine (T), cytosine (C), and uracil (U).
    • Thymine: 5-methyl-2,4-dioxo derivative of pyrimidine.
    • Cytosine: 4-amino-2-oxo derivative of pyrimidine.
    • Uracil: 2,4-dioxo derivative of pyrimidine.
  • Two are purines (bicyclic): adenine (A) and guanine (G).
    • Adenine: 6-amino derivative of purine.
    • Guanine: 2-amino-6-oxo derivative of purine.

• Pyrimidines are monocyclic, six-membered rings.

• Purines are bicyclic, consisting of fused five- and six-membered rings.

Nucleotide Structure

• Phosphate group is attached to the pentose sugar.
• Pentose sugar can be ribose or 2-deoxyribose.
• The nitrogenous base is attached to the pentose sugar (N1 for pyrimidines and N9 for purines).

Nucleic Acid Structure

• The alternating sugar-phosphate chain forms the nucleic acid backbone.
  • In DNA, the backbone consists of alternating phosphate and deoxyribose.
  • In RNA, the backbone consists of alternating phosphate and ribose.
  • Each nonterminal phosphate carries a -1 charge.

Primary Structure

• The sequence of nucleotides in a nucleic acid.
• For DNA, the sequence of bases determines the information contained.
• 5' end normally has a free phosphate group; 3' end has a free hydroxyl group.

Base Pairing

• Complementary base pairing is essential for DNA structure and replication.
• A pairs with T; G pairs with C; in the DNA double helix.
• Purines pair with pyrimidines to maintain the consistent width of the helix.

Replication

• DNA replication is the process of creating exact duplicates of DNA molecules.
• The DNA double helix unwinds; hydrogen bonds between bases break.
• Each strand acts as a template for a new complementary strand.
• DNA polymerase verifies correct base pairing and catalyzes new phosphodiester linkages in the 5'→3' direction.
• Leading strand grows continuously; lagging strand grows in fragments (Okazaki fragments). DNA ligase connects these fragments.

Other Key Concepts

• Chromosomes consist of DNA and proteins (histones).
• Four major types of RNA: messenger RNA (mRNA), ribosomal RNA (rRNA), transfer RNA (tRNA), and heterogeneous nuclear RNA (hnRNA).
• Transcription is the process of synthesizing RNA from a DNA template.
  • Involves RNA polymerase.
  • RNA nucleotides are added to the growing RNA strand, complementary to the template DNA strand.
• Translation is the process of protein synthesis directed by mRNA.
  • Ribosomes read mRNA codons.
  • tRNA molecules bring amino acids corresponding to the codons.
  • Polypeptide chain forms.
• Splicing involves removing introns and joining exons from hnRNA to produce mature mRNA.