Nucleic acids

• Nucleic acids


  Large biomolecules or biological polymers that play essential roles in all cells or life and viruses
• Nucleic acids


  • Consist of nitrogenous bases, 5-carbon (pentose) sugar, and phosphate groups
  • Major function involves the storage and expression of genomic information
• Types of nucleic acids


  • DNA
  • RNA
• DNA


  Encodes the information cells need to make proteins
• RNA


  Acts as a messenger between DNA and ribosomes to make amino acids and proteins
• Nucleic acids carry the genetic information of all organisms and direct protein synthesis
• All living organisms and viruses use nucleic acids
• Major classes of nucleic acids


  • 2-deoxyribonucleic acid (DNA)
  • Ribonucleic acid (RNA)
• DNA and RNA


  • Differ according to their structure, sugar (2'-deoxyribonucleic in DNA and ribose in RNA), and bases (adenine, thymine, guanine, cytosine in DNA and uracil instead of thymine in RNA)
• Nucleic acids code genetic information and perform other functions in cells
• Types of DNA


  • A-DNA
  • B-DNA
  • Z-DNA
• Types of RNA


  • mRNA
  • tRNA
  • rRNA
• Biochemists also synthesize artificial nucleic acid analogs that mainly differ from DNA or RNA by the composition of their backbones
• Nucleic acid monomers


  Nucleotides consisting of nitrogenous base, pentose sugar, and phosphate group
• Nucleic acid structure


  • Helical shape (with some exceptions in RNA)
  • DNA forms a double helix, while RNA mostly forms a single helix
  • Phosphate of one nucleotide connects to the OH group on the 3' carbon of the sugar of the next nucleotide, forming a backbone of alternative phosphate and sugar subunits
  • Purines and pyrimidines branch off of the backbone
  • Backbone has a "direction" with a free sugar (3' end) and a free phosphate group (5' end)
  • Two strands of a DNA helix are antiparallel
• By convention, chemists read the code of a nucleic acid starting with the 5' end
• DNA bases


  • Adenine
  • Thymine
  • Guanine
  • Cytosine
• RNA bases


  • Adenine
  • Uracil
  • Guanine
  • Cytosine
• Functions of nucleic acids


  • Responsible for heredity, transmission of genetic material from one generation to the next
  • Control protein synthesis in a cell, with DNA coding the message and RNA performing the actual synthesis
• Similarities between DNA and RNA


  • Both store genetic information
  • Both are large biological polymers
  • Both consist of sugar, nitrogenous bases, and a phosphate backbone
  • Guanine and cytosine pair with each other, connected by hydrogen bonding
• Differences between DNA and RNA

  • DNA uses deoxyribose sugar, RNA uses ribose sugar
  • DNA is usually double-stranded, RNA is single-stranded
  • DNA uses adenine, thymine, guanine, and cytosine bases, RNA uses adenine, uracil, guanine, and cytosine
  • DNA stores and transfers genetic information, RNA acts as a messenger between DNA and ribosomes
  • DNA is more stable and less vulnerable to mutation and attack than RNA
• DNA is susceptible to UV damage, while RNA is relatively resistant to UV damage
• DNA is more stable than RNA and resists alkaline conditions, while RNA is more reactive and not stable in alkaline conditions
• DNA helix families


  • A-DNA, B-DNA, and Z-DNA
  • A-DNA and B-DNA are right-handed helices, Z-DNA is left-handed
• DNA helix families

  • A-DNA
  • B-DNA
  • Z-DNA
• 1. DNA and B-DNA


  Right-handed helices
• 1. DNA


  Left-handed helix
• 1. DNA, B-DNA and Z-DNA have different helical pitches and diameters
• Sequences favoured by different DNA helix families


  • A-DNA: certain stretches of purines (e.g. GAGGGA)
  • B-DNA: mixed sequences
  • Z-DNA: alternating pyrimidine-purine steps (e.g. CGCGCG)
• The phosphate backbone consists of deoxyribose sugar molecules linked together by phosphate groups
• The phosphate groups have a negative charge, giving the outside of the DNA an overall negative charge
• DNA strands are antiparallel, going in opposite directions
• 5' to 3' direction


  The convention for writing DNA sequences, as this is the direction enzymes synthesize DNA
• Base pairing


  A pairs with T, G pairs with C through hydrogen bonding
• 1. C base pairs have 3 hydrogen bonds, A-T base pairs have 2 hydrogen bonds</b>
• Base stacking

  • The most important contribution to DNA helix stability
  • Sequence dependent stacking energies
• Propeller twist


  Angle between planes of base pairs, related to helix rigidity
• Twist angle

  Angle between adjacent base pairs, determines helix pitch
• Twist angles can vary from 27.9° to 40°, resulting in 9-12.9 base pairs per turn
• Major groove

  Larger groove in B-DNA, where proteins often bind