Nucleic acids

• DNA as the genetic material of living organisms
• Ribonucleic acid (RNA) is another type of nucleic acid which is the main component of ribosomes, which play an important role in protein synthesis

  
• Certain viruses (such as SARS-CoV-2) contain RNA as their genetic material instead of DNA
• Viruses are not considered to be living organisms, since they are unable to replicate by themselves
• Viruses also lack a cellular structure, which is another reason they are not considered to be living
• Nucleotide

  A unit made up of a pentose sugar, a nitrogen-containing organic base, and a phosphate group

  
• Nitrogenous bases in DNA

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Thymine (T)

  
• Nitrogenous bases in RNA

  • Adenine (A)
  • Guanine (G)
  • Cytosine (C)
  • Uracil (U)

  
• Purine bases

  Adenine and guanine
• Pyrimidine bases

  Cytosine, thymine (in DNA) and uracil (in RNA)

  
• Forming the sugar-phosphate backbone

  1. Phosphate group of one nucleotide forms a covalent bond to the pentose sugar of the next one

  2. This carries on to form a large polymer

  
• Polynucleotide

  A polymer of nucleotides

  
• DNA is double-stranded, RNA is usually single-stranded
• RNA structure

  • RNA molecules are relatively short with lengths of between a hundred to a few thousand nucleotides
  • RNA usually forms a single-stranded polynucleotide with ribose as the pentose sugar in each nucleotide

  
• Formation of an RNA polymer

  1. Adjacent RNA nucleotides are linked together by condensation reactions, during which a molecule of water is released

  2. This forms a phosphodiester bond between the pentose sugar of one nucleotide and the phosphate group of the next nucleotide

  
• DNA structure

  • DNA is a double helix made of two antiparallel strands of nucleotides linked by hydrogen bonding between complementary base pairs
  • The nitrogenous bases of each nucleotide project out from the backbone towards the interior of the double-stranded DNA molecule

  
• Complementary base pairing

  Adenine (A) always pairs with thymine (T), and guanine (G) always pairs with cytosine (C)

  
• DNA is described as a double helix
• Coding strand

  One of the strands of a DNA molecule that will carry the base sequence that will be read by enzymes

  
• Genetic code

  The sequence of nitrogenous bases in the nucleotides of DNA molecules that carries the genetic information

  
• Genetic code

  DNA molecules carry the genetic code as a sequence of nitrogenous bases in the nucleotides

  
• Nitrogenous bases in DNA

  • Adenine
  • Guanine
  • Cytosine
  • Thymine

  
• Codon

  A sequence of three bases that codes for one amino acid
• There are 20 different amino acids that could be coded for</b>
• The sequence of amino acids will determine the shape and function of the protein that is synthesised from the code
• The genetic code is universal, meaning that almost every organism uses the same code
• The universal nature of the genetic code is why genetic engineering is possible
• The similarity in conserved sequences indicate that living organisms share a universal ancestry
• DNA nucleotide

  Contains the pentose sugar deoxyribose

  
• RNA nucleotide

  Contains the pentose sugar ribose and the nitrogenous base uracil instead of thymine

  
• RNA molecules are only made up of one polynucleotide strand (they are single-stranded)

  
• RNA polynucleotide chains are relatively short compared to DNA
• Differences between DNA and RNA

  • DNA contains thymine, RNA contains uracil
  • DNA contains deoxyribose, RNA contains ribose
  • DNA is double-stranded, RNA is single-stranded
  • DNA strands are longer than RNA strands

  
• Complementary base pairing

  Adenine (A) pairs with Thymine (T), Cytosine (C) pairs with Guanine (G)

  
• Complementary base pairing allows DNA to be copied very precisely during DNA replication
• DNA has an almost limitless capacity for storing genetic information in living organisms
• The DNA in the nucleus of a human cell contains about 3.2 gigabases (10^9 base pairs)

  
• Directionality of DNA/RNA

  One strand runs from 5' to 3', the other strand runs from 3' to 5' (antiparallel)
• Directionality of DNA/RNA

  • Crucial for ensuring the genetic code is copied, transcribed and translated correctly
• Purines (A, G) are larger than pyrimidines (T, C) due to their two carbon ringed structure