1.4

• Nucleic acids

  Polymers of nucleotides
• Nucleotides

  Consist of pentose (5 carbon sugar), a nitrogen-containing organic base, and a phosphate group
• DNA nucleotide

  Components are deoxyribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or thymine
• RNA nucleotide

  Components are ribose, a phosphate group and one of the organic bases adenine, cytosine, guanine or uracil
• What are Pyrimidines?

  Thymine, uracil and cytosine, have a single ring structure
• What are Phosphodiester bonds?

  Bonds that join nucleotides together, formed in condensation reactions
• What is a DNA molecule?

  Double helix composed of two polynucleotides joined together by hydrogen bonds between complementary bases
• What is RNA?

  Single-stranded, comes in multiple different forms such as mRNA, tRNA and rRNA, involved in protein synthesis
• Semi-conservative replication


  Ensures genetic continuity between generations of cells, meaning genetic information is passed on from one generation to the next
• Steps of semi-conservative DNA replication

  1. DNA double helix unwinds as hydrogen bonds are broken between complementary bases

  2. One of the strands is used as the template, free nucleotides line up and complementary base pairing occurs between the template strand and free nucleotides

  3. Adjacent nucleotides are joined by phosphodiester bonds formed in condensation reactions, catalysed by DNA polymerase

  4. The new DNA molecules automatically fold into double helices as hydrogen bonds are formed within the molecules
• DNA replication is said to be semi-conservative because the new DNA molecules contain one original strand of DNA and one newly-synthesised strand of DNA
• Genetic code

  Consists of triplets of bases called codons, each codon codes for an amino acid
• Gene

  A sequence of bases on a DNA molecule coding for a sequence of amino acids in a polypeptide chain
• Introns

  Non-coding regions of DNA
• Exons

  Coding regions of DNA
• Features of the genetic code

  • Non-overlapping - each triplet is only read once and triplets don't share any bases
  • Degenerate - more than one triplet codes for the same amino acid, reducing the effect of mutations
  • Contains start and stop codons which either start or stop protein synthesis
  • Universal - the same in all organisms and species
• Transcription

  The process where a molecule of mRNA is made in the nucleus from a DNA template
• Steps of transcription


  1. The hydrogen bonds between the complementary bases of the DNA double helix break and DNA uncoils, catalysed by DNA helicase

  2. One of the DNA strands is used as a template to make the mRNA molecule, this is called the antisense strand

  3. Free nucleotides line up on the template strand by complementary base pairing and adjacent nucleotides are joined together by phosphodiester bonds, catalysed by RNA polymerase

  4. mRNA then moves out of the nucleus through a nuclear pore and attaches to a ribosome in the cytoplasm
• Translation

  The process where amino acids join together to form a polypeptide chain
• Translation

  1. mRNA attaches to a ribosome on the rough endoplasmic reticulum

  2. A tRNA molecule with a specific amino acid attached binds to the mRNA via its anticodon

  3. Hydrogen bonds form between the anticodon of the tRNA and the codon of the mRNA

  4. A second tRNA molecule binds to the next codon of the mRNA and the two amino acids form a peptide bond

  5. A third tRNA molecule joins and the first one leaves the ribosome

  6. This process is repeated until a stop codon is reached on mRNA, forming a polypeptide chain
• Adenine and guanine both have two nitrogen-containing rings and are classified as purine bases
• Thymine, uracil and cytosine all have a single ring structure and are classified as pyrimidines. Pyrimidines are smaller than purines as they only contain one nitroge-containing ring