Nucleic acids

• Purines

  Larger, double-ringed molecules (adenine, guanine)

  
• Pyrimidines

  Smaller, single-ringed molecules (thymine, cytosine, uracil)
• Formation of polynucleotides DNA and RNA


  1. Nucleotides are linked into a long chain

  2. Nucleotides are linked together by covalent bonds called phosphodiester bonds

  3. Takes place inside the nucleus during interphase

  4. Purines combine with pyrimidines

  
• In RNA, adenine pairs with uracil
• DNA molecule


  • Sides of the ladder are made up of alternating molecules of phosphate and deoxyribose
  • Nitrogenous bases that make up rungs of ladder have hydrogen bonds between them
  • They link bases and hold 2 strands together
  • DNA molecules are made of 2 polynucleotide strands lying anti-parallel to each other held by hydrogen bonds between bases
• Semi-conservative replication of DNA

  1. Double helix of DNA is unwound by enzyme helicase

  2. Enzyme primase synthesises a short piece of RNA called primer which marks the starting point for synthesis of new strand

  3. DNA polymerase uses the primer and synthesises new strand

  4. One of the new strands, the leading strand, is made continuously

  5. DNA polymerase progresses down the strand adding bases in a 5' to 3' direction

  6. When using the lagging strand as a template, DNA polymerase adds nucleotides in short stretches called okazaki fragments to overcome directionality problem

  7. DNA ligase seals up fragments of DNA in both to form a continuous double stranded helix

  
• Polypeptide


  Coded for by a gene, which is a sequence of nucleotides that forms part of a DNA molecule

  
• DNA controls protein synthesis by determining the order of amino acids when proteins are synthesised in cells

  
• Codon


  A sequence of 3 bases that codes for 1 amino acid
• Gene

  Part of a DNA molecule where the nucleotide sequence codes for just one polypeptide
• Gene mutation

  A change in the sequence of nucleotides that may result in an altered polypeptide
• Types of gene mutations

  • Substitution
  • Deletion
  • Insertion
  • Inversion
  • Frameshift
• Alleles

  Variants of genes (caused by mutations)
• Sickle cell anaemia


  • Caused by a change in the base sequence of amino acids in the β-polypeptide chain
  • Adenine replaces thymine in CTT triplet forming CAT
  • This type of mutation is called substitution
  • During synthesis of sickle cell haemoglobin, the amino acid valine (GTG), which is non-polar, is incorporated instead of glutamic acid (GAG)
  • Having this non-polar R group on the outside of the molecule makes the cell less soluble
  • Individuals with 2 copies of HbS allele inherit the disease (recessive)
• Transcription


  1. RNA polymerase binds to a region of gene called promoter

  2. The strand that's read is the sense-strand

  3. RNA polymerase reads sense strand in a 3' to 5' direction and generates mRNA from 5' to 3'

  4. When RNA polymerase has reached the terminator sequence at the end of the gene, transcription stops

  5. Last triplet transcribed to mRNA is a DNA triplet coding for STOP

  
• STOP codons

  • Codons that terminate translation
  • Do not specify any amino acid
  • Have no complementary tRNA/anti-codon
  • Causes the release of a completed polypeptide chain

  
• Translation


  1. mRNA leaves the nucleus and binds to the smaller ribosomal unit

  2. Every 3 bases (a codon) on mRNA codes for a specific anti-codon which is carried by a transfer RNA (tRNA) molecule

  3. Each different tRNA is covalently linked to a particular amino acid

  4. An initiator tRNA adheres to a START codon

  5. The tRNA that corresponds to the next codon after the START codon enters the ribosome carrying an amino acid with it which becomes covalently bound to methionine from the initiator tRNA

  6. The first tRNA detaches and leaves the ribosome which has shifted over making room for the next tRNA molecule

  7. New amino acid from new tRNA links the first two

  8. This process continues all the way down the mRNA strand until a STOP codon is reached