Genetics

• DNA


  Deoxyribonucleic acid has a sugar-phosphate backbone, with information contained in the sequence of bases
• DNA double helix


  • Two strands running in opposite direction
  • Hydrogen bonds between bases link the strands
  • Adenine pairs with Thymine
  • Guanine pairs with Cytosine
• Purines


  • Adenine
  • Guanine
• Pyrimidines


  • Thymine
  • Cytosine
• RNA


  • Ribonucleic acid is usually a single stranded molecule
  • The sugar in the backbone is Ribose
  • Uracil is used instead of Thymine
  • Has a number of different functions in the cell
• The human genome has approximately 3 billion bases, although only a small proportion of them (1-2%) encode genes
• The DNA helix is wound around proteins called Histones, and interacts with other proteins to make a structure within the nucleus
• Genome structure


  • DNA wraps around a histone octamer
  • DNA is then coiled further to make the chromosome structure
  • Condensation of DNA controls gene function
  • DNA that is tightly coiled is less transcriptionally active
• Each cell has 22 pairs of chromosomes and the sex chromosomes
• DNA replication


  1. Leading strand has bases added in 5' to 3' direction

  2. Lagging strand has Okazaki fragments synthesised as it unwinds

  3. Okazaki fragments are then joined by DNA ligase
• Mitosis


  1. Interphase

  2. Prophase

  3. Metaphase

  4. Anaphase

  5. Telophase
• Meiosis


  1. Interphase

  2. Meiosis I (Prophase I, Metaphase I, Anaphase I, Telophase I)

  3. Meiosis II (Prophase II, Metaphase II, Anaphase II, Telophase II)
• Chromosome structure


  • Short arm (p arm)
  • Long arm (q arm)
  • Centromere
  • Telomere
  • Sister chromatids
• Chromosome types


  • Submetacentric
  • Metacentric
  • Acrocentric
• Chromosome banding


  • Dark bands show gene poor regions
  • Light bands show gene rich areas
• The normal chromosome complement is 22 pairs of chromosomes and 2 sex chromosomes, either 2 X chromosomes, or an X and a Y
• Balanced chromosome complement


  Normal amount of each chromosome
• Unbalanced chromosome complement


  Extra or missing chromosomal material
• Unbalanced chromosomes


  • Down syndrome (47,XY,+21)
• Robertsonian translocation


  • Two acrocentric chromosomes joined end to end
  • Short "p" arms are lost
  • Increased risk of child inheriting unbalanced chromosomes
• Reciprocal translocation


  • Swap of genetic material between chromosome arms
  • High risk of children with unbalanced chromosomes
• Chromosome deletion


  • Deletion or insertion of genetic material
  • Changes smaller than 5 million base pairs unlikely to be visible by microscopy
• Fluorescence in-situ hybridisation (FISH)

  1. Probe DNA

  2. Denaturation

  3. Hybridisation
• Chromosome microarray


  • Allows analysis of chromosomes at much higher resolution than karyotyping
  • Can identify tiny deletions in the genome
• 1. inactivation


  1. Regulation (one X chromosome inactivated at random)

  2. Spreading (Xist gene expression and chromosome condensation)

  3. Maintenance (inactivation pattern remains constant)
• DNA transcription


  1. Transcription factors bind to promoter

  2. DNA polymerase targets DNA

  3. Pre-mRNA synthesised using DNA as template
• Central dogma


  DNA -> Transcription -> Pre-mRNA -> Splicing -> mRNA -> Translation -> Protein
• Splicing occurs to remove introns from the precursor RNA
• 1. Inactivation


  50% of cells will have the maternally derived X chromosome active and the other 50% have the paternally derived X chromosome active
• 1. Inactivation Maintenance


  1. The X-Inactivation pattern remains constant throughout the life of the cell, and is maintained during cell division

  2. It is only removed in germ cell formation
• DNA Transcription


  1. Transcription factors bind to a gene promoter

  2. DNA polymerase targets the stretch of DNA

  3. A precursor RNA (Pre-mRNA) is synthesised using the DNA strand as a template
• Pre-mRNA


  Includes sequences from the exons of the gene (which encode protein) and the introns (the sequences between which do not)
• Splicing


  Introns are removed and the mature messenger RNA (mRNA) is left
• Splicing


  • Occurs in the nucleus
  • Small Nuclear Ribonucleoproteins (SNRPs) recognise specific RNA sequences (called Motifs), including Splice Acceptor, Splice Donor and Lariat sequences, forming a spliceosome
• Translation

  1. Occurs at the ribosome

  2. Transfer RNAs (tRNAs) carry an amino acid, and bind to the mRNA

  3. The mRNA moves along the ribosome with a new amino acid being included in the peptide chain for each 3 base codon

  4. A release factor binds to the ribosome and causes the peptide to be released for further processing when a stop codon is reached
• Post-Translational Modification

  The completed polypeptide chain undergoes folding, addition of extra side chains, and transport to its specific subcellular location
• 1. Linked Glycosylation

  Addition of a glycan/polysaccharide to the nitrogen of an asparagine amino acid in the protein
• DNA Repair Pathways

  • Single Strand Repair
  • Base Excision Repair
  • Nucleotide Excision Repair
  • Mismatch Repair
  • Homologous Recombination Repair
  • Non-Homologous End Joining
• DNA Repair


  • Repairs chemical crosslinks, single and double stranded breaks, and incorporation of mismatched bases
• Polymorphisms


  Variations in DNA sequence, including Single Nucleotide Polymorphisms (SNPs), deletions, and duplications