Genetics

• Nucleotide monomers are comprised of a phosphate, a deoxyribose sugar, and a nitrogenous base (Adenine, Thymine, Guanine, or Cytosine)
• The purines are adenine and guanine (double ring) while the pyrimidines are cytosine and thymine (single ring)
• The double helix is made up of the sugar-phosphate backbone, anti-parallel strands (5' to 3'), and base pairs with hydrogen bonds
• DNA replication follows a semi-conservative model; strands separate and each is a template for synthesis of a new daughter
• Principles of DNA replication:

  1. Helicase separates DNA strands & single-strand binding proteins prevent rejoining
  2. Primase makes RNA primer; synthesis occurs 5' to 3'
  3. Continuous synthesis of leading strand by DNA pol III
  4. Discontinuous synthesis of lagging strand resulting in Okazaki fragments; DNA pol I must replace primers & DNA ligase joins fragments
• A karyotype is an aligned chromosome set in a cell
• Autosomes are non-sex chromosomes
• A diploid cell has homologous chromosomes pairs
• Gametes transmit gene copies between generations and arise in germ cells by meiosis
• Meiosis is the specialised cell division which produces 4 gametes. It occurs in 2 divisions: meiosis I which separates homologous pairs and meiosis II which separates chromosomes
• Crossing over occurs in prophase 1
• In asexual reproduction, genetic makeup is identical until mutation. In sexual reproduction, there is genetic variation due to unique gene combinations.
• The basis of variation:

  1. Independent assortment of chromosomes in gametes
  2. Crossing over
  3. Random combination at fertilisation
• Mendel's hypothesis:

  1. Alleles explain character variations
  2. Organisms inherit 2 alleles, 1 from each parent
  3. Gametes carrying alleles are separated
  4. Dominant alleles prevail over recessive
• Law of segregation: 2 alleles for a heritable character segregate during gamete formation & end up in different gametes
• Law of independent assortment: 2 or more genes segregate independently of each other during gamete formation
• Complex causes in relation to mendelian genetics:

  1. Incomplete dominance: Heterozygote has intermediate character
  2. Pleiotropy (1 gene has multiple effects) / Polygenic Inheritance (Multiple genes contribute to the phenotype) / Nurture (Environmental Factors)
  3. More than 2 possible alleles
  4. Epistasis: Gene at 1 locus affects gene at another locus
• Linked genes are located nearby on the same chromosomes and tend to be inherited together though this can break with crossovers and dilutes with distance
• Pedigree Analysis uses retrospective family history
• Albinism is a recessive disorder while achondroplasia and Huntington's disease are dominant disorders
• Non-disjunction is when chromosomes don't separate correctly in meiosis 1 or 2 resulting in aneuploidy eg. down syndrome
• Enzymatic proteins provide selective acceleration of chemical reaction eg. digestive enzymes catalyse the hydrolysis of bonds in food molecules
• Storage proteins provide storage of amino acids eg. casein for baby mammals
• Biochemical properties of proteins determine their structure & enable their function eg. antibody protein surface matches target antigen
• The levels of protein structure:

  1. Linear order of amino acids
  2. Hydrogen bonding
  3. Stabilisation by interaction between sidechains
  4. Close association of 2+ polypeptide chains
• Amino acids are made up of an amino group, a carboxyl acid group, a central alpha carbon, and a sidechain (R chain)
• A peptide bond is the covalent bond between amino acids
• The 20 sidechains corresponding to the 20 amino acids can be hydrophobic, uncharged hydrophilic, or charged hydrophilic
• The steps of transcription are:

  1. Initiation; RNA polymerase binds to start
  2. Elongation; It moves from 5' to 3' and polymerises RNA based on the DNA template
  3. Termination; The RNA polymerase detaches
• Post-transcriptional processing:

  1. mRNA protective capping
  2. poly-A tail for stabilisation
  3. Splicing; removes introns & concatenates exons
• A gene is region of DNA that can be expressed to produce a final functional product - either a polypeptide or RNA molecule
• Steps of translation:

  1. tRNA matches codon at A site
  2. Peptide bond forms at P site
  3. Empty tRNA leaves at E site
• Types of mutations include substitution and frameshift
• Substitution mutations involve a change of nucleotides. Outcomes:

  1. Silent : Codes for same amino acid
  2. Missense : Codes for different amino acid
  3. Nonsense : Stop codon
• Frameshift mutations involve insertion or deletion. Outcomes:

  1. Premature stop
  2. Many incorrect amino acids
  3. Minor effect is shift is a multiple of 3
• Enzyme activity is regulated by feedback inhibition and enzymes production is regulated by regulation of gene expression
• An operon is a cluster of related genes regulated by a single promoter in prokaryotes
• Transcription regulation in prokaryotes case 1:

  • Involves trp operon, trp repressor protein, and corepressor molecule tryptophan
  • Negative gene regulation; transcription factor only binds, blocking RNA polymerase, when corepressor molecule is present
• Transcription regulation in prokaryotes case 2:

  • Involves lac operon, lac repressor protein, and inducer molecule allolactose
  • Negative gene regulation; transcription factor only binds, blocking RNA polymerase, when inducer molecule is absent
• Transcription regulation in prokaryotes case 3:

  • Involves lac operon, CRP activator protein, and coactivator molecule cAMP
  • Positive gene regulation; transcription factor only binds, activating RNA polymerase, when coactivator molecule is present