Biology

• Exons


  Coding regions of the primary mRNA transcript
• Transcription


  1. RNA polymerase moves along the DNA, unwinding the double helix and breaking the hydrogen bonds between bases

  2. RNA polymerase synthesises a primary transcript of mRNA from RNA nucleotides by complementary base pairing
• mRNA splicing


  1. The primary mRNA transcript is composed of introns and exons

  2. The introns are removed

  3. The exons are joined together to form the mature mRNA transcript

  4. The order of the exons remains unchanged during splicing

  5. The mature mRNA transcript leaves the nucleus and travels to a ribosome in the cytoplasm
• Alternative RNA splicing


  Different mature mRNA transcripts can be produced from the same primary transcript depending on which exons are retained. In this way, one gene can code for more than one protein.
• Introns and exons


  • Introns are removed and exons are retained during RNA splicing to produce the mature mRNA transcript
• RNA splicing


  1. Primary transcript

  2. Splicing

  3. Mature transcript
• Cellular differentiation


  The specialisation of cells to produce proteins characteristic of that cell type, allowing the cell to carry out specialised functions
• Meristems in plants


  • Regions of unspecialised cells in plants that can divide (self-renew) and differentiate
• Stem cells


  Unspecialised cells in animals that can divide (self-renew) and/or differentiate into one of many cell types
• Types of animal stem cells

  • Tissue stem cells
  • Embryonic stem cells
• Tissue stem cells


  • Involved in the growth, repair and renewal of cells found in a particular tissue, and are multipotent
• Embryonic stem cells


  • Found in the very early embryo, can self-renew and are pluripotent
• Embryonic stem cells can offer effective therapeutic treatment for both disease and injury
• Current research uses of stem cells


  • As model cells to study how diseases develop
  • For drug testing in the laboratory
• Stem cell research provides information on how cell processes like cell growth, cell differentiation and gene regulation work
• Therapeutic uses of stem cells


  • Repair of diseased organs or tissues
  • Corneal repair
  • Skin regeneration
• The use of embryonic stem cells raises ethical issues, including the fact that it involves destruction of embryos
• Genome


  The entire hereditary information of an organism encoded in DNA
• Genes


  DNA sequences that code for proteins
• Non-protein coding sequences


  DNA sequences in the genome that do not code for proteins, but may regulate transcription or be transcribed into non-coding RNA
• Mutation


  An irreversible change in the sequence of nucleotides within a gene or chromosome, resulting in either no protein or an altered protein being synthesised
• Main categories of mutations


  • Single gene mutations
  • Chromosome structure mutations
• Single gene mutation


  The alteration of a DNA nucleotide sequence as a result of substitution, insertion or deletion of nucleotides
• Types of single gene mutations


  • Substitution mutations (missense, nonsense, splice-site)
  • Insertion mutations (frame-shift)
  • Deletion mutations (frame-shift)
• Substitution mutation - missense


  One base within a triplet of DNA bases is substituted for another, resulting in a change to the amino acid sequence but not necessarily the function of the protein
• Substitution mutation - nonsense


  A premature stop codon is produced, resulting in a shorter protein
• Substitution mutation - splice-site


  A substitution at a splice-site results in some introns being retained and/or some exons not being included in the mature mRNA transcript
• Frame-shift mutation


  An insertion or deletion that shifts the reading frame of the genetic code, resulting in a major effect on the protein produced
• Chromosome structure mutation

  A mutation that affects many genes on a chromosome, such as deletions, duplications, inversions or translocations
• Chromosome number mutations can result in conditions like Down's syndrome
• Chromosome structure mutation


  Whole sections of chromosomes containing many genes are broken, rearranged or lost
• Chromosome structure mutations


  • Substantial changes often make them lethal
• Deletion


  1. A section of a chromosome is removed

  2. Two breaks occur along the length of the chromosome

  3. The middle segment containing many genes is lost

  4. The broken ends of the remaining chromosome join together
• Duplication


  1. A section of a chromosome is added from its homologous partner

  2. A broken segment from a similar neighbouring chromosome is inserted, duplicating a specific set of genes
• Inversion


  1. A section of chromosome is reversed

  2. Two breaks occur along the length of a chromosome

  3. The segment between the breaks rotates 180 degrees and reattaches, reversing the gene sequence
• Translocation


  1. A section of a chromosome is added to a chromosome, not its homologous partner

  2. A segment breaks off the end of one chromosome and attaches to the end of a neighbouring chromosome, adding additional genes
• Chromosomes are arranged in pairs. An homologous pair contains one chromosome from each parent. Both chromosomes in a pair are the same length and contain similar locations
• Importance of duplication in evolution


  • Mutations can be harmful or beneficial
  • Duplication of genes allows potential for beneficial mutations to occur in a duplicated gene
  • The second copy of the gene has the potential to undergo random mutation itself and may produce a protein that could confer a survival advantage
  • The original gene is still expressed to produce its protein
• Evolution


  The changes in organisms over generations as a result of genomic variation
• Ways genomic material can be inherited


  • Vertical gene transfer
  • Horizontal gene transfer