Genetics

• RNA


  Transfers genetic information from DNA to the ribosomes
• Types of RNA


  • mRNA (messenger)
  • tRNA (transfer)
  • rRNA (ribosomal)
• mRNA


  Single stranded copy of one gene of the DNA, takes the copy of the gene out of the nucleus to the ribosomes for protein synthesis
• tRNA


  Single stranded molecule that folds using complementary base pairing, carries an amino acid to the ribosome to be made into a protein
• rRNA


  Forms part of the ribosomes
• Chromosomes


  Found in the nucleus, carry genes and made up of large molecules of DNA
• Gene


  Section of DNA on chromosomes, controls the formation of a specific protein
• Allele


  Alternative forms of a particular gene with different base sequences, coding for different versions of the same protein
• Structure of chromosomes


  • DNA wrapped around nucleosomes of histone proteins to form chromatin, which is wrapped around each other to make a chromosome
  • Chromosomes located in the nucleus of every cell, only visible during cell division
• Homologous pairs


  Half chromosomes from sperm, half chromosomes from egg
• Genetic code


  • Triplet code (3 bases = 1 amino acid)
  • 64 possible amino acids
  • Degenerate code (most amino acids coded for by multiple triplets)
  • Non-overlapping code (each base only appears in one triplet)
  • Universal code (same in all known living things)
• DNA organisation


  • Coding DNA (genes that code for proteins, some code for functional RNA)
  • Non-coding DNA (regions of repeated sequences, introns within genes)
• Eukaryotic DNA


  Large, linear, associated with proteins, form chromosomes
• Prokaryotic DNA


  Short, circular, not associated with proteins, no chromosomes
• Genome


  Complete set of genes in a cell, including that in the mitochondria and chloroplasts
• Proteome


  Full range of proteins produced by the genome
• Transcription


  1. DNA unwound

  2. DNA helicase breaks hydrogen bonds

  3. RNA polymerase attaches activated RNA nucleotides along template strand to form pre-mRNA

  4. DNA reforms double helix behind RNA polymerase

  5. Stop codon reached, RNA polymerase detaches, completed pre-mRNA released
• RNA splicing


  Pre-mRNA consists of exons and introns, only exon sequences code for amino acids, introns are removed and exons joined together to form mature mRNA
• Alternative splicing


  Exons can be spliced in different ways, allowing multiple proteins to be made from a single gene
• Mutation may result in faulty splicing, leading to non-functional proteins
• ribosome

  attaches to mRNA at start codon
• Translation/Protein synthesis


  1. tRNA with complementary anticodon comes

  2. Bringing a specific amino acid

  3. Peptide bonds form between the amino acids (requires ATP)

  4. Empty tRNA leaves to pick up another specific amino acid

  5. Ribosome moves along mRNA to the next codon

  6. Repeats until ribosome reads a stop codon
• mRNA attaches to ribosome at start codon
• Meiosis - Prophase I


  1. Longest and most complex phase (90%)

  2. Chromosomes condense by supercoiling

  3. Homologous chromosomes come together to form a tetrad

  4. Tetrad is two chromosomes at four chromatids (sister non-sister)

  5. Crossing over occurs
• Meiosis - Anaphase I


  1. Homologous chromosomes separate and move towards poles

  2. Sister chromatids remain attached at their centromeres
• Meiosis - Prophase II

  Same as prophase in mitosis
• Meiosis - Metaphase II

  Same as metaphase in mitosis, chromosomes line up in centre
• Meiosis - Anaphase II

  Same as mitosis, sister chromatids separate
• Meiosis - Telophase II

  Same as mitosis, cytokinesis, four haploid daughter cells produced
• Crossing over


  • In prophase, homologous chromosomes pair up
  • During crossing over, a structure called a chiasma randomly forms and creates a point of contact between non-sister chromatids
  • This allows for an exchange of genetic material to occur between non-sister chromatids at the chiasmata
  • Segments or non-sister chromatids break and reattach to the other chromatid
• Independent assortment


  • Orientation of homologous pairs to poles is random, resulting in the production of genetically different gametes
  • As four gametes are formed from meiosis, they have different combinations of chromosomes
  • Each gamete has a combination of chromosomes from both parents
• Mutation

  A change to the quality or structure of an organism's DNA
• Mutagenic agents

  • Radiation
  • X-rays
  • UV light
  • Alcohol
  • Cigarettes
• Types of mutations

  • Substitution
  • Silent mutation
  • Missense mutation
  • Nonsense mutation
  • Insertion
  • Deletion
• Silent mutation


  Substitution of a base that codes for the same amino acid as the original base (due to degeneracy), has no effect
• Missense mutation


  Change in base leads to a different amino acid being coded for, polypeptide has a different amino acid
• Nonsense mutation


  Substitution leads to a premature stop codon being coded for, leads to premature termination of polypeptide synthesis or protein won't function normally
• Frameshift mutation


  Insertion or deletion mutations move the reading frame, making the rest of the code unreadable
• Chromosomal mutations

  • Gene deletion
  • Gene duplication
  • Translocation
  • Inversion
• Nondisjunction

  Chromosomes don't separate properly during meiosis