Biology Topic 4

• DNA is stored in the nucleus of eukaryotic cells
• Chromosome

  Tightly coiled DNA
• Histone proteins

  Proteins that DNA is wrapped around to prevent tangling
• Nucleosome

  Complex of DNA wrapped around histone proteins
• In prokaryotic cells, DNA is circular and not wrapped around histones
• Mitochondria and chloroplasts contain their own circular, histone-free DNA
• Mitochondrial and chloroplast DNA is similar to prokaryotic DNA
• Gene

  Short section of DNA that codes for a polypeptide and functional RNA
• Allele

  Different form/version of the same gene
• Locus

  Location of a gene on a chromosome
• Homologous pair

  Pair of chromosomes with the same genes
• Humans have 23 pairs of chromosomes
• The 23rd chromosome pair are the sex chromosomes (X and Y)
• Compacting DNA into chromosomes

  DNA is tightly coiled around histone proteins
• Function of histone proteins

  Allow DNA to be tightly coiled into chromosomes
• The DNA in one human muscle cell is 2.3 meters long
• Genetic code

  • Degenerate
  • Universal
  • Non-overlapping
• Start codon

  The first three bases on DNA/mRNA that initiate translation
• Stop codon

  The final three bases on DNA/mRNA that cause the ribosome to detach and translation to stop
• Degenerate

  Each amino acid is coded for by more than one triplet of bases
• The genetic code has 64 possible triplet combinations, more than the 20 amino acids needed
• Degenerate genetic code means a point mutation may not change the amino acid coded for
• Universal

  The same triplet of bases codes for the same amino acid in almost all living things
• Universal genetic code allows genetic engineering between species
• Non-overlapping

  Each base is only part of one triplet of bases coding for an amino acid
• Non-overlapping means a point mutation only affects one amino acid
• Introns

  Sections of DNA that do not code for amino acids, found in eukaryotes
• Exons

  Sequences of DNA that do code for amino acids
• Genome

  An organism's complete set of DNA within one cell
• Proteome

  The full range of proteins in one cell
• Genome does not change (except for mutations), proteome changes often between cells and over time
• Genomes vary greatly in size between organisms, e.g. humans have 3 billion base pairs, bacteria have 600,000
• Protein synthesis

  The process where proteins are made
• Protein synthesis

  1. Transcription

  2. Translation
• Transcription

  The first step of protein synthesis, happening inside the nucleus, where a copy of a gene is made as mRNA
• Translation

  The second step of protein synthesis, happening in the cytoplasm, where the mRNA attaches to a ribosome and tRNA molecules bring specific amino acids to build the polypeptide chain
• Transcription

  1. Unwind DNA double helix

  2. Expose DNA strand to act as template

  3. RNA nucleotides align complementary to exposed DNA strand

  4. RNA polymerase joins nucleotides

  5. Pre-mRNA formed

  6. Introns spliced out by spliceosome
• Introns

  Sequences of bases in DNA that do not code for amino acids, removed by splicing
• Exons


  Coding regions of DNA that remain after introns are spliced out
• Translation

  1. mRNA attaches to ribosome at start codon

  2. tRNA molecules bring complementary amino acids

  3. Ribosome holds tRNA and mRNA in place

  4. Peptide bonds form between amino acids

  5. Ribosome moves along mRNA

  6. Process continues until stop codon reached