Topic 4

• DNA in prokaryotic cells


  Short, circular and not associated with proteins
• DNA in eukaryotic cells

  Very long, linear and associated with proteins called histones
• Chromosome


  DNA molecule and its associated proteins
• DNA in mitochondria and chloroplasts of eukaryotic cells


  Short, circular and not associated with protein
• Gene


  Base sequence of DNA that codes for amino acid sequence of a polypeptide or a functional RNA
• Locus


  Fixed position of a gene on a DNA molecule
• Triplet


  Sequence of three DNA bases that codes for a specific amino acid
• Genetic code


  • Universal
  • Non-overlapping
  • Degenerate
• Non-coding DNA in eukaryotes

  Multiple repeats of base sequences between genes, non-coding sequences called introns within genes
• Genome


  Complete set of genes in a cell
• Proteome


  Full range of proteins that a cell is able to produce
• Messenger RNA (mRNA)


  Carries the sequence of codons for translation into polypeptides
• Transfer RNA (tRNA)


  Brings specific amino acids to the ribosome during translation
• Transcription


  1. Production of mRNA from DNA

  2. In prokaryotes, mRNA is directly produced from DNA

  3. In eukaryotes, pre-mRNA is produced and then spliced to form mRNA
• RNA polymerase


  Enzyme that joins mRNA nucleotides
• Translation


  1. Production of polypeptides from the sequence of codons carried by mRNA

  2. Roles of ribosomes, tRNA and ATP
• Students should be able to relate the base sequence of nucleic acids to the amino acid sequence of polypeptides, when provided with suitable data about the genetic code
• Students should be able to interpret data from experimental work investigating the role of nucleic acids
• Students will not be required to recall in written papers specific codons and the amino acids for which they code
• Gene mutations involve a change in the base sequence of chromosomes
• Gene mutations can arise spontaneously during DNA replication and include base deletion and base substitution
• Due to the degenerate nature of the genetic code, not all base substitutions cause a change in the sequence of encoded amino acids
• Mutagenic agents


  Can increase the rate of gene mutation
• Mutations in the number of chromosomes can arise spontaneously by chromosome non-disjunction during meiosis
• Meiosis produces daughter cells that are genetically different from each other
• Meiosis


  1. Two nuclear divisions result usually in the formation of four haploid daughter cells from a single diploid parent cell

  2. Independent segregation of homologous chromosomes results in genetic variation among daughter cells

  3. Crossing over between homologous chromosomes results in further genetic variation among daughter cells
• Students should be able to explain the different outcome of mitosis and meiosis
• Students should be able to recognise where meiosis occurs when given information about an unfamiliar life cycle
• Students should be able to explain how random fertilisation of haploid gametes further increases genetic variation within a species
• Genetic diversity


  The number of different alleles of genes in a population
• Genetic diversity is a factor enabling natural selection to occur
• Natural selection


  1. Random mutation can result in new alleles of a gene

  2. Many mutations are harmful but, in certain environments, the new allele of a gene might benefit its possessor, leading to increased reproductive success

  3. The advantageous allele is inherited by members of the next generation

  4. As a result, over many generations, the new allele increases in frequency in the population
• Directional selection


  Exemplified by antibiotic resistance in bacteria
• Stabilising selection


  Exemplified by human birth weights
• Natural selection results in species that are better adapted to their environment
• Adaptations may be anatomical, physiological or behavioural
• Substitution mutation


  A base in the DNA sequence is replaced by a different base
• Substitution mutation


  • ATGCCT becomes ATTCCT
• Deletion mutation


  A base is removed from the DNA sequence
• Deletion mutation


  • ATGCCT becomes ATCCT