Genetics and evolution

• Mutation


  Change in the sequence of nucleotides in DNA molecules
• Types of mutations


  • Insertion/deletion
  • Point mutation/substitution
  • Nonsense
  • Missense
  • Silent
• Insertion/deletion mutations


  • One or more nucleotide pairs are inserted or deleted from the sequence, altering the sequence of nucleotides after the insertion/deletion point (frameshift)
• Point mutation/substitution


  • One base pair is replaced by another
• Nonsense mutation


  • Translation is stopped early, resulting in a truncated polypeptide due to premature introduction of a stop codon
• Missense mutation


  • Codon change results in the production of a different amino acid, thus altering the tertiary structure of the protein
• Silent mutation


  • Codon change does not affect the amino acid sequence produced, due to the degenerate nature of the genetic code
• Mutations can have neutral effects where the mutation causes no change to the organism
• Mutations can be beneficial, for instance, humans developed trichromatic vision through a mutation
• Harmful mutations include a mutation in the CFTR protein which causes cystic fibrosis
• Whether a mutation proves to be beneficial or detrimental to an organism will depend on the environment of the organism
• Gene expression


  Can be controlled at the transcriptional, post-transcriptional, and post-translational levels
• Lac operon


  1. Promoter region

  2. Operator region

  3. Structural genes
• When glucose concentration is high and lactose concentration is low


  Transcription of structural genes is inhibited due to binding of the repressor to the operator region
• When glucose concentration is low and lactose concentration is high


  Lactose binds to the repressor, changing its DNA binding site and making it ineffective, allowing RNA polymerase to bind to the promoter region and transcription of structural genes to take place
• Transcription factors


  • Have the ability to switch genes on and off through interaction with the promoter sequence of DNA to either initiate or inhibit transcription
• Post-transcriptional control


  Editing of the primary mRNA transcript, removing non-coding regions (introns) to create a mature transcript consisting only of protein-producing regions (exons)
• Post-translational control


  Proteins such as adrenaline can be activated with the help of cyclic AMP, which starts a cascade of enzyme reactions within the cell to activate the protein
• Homeobox genes


  Involved in controlling the development of body plan of organisms, coding for transcription factors that bind to DNA to regulate transcription by switching genes on and off when required at particular stages of development
• Apoptosis


  A form of programmed cell death which can act as a mechanism to control the development of body plans, an ordered, controlled series of biochemical events leading to cell death, the opposite of necrosis
• Apoptosis is a means of controlling the number of cells and ensuring that it remains constant to prevent cancer
• Discontinuous variation


  Variation which can be assigned to a particular category, e.g. shoe size or blood type
• Continuous variation


  Variation where the differences between phenotypes are quantitative, e.g. height or weight
• Variation can be influenced by both environmental factors and genetic factors
• Meiosis


  A form of cell division that gives rise to genetic variation, producing haploid gametes with half the number of chromosomes

  
• Mechanisms of genetic variation in meiosis

  1. Crossing over of chromatids

  2. Independent assortment of chromosomes

  
• Allele


  Alternative form of a gene
• Locus


  The specific position of a gene on a chromosome, the two alleles of a gene are found at the same loci on the chromosome pairs
• Phenotype


  Observable characteristics of an organism which are as a result of genotype and environment
• Genotype


  The alleles present within cells of an organism, for a particular trait or characteristic
• Dominant


  Only a single allele is required for the characteristic to be expressed, that allele is always expressed in the phenotype
• Recessive


  The characteristic is only expressed if there is no dominant allele present
• Homozygous


  Two identical alleles
• Heterozygous


  Two different alleles
• Codominance


  Both alleles contribute to the phenotype
• Linkage


  Genes for different characteristics are located at different loci on the same chromosome and so are inherited together
• Monogenic inheritance


  A phenotype or trait is controlled by a single gene
• Dihybrid cross


  Inheritance of two genes
• Sex linkage


  Expression of an allele dependent on the gender of the individual as the gene is located on a sex chromosome
• Autosomal linkage


  Genes located on the same chromosome (not a sex chromosome) and tend to be expressed together in the offspring