Evolution

• Individuals with advantageous traits are more likely to survive and reproduce, passing on those traits to future generations.
• Mutation is any spontaneous/random change in the genetic material of an organism
• Mutation occurs randomly and spontaneously, any gene can undergo mutation at any time without any known cause
• Mutation can be either spontaneous or induced
• Spontaneous mutations have no known cause to occur
• Induced mutations can be caused by accidental or deliberate exposure of organisms to mutagenic agents
• The agent that causes mutation is called mutagens
• Mutagens include radiation (UV-rays, X-ray, nuclear radiation) and chemicals (carcinogenic chemicals such as mustard gas, formaldehyde, cochicines, certain constituents of tobacco, some drugs, food preservatives, and pesticides)
• Mutations occur at a very slow rate, with an estimated rate of 1 in 50 million base pairs
• Most mutations are detected and repaired, and because 95% of our DNA is non-coding, most mutations are unlikely to affect coding genes
• Types of mutations
• Point/gene mutation: A change in only a single base in the nucleotide sequence of the DNA molecule
• Point mutations occur randomly during DNA replication and may affect the process of protein synthesis by changing the amino acid sequence
• Subtypes of point mutations include substitution, addition, and deletion of a base in the gene
• Chromosomal mutation: Any change in the arrangement or structure of the chromosomes
• Structural chromosomal mutations involve inversion, duplication, deletion, and translocation
• Numerical chromosomal mutations involve euploidy and aneuploidy
• Consequences of gene mutations
• Somatic mutations occur in normal body cells and can be harmless, damaging, or cancerous, but they are not passed to the next generation
• Germinal mutations occur in sex cells and are heritable
• Proto-oncogenes and tumour suppressor genes play important roles in regulating cell division and preventing the formation of tumors
• Benefit of mutation
• Mutations are the raw material of evolution and create new genes
• If a mutated allele gives an organism an advantage, Natural Selection will act to increase the frequency of that allele with successive generations
• Mutations in bacteria can confer resistance to specific antibiotics, providing an advantage in environments where those antibiotics are used
• Chromosome mutations
• Structural chromosomal mutations alter the structure of chromosomes and can result in inversion, duplication, deletion, and translocation
• Numerical chromosomal mutations involve changes in the number of whole chromosomes, such as aneuploidy and polyploidy
• Aneuploidy can result from chromosome non-disjunction and lead to conditions like Down syndrome
• Down's syndrome is characterised by:

  • Mental retardation
  • Heart defects
  • Stunted growth
• Meiotic nondisjunction occurs when chromosomes fail to separate from each other during meiosis
• Aneuploidy results in chromosomal disorders such as:

  • Trisomy 21: Down Syndrome
  • Trisomy 18
  • Trisomy 13
  • Klinefelter Syndrome (XXY): abnormal male with feminised features and sterility
  • XYY Syndrome: very tall male
  • Triple X Syndrome (47,XXX)
  • Monosomy: Turner Syndrome (XO) - abnormal female with slightly masculinised features and sterility
• Polyploidy is a type of euploidy where an organism has more than two sets of homologous chromosomes (2x)

  • Triploidy: 2n=3x - three sets of chromosomes per cell
  • Tetraploid: 2n=4x - four sets of chromosomes per cell
  • Pentaploid: 2n=5x - five sets of chromosomes per cell
  • Human liver cells have 92 chromosomes per cell (2n=4x=4 x 23=92)
• Polyploidy does not add new genes to a gene pool, but gives rise to new combinations of genes

  • Polyploidy can exist in two forms: Autoployploidy and Allopolyploidy
• Autopolyploidy:

  • The number of chromosomes is increased within the same species either naturally or artificially
  • It happens during chromosome replication (interphase) when chromatids fail to align at the metaphase plate and remain scattered in the cytoplasm, resulting in a doubling of the chromosome number
  • This is due to the disruption of the formation of spindle fibers by cochicines
• Allopolyploidy involves the formation of an additional set of chromosomes that come from more than one species
• Evolution can be defined as the change in genetic composition of a population over successive generations, caused by meiosis, hybridisation, natural selection, or mutation

  • This leads to a divergence from other populations of the same species and may lead to the origin of a new species
• The theory of evolution describes how various forms of life on Earth emerged and developed

  • The origin of life on Earth has five main theories:
  • Special creationism
  • Spontaneous generation
  • Eternity of life
  • Cosmozoan theory
  • Biochemical origin
• The theory of special creationism states that at some stage, some supreme being created life on Earth

  • Different versions of special creationism are linked with different religions such as Young Earth creationism, Old Earth creationism, Day-age and gap creationism, Progressive creationism, Theistic evolution/Evolutionary creationism, and Intelligent design
• The theory of spontaneous generation/abiogenesis states that life originated from non-living matter without the intervention of living things