Bio A2.1 Origin of Cells

• The endosymbiotic theory suggests that mitochondria originated from bacteria.
• Chloroplasts are organelles found only in plant cells and some protists.
• Mitochondrial DNA is circular, double-stranded, and has no introns or exons.
• Before life evolved on Earth, our planet is thought to have had an atmosphere consisting mostly of reducing gases such as ammonia, methane and carbon dioxide
• There were high levels of ultraviolet and solar radiation
• Temperatures were much higher than today, probably exceeding 100 ℃
• These conditions would have resulted in extreme weather events, including frequent electrical storms
• Cells


  The smallest self-sustaining units of life
• Processes of life


  • Metabolism
  • Response to stimuli
  • Homeostasis
  • Movement
  • Growth
  • Reproduction
  • Excretion
  • Nutrition
• Viruses are not considered to be living organisms because they cannot carry out these processes of life independently
• Cells can only be produced by the division of pre-existing cells
• Necessary requirements in the evolution of the first cells


  • Catalysis
  • Self-replication of molecules
  • Self-assembly
  • Emergence of compartmentalisation
• Under the conditions present on early Earth, it is thought that biological molecules formed from simple, inorganic molecules
• Miller and Urey carried out an investigation to model the formation of biological models in the conditions they thought existed on early Earth, although we now think the conditions are different to the ones simulated in this investigation
• The formation of a lipid bilayer was a necessary stage in the origin of the first cells, separating the cell's interior from the external environment
• RNA


  The first molecule of life, exhibiting both a catalytic function and acting as the genetic material of cells
• Proteins


  Have taken over as the main catalytic molecule in cells because they are capable of more variability
• DNA


  Has taken over as the primary genetic material because it is more stable
• The last universal common ancestor (LUCA) is a hypothetical organism from which all organisms living on Earth today have evolved
• Using phylogenetic analysis and fossil records, scientists think that LUCA was a simple single-celled microorganism that existed between 2.5 and 3.5 billion years ago in hydrothermal vents
• Mitochondrial DNA is circular, like bacterial chromosomes.
• Both mitochondria and chloroplasts have their own ribosomes (70S) which are similar to those found on the surface of prokaryotes.
• Mitochondrial DNA is inherited maternally (from the mother).
• Mutations in mtDNA can lead to diseases such as Leber's hereditary optic neuropathy (LHON) and MELAS syndrome.
• Both mitochondria and chloroplasts have their own DNA and ribosomes, suggesting they were once free-living prokaryotes.
• Chloroplasts contain thylakoids, which resemble the internal membranes of photosynthetic bacteria.
• Mitochondrial DNA mutates faster than nuclear DNA due to its lack of histones and repair mechanisms.
• Mitochondria are responsible for cellular respiration, producing ATP through the breakdown of glucose.
• Bacteria were once thought to be the ancestors of eukaryotes, but now we know that mitochondria are descended from bacteria.
• Chloroplasts contain thylakoids stacked into grana, while mitochondria contain cristae.