The Evolution of the Eukaryotic Cell

Evolutionary Context

  • Timeline: Evolutionary evidence includes Grypania spiralis fossils, which are distinctive spirally coiled photosynthetic algae dating back 2.1 billion2.1\,billion years.

  • Indigenous Connection: The University acknowledges Aboriginal and Torres Strait Islander histories of continuous connection to the land dating back more than 60,00060,000 years.

Structure and Function of the Nucleus

  • Anatomy: Surrounded by a double membrane (nuclear envelope) containing nuclear pores approximately 50 nm50\,nm in diameter to control molecular movement.

  • Genetic Material: Contains most of the cell's DNA organized into chromatin (DNA and protein complexes) and chromosomes.

  • Nucleolus: A subregion responsible for transcribing ribosomal genes and ribosomal biogenesis.

  • Functions: Primary site for DNA replication and gene transcription (on/off).

  • Origin: Likely formed through the invagination of the plasma membrane around the nucleoid of an ancient prokaryote.

Structure and Function of Mitochondria

  • Anatomy: Consists of an outer membrane and a highly convoluted inner membrane forming cristae; the internal space is the mitochondrial matrix.

  • Function: Site of aerobic respiration, transforming fuel molecules like glucose into ATPATP.

  • Aerobic Respiration Equation: C6H12O6+6O2→6CO2+6H2O+ATPC_6H_{12}O_6 + 6O_2 \rightarrow 6CO_2 + 6H_2O + ATP

  • Autonomy: Contains its own DNA and ribosomes; can divide independently of the nucleus.

Structure and Function of Chloroplasts

  • Anatomy: Double-membrane organelles containing thylakoids (site of chlorophyll) and stroma.

  • Function: Site of photosynthesis, converting light energy into chemical energy.

  • Autonomy: Like mitochondria, they retain their own genome and can divide independently.

Endosymbiotic Origins and Evidence

  • Primary Endosymbiosis:

    • Mitochondria: Evolved from a single event involving the endosymbiosis of a purple bacteria.

    • Chloroplasts: Evolved from the endosymbiosis of photosynthetic cyanobacteria.

  • Secondary Endosymbiosis: Occurs when a eukaryote engulfs another eukaryotic cell containing a chloroplast (e.g., in euglenoids, haptophytes, and "Protistan pirates"). This results in plastids with three or four membranes.

  • Evidence for Endosymbiosis:

    • Organelles are morphologically similar to bacteria.

    • Presence of double membranes (innerinner and outerouter).

    • Retention of semi-autonomous genomes (DNA/RNA) and protein synthesis machinery (ribosomes).

    • The presence of a bacterial peptidoglycan wall in the plastids (cyanelles) of some species like Cyanophora paradoxa.