Origins of Multicellularity Notes

This lecture will build upon understanding of:
- Prokaryotic cells.
- Gene expression regulation in prokaryotic cells.
- Differences between prokaryotic and eukaryotic cells.
- Why eukaryotes evolved into multicellular organisms.
- The essential role of cell signalling in multicellularity.
- Types of cellular responses to intercellular signals.
Leading to understanding:
- How eukaryotic gene expression is regulated.
- Endpoint and RT-qPCR for measuring mRNA.

The earliest life forms on Earth were single-celled organisms.
Examples: Bacteria, Archaea, single-cell Eukaryotes.
Single-celled organisms are the foundation of all life, providing insights into the origins of multicellularity.

Contain DNA/RNA for genetic information.
Cell membrane for protection and interaction with the environment.
Cytoplasm for metabolic activities.
Highly varied metabolism, including photosynthesis (cyanobacteria) and chemosynthesis (some archaea).
Can grow at all sizes.

Organisms evolved colonial formations as a step towards multicellularity.
Example: Cyanobacteria forming long chains that are colonies of individual organisms.

Multicellularity has arisen multiple times during eukaryotic evolution.
Evolution of animals, plants, multicellular fungi, and cellular slime moulds occurred independently.
The driving force behind this repeated evolution.
Evolved from the last eukaryotic common ancestor (LECA).

The last common ancestor of modern eukaryotes lived during the Mesoproterozoic era, approximately 1.6 to 1 billion years ago, or possibly earlier.
This coincides with the age of the oldest multicellular algae (1.6 billion years old).

Protection: Larger size provides better protection from predation.
Buffering: Larger organisms are buffered more effectively from the external environment.
Specialization: Multicellularity allows the development of cell types with specialized functions.
- Primitive colonial forms have few specialized cells.
- Humans contain over 200 cell types.

Cells with flagella enable movement in simple multicellular organisms.
The microtubule organizing machinery needed for flagella formation is also required for the spindle apparatus in cell division.
Competition exists for this machinery between cell movement and cell division processes.
The presence of both specialized flagellated and non-flagellated cells in a colony allows simultaneous movement and growth.

Proposed in the 19th century that cell nuclei and chromosomes contained heritable genetic information.
August Weisman proposed that the nucleus contained “determinants”.
Unequal division of determinants to daughter cells accounting for differences between cells.

Hans Driesch’s experiments with sea urchins argued against mosaic development.
He claimed that all cells contain all the necessary information for normal development.

Dolly the Sheep was the first cloned mammal (following Prof Sir John Gurdon’s work in cloning Xenopus frogs).
Dolly was created by nuclear transfer of a mammary cell nucleus into an oocyte.
Dolly challenged the idea of the irreversibility of the differentiated state of a somatic cell from adult individuals.
Gene expression, not which genes the cell has, is vital to cell specialization.

Hans Spemann and Hilde Mangold demonstrated that transferring cells from the dorsal lip of an organism to another location induced the formation of a secondary embryonic axis.
This showed that cell specialization is influenced by interactions between neighboring cells and tissues, not solely determined by the genome.
These concepts underpin developmental biology.

The balance between differentiated cell types is crucial within a multicellular organism (e.g., the flagella constraint hypothesis). An imbalance prevents the organism from functioning correctly.
Cell lineage is due to different genes being switched on in different cells, despite cells having the same genetic material (Driesch, Spemann and Hilde Mangold, Dolly the Sheep).
A key process must regulate which genes are activated at which points during development.

Homeostasis: Maintenance of the internal state of the adult. Multicellularity buffers the organism against changes in the external environment.
Development Regulation: Cell signalling regulates the process of cell cycle, cell movement, differentiation and patterning (cells need to be in the right place at the right time).

Self/Non-Self Recognition: Ability to recognize self and non-self to organize into a multicellular structure.
Cell Adhesion: Cells need to adhere to one another to form structures and extracellular matrices.

Genome analysis indicates a close relationship between animals and choanoflagellates.
Choanoflagellates are unicellular and colonial eukaryotes.
They have many genes found in animals, including genes coding for components of signalling pathways.
Intercellular signalling occurs for colony formation.

Gene expression involves the process of activating a gene.
Formally defined as “the process by which a gene’s coded information is converted in the structures present and operating in the cell”.
Expressed genes include those transcribed into mRNA (then translated into protein) and those that do not make protein (e.g., transfer RNAs, ribosomal RNAs, and regulatory RNAs).

The Central Dogma: $DNA \rightarrow RNA \rightarrow Protein$ . This describes the flow of genetic information within a biological system. DNA is transcribed into RNA, which is then translated into protein. This process is essential for gene expression and cellular function.