Origins of multicellularity

Single cell ancestors

The earliest life forms on earth were single celled organisms

Single cell ancestors examples

bacteria, archaea, single cell eukaryotes

Single cell organisms

Contain DNA/RNA for genetic information and have a cell membrane for protection and interaction with the environment. Cytoplasm for metabolic activities.

Metabolism in single celled organisms

Can be highly varied: for example, photosynthesis (cyanobacteria) and chemosynthesis (some archaea)

When is the last common ancestor of modern eukaryotes believed to have lived?

during the Mesoproterozoic era, about 1.6 to 1 billion years ago, or earlier

What drives multicellularity?

Multicellular organisms are bigger and therefore protected from predation. More efficient as multiple cells help to buffer against the external environment. Allows the development of different cell types with specialised functions.

How many cell types in a human

Over 200

Flagella synthesis constraint hypothesis

Constraint that cells can either divide or have a flagella. a cell can either reproduce or be able to move around. This caused some cells to do one part and some to do the other part, so they organised in a coherent way and allowed communication between cells. This meant we can have organisms which can both reproduce and move around.

Mosaic development

Idea proposed in the 19th century that cell nuclei and chromosomes contained heritable genetic information. August Weisman proposed that the nucleus contained "determinants" and when cells divided, these got split between cells, and unequal division of determinants would account for differences between cells.

Regulative development

Hans Driesch's experiments with sea urchins argued against mosaic development. His results showed that all cells contained all information required for normal development.

Dolly the sheep

First cloned mammal. Showed that putting cellular tissue into an oocyte would cause it to grow into an adult. Challenged the idea of irreversibility of the differentiated state of a stomatic stem cell taken from adult individuals.

How was dolly the sheep created?

Nuclear transfer of a mammary cell nucleus into an oocyte.

What did the dolly the sheep experiment demonstrate?

Cell specialisation during development is not predetermined by the genome alone but can be influenced by interactions between neighbouring cells and tissues.

Signalling regulates...

The cell cycle, cell movement, differentiation, patterning.

What initiates cell signalling?

Different genes being switched on in different cells as the cells all have the same genetic material.

Choanoflagellates

Unicellular and colonial eukaryotes. Genome analysis indicates a close relationship between these and animals.

To regulate development, cells need to be able to differentiate between..

self and non-self

P53

Protein known to suppress cancer and exists in nearly every organism, despite the fact that single celled organisms can't get cancer.

Slime moulds

Single celled organism that can become multicellular when it needs to. They do this with signalling molecule cyclic AMP.

Types of cellular response

Cells may move, cells may contract

Example of cells that move in the human body

Immune system cells

Example of cells that contract in the human body

Mammary glands contract to push milk out. Calcium ions act as a messenger to tell the glands that they need to contract.

Gene expression

The process by which a gene's coded information is converted in the structures present and operating in the cell. Expressed genes include genes which are transcribed into mRNA, which are then translated into protein.

Ways genes can be expressed that do not make protein

transfer RNAs, ribosomal RNAs, regulatory RNAs

Central dogma

The idea that DNA can go forward to RNA and the RNA is picked up by ribosomes and made into protein.

What does the link from DNA to mRNA to protein show?

Gene expression can be regulated at multiple levels to enable development.

How can mRNA be regulated?

Processes before translation such as splicing.

Post-translational modifications

Proteins encoded by genes may be regulated by post-translational modifications that alter activity and stability of the protein