THE ORIGIN OF CELLS
The essential concept (proposal?) is that cells can only be formed by division of pre-existing cells so therefore there must be an unbroken chain of life from the first cells on Earth, to all cells in existence today.
The essential unknown is how did those first cells originate! This is an intriguing question and sets us on the way to the nature of Science (NoS) statement which introduces Topic 1.5: “Testing the general principles that underlie the natural world – the principle that cells only come from cells needs to be verified.” What do we have at present? We have:
- Louis Pasteur’s denouncement of spontaneous generation.
- Urey and Miller’s experiments which showed that organic compounds could be derived from inorganic chemicals.
- The endosymbiosis theory of the evolution of eukaryotic cells.
In truth this ‘evidence’ does not amount to much. Nevertheless we make a big deal of the statements that cells are derived from other pre-existent cells and so all cellular life must have developed from the first cells.
THE SYLLABUS 1.5 Origin of cells
How did life get going on planet Earth? Are the conditions necessary for life in existence elsewhere in the solar system? Are archaea the first forms of life? How did eukaryotic plant and animal cells originate, containing organelles with special functions? Some of these questions are well answered by the University of Utah Genetic Science Learning Center – always a good source of information about anything to do with cells, DNA, genetics and evolution. (Follow this link for some specific answers to the questions above, but also have a good look around the web site.)
A link to a brief but specific summary of how early cells probably evolved: http://learn.genetics.utah.edu/content/cells/organelles/
University of California, Berkely summary:
THE ENDOSYMBIOTIC THEORY
The ENDOSYMBIOTIC THEORY for the evolution of modern-day eukaryotic cells proposes that:
- Early, simple prokaryotic cells, containing DNA and ribosomes, became heavily invaginated (perhaps to increase their surface area), until eventually these membrane invaginations became recognisable as membranes around the DNA – nuclear membrane – and as endoplasimic reticulum, where some of the ribosomes were located.
- These early cells essentially could be called eukaryotic cells but without organelles such as mitochondria and chloroplasts.
- Specialised, aerobic prokaryotic cells, exploiting the increasing amounts of oxygen in the atmosphere some 2.5 bya to metabolise ATP, entered the invaginations in these early eukaryotes, and the two eventually became symbiotic systems, until neither the aerobic prokaryotic cells nor the host eukaryotic cells could survive without the other.
- The aerobic prokaryotic cells could now be identified as mitochondria inside the eukaryotic cells.
- Some of these symbiotic systems evolved to become animal cells as we now know them.
- Others of these symbiotic systems went on to host more prokaryotic cells of a different type – cyanobacteria, which are photosynthesisers – and these new symbiotic systems evolved into what we now know as plant cells, containing chloroplasts.
It is all so very plausible. A good theory! What is the evidence? How are mitochondria, chloroplasts and prokaryotic cells similar? The evidence is good!
- They all have their own DNA, which is circular and ‘naked’ (contains no protein).
- Prokaryotic ribosomes (70S) are similar to mitochondrial and chloroplast ribosomes.
- All – prokaryotic cells, mitochondria and chloroplasts – are roughly the same size.
- Mitochondria and chloroplasts are, like bacteria, susceptible to the antibiotic chloramphenicol.
- All transcribe their DNA and use mRNA to synthesis proteins.
- All can only result from binary fission (division) of pre-existing mitochondria or chloroplasts (or prokaryotic cells).
- Mitochondria and chloroplasts contains enzymes and transport systems which are similar and homologous to those in today’s prokaryotic cells.
LOUIS PASTEUR’S EXPERIMENTS
Louis Pasteur refuted the theory of spontaneous generation in a classic series of experiments:
Urey & Miller’s experiments in 1953
So … since cellular life began, a-sexual cell division has continued.Before a cell can divide into two, the genetic material (DNA) must be replicated.
- All cells start out their life as relatively small and undeveloped, but with a full complement of DNA typical of their species (pairs of homologous chromosomes in eukaryotic cells and a ring of DNA in prokaryotic cells).
- The cells then grow – get bigger and become more complex, with more organelles.
- Now there are sufficient resources, the DNA can be replicated. Each chromosome then consists of chromatids, remaining joined together.
- The cell continues its routine life and prepares for cell division.
- Finally mitosis can occur and the two new ‘daughter’ cells can start the same cycle.
Described above and in diagrams below, is what is known as the CELL CYCLE
The CELL CYCLE is split into the 3 phases of INTERPHASE, followed by MITOSIS.
Cells exploit cell division for:
- REPAIR & REPLACEMENT of damaged cells
- A-SEXUAL CELL REPRODUCTION of an entire organism
CHECK POINTS & CONTROL OF THE CELL CYCLE – CYCLINS
Of course cells do not race through the cell cycle and divide as soon as they are able. There are controls exerted on the cell cycle, so that cells divide at appropriate times in the life of cells. Cells may pass ages in G1 or G2, before at last dividing. These controls or check-points were not well understood until 1982, when a biochemist, Tim Hunt, and his team accidentally stumbled upon cyclins.
The new IB syllabus highlights this discovery as something serendipitous. Scientists are encouraged to follow a formal process when making investigations – what is known as THE SCIENTIFIC METHOD. The method involves careful planning and hypotheses, repetitions and the collection of huge amounts of data, stringent treatment of the results and subsequent analysis, justifiable conclusions and a final evaluation. The intention is that research can be repeated in order to arrive at the same conclusions. all well and good but often very, very tedious.
Tim Hunt was working with the eggs of sea urchins on something entirely different. he was working very rigorously but accidentally he discovered several new proteins, including cyclins. (He named them cyclins because he was ‘into’ cycling at the time!) In other words he was not following any scientific methodology in the context of his discovery. Pure accident! That is how science (and life as a whole!) often works itself out!
Cyclins are enzyme inhibitors which work in the G1/S phase of the cell cycle and also in metaphase of mitosis.
What happens when the controls or check-points in the cell cycle do not work? Often the result is cancer. A cancerous tumour is formed by rapidly dividing cells and can occur in any tissue or organ.
There are many so-called carcinogens or cancer-inducing risk factors – mutagens. Some are very well known, for instance UV light and some of the contents of tobacco smoke. There is also possibly a genetic susceptibility to cancer. Common forms of cancer are breast, prostate, lung and skin cancer.
In healthy tissue, if there is uncontrolled cell growth, the body itself will perform APOPTOSIS and destroy the resultant cells. Sometimes apoptosis fails and this seems to have something to do with the expression of ONCOGENES, so the tumour continues to grow.
This primary cancerous tumour might be removable or treatable but if it disintegrates, especially if it is growing inwards in the tissue, towards the capillaries and blood supply in a process known as METASTASIS, then tumours, now known as secondary tumours, may grow elsewhere in the body. These are difficult to treat and control.
THE CORRELATION BETWEEN SMOKING AND INCIDENCE OF LUNG CANCER
The end of interphase is marked by the onset of MITOSIS, before the cell cycle will be completed by cytokinesis and the appearance of two new cells. Here is what you need to know:
Notice the difference between mitosis in plant and animal cells:
- centrioles are only present in animal cells (although the spindle fibres are found in plants and animal cells)
- cytokinesis (after the mitotic division of chromosomes):
- Plants – a cell plate (the intiation of a cell wall) grows down between the two new cells
- Animals – a cleavage plate separates the two new cells. This is a constriction around the middle of the original, parent cell.
MICROSCOPIC OBSERVATION OF THE PHASES OF MITOSIS & THE MITOTIC INDEX
It is neat to do a root tip squash preparation in onion or garlic roots. They have a small number (8) of large chromosomes which are visible during mitosis. The cells must be carefully teased apart and the DNA stained up. It is fun but tricky to accomplish successfully so many labs do not bother! Here is what a preparation might look like:
By examining such a slide preparation (or from a commercially prepared slide), and counting the cells in each phase, it is possible to work out the MITOTIC INDEX, which is an indication of the time spent in interphase or in each of the stages of mitosis. Here are two typical pie charts:
So what? Well, the mitotic index is a fundamental tool for cancer specialists in their evaluation of the state of tumour growth.
PRACTICE QUESTIONS (from Biology in Context for Cambridge International A Level, Glenn and Susan Toole)