Draft:History of the Cell Cycle

The discovery of cells and the cell cycle is critical to our modern understanding of living systems, and the fields of biology, biochemistry, biophysics, and medicine, as well as many others. While Robert Hooke initially discovered cells in 1665, research in the cell cycle has made most of its notable progress in the last two centuries. This includes the first descriptions of mitotic progression, the description of the entire cell cycle including the different parts of interphase, a new molecular understanding of the drivers of the cell cycle, and the discovery of the essential checkpoints.

Discovery of the Cell Cycle and Its Phases

One of Walther Flemming's drawings of mitosis from Kern und Zelltheilung, 1882

Robert Hooke in 1665 coined the term cell to describe the small walled structures he saw within a cork sample. This led to Schleiden and Schwann proposing cell theory that stated all living beings are made of cells and that cells formed de novo by some sort of crystallization, also known as free-cell formation . This was widely accepted for almost three decades until the 1870s where Rudolf Virchow and others showed that cells divide to create new cells . Walther Flemming then began his work which would end up describing in great detail the process of mitosis and mark the beginning of the cell cycle as we know it today. He developed his own methods for staining chromosomes and observed them in cells, documenting their shape and size changes as nuclear division occurred . This led to the two phase cell cycle, mitosis where the nucleus is split, and interphase which encompasses the rest of the time where it appeared to Flemming that the cell was relatively inactive but grew in size.

Research into interphase was not seriously undertaken until the mid 1900s when Alma Howard and Stephen R. Pelc observed the incorporation of of radiolabeled phosphorus into dividing bean cells. They observed that it occurred only in dividing cells and only during interphase. From this, they then observed that there was 3 distinct parts of interphase, G1 (Gap1), S (for DNA synthesis), and G2 (Gap2). In 1954, Laszlo Lajtha showed bone marrow cells obeyed this 4 stage cycle, and also proposed a G0 (Gap0) phase for cells which exit this cycle and enter a resting phase. With this final phase described, the cell cycle as we know today was complete.

Cell cycle diagram showing both the two phase cycle on the outer edge (I=interphase, M=metaphase) and the 4 phase on the inner ring (G1=Gap1, S=Synthesis, G2=Gap2, M=mitosis) as well as the rest phase (G0)

Basic Mechanisms of the Cell Cycle

In the 1970s, research into how the cells transitioned between these phases began. Many different fields were working on this simultaneously. In cell biology, Rao and Johnson’s fusion experiments gave early evidence for the idea that there was some factor which triggers transitions between phases. They showed that the fusion of a cell in S phase with a cell in G1 will produce an S phase cell, whereas a fusion of an S phase cell and a G2 cell will produce a G2/M phase cell. These imply that there is something which causes a cell to go from G1 to S and another to go from S to G2. They also performed a fusion experiment of an M (mitosis) phase cell and a G1 phase cell which surprisingly led to early progression into mitosis, showing that there must be a third factor that causes cells to enter M phase.

In the field of zoology, Yoshio Masui’s lab supported these finding by observing in frog oocytes (Xenopus Laevi) that adding progesterone to prophase arrested oocytes triggered their maturation to metaphase arrest. This, along with their observation that the addition of cytoplasm from a metaphase arrested egg into a prophase arrested oocyte can produce the same maturation in the oocyte, led them to propose the presence of a factor called MPF (Maturation Promoting Factor) in 1971.

At the same time, genetic work in yeast was being done to identify key genes related to the cell cycle. Lee Hartwell took to identifying thermosensitive cell division cycle (cdc) mutants of budding yeast (S. cerevisiae). These then could be categorized and sequenced to identify the mutated genes. Similarly, Paul Nurse worked with fission yeast (S. pombe) to also identify and sequence cdc mutants. These experiments were able to be performed, as the number of buds or the size of the yeast is directly related to the phase of the cell cycle they were in. From these two different systems, two different cdc mutants were identified to be deficient in cell cycle progression, cdc28 in budding yeast and cdc2 in fission yeast. Paul Nurse discovered each of these mutants could be rescued with the other species gene library, leading to the hypothesis that the mechanism to drive the cell cycle is highly conserved and that cdc2/cdc28 could code for that mechanism .

Diagram showing how cyclin and CDK interact. CDK is phosphorylated by cyclin and forms a new complex which is then active.

After the proposal of MPF, the next challenge was to try and purify this molecule. The successful purification of MPF actually produced two different proteins, one 34kDa and one 45kDa . The former turned out to be the cdc2 equivalent in Xenopus Laevi, however the identity of the other protein remained unknown. While this work was ongoing, Tim Hunt was working on protein synthesis and degradation during the cell cycle, and had identified a cycling protein he named cyclin. Cyclin was synthesized before each cell division and then completely degraded after the cells exited mitosis. Once this was identified, it was quickly found that it associates closely with cdc2, and thus the 45kDa protein was identified. This completed the picture of how the cell cycle is driven, the cdc2-type kinase is present throughout the cell cycle while the cyclin levels fluctuate, this allows the cdc2 kinase to activate and deactivate when needed to transition between phases of the cell cycle. Since then, there have been other cdc kinases and cyclins found in other organisms and some organisms have multiple versions of each. The cdc kinases were then renamed CDKs for cyclin-dependent kinases.

Cell Cycle Checkpoints

Now that the logic of the cell cycle and some of the major factors in driving it had been identified, how these transitions were regulated was the main focus of research. The idea of checkpoints and that each phase is dependent on the previous one being sufficiently completed was proposed by Lee Hartwell in the 1970s. This led to the discovery of the 4 main checkpoints in the cell cycle. The G1/S transition was described first by Howard Temin in the early 1971 in chicken embryos and then further elucidated by Arthur Pardee in 1974. It ensures proper that the growth has occurred and that there are sufficient nutrients present. The G2 or DNA damage checkpoint ensures there is no damage to the DNA after replication, and was described by Lee Hartwell in 1989. He identified rad9 as one of the essential genes in the arrest to repair DNA damage. The last two checkpoints both occur in M phase. The first is the mitotic checkpoint, and requires that the spindle is formed properly. This was discovered in the 1970s by Raymond Zrikle. The last checkpoint was discovered in 2006 by Yves Barral and is the cytokinetic checkpoint and determines whether the cell divides.

In 2001, the Nobel Prize in Physiology or Medicine was awarded to Lee Hartwell, Paul Nurse, and Tim Hunt for their discoveries and work with checkpoints, CDKs, and cyclins respectively.