Figure 25 is a diagram of the cell cycle. Try to imagine the cell cycle
as a race track and individual cells as cars that race around it. You
are sitting at the finish wire, which is mitosis (we chose mitosis
because it is easy to recognize when the cell is observed with the aid
of a microscope). At a certain time during the race, all the cars in a
portion of the track, say a 200-yard sector of the backstretch, are
sprayed with a blue dye as they race by. These cars are now marked, just
as cells synthesizing DNA are marked if briefly exposed to tritiated
thymidine, the common radioactive precursor of DNA. As soon as these
cars have been sprayed, you observe all the cars as they pass the finish
line in front of you. At first, you will see cars that were nearest the
wire and were not sprayed; then the dye-marked cars will pass; and
finally more unmarked cars, those that had passed the finish line but
had not reached the spray area when the marking was done, will come by.
If you replace the words spray, cars, and wire with the words
radioactivity, cells, and mitosis, you have described the cell cycle and
the flow of cells in the cycle.
Now, if all cars were going at the same speed, you could calculate with
great accuracy the time taken for any one car to go around the track, or
from the finish line to the backstretch, or through the spray sector,
and so on. However, since cars move at different speeds, you can only
obtain an average time for all sprayed cars. Similarly, since individual
cells behave differently, you can only obtain averages of the times
these cells spend in the various portions of the cell cycle.
[Illustration: Figure 25
THE CELL CYCLE]
These cell-cycle portions are four in number, according to nomenclature
originated by A. Howard and S. R. Pelc, two English investigators who
first described the cycle: (1) mitosis; (2) G₁, which is the period
between mitosis and DNA synthesis; (3) S phase, which is the period
during which DNA is replicated; and (4) G₂, which is the period between
DNA synthesis and the next mitosis. Only cells in the S phase (DNA
synthesis) are marked when exposed to a radioactive precursor of DNA.