If someone gave me a penny every time one of my cells died, I'd be a millionaire in less than 30
seconds. That's a lot of money! But that's also a lot of dead cells. Now, I know what you're
thinking: At that rate, shouldn't most of my cells have died by now? And the answer is, yes! So,
how am I still here? Luckily for us, our body has a way of replacing our hard-working cells
so we can live much longer on average than they do. After today's lesson you'll be able
to define and identify the steps of the cell cycle, explain how the cell cycle is
important to living things, and describe a few of its limitations. Let's get into it!
The cell cycle is a process by which cells grow and divide to create new cells,
typically with the goal of replacing cells that are worn out or damaged.
Think about a time when you've bruised or scraped your skin. The injury is the result of damaged
cells, and the cell cycle makes copies of healthy cells to replace the damaged ones. It is called a
cycle because this process is constantly repeating itself to keep you healthy. Cells are born from
the division of other cells, and then eventually, those cells divide and create more new cells.
The cell cycle has four distinct phases, and the first we will discuss is G1. G1,
or the first gap phase, is the phase of the cell cycle where cells grow and prepare to divide.
If cells just divided in half without growing, we'd end up with tiny and ineffective cells.
Because of this, cells in G1 are essentially "eating for two" as they prepare for division,
building a whole new set of organelles, or working parts, and doubling their cytoplasm.
This can take up to 10 hours, making G1 the most time-consuming phase of the cell cycle.
Next, the cell enters the S phase. In this phase, cells synthesize, or create, an identical copy of
their DNA. Remember, DNA is the genetic blueprint of our cells, so S is essentially making a copy
of this blueprint so both future cells will have instructions that tell them what to do.
From here, we move into G2, or gap 2, which is the phase of the cell cycle
where cells continue to grow and make final preparations to divide.
Once the cell has decided that everything is just right for division, the cell enters
the final stage: M, or mitosis. Mitosis is the phase of the cell cycle where a cell actively
divides into two identical cells. In this process, the nuclear envelope,
or protective barrier surrounding the nucleus, dissolves, and the cell's two identical sets
of DNA are lined up in the middle of the cell by specialized organelles called centrosomes.
The centrosomes then pull the two sets of DNA to opposite sides of the cell.
Once there is an identical set of DNA on each side, two nuclear envelopes form around them,
creating two identical nuclei. Finally, in a process called cytokinesis,
the cell pinches together in the middle to separate into two individual cells.
All of this typically happens in roughly two hours. From start to finish,
most cells in your body complete the entire cell cycle in about 24 hours.
Okay, so in just one day, we've made two new cells out of an existing one. Now what? Well, it
depends. If you still need more cells to replace old ones or if you're just a growing teenager,
these two new cells might immediately enter the cell cycle all over again to fill that need.
However, if division is no longer required, these new cells will exit the cell cycle and
hang out in a phase called G0. G0 is a state where cells perform their normal functions
without preparing to divide. From here, they can easily enter the cell cycle again when needed.
Sometimes, however, cells miss their cue to exit the cell cycle. This leads to a very serious
disease called cancer, which is uncontrolled cell growth. When this occurs in solid tissue,
like organs or bones, tumors form, which are masses of rapidly dividing cells that can
impede normal function in the body. Studying the cell cycle has helped scientists understand more
about this disease, which has led to a broader range of treatment options for cancer patients.
Now, you might be thinking, serious disease or accidents aside, if our
cells can continuously replace themselves, shouldn't we live
forever? We can replace all of our older damaged parts with brand-new ones, right?
Well, sort of. Whenever our cells copy our DNA, they don't always copy this information perfectly.
As a result, errors in our DNA occur over time, compounding as we get older.
Our skin starts to wrinkle, our muscles lose their strength, and our organs become less effective at
their respective jobs. These are all unfortunate side effects of relying on the cell cycle for
a lifetime. Of course, without it, we wouldn't live long enough to age much in the first place.
Alright, we have now covered the basics of the cell cycle, which is a process of cellular growth
and division. It has four main phases, including G1, S, G2, and mitosis. Without the cell cycle,
our bodies wouldn't be able to heal or perform routine maintenance. There are, however,
some limitations. These include cancer and aging, which are caused by imperfections in the process.
And that, fellow biologists, concludes our unit over cellular processes. In our next unit, we'll
explore the field of genetics as we examine the structure and expression of DNA. Until then, I'm
Caroline, and remember, life is full of wonders, so keep learning, and don't ever stop wondering!