How and why do cells divide? The cell cycle!

You started off as one cell: one tiny little zygote containing a full set of DNA (23 pairs of chromosomes).  As an adult human being, you are now made up of over 37 trillion cells. This means that that one cell  divided to make two cells, each of those cells divided to make 4 cells, those 4 cells divided to make 8 cells and on and on until the 37 trillion cells that make up you today. Even now, your body makes around 60 billion cells each day to create new skin cells, intestine cells, hair cells and and nail cells. When you cut yourself, the body needs to make new cells to heal.  And if your cells divide out of control, this can cause cancer and if they stop diving this causes of aging. So understanding how cells divide is super important!

The cell cycle, which is the process of one cell and one set of DNA turing into two cells with two sets of DNA.  There are three main parts of the cell cycle:

1.  To make two cells from one, you can imagine that a few important things need to happen.  First, you need the cell to grow to get bigger and to accumulate enough nutrients to support two cells.  Second, you need to replicate the DNA so that when the cell divides, each “daughter” cell gets one copy of the DNA. These two things happen in the interphase part of the cell cycle.  Interphase is separated into 3 parts

  • Gap 1 (usually just called G1 phase) where the cell grows
  • Synthesis (usually just called S phase) where the DNA is copied so that two complete copies of DNA are now in the cell
  • Gap 2 (usually just called G2) where the cell grows some more


The chromosomes (shown in blue) condense and line up before being pulled into two cells by microtubules (shown in green)
By Roy van Heesbeen (Roy) [Public domain], via Wikimedia Commons 

2. Once the cell has copied the DNA and grown big enough to split into two cells, the cell undergoes mitosis.  Mitosis is when the copied chromosomes are separated into two different cells.  Remember that if you took all the DNA in a cell and stretched it out from end to end that it would be 6-10 feet long? Since this DNA is already replicated by the time the cell gets to mitosis, there are 92 chromosomes (two copies of the two pairs of 23 chromosomes) and 12-20 feet of DNA that needs to be organized and sorted into two separate cells.  How does the cell make this nearly impossible sounding task happen?  First, when each chromosome makes a copy of itself, it stays connected to the orignal (kind of like if there were little protein magnets holding them together). mitosis Second, when the chromosomes are ready to separate into different cells they “condense”, getting much, much smaller (see the blue DNA in the photo above).  Third, there are mechanisms in the cell that make the chromosomes line up.  So what you end up with are all of the chromosomes in tight little bundles lined up in a row.  At that point, the cell creates “ropes” out of a protein called microtubules that pull the copied chromosomes apart into the two separate cells.

cytokinesis3.  Finally, now that the DNA is separated into the two new cells, these cells have to officially split into two in a process called cytokinesis.  You can imagine this is like pulling a drawstring closed to pinch the space between the two cells until they have completely split apart.

If this sounds like a complicated process, you’re right.  It is.  But it happens flawlessly 10,000 trillion times in a lifetime.  Part of the reason why this is a flawless process is because the cell puts checkpoints into the process.  It’s like when your bank calls you because they observed a strange transaction on your credit card and they put your card on hold.  If the cell sees something strange happening when the cell is trying to undergo cell division, it puts a hold on the whole process until it gets fixed.  We’ll discuss this a lot more in the future because when the cell cycle isn’t running flawlessly and these checkpoints aren’t working, this contributes to causing cancer and other diseases.

4 thoughts on “How and why do cells divide? The cell cycle!

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