# The Cell Cycle: Definition, Phases & Sequence

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• 1:06 Cell Cycle
• 1:30 Interphase
• 6:30 Cell Division
• 8:58 Summary

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Lesson Transcript
Instructor: Greg Chin
Learn about the dividing and non-dividing states of the cell and discover the different phases of the cell cycle, including interphase, cytokinesis, and the stages of cell division.

## Defining the Cell Cycle

Alright, it's almost time for us retail employees to be decorating for Christmas, but gosh darn it, I got that biology midterm hanging over my head. I wonder if these Christmas tree decorating job can help me study for the cell cycle.

Let's see what Christmas tree decorating can tell us about the cell cycle. Say we have some Christmas tree displays that we have to decorate, but unfortunately, we only have one box of ornaments. We want to decorate both of these trees identically. Luckily though, we are in the future, and we're going to use a replicator device to make copies of the ornaments. By using this replicator device, we'll be able to decorate both of the trees. This idea about making copies of things is a central idea in the cell cycle.

The cell cycle is basically all the events that can occur during the lifetime of a cell. The cell can be thought of as being in one of two states; it can be not dividing or dividing. Let's layer on top of that a few terms. Whenever the cell isn't dividing, that's referred to as interphase, and cell division is just cell division.

There are three different phases that interphase can be divided into. You can have what's called Gap 1, which is also abbreviated as G1. You can have what's called S phase, and that S just stands for synthesis - we'll talk a little bit about what's getting synthesized in a minute. And then finally, interphase also involves Gap 2 or G2.

By the same token, cell division can also be further subdivided. You can have M phase, and during M phase mitosis occurs. Mitosis is then followed by cytokinesis.

## Phases of the Cell Cycle

Let's look a little more closely at the cell cycle phases, and let's start out with G1. G1 is kind of the everyday life of the cell. If we think about it in terms of our Christmas tree decorating, then normally the employee for the normal part of the year is doing things like stocking the shelves, selling items and that sort of thing. The same thing can be said for the G1 phase. Normal things are happening. Cell growth is happening, and protein synthesis is happening. But toward the end of G1 phase, what's going to be happening is the cell is going to be preparing for the S phase. Preparations need to be made. Just as with Christmas time approaching, the store employee needs to start preparing for the decorating that's going to occur.

We're going to need to decorate those two trees, and we're going to use the replicator to make copies of all those ornaments for the trees. By the same token, the cell wants to make copies of its chromosomes in a process known as replication. This part of the the cell cycle is called S phase because the cell is going to synthesize new chromosomes.

As the store clerk tries to make duplicate displays, we want both of the displays at each end of the store to look the same. Unfortunately, since we're only starting with one box of ornaments, we have to use this replicator to accomplish that. Now, we're talking about interphase as the non-dividing part of the cell cycle. But we can basically think of interphase, at least in one sense, as being preparation for cell division.

If we have our one cell, as a multicellular organism, and we need to make two cells, we want those two cells to be duplicate copies of one another. As part of that process, we need to make duplicate copies of our chromosomes. Basically the, point of replication in S phase, that event in the cell cycle, is to make duplicate chromosomes. As the cell is making copies of the chromosomes, it's synthesizing what are called chromatids. A chromatid is just one of the two copies of the chromosome. So, we have the chromosome here, and once replication occurs, we have two copies, and we refer to these copies as chromatids. So, we have two copies of the chromosome (i.e. two chromatids).

Now that the cell has made copies of its chromosomes, it's almost ready for cell division. Just as we've made copies of the ornaments, we're almost ready to decorate the tree. Let's say our trees are pretty tall - they're like those super big trees you see out in front of a department store - and you're going to need a ladder to put all those ornaments on, especially the star that's going to go at the top of the Christmas tree. We're going to have to trudge on down to the basement to be able to get the ladder to be able to complete the christmas tree set-up.

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