In this section of the AP Biology curriculum, we start to look at the different parts of the cell cycle. Specifically, we’ll see how the cell cycle is broken down into two main parts – interphase and cell division. We’ll start by examining the 3 different parts of interphase – G₁ phase, S phase, and G₂ phase. Plus, we’ll see how some cells can temporarily or permanently exit the cell cycle via quiescence or the G₀ phase. After this, we’ll look at the 3 different types of cell division including binary fission, mitosis, and meiosis. We’ll learn when the different types are used and for what purpose. Finally, we’ll dive into the cell cycle checkpoints that regulate the cell cycle, and we’ll take a specific look at the stages of mitosis and the actions they accomplish for a dividing cell!

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Resources for this Standard

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• Crossword Puzzle

• PowerPoint

ENDURING UNDERSTANDING
IST-1
Heritable information provides for the continuity of life.

LEARNING OBJECTIVE
IST-1.B
Describe the events that occur in the cell cycle.

IST-1.C
Explain how mitosis results in the transmission of chromosomes from one generation to the next.

ESSENTIAL KNOWLEDGE
IST-1.B.1
In eukaryotes, cells divide and transmit genetic information via two highly regulated processes.
IST-1.B.2
The cell cycle is a highly regulated series of events for the growth and reproduction of cells–

1. The cell cycle consists of sequential stages of interphase (G1, S, G2), mitosis, and cytokinesis.
2. A cell can enter a stage (G0) where it no longer divides, but it can reenter the cell cycle in response to appropriate cues. Nondividing cells may exit the cell cycle or be held at a particular stage in the cell cycle.

IST-1.C.1
Mitosis is a process that ensures the transfer of a complete genome from a parent cell to two genetically identical daughter cells–

1. Mitosis plays a role in growth, tissue refrain, and asexual reproduction.
2. Mitosis alternates with interphase in the cell cycle.
3. Mitosis occurs in a sequential series of steps (prophase, metaphase, anaphase, telophase.)

4.6 Cell Cycle Overview

What do a baby and the mold on a tangerine have in common? Cells in both of these organisms are rapidly going through the cell cycle, undergoing mitosis, and producing new cells in order to grow and survive. Regardless of the fact that a human baby and the mold on a tangerine are part of completely different taxonomic kingdoms, they follow the exact same cell cycle.

Cells in both of these organisms contain DNA, which houses the heritable information that provides for the continuity of life. Cells in all domains of life follow the same basic cell cycle. But, do you know the phases of this cycle and the stages of mitosis that allow cells to undergo cell division? This information will definitely be on the AP Test. So, stick with us as we cover everything you need to know about the cell cycle!

Let’s start with an overview of the cell cycle, and see where mitosis fits into the process. The first thing you should know about this topic is that there is no universally accepted name for the different parts of the cell cycle or the different parts of mitosis. Some people call them stages, while others call them phases. The terms are interchangeable.

Let’s take a look at a broad overview of what the cell cycle is. Essentially, every cell must grow, replicate its DNA, and divide into two new cells. This cell cycle is broken into two parts – interphase and cell division (specifically mitosis in eukaryotes). Both mitosis and interphase are broken into smaller sections. A cell that has just divided starts into the G₁ phase. During this phase, the cell mainly metabolizes energy, creates new proteins and lipids, and grows in size. After the cell has reached a certain size and has amassed enough nutrients, the cell will enter the S or synthesis phase. During this phase, a new strand of DNA is synthesized via the enzyme DNA polymerase and the process of DNA replication. After S phase, the cell enters the G₂ phase. During this phase, the cell continues growing, replicates important organelles and cellular components in preparation for cellular division.

After this, the cell suspends important metabolic processes and enters the complex process of mitosis. We will go over the specifics of mitosis in a few slides. For now, all you need to know is that the cell separates DNA molecules into two new daughter cells, and the process starts over.

Some cells take a detour from the normal cell cycle and enter the G₀ phase, also known as quiescence. In this state, a cell is not working to grow or divide, it is simply functioning. A good example of a cell in the G₀ phase is a nerve cell, which can function for an organism’s entire lifetime without reentering the cell cycle.

Think about this… Have you ever gotten a bad sunburn that resulted in dead layers of skin peeling off a few weeks later? Well, the sun really just sped up a process that is naturally occurring all the time. Scientists estimate that our bodies replace almost every single cell over the course of about 10 years – with some cells being replaced much more often than this. So, if you ever want to be a new person, just wait a decade – you will be!

There are three types of cell division, depending on the type of organism that is dividing. Binary fission is the process used by most bacteria and single-celled organisms. Since their DNA consists of a single, circular chromosome, there is no need for complex processes to sort various chromosomes into the appropriate cells. The overall process starts with DNA replication in S phase of the cell cycle. Then, the cell enters the binary fission stage and simply separates each strand of circular DNA into one of the two new daughter cells.

By contrast, the process of mitosis is much more complex. The process of mitosis has many more steps for several reasons. First off, eukaryotes typically have more than 1 chromosome. As these chromosomes duplicate, this means there are twice as many chromosomes the cell must properly separate in order to divide into two functional cells. This is why sister chromatids get bound at the centromere during prophase and into metaphase. This ensures that each new cell will receive 1 copy of each chromosome. Besides this, eukaryotic cells also have a nuclear envelope and other organelles to deal with, adding to the complexity of mitosis.

Mitosis is the main process used by single-celled eukaryotes to carry out asexual reproduction. However, it is also responsible for building and maintaining the bodies of multicellular organisms. Some multicellular organisms that can reproduce asexually (like jellyfish) use the process of mitosis to do so.  We’ll take a closer look at the stages of mitosis in a bit.

The final form of cell division is meiosis. Organisms that reproduce sexually need to reduce the amount of DNA in each gamete cell (such as a sperm or egg). Otherwise, the genome would double in size every time two cells merged. We’ll cover this complex form of cell division in section 5.1.

The cell cycle does not just happen by coincidence. It is a highly regulated process with a number of checkpoints that ensure things are proceeding as they are supposed to. We will cover exactly how these cell cycle checkpoints work in the next section. But, for now, let’s take a look at the three most important checkpoints in the cell cycle.

The G₁ checkpoint happens right before the cell enters the S phase and replicates the DNA. During this checkpoint, the cell checks for several important things. The cell checks that it is an appropriate size to divide, that it has enough nutrients to supply both daughter cells with sufficient energy to get started, and it checks the DNA to ensure that there is no DNA damage. If any of these conditions are not met, or if the cell receives a signal to go into quiescence, it will enter the non-dividing G₀ phase.

The next checkpoint, the G₂ checkpoint, comes near the end of the DNA phase. During this checkpoint, the cell ensures that DNA replication has been completed and that the DNA has not become damaged. If the cell passes this checkpoint, it can proceed into mitosis. If there has been DNA damage, the cell will enter the process of apoptosis (a.k.a. cell death) to ensure that the genetic alterations are not passed on. If this process fails, this usually means that the genes controlling the checkpoint signal transduction pathways are damaged. This is sometimes what leads to cancer cells dividing out of control!

The final checkpoint occurs during metaphase of mitosis. Sometimes called the M checkpoint and sometimes called the Spindle Checkpoint, this checkpoint takes place as the chromosomes line up on the metaphase plate. Essentially, this checkpoint ensures that the chromosomes are going to be evenly divided so both new cells have a full genetic code!

We’ll see exactly how these checkpoints take place and the regulation of the cell cycle in section 4.7.

Let’s look at the specific stages of mitosis and what happens in each stage. As the cell enters the process of mitosis from interphase, two features are discernable: the chromosomes have been duplicated and there are now 2 centromeres that will organize the spindle fibers needed to separate the chromosomes.

This brings the cell to the start of mitosis, a stage known as prophase. During prophase, two important events take place. First, the centrosomes migrate to opposite poles within the cell. Then, the nuclear envelope breaks down and spindle fibers begin to seek out the centrosomes of each chromosome to pull them apart. As the spindle fibers push and pull the chromosomes from each pole, the cell enters metaphase. During metaphase, the chromosomes line up on the metaphase plate, where the sister chromatids of each chromosome begin to separate.

As the cell enters anaphase, each sister chromatid fully separates into an individual chromosome, and the chromosomes are pulled to either pole of the cell. After anaphase comes telophase. During telophase, the nuclear envelope reforms around each new nucleus, and a cleavage furrow forms a band around the cell and starts to pinch the cell into two. The final process, cytokinesis, is simply the full separation of the two new daughter cells. Each of these new cells will restart at interphase, and the cell cycle will start again.