The Cell Cycle for Dummies
Did you know that the body contains 37.2 trillion cells?
I know what came to my mind when I discovered this crazy fact. In other words, I was absolutely mind-blown that our body contains such a gigantic number of individual cells. It’s truly a wonder to consider how these cells are able to work in cooperation and perform the necessary functions that allow for us to survive. Regardless of type, each cell is critical for human function due to different functionalities, allowing for varied capabilities.
To truly understand the importance of a cell, let’s take a look at its cycle.
The cell cycle is split into four critical sections, each of which allows for a cell to develop itself to perform its necessary functions. These “phases” are G1, S, G2, and M. While G1, S, and G2 are part of a larger subset called interphase, the M stage is entirely separate and thus serves a different purpose for the cell.
The Phases
The G1 Phase: Being one of the initial stages for a cell’s growth, the G1 phase (also called the “Gap 1” phase) is characterized by a cell preparing for DNA replication. Firstly, the cell will grow in size to provide ample space for organelles and to perform DNA synthesis (the S phase) later on. In addition, proteins and RNA will be synthesized within the cell (for it to function) and additional functions such as the development and replication of centromeres will be performed.
Sounds cool, right? Wait ‘till you see the next phase!
The S Phase: During the S phase, DNA replication will be performed within the cell. The primary purpose for this is that when cells divide later on (during mitosis), each newly formed cell will have an identical copy of the same DNA, preventing any problems.
DNA Replication
DNA replication can be split into four major steps:
1.) Replication Fork: To replicate DNA, its natural “double helix” shape will be split into single strands. If you remember from your high school biology teacher (don’t worry if you’re not in high school yet, I’ll help you with this article), this process was commonly called “unzipping” and is done by an enzyme called helicase. As shown in the picture above, helicase will not unzip the entire DNA complex at once, and only does sections at a time; this is why the resulting figure is called the replication fork.
An important thing to note is the direction(s) of the DNA complex during replication. At this point DNA in the replication fork is bi-directional, meaning its two strands will run in opposite directions. While the strand going from 3' to 5' is called the leading strand, the 5' to 3' section is called the lagging strand.
2.) Primase for Replication: From steps 2–4, the actual DNA replication will begin (step 1 is purely for preparation purposes). For the actual replication to begin, there needs to be an indication on the DNA strands of where to begin. Primers (which is made by DNA primase) helps by attaching a small strand of RNA to the 3' ends of each strand, which will kickstart DNA polymerase’s job of replicating each strand.
3.) Elongation: At this point, the DNA complex is split and contains a leading (3' to 5') and a lagging (5' to 3') strand. Replicating the leading strand is extremely easy and is done with DNA polymerase, which will replicate each strand by adding complementary base pairs (A-G and C-T) to the strand. At this point, you might be thinking: if DNA polymerase only replicates from 3' to 5', how does it replicate the lagging strand? That’s the perfect question to ask, because DNA polymerase has a little trick up its sleeve.
Since DNA polymerase only replicates in the 3' to 5' direction, it jumps forward on the lagging strand and will replicate the DNA backwards, so that primase is technically operating in the correct direction. With this peculiar replication method, primase will end up creating these fragments of replicated DNA at a time, which are called Okazaki fragments. Since this process takes a much longer time to complete, the 5' to 3' strand was coined as the “lagging strand.”
4.) Termination: This is the final step for DNA replication, and a lot is going down during this stage. Firstly, the RNA snippets that primase initially created (see step 2) will be removed by exonuclease and are replaced with the correct complementary base pairs.
Remember the Okazaki fragments from replicating the DNA of the lagging strand? Right now, the replicated lagging strand is currently broken into several fragments, which need to be put together. Luckily, DNA ligase solves this conundrum by gluing the fragments together and piecing the replicated lagging strand together.
Finally, the “parent” strands and their complementary replications will intertwine into that double helix shape, thus resulting in two separate strands of DNA.
I know that was a lot of information, but understanding DNA replication is super important (and can be fascinating to learn about)! Let’s take a look at the next phase.
The G2 Phase: Think of the G2 phase as a student checking their work before handing in a math test. Being the last section of interphase, the G2 phase is a cell’s final opportunity to check its inner workings and ensure that its ready to move forward in the cell cycle. During this phase, the cell will produce any additional material and grow a little more in order to fully prepare for the following stage: mitosis.
The Basics of Mitosis
We’ve reached the final stage of the cell cycle! Mitosis is a form of cell division and allows for the cell developed during interphase to become two daughter cells. This process is a lengthy one and is split into five stages: prophase, metaphase, anaphase, telophase, and cytokinesis.
Prophase
During DNA replication, the resulting genetic material is known as chromatin and is loosely packaged in the cell. However, it’s later condensed into chromosomes during mitosis to make the cell replication less “messy” (this will be more understood in later stages). Structures called microtubules also develop during prophase and form mitotic spindles; they allow for the organized separation and movement of chromosomes. These centromeres will also move to opposite sides of the cell for later parts of mitosis.
Metaphase (Both Pro- and Metaphase)
Prometaphase refers to the events occurring in a cell prior to metaphase. During that space, the mitotic spindles will attach to the chromosomes in the center of the cell while the nuclear envelope (the structure that encloses genetic material) dissolves entirely.
In metaphase, the chromosomes will gravitate towards the center of the cell and align on the metaphase plate, where the chromatids (half of a chromosome) prepare for separation.
Anaphase
In this “separation” stage, the chromatids are separated at the metaphase plate by the mitotic spindles and their respective sister chromatids migrate to to opposite ends of the cell. To complete this process, the spindles will elongate and attach to centromeres of the chromosomes; they soon contract as the sister chromatids are moved.
Telophase and Cytokinesis
Being the final stages of mitosis, the cell will elongate to such a point that the resulting figure will almost appear as two cells. A nuclear envelope will form on the outer regions of the separated sister chromatids, but the cell(s) haven’t separated entirely yet. This is where cytokinesis comes into play.
During cytokinesis, a protein called actin is involved as it “pinches” the cytoplasm along the cleavage furrow (a crease that forms when two cells are about to separate).
Key Takeaways
- The cell cycle is split into four stages: G1, S, G2, and M. While G1, S, and G2 are part of a larger section called interphase, M is an entirely separate process.
- DNA replication is performed during the S phase and consists of four critical steps.
- During mitosis, the cell experiences five different phases to fully divide into two daughter cells.
