Cell Cycle Regulators

You have probably heard before that some cancer cells might arise from activating mutations in enzymes known as cyclin-dependent kinases (Cdks). Cdks play a significant role in regulating the cell cycle. But how do they do it? Let's explore the world of cell cycle regulators!

Regulation of the eukaryotic cell cycle

First, let's define what the cell cycle is.

In eukaryotes, the cell cycle involves two phases: interphase and mitosis.

Let's take a look at interphase. Here, the cell goes through all the necessary preparation for cell division. Interphase is divided into three stages: G₁ phase, S phase and G₂ phase. The first stage of interphase is the G₁ phase or the first gap phase. In this stage, the cell grows and prepares for DNA replication. In addition, it is the longest phase of the cell cycle.

The S phase is the stage where DNA replication occurs. DNA replication can be divided into three steps: initiation, elongation and termination.

Step 1: Initiation - In this step, a pre-replication complex look for the origins of replication, which are specific nucleotide sequences along the DNA strand. Unlike prokaryotes, eukaryotes have multiple origins of replication.

Then, the helicase enzyme binds to the origins of replication and unwinds/separates the DNA strand, creating a replication fork. Both DNA strands will then be used as a template for synthesizing a new, complementary strand. In order to prevent single-stranded DNA from becoming unstable, single-stranded DNA binding proteins bind to each strand.

As DNA helicase unwinds the DNA strand, DNA topoisomerase works ahead to the DNA helicase to prevent supercoiling downstream of the replication fork.

Step 2: Elongation - Now that we have the DNA strands ready for synthesis, it's time to recruit the enzyme RNA primase. Its job is to randomly synthesize small lengths of RNA, called RNA primers. Luckily, one of those RNA primers will be complementary to the newly opened DNA strand and will bind to it!

Then, DNA polymerase binds to this new double-stranded location, and starts adding complementary nucleotides to the template strand. As DNA polymerase synthesizes DNA in a 3' to 5' direction, the complementary strand is formed in the opposite direction (5' - 3'). This newly formed DNA strand is called the leading strand.

Now for the other DNA strand, DNA replication does not go as smooth. In this case, the RNA primase creates many RNA primers that bind to different spots along the length of the DNA strand, basically offering DNA polymerase many 3' ends for it to bind to and add nucleotides! These growing fragments of DNA are called Okazaki fragments.

Step 3: Termination - When DNA polymerase reaches a region of DNA where there are repeating sequences of TTAGGG (also known as the telomere region of the chromosome), then DNA polymerase knows it is time to leave!

The third stage of interphase is the G₂ phase, or gap two phase. In this phase, organelles are duplicated, and the cells prepares for mitosis, or the M phase.

• Mitosis is referred to as the division of a cell into two identical daughter cells.

Proteins that regulate cell cycle

Different steps in the cell cycle can be switched on/off by proteins and enzymes. These cell cycle regulators are cyclins and cyclin-dependent kinases (Cdk).

Cyclins can be categorized as cyclins D, E, A or B, and each cyclin regulates a different part of the cell cycle.

Basically, cyclin-dependent kinases (Cdk) regulate the events happening in the cell cycle by phosphorylating (adding a phosphate group) and de-phosphorylating (taking away a phosphate group) target proteins as a way of activating/deactivating them. However, a Cdk can only be activated if a cyclin binds to it, creating a cyclin-Cdk complex.

The concentration of Cdk remains relatively constant during the cell cycle, but the concentration of cyclin protein varies depending on the stage of the cell cycle, since they control the activation/deactivation of Cdks.

Cell cycle checkpoints and regulation

To ensure that all the events in the cell cycle happen at the correct time, the cell cycle has four checkpoints.

The first checkpoint is the G1 checkpoint, also known as the restriction point. This checkpoint looks for DNA damage and favorable conditions before entering the S phase. If it finds any DNA damage, it will try to repair it. But, if that is not possible, it will trigger apoptosis. Now, if conditions are not favorable, then it will send the cell into a special phase called the G₀ phase.

If everything is correct, then the cell will progress to the S phase. In this phase, we have the S checkpoint, whose job is to check for any DNA damage that may happen before or during DNA replication. The S checkpoint also prevents the reduplication of DNA, and after it gives a green light, the cell passes to the G₂ phase.

In the G₂ phase, the G₂ checkpoint makes sure all DNA has been duplicated, and also checks for DNA damage. Finally, in the Mitosis phase, the spindle-assembly (M) checkpoint ensures that all chromosomes at aligned at the metaphase plate and attached to spindle fibers before going into anaphase.

Cyclin-cdk cell cycle regulation

Cyclin-dependent kinases (Cdks) help in the regulation of the cell cycle checkpoints. The table below shows some important cyclin-Cdk complexes involved in cell cycle regulation.

Describe how the cell cycle is regulated

Now that we learned about the cell cycle, its different checkpoints, and the proteins that regulate the cell cycle, let's put it all together and look at the cyclin-Cdk complexes, and how they help regulate the cell cycle.

First, different cyclin-cdk complexes are formed (Cdk 4,6-cyclin D, Cdk 2-cyclin E). Rb protein (pRb), the product of the retinoblastoma gene (RB), is phosphorylated to initiate G₁ to S phase transition. In turn, this activates several genes required for DNA replication in the S phase.

• Retinoblastoma is a tumor suppressor that also helps regulate the cell cycle.

When the cell enters the S phase, cyclin D gets degraded, and the S-Cdk complex is formed. This complex triggers the S phase. After DNA replication, the S-Cdk complex gets destroyed.