Welcome to My AP Biology Thoughts podcast, my name is Nidhi and I am your host for episode #95 called Unit 4 Cell Communication and Cell Cycle: CDK and Cyclins. Today we will be discussing what cyclins and CDK are and why they're important. 

• Cyclins are a group of related proteins, and there are four basic types found in humans and most other eukaryotes. These include G1cyclins, G1/S cyclins, S cyclins, and M cyclins. Each cyclin is associated with a particular phase, transition, or set of phases in the cell cycle and helps drive the events of that phase or period. For instance, M cyclin promotes the events of the Mitosis phase, such as nuclear envelope breakdown and chromosome condensation. Cyclin-dependent kinases, or CDKs, are enzymes that catalyze the phosphorylation of target proteins in the cell cycle. The attached phosphate makes the target protein more or less active. The CDKs are activated when attached to cyclin because the cyclin changes the shape of the enzyme. When a cyclin attaches to a Cdk, it has two important effects: it activates the Cdk as a kinase, but it also directs the Cdk to a specific set of target proteins ensuring that those are proteins appropriate to the cell cycle period controlled by the cyclin. After the phosphorylation of proteins is complete, cyclin breaks down and CDK is inactive. CDK-Cyclins also act as a control or regulator for the cell cycle. Cdk activity and target proteins change as levels of the various cyclins rise and fall. In addition to needing a cyclin, Cdks must also be phosphorylated on a particular site in order to be active.

•  An example of how cyclins and cdks work is the mitosis-promoting factor. A MPF molecule is a CDK bound to an M cyclin. As the cell approaches the G2/ Mitosis transition phase in the cycle, the levels of the M cyclin increase. It then binds to CDKs present in the cell and together they cause the Mitosis phase to begin. The MDF adds phosphate to protein in the nuclear envelope, causing it to break down, and activates chromosome condensation promoting targets. In addition to driving the events of M phase, MPF also triggers its own destruction by activating the anaphase-promoting complex/cyclosome or APC/C, a protein complex that causes M cyclins to be destroyed starting in anaphase. The destruction of M cyclins pushes the cell out of mitosis, allowing the new daughter cells to enter G1. 
• CDKs and cyclins often respond to cues from the cell to regulate. Positive cues, like growth factors, increase activity of Cdks and cyclins, while negative ones, like DNA damage, usually decrease activity. When DNA damage occurs, a protein called p53 triggers the production of CDK inhibitor proteins. These proteins bind to Cdk-cyclin complexes and block their activity, allowing time for DNA repair to occur. By ensuring that cells don't divide when their DNA is damaged, mutations are not being passed onto daughter cells. When p53 is defective or missing, mutations can accumulate, potentially leading to cancer 

•  Cylins and CDK can connect to the principles of evolution. Cyclins and Cdks are found in...