The cell cycle consists of interphase, mitosis and cytokinesis.
The cell cycle consists of interphase, mitosis and cytokinesis.(1:33) There are a number of internal checkpoints that regulate a cell's progression through the cell cycle, including cyclins(4:23) and Cdk complexes(5:23). These couple with phosphorylation to activate target proteins throughout the cell cycle.
The Question of the Day asks (7:38) Some cells, like neurons, exit the cell cycle prior to the S phase - what phase are they in?
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Hi and welcome to the APsolute Recap: Biology Edition. Today’s episode will recap the Cyclins and CDKs - oh my!
Zoom out:
Unit 4 - Cell communication and cell cycle
Topics 4.7 Regulation of Cell Cycle
Big idea - Information storage and transmission
Do not pass go, do not collect $200, go straight to jail. I don’t know about you, but I always hated that chance card in Monopoly. I’m going around the board, buying up properties, stopping to occasionally pay rent to my competition and always hoping to roll doubles. When just like that, my luck changed. The cell cycle is similar! Cells move around the cycle game board, passing different stopping points and accomplishing different tasks. Following the rules and making sure nothing goes out of turn Thankfully, eukaryotic cells have better game pieces than the thimble and top hat. They have Cyclins and CDKs, oh my! Don’t be scared, these are just proteins. And like most proteins, they also have specific structures for specific functions.
Let’s Zoom in:
Before we begin with regulation, let’s recap the cell cycle first.
The purpose of cell division by mitosis is to create identical daughter cells for growth, repair, and asexual reproduction. In order of Cell Cycle event: Interphase with G1 (growth), S (DNA synthesis), and G2 (more growth). Next is mitosis with Prophase, Metaphase, Anaphase, and Telophase - all steps of nuclear division (and recapped in other episodes). Lastly, cytokinesis (cell or cytoplasmic division) Timing of the cell cycle varies from one cell type to another, but most of a cell’s lifespan is spent in interphase. Regulation of the cell cycle is essential for normal cellular function. Any disruptions to the cell cycle may result in cancer or apoptosis.
There are a number of internal controls or checkpoints that regulate a cell's progression through the cell cycle. Some checkpoints include checking for DNA damage, incomplete DNA replication, or unattached spindle fibers. Once a cell goes past certain checkpoints, it may enter the place of no return. For example, a cell that has finished G1 should have passed the test for no DNA damage, sufficient size and nutrients, as well as appropriate growth factors - which are often signals from neighboring cells. The other two major checkpoints occur during the S phase (to ensure that DNA was copied correctly and completely) as well as the M phase (in particular that spindle fibers are attached properly to separate sister chromatids). The cell cycle is further regulated by a set of regulatory proteins and protein complexes including cyclins and cyclin dependent kinases (Cdks). These complexes form and denature to activate target proteins throughout the cell cycle.
Let’s chat cyclins first - arguably the most important cell cycle regulatory protein. There are four specific cyclins responsible for transitional events through each stage of the cell cycle in eukaryotes. Since they are “stage specific,” the relative concentration of the cyclin peaks at the transition. For example, M cyclin is found in greatest concentration at the start of prophase when chromosomes condense and the nuclear membrane breaks down. S cyclin gradually increases in concentration throughout the synthesis phase. The black sheep of the cyclin family is G1 Cyclin, which is found at fairly constant concentration through the cell cycle (must be important). And since we’ve already mentioned 3 out of 4, the last one is G1/S cyclin - and you guessed it - peaks at the juncture between those phases. Not terribly creatively named, but you can rely on the correct assumption that they regulate main events unique to each cell cycle phase.
Cyclins don’t act alone, but instead bind to CDKs, another group of proteins - more specifically, a type of kinase enzyme. Cyclin dependent kinases are activated by this pairing with cyclin and work to modify specific proteins in the cell through phosphorylation. Which proteins are targeted you might ask? Well, that depends on which cyclin is in charge during that phase of the cell cycle. For example, if the G1/S cyclin binds to a CDK, then the proteins activated would be involved in DNA replication for the synthesis phase. Phosphorylation is a quick on/off switch. Not only will the target proteins be activated by the addition of the phosphate, but often CDKs themselves can be similarly activated, or inactivated. Wax on/wax off - or in biology - phosphate on, phosphate off. Remember, enzymes are not used up in a chemical reaction. As such, CDKs stay at the same relative concentration throughout the cell cycle.
Time for unit connections. The only mention of cyclins and CDKS in the AP bIology course exam description is in this exact section, Unit 4 - section 7. However, its a good idea to refresh your knowledge on protein structure and function and ATP from Unit 1.
Alright - what about the exam? You do not need to know specific Clyclin/CDK protein pairs or growth factors for the AP Exam. But, just because you don’t need to know them, doesn’t mean they won’t throw specific examples into a larger question. Think of these types of proteins like algebraic stand-ins. Cyclin X and CDK y regulate G1. It doesn’t matter what they are called. Be prepared to describe the effect of cyclins/CDK pairings from a prompt, particularly if they are disrupted. Many questions will be coupled with cell communication.
To recap……
The cell cycle consists of interphase, mitosis and cytokinesis. There are a number of internal checkpoints that regulate a cell's progression through the cell cycle, including cyclins and Cdk complexes. These couple with phosphorylation to activate target proteins throughout the cell cycle.
Coming up next on the Apsolute RecAP Biology Edition: Homeostasis
Today’s question of the day is about the cell cycle
Question of the day: Some cells, like neurons, exit the cell cycle prior to the S phase - what phase are they in?