Teaching Module Entire Thing

*Use the Edit button to enter your Module *

Key	Value
Module Name:	Make this a look up of existing pages
Physiology:	The cell division cycle is the universal mechanism controlling DNA replication and cell division in all eukaryotes. Progression through the cell cycle is unidirectional. The cell cycle consists of four phases: G1 (gap or growth), S (DNA replication), G2 (gap or growth), and mitosis. Checkpoints within the system ensure that one phase of the cell cycle is complete before the cell progresses to the next phase. Checkpoints also monitor the cell for DNA damage and delay cell cycle progression to allow for repair of DNA damage. The restriction point (or START) regulates entry into the cell cycle in the presence of growth factors. In the absence of growth factors, the cell withdraws to a state of quiescence called G0. Behaviors of even the simplest cell cycles (such as those of frog egg extracts, described here) cannot be explained by simple intuition without a lot of hand-waving arguments. Why is the cell cycle unidirectional? Once a cell initiates mitosis, why does it never slip back into S or G2? What controls the timing of cell cycles, which can range from eight minutes (in fly embryos) to more than 24 hours (in adult mammals) in length? Mathematical models of the cell cycle offer a systems-level view that can answer these questions by revealing fundamental behaviors of the system. In this tutorial, we will explore the concept of hysteresis, which explains the irreversible switch-like behavior of the cell cycle, the concepts of lag times and “critical slowing down”, which contribute to regulation of cell cycle timing, and the feedback loops that generate autonomous oscillations through the cell cycle (Sible and Tyson, 2007). In the exercises, we will explore several specific behaviors that were discovered by experimentation and can be better understood in the context of a mathematical model. First, we will explore the observation by Murray and Kirschner that synthesis and degradation of cyclin is all that is needed to drive cell cycle oscillations in frog egg extracts (Murray and Kirschner, 1989; Murray et al., 1989). Second, we will investigate the observation by Solomon that a threshold amount of cyclin was required to drive an extract into mitosis (Solomon et al., 1990). We will also discover the role of positive feedback in cell cycle progression, the bistable nature of the molecular control network and the affect of unreplicated DNA on cell cycle progression. Insert Figure 1: The eukaryotic cell cycle figure 1
Attach slides
Molecular Biology:	Progression through the eukaryotic cell cycle is driven by a family of enzymes called cyclin - dependent kinases (Cdks). Cdks consist of a catalytic subunit, which attaches a phosphate group to serines or threonines on target proteins, and a regulatory cyclin subunit, which is generally an unstable protein that is synthesized and degraded periodically during the cell cycle. Different Cdk complexes are responsible for progression through specific phases of the cell cycle. For example, cyclin E-Cdk2 drives the cell through S phase by phosphorylating components of the DNA origin replication complex to trigger origin firing and subsequently to block rereplication of DNA (Furstenthal et al., 2001). A simplified view of the Cdk complexes that drive the eukaryotic cell cycle is depicted in Figure 2. Insert Figure 2. Cyclin – Cdk complexes present in the frog egg extract. In this exercise, we focus on cyclin B-Cdk1, which catalyzes the G2/M - phase transition. This Cdk complex is also known as cyclin B-Cdc2 or MPF (M - phase promoting factor) for historical reasons. Like most Cdk complexes, cyclin B-Cdk1 activity is regulated by multiple mechanisms: 1) cyclin synthesis and degradation: cyclin levels accumulate as the cell cycle progresses, peaking at M-phase, coincident with the peak in Cdk activity. Then, cyclin is rapidly degraded and the cell exits mitosis. Cyclin degradation is catalyzed indirectly by cyclin B-Cdk1 itself, forming a time delayed negative feedback loop. 2) stoichiometric inhibitors: A family of proteins called the cyclin – dependent kinase inhibitors (CKIs) bind to and repress Cdk activity. Cyclin B-Cdk1 is susceptible to these inhibitors, but as they are not present in the frog egg extracts, they will not be introduced into our model. 3) activating phosphoryation. Phosphorylation of the catalytic subunit by Cdk – activating kinase (CAK) is required for activation (Solomon et al., 1992). Because this phosphorylation event does not appear to be regulated in frog egg extracts, it will be ignored in our model. 4) Inhibitory phosphoryations. When cyclin B/Cdk1 is phosphorylated on threonine 14 and tyrosine 15 by Wee1/Myt1 (Mueller et al., 1995a; Mueller et al., 1995b), its kinase activity is inhibited. The opposing activating phosphatases are members of the Cdc25 family (Gautier et al., 1991; Kumagai and Dunphy, 1991). These two groups of enzymes regulating the phosphorylation states of Thr 14 and Tyr 15 play a central role in our mathematical model as they are engaged in feedback loops with cyclin B/Cdk2. Figure 3. Wiring diagram for the regulation of cyclin B-Cdk1 activity in frog egg extracts. Solid lines represent biochemical transitions; dashed lines represent catalytic effects. IE = intermediate enzyme (Cdc20/Fizzy). APC = anaphase promoting complex that tags cyclin B for degradation with a polyubiquitin tail. (Show rate contant placings on this diagram?)
Attach Slides
Exercises:	Create Exercise Page
Module Created:	2007/9/22

-- RaquellHOlmes - 17 Sep 2007

Sandbox

Sandbox Web

Copyright © by the contributing authors. All material on this collaboration platform is the property of the contributing authors.
Ideas, requests, problems regarding TWiki? Send feedback