Cell Cycle and its Control

Lecture 37 BSCI 420,421, 620Nov 27, 2002

“ Cells come from previous cells” – R. Virchow

1. Intro to the cell cycle2. The cell cycle control system

a. MPF in frogsb. Cdc mutants in yeastc. The mammalian cell cycle

1. Intro to the cell cycle

When you look at cells under a microscope, you see 2 states:Dividing cells and cells in an Interphase between divisions.With the introduction of DNA labeling methods like 3H-TdRand ARG or BrdU and Fl-anti-BrdU-DNA, 4 phases were IDd.

These two methods allow progression thru the cell cycleTo be studied. Eg The Labeling index after a pulse tells you how many cells are in S (a fx of how fast/many cells are cycling).%S = tS/tT if all cells cycling%M= tM/tT “ “ “ (Fig. 17-11)

Flow cytometry gives a histogram of no. of cells vs. Amount of DNA/cell. Gives % cells in cycle phases.

The cell cycle is a set of biochemical events driven by a control system that tells cells whether they can enter the next phase of the cell cycle.

How does this CC control system work?

By activation and inactivation of a set of protein kinases called Cyclin-dependent protein Kinases (CdKs).The two key components are Cyclin, a protein whose levels goup & down during the cell cycle, and the CdK enzyme itself.

1. The cell cycle control systema. Maturation Promoting Factor (MPF) in frog eggs

Exposure to maleGonadotropin

(Pit) Progesterone (Foll.

Cells)

Masui & Markert found that a small amt. of cytoplasm from mature, metaphase-arrested eggs injected into interphase-arrested immature oocytes induced meiotic divisions and Maturation to metaphase II. (in absence of progesterone)

This factor they named MPF

MPFAct

imm Meiosis Meiosis Mit 1 Mit 2 oocyte I II fert

time

Using this activity when inj into imm oocytes as a Bioassay,M & M purified MPF down to 2 proteins:

A 34 kD protein kinase that stays relatively constant &A 56 kD protein that increases and decreases during cycle.

They named the cycling protein “Cyclin” & the enzyme CdK.

b. Cdc mutants in yeast

Yeasts are single-celled fungi with cell cycles very sim to ours.2 diff species have been extensively studied:

Temperature-sensitive mutants can be selected that arrest inParticular phases of the cell cycle when switched to restrictive(high) temps. These are called cell-division-cycle (cdc) genes.

E.g., when grown at restrictive temp the normal yeast (A)Show all cell cycle stages

The cdc 15mutants (B)Are blocked at the end of M

Yeast mutations demonstrated genes for:

One Cdk enzymeThree classes of cyclins:G1/S-cyclin, necessary for cells to pass the Start point & enter SS-cyclin, required to initiate DNA replicationM-cyclin, required to enter Mitosis

Several proteins that regulate Cdk activity:CAK, a Cdk-activating kinase

Also, Wee 1 a kinase that inhibits Cdk by P04ing a tyr, &Cdc 25, a phosphatase that stimulates Cdk by removing thatP04

p27 a direct-binding inhibitor of Cdk

c. The mammalian cell cycle

Human cells were fused together in diff CC phases:

Overview of cell cycle checkpoints: