CELL CYCLE OF XENOPUSDIGITAL ASSIGNMENT

CELL BIOLOGY and BIOCHEMISTRYPROF. GAYATHRI M

SLOT G2

INTRODUCTION TO CELL CYCLE

Organisms consist of cells that multiply through cell division. Before a cell can divide it has to grow in size, duplicate its chromosomes and separate the chromosomes for distribution between the 2 daughter cells. These different processes are coordinated in the cell cycle.

The cell cycle consists of several phases. In the first phase G1, the cell grows. When it has reached its appropriate size, it enters the phase of DNA-synthesis (S), where the chromosomes are duplicated. During the next phase (G2) the cell prepares for division. In Mitosis (M) the chromosomes, separate, and the cell divides into two daughter cells. Through this mechanism, the daughter cells receive identical sets of chromosomes. After division, the cells are back in G1 and the cell cycle is completed.

The division of cells was first studied by cytologists (people who look at cells). Microscope observations in

living and fixed cells revealed the process of mitosis, in which chromosomes con- dense and divide by attaching

to the two poles of the mitotic spindle. The stages of mitosis get their names from the appearance of

chromosomes.Interphase was initially regarded

as a “resting” phase, when the chromosomes were not

individually visible and the cell did not appear to be doing much. We

now know that this is a very active time for the cell - it’s when DNA

replication and most transcription and growth take place.

1. The eucaryotic cell cycle is divided into four phases

2. Cell-cycle control is similar in all eucaryotes

3. Cell-cycle control system dissected genetically in yeasts

4. Cell-cycle control system analyzed biochemically in animal embryos

5. Cell-cycle progression studied in various ways

Overview of Cell Cycle

The four phases of the cell cycle

The major events of the cell cycle

The molecular basis for cell cycle control was worked out using complementary approaches in a number of different experimental systems. Paul Nurse in England and Lee Hart- well in the U.S. used genetics to identify mutations in single- celled fungi that showed defects in cell cycle progression. Identification of many of these “cdc” (cell division control) mutations has provided enormous molecular clues about how cell cycle regulation works in all eukaryotes. Tim Hunt, working both in Cambridge, England and at Woods Hole, MA, showed biochemically that certain proteins oscillated in con- centration during each cell cycle in early embryos from sea urchins and clams. He named these proteins “cyclins.” These proteins turned out to be encoded by some of the cdc genes discovered by Nurse, Hartwell, and others.

Yoshio Masui developed the frog egg extract system for the analysis of cell cycle events.Frog eggs are arrested at metaphase of meiosis II when they are laid by the female frog. They can

be released from M-phase into interphase by fertilization, or experimentally by treatment with calcium ionophores (which cause a sudden increase in cytoplasmic calcium). Masui found that

cytoplasm from an M-phase arrested cell could induce M-phase like behavior when it was injected into an egg in inter- phase. By fractionating the M-phase extract, he identified the molecular

nature of this Mitosis-promoting factor (also called Maturation-promoting factor), and showed that it had kinase activity.

Studying cell cycle in a cell-free system

Cell-cycle control system analyzed biochemically in animal embryos

A mature Xenopus egg, ready for fertilization

FERTILISATION

Oocyte growth and egg cleavage in Xenopus

FIRST CLEAVAGES

SPLITTING OF EMBRYO

1. Cell-cycle control system triggers the major events of the

cell cycle 2. The cell-cycle control system depends on cyclically

activated cyclin-dependent protein kinases (Cdks) 3. Inhibitory phosphorylation and Cdk inhibitory proteins

(CKIs) can suppress Cdk activity 4. The cell-cycle control system depends on cyclical

proteolysis 5. Cell-cycle control also depends on transcriptional

regulation 6. The cell-cycle control system functions as a network of

biochemical switches

Control Of Cell Cycle

Two key components of the cell-cycle control system

The two central events of the cell cycle are:- replication of DNA during the S phase

- chromosome segregation and cell division during the M phase

Both these events are controlled by the cyclin-Cdk complexes

Thank you!

Prashance Pandey 15BCB0018Ria Arora 15BCB0066

Rishabh Mittal 15BCB0076