cel cycle regulation

8/7/2019 cel cycle regulation

1/17

REPORT ON

CONTROL OF CELL CYCLE

TABLE OF CONTENT

1). Abstract

2). Introduction

3). The cell cycle

4). Steps involved in cell cycle

5). Check points in cell cycle

6). Effect of loss of any one check point

7). Cyclins

8). Complex proteins

9). Regulation of cell cycle

10). Poster representing the regulation process

11). Tumour suppressors and regulation

12). Refrences


2/17

ABSTRACT

The aim of the report was to make the people aware much more about the cell

cycle, the process with which every single organism is related , the cell cycle is

the process with which the cell divides and duplicates(replicates). The details given

here are about the phases in which it is divided and the significance of these phases

along with the fact that what are cell cycle check points, what happens when

anyone looses a check point ,all these important phenomenon are given in detail in

this report along with a schematic diagram showing the various steps that controls

the cell cycle.


3/17

INTRODUCTION

Cells are the basic unit of all the living organisms. These cells can divide and multiplyundergoing series of events and these series of events are termed as cell cycle.

The cell is of two types prokaryotic cell and eukaryotic cell. Prokaryotic cells are withoutnucleus and in these cell cycle takes place by the process of binary fission. Whereas in

eukaryotic cell the cell cycle is somewhat typical and takes place in five different phases that are

given below :-

1. G0 Phase

2. G1 Phase

3. S Phase

4. G2 Phase

5. M Phase

For the proper functioning of the phases it is essential that the different phases are coordinated

and controlled so that one phase is completed then only the next can begin.


4/17

Cell grow during G1 phase and the genome present in it replicate during the S phase. During G2phase cell continue to grow and prepare for cell division. During M phase mitosis occurs and

cytokinesis is the phase when cytoplasm divides giving two daughter cells. Three principlecheck points control the cell cycle in eukaryots. The G1 check point makes key decision is towhether cell should go division or remain in resting stage. The G2 check point accesses the

success of DNA replication and triggers the start of mitotic phase M phase. If this check point

is passed the cell initiates the beginning of mitosis. The accuracy of the M phase is accessed at Mcheck point. This check point triggers the exit form mitosis and cytokinesis and enters the G1.

At G2 check point Cyclin Dependent Kinases phosphorylate histones and other proteins. During

G2 there is gradual accumulation of G2 cyclin also called as mitotic cyclin. These cyclins binds

to CDKases to form complex called Mitosis promoting factor (MPF). When the level of MPFexceeds the thrashold amount, it triggers mitosis and G2 phase ends. One of the prime fuction of

MPF is to activate the proteins that destroy Cyclin. As mitosis preceeds to the end of metaphaseCDK level stay constant while the G2 cyclin degrades causing less MPF to be available andinitiate the events that end mitosis.

After mitosis, the gradual accumulation of new cyclin starts the next phase of cell cycle. The G1cyclin acts same as that of G2, the level of of G1 cyclin increases and associates to CDK and its

thrashold amount triggers the DNA replication and cyclin is degraded and cycle begins again.

The Cell CycleThe cell cycle in eukaryotes includes in the manner shown in the diagram.


5/17

Of the four phases depicted in the Figure, the two critical steps are DNA replication, which

occurs during S-phase, and the physical process of cell division which occurs during M-phase(for mitosis). If we start at the beginning of the process, a cell undergoes a period where all of

the necessary machinery for the process of DNA replication is synthesized. This process occurs

during what is referred to as a gap between S-phase and M-phase and is termed G1. Following

DNA replication, the cell pauses in another gap phase termed G2 where all the machinerynecessary for cell division is synthesized. M-phase is composed of two discreet steps: mitosis,

which constitutes the pairing and separation of the duplicated chromosomes, and cytokinesis

which is the physical process whereby the cell splits into two daughter cells. Not all cellscontinue to divide during the life-span of an organism. Many cells undergo what is referred to as

terminal differentiation and become quiescent and no longer divide. Cells in this phase of their

life-cycle are said to reside in another gap phase called G0. Under certain conditions, such as thatresulting from an external signal stimulating cell growth, cells can exit the quiescent state and re-

enter the cell cycle.

Steps involved in cell cycle

G0: It is the resting phase or the quiescent or arrested phase.


6/17

G1: The first gap in the normal cell cycle is called G1 and is the period when the necessaryproteins for DNA replication are synthesized. However, this phase of the cell cycle is not onlycharacterized by synthesis of replication machinery. During this period the cell must monitor

both the internal and external environments to ensure that all the preparations for DNA synthesis

have been completed and that overall conditions for cell division are favorable. As discussed

below, there is a major check-point in a normal cell cycle that is critical for ensuring that all iswell for the cell to enter S-phase.

S-phase : The duplication of the cellular content of DNA occurs during S-phase, so-calledbecause this is the phase when DNA is synthesized. This phase of the cell cycle is the longest

taking 1012 hours of a typical 24hr eukaryotic cell cycle.

G2: During the second gap phase of the cell cycle the cell undertakes the synthesis of theproteins required to assemble the machinery required for separation of the duplicated

chromosomes (the process called mitosis) and ultimately division of the parental cell into two

daughter cells (the process termed cytokinesis). Like the G1 phase, the G2 phase is also a stage

when the internal and external environments are monitored to ensure that faithful replication ofthe DNA has occurred and that conditions are favorable for cytokinesis. In addition, as for the G1phase there is a major check-point at the end of the G2 phase that controls the entry into M-phase.

M-phase: During M-phase there is an ordered series of events that leads to the alignment andseparation of the duplicated chromosomes (called sister chromatids) This process is divided into

distinct steps that were originally identified and characterized through light microscopic

observations of dividing cells. The steps of mitosis are termed prophase, prometaphase,metaphase, anaphase and telophase. Although cytokinesis is the process by which the parental

cell is physically separated into two new daughter cells, it actually begins during anaphase. The

processes that occur during M-phase require much less time than those of S-phase, generallylasting only 12hrs.

During prophase the duplicated chromosomes condense while outside the nucleus the mitotic

spindle assembles between the two centrosomes. The centrosome is an organelle that serves as

the main microtubule organizing center that is involved in the attachment of microtubules to thesister chromatids.

During prometaphase the nuclear membrane breaks apart and the chromosomes can attach to

spindle microtubules and begin active movement.

During metaphase the chromosomes are aligned at the equator of the spindle midway betweenthe spindle poles. The sister chromatids are attached to opposite poles of the spindle.

During anaphase the sister chromatids synchronously separate to form the two sets of daughter

chromosomes. Each sister chromatid is slowly pulled towards the spindle pole it faces.


7/17

During telophase the two daughter chromosomes arrive at the spindle poles and decondense. A

new nuclear envelope forms around each set of chromosomes which forms the two new nuclei.

This process marks the end of mitosis and sets the stage for cytokinesis.

Check points in cell cycle

Checkpoint controls function to ensure that chromosomes are intact and that critical stages ofthe cell cycle are completed before the following stage is initiated. The various checkpoints incell cycle are as follows

G1 (restriction) checkpoint: where the decision is made whether the cell will bedivided, delayed division, or enter the resting stage

G2 checkpoint: which checks the success of DNA replication from the S phaseM checkpoint: the process of mitosis is assessed (whether it was a success).


8/17

What happens if you lose one of checkpoints?DNA replication and chromosome distribution are indispensable events in the cell cycle control.Cells must accurately copy their chromosomes, and through the process of mitosis, segregate

them to daughter cells. The checkpoints are surveillance mechanism and quality control of the

genome to maintain genomic integrity. Checkpoint failure often causes mutations and genomicarrangements resulting in genetic instability. Genetic instability is a major factor of birth defects

and in the development of many diseases, most notably cancer. Therefore, checkpoint studies are

very important for understanding mechanisms of genome maintenance as they have direct impacton the ontogeny of birth defects and the cancer biology.

Cyclinscyclin are the proteins and are key regulators of the cell cycle. Cyclins bind and activatemembers of the cyclin-dependent kinase (Cdk) family to effect cell cycle progression. Cell cycle

progression is controlled by the relative levels of individual cyclin family members. Progression

through the G1-S-G2-M cycle follows successive oscillations in the levels of cyclins, D, E, A

and B.

Cyclins are grouped into classes that relate to the phase of the cell cycle they regulate. Cyclin D

family members are G1 phase cyclins that regulate the entry of cells into G1 from Go. Cyclin D


9/17


10/17

a)detecting DNA damage, especially double-strand breaks;

b)interrupting (with the aid of p53) the cell cycle when damage is found;

c)maintaining normal telomere length.

3)MAD : MAD (="mitotic arrest deficient") genes (there are two) encode proteins thatbind to each kinetochore until a spindle fiber (one microtubule will do) attaches to it. If there isany failure to attach, MAD remains and blocks entry into anaphase (by inhibiting the anaphase-

promoting complex).


11/17

Regulation of cell cycle

As indicated above, there is the need for cell cycle control mechanisms to exert their influences

at specific times during each transit through a cell cycle. The heart of this timing control is the

responsibility of a family of protein kinases that are called cyclin-dependent kinases, CDKs.

The kinase activity of these enzymes rises and falls as the cell progresses through a cell cycle.Different CDKs operate at different points in the cell cycle. As would be expected, the

oscillating changes in the activity of CDKs leads to oscillating changes in the phosphorylation ofvarious intracellular proteins. These phosphorylations alter the activity of the modified proteins

which then effect changes in events of the cell cycle. The cyclical activity of each CDK is

controlled by a complex series of proteins, the most important of which are the cyclins, hence the

name of the enzymes as cyclin-dependent kinases. The CDKs are absolutely dependent upontheir interaction with the cyclins for activity. Unless they are tightly bound CDKs have no kinase

activity. The cyclins were originally idenitified because they undergo a cycle of synthesis and

degradation at specific points in each cell cycle. Thus, whereas the levels of the various CDKsremain fairly constant throughout the cell cycle, their activities changes in concert with the

fluctuations of the cyclins.

And as discussed above the Four different classes of cyclins have been defined on the basis of

the stage of the cell cycle in which they bind and activate CDKs. These four classes are G1-cyclins, G1/S-cyclins, S-cyclins, and M-cyclins. The cyclin nomenclature and associated CDK in

mammalian cells are listed in the following Table.

Cyclin-CDK Complex Cyclin CDK Partner

G1-CDK cyclin D* CDK4, CDK6

G1/S-CDK cyclin E CDK2

S-CDK cyclin A CDK2

M-CDK cyclin B CDK1**

*There are three D cyclins in mammals: D1, D2, and D3

**CDK1 is the same as CDC2 in fission yeast and CDC28 in budding yeast

The G1-cyclins are not found in all eukaryotic cells but in those where they are synthesized they

promote passage through a restriction point in late G1 called Start. The G1/S-cyclins bind to theircognate CDKs at the end of G1 and it is this interaction that is required to commit the cell to the

process of DNA replication in S-phase. The S-cyclins bind to their cognate CDKs during S-

phase and it is this interaction that is required for the initiation of DNA synthesis. The M-cyclinsbind to their cognate CDKs and in so doing promote the events of mitosis.


12/17

Although CDKs are inactive unless bound to a cyclin, there is more to the activation process than

just the interaction of the two parts of the complex. When cyclins bind to CDKs they alter the

conformation of the CDK resulting in exposure of a domain that is the site of phosphorylation byanother kinase called CDK-activating kinase (CAK). Following phosphorylation the cyclin-

CDK complex is fully active.

In addition to control of CDK kinase activity by cyclin binding and CAK phosphorylation,

control is exerted to inhibit CDK activity through interaction with inhibitory proteins as well asby inhibitory phosphorylation events. Thus, there is extremely tight control on the overall

activity of each CDK. One of the inhibitory kinases that phosphorylates CDKs is called Wee1.

The inhibitory phosphorylations are removed through the action of a phosphatase called CDC25.The action of these two regulatory enzymes on CDK activity is most important at the level of the

M-CDK activity at the onset of mitosis. Proteins that bind to and inhibit cyclin-CDK complexes

are called CDK inhibitory proteins (CKI, for cyclin-kinase inhibitor). Mammalian cells expresstwo classes of CKI. These are called CIPs for CDK inhibitory proteins and INK4 for inhibitors

of kinase 4. The CIPs bind and inhibit CDK1, CDK2, CDK4, and CDK6 complexes, whereas the

INK4s bind and inhibit only the CDK4 and CDK6 complexes. There are at least three CIPproteins in mammalian cells and these are identified as p21, p27 , and p57 . The expression ofeach of these CIPs is controlled by specific events that may have occurred during cell cycle

transit. For example p21Cip1 expression is induced in response to DNA damage. This induction is

under the control of the action of the tumor suppressor protein p53. There are at least four INK4proteins that are each identified by their molecular weights: p15INK4B, p16INK4A, and

p18INK4C (these were the first 3 characterized) as well as p19INK4. The p16INK4A protein is

also a tumor suppressor since loss of its function leads to cancer. All the INK4 proteins contain 4tandem repeats of a sequence of amino acids that were first identified in ankyrin and are thus

referred to as ankyrin repeats.

As indicated above, many cells reside in a resting or quiescent state but can be stimulated byexternal signals to re-enter the cell cycle. These external growth promoting signals are the resultof growth factors binding to their receptors. Most growth factors induce the expression of genes

that are referred to as early and delayed-response genes. The activation of early response genes

occurs in response to growth factor receptor-mediated signal transduction resulting inphosphorylation and activation of transcription factor proteins that are already present in the cell.

Many of the induced early response genes are themselves transcription factors that in turn

activate the expression of delayed-response genes. In the context of the cell cycle, these delayed-

response genes encode proteins of the G1-CDK complexes.

One such early response gene is the proto-oncogene MYC. With respect to the cell cycle some of

the genes turned on by activation of MYC are cyclin D, proteins of the ubiquitin ligase complex

called SCF (Skp1/cullin/F-box protein) and the members of the E2F transcription factor family.There are six members of the E2F family: E2F1 through E2F6). The synthesis of cyclin D will

result in the activation of G1-CDK complexes. The synthesis of components of SCF leads to the

degradation of p27 which normally inhibits G1-CDK complexes. The synthesis of E2F family

members results in increased synthesis of proteins involved in DNA synthesis as well as thesynthesis of the S-phase cyclins A and E and CDK2. Regulation of E2F activity by the tumor

suppressor pRB will be discussed below.


13/17

The cyclical degradation of the cyclins is effected through the action of several different

ubiquitin ligase complexes. The action of ubiquitin ligases in protein turn-over is discussed in

more detail in the Protein Modifications page. There are two important ubiquitin ligasecomplexes that control the turn-over of cyclins and other cell cycle regulating proteins. One is

the SCF complex which functions to control the transit from G1 to S-phase and the other is

called anaphase promoting complex (APC) which controls the levels of the M-phase cyclins aswell as other regulators of mitosis.

One important function of APC is to control the initiation of sister chromatid separation which

begins at the metaphase-anaphase transition. The attachment of the sister chromatids to the

opposite poles of the mitotic spindles occurs early during mitosis. The ability of the sisterchromatids to be pulled apart is initially inhibited because they are bound together by a protein

complex termed cohesin complex. The cohesin complex is deposited along the chromosomes as

they are duplicated during S-phase. Anaphase can only begin with the disruption of the cohesincomplex. The breakdown of the cohesin complex is brought about as a consequence of the

activation of the ubiquitin ligase activity of the APC. APC targets a protein called securin.

Securin functions to inhibit the protease called separase and the action of separase is to degradethe proteins of the cohesin complex, thus allowing sister chromatid separation.


14/17


15/17

Tumor Suppressors and Cell Cycle RegulationTumor suppressors are so called because cancer ensues as a result of a loss of their normalfunction, i.e. these proteins suppress the ability of cancer to develop. It would seem obvious,

therefore, that one import function of tumor suppressors would be control of the progression of a

cell through a round of the cell cycle. If cells are able to synthesize damaged DNA before it isrepaired or to divide when the DNA is damaged then the resulting daughter cells can pass on the

resultant DNA damage to their progeny. The result can be catastrophic resulting in cancer. For

this reason, the two most important check points in the eukaryotic cell cycle are the G1-S

transition and the entry into mitosis. The former prevents DNA replication prior to repair ofdamaged DNA and the latter prevents damage that may have occurred to the DNA during

replication to propagated into daughter cells during mitosis. Following the isolation and

characterization of two tumor suppressor genes in particular it was found that they function tocontrol the ability of cells to progress through these two important checkpoints. The protein

encoded by the retinoblastoma susceptibility gene (pRB) and the p53 protein are both tumor

suppressors. The function of pRB is to act as a brake preventing cells from exiting G1 and that of

p53 is to inhibit progression from S-phase to M-phase.

The best understood effect of G1-CDK activity is that exerted on transcription factors of the E2F

family, hereafter referred to simply as E2F. In the context of the cell cycle regulation, E2F

activates the expression of cyclin A, cyclin E and CDK2. These proteins are components of theS-CDK complexes necessary for progression through S-phase. The activity of E2F is itself

controlled via interaction with pRB. When pRB binds E2F it can no longer function as a

transcription factor as it is sequestered in the cytosol. Interaction of pRB and E2Fcorrelates to thestate of phosphorylation of pRB and the affinity between the two proteins is highest when pRB is

hypophosphorylated. Phosphorylation of pRB is maximal at the start of S phase and lowest after

mitosis and entry into G1. Stimulation of quiescent cells with mitogen induces phosphorylation

of pRB, while in contrast, differentiation induces hypophosphorylation of pRB. One of the mostsignificant substrates for phosphorylation by the G1 cyclin-CDK complexes is pRB. When pRB

is phosphorylated by G1 cyclin-CDK complexes it releases E2F allowing E2F to transcriptionally

activate its target genes. When E2F activates the expression of S-CDK complex proteins thesecomplexes also target pRB for phosphorylation, thus maintaining the cell in a pro cell cycle

progression state.


16/17

Regulation of E2F by pRB:One major function of the p53 protein, which is active as a homotetrameric transcription factor,

is to serve as a component of the checkpoint that controls whether cells enter as well as progress

through S-phase. The action of p53 is induced in response to DNA damage. Under normal

circumstances p53 levels remain very low due to its interaction with a member of the ubiquitin

ligase family called MDM2. MDM2 is so named since it was isolated as an amplified gene in the

tumorigenic mouse cell line 3T3DM. In response to DNA damage, e.g. as a result of uv-

irradiation or -irradiation, cells activate several kinases including checkpoint kinase 2 (CHK2)

and ataxia telangiectasia mutated (ATM). One target of these kinases is p53. ATM also

phosphorylates MDM2. When p53 is phosphorylated it is released from MDM2 and can carry

out its transcriptional activation functions. One target of p53 is the cyclin inhibitor p21 gene.

Activation of p21 leads to increased inhibition of the cyclin D1-CDK4 and cyclin E-CDK2

complexes thereby halting progression through the cell cycle either prior to S-phase entry or

during S-phase. As a consequence of p53-induced synthesis of p21 expression, there is aconvergence between the roles of p53 and pRB (as outlined above) in regulation of cyclin-CDK

complexes. In either case the aim is to allow the cell to repair its damaged DNA prior to

replication or mitosis.


17/17

REFRENCES:

http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A3432

wormbase.net/chapters/www_cellcyclereguln/cellcyclereguln.html

http://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-

overviews/cyclins-and-cell-cycle-regulation.html

http://www.sparknotes.com/biology/cellreproduction/cellcycle/section3.rhtml

http://themedicalbiochemistrypage.org/cell-cycle.html

http://www.garlandscience.com/textbooks/081533480X/pdf/ch18.pdf

Hartwell, L. H. & Weinert, T. A. Checkpoints: controls that ensure the order of cell

cycle events. Science 246, 629-34 (1989).

http://image.tutorvista.com/content/cell-reproduction/cell-cycle.jpeg
http://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A3432http://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-overviews/cyclins-and-cell-cycle-regulation.htmlhttp://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-overviews/cyclins-and-cell-cycle-regulation.htmlhttp://www.sparknotes.com/biology/cellreproduction/cellcycle/section3.rhtmlhttp://themedicalbiochemistrypage.org/cell-cycle.htmlhttp://www.garlandscience.com/textbooks/081533480X/pdf/ch18.pdfhttp://image.tutorvista.com/content/cell-reproduction/cell-cycle.jpeghttp://www.ncbi.nlm.nih.gov/bookshelf/br.fcgi?book=mcb&part=A3432http://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-overviews/cyclins-and-cell-cycle-regulation.htmlhttp://www.sigmaaldrich.com/life-science/your-favorite-gene-search/pathway-overviews/cyclins-and-cell-cycle-regulation.htmlhttp://www.sparknotes.com/biology/cellreproduction/cellcycle/section3.rhtmlhttp://themedicalbiochemistrypage.org/cell-cycle.htmlhttp://www.garlandscience.com/textbooks/081533480X/pdf/ch18.pdfhttp://image.tutorvista.com/content/cell-reproduction/cell-cycle.jpeg

Documents

cel cycle regulation