AH Biology: Unit 1 Control of the Cell Cycle

Control of the cell cycle LOs 1 Checkpoints are critical control points where stop and go ahead signals regulate the cycle. For many cells the G1 checkpoint is the most important. If a go ahead signal is not reached at the G1 checkpoint the cell switches to a non-dividing state called the G0 phase. As the cell size increases during the G1 cyclin proteins accumulate and combine with kinases to form regulatory protein molecules known as cyclin-dependent kinases (Cdks). Cdks cause the phosphorylation of proteins that stimulate the cell cycle.

Control of the cell cycle LOs 2 If a sufficient threshold of phosphorylation is reached the cell cycle moves on to the next stage. If an insufficient threshold is reached, the cell is held at a checkpoint. The G1 Cdk phosphorylates a transcription factor inhibitor, retinoblastoma (Rb) protein, allowing DNA replication in the S phase. DNA damage triggers the activation of several proteins including p53 that can stimulate DNA repair, arrest the cell cycle or cause cell death.

The cell cycle: summary G1 G2 S Interphase M Cytokinesis Mitosis

The cell cycle: summary G1 G2 S Interphase M Cytokinesis Telophase Anaphase Metaphase Prophase Mitosis

What features should an effective cell cycle control system possess? Why does the progress of a cell through the cell cycle need to be monitored and regulated? What features should an effective cell cycle control system possess? Monitoring to ensure that cell cycle events occur in the correct sequence and do not take place unless the previous ones have been completed, eg chromosomes should only be segregated after their DNA has been replicated, cell growth must be coordinated with a cell’s progress through the cell cycle so the size of the cells is maintained. Regulation is particularly important in a multicellular organism to ensure that cells divide at the correct rate to allow growth and development to occur correctly and regulate the size of cell populations. Design features: mechanisms for triggering the cell cycle events in the correct order at the correct time and making sure events are not repeated more than once in the same cycle. Events triggered in ‘all or none’ (binary) fashion so that once an event is switched on: it runs to completion. System should be adjustable to cope with different environmental conditions and suit different types of cells. System should be able to respond to internal and external feedback.

The cell cycle control system can be studied using model organisms Yeast: Identification of mutations that arrest the cell cycle at specific points. Affected genes are known as cell-division-cycle (cdc) genes. http://homepage.mac.com/enognog/pombeDIC.jpg

The cell cycle contains control points G1 G2 S I M G1 checkpoint (entry to S phase) M checkpoint (exit from M phase) G2 checkpoint (entry to M phase)

The cell cycle contains control points G1 G2 S I M G1 checkpoint (initiation of DNA replication) M checkpoint (initiation of anaphase) G2 checkpoint (assembly of spindle fibres)

The control points are checkpoints for the cell cycle control system G1 G2 S I M G1 checkpoint: Has the cell reached a sufficient size? Are environmental conditions favourable? M checkpoint: Are all chromosomes attached to spindle fibres? G2 checkpoint: Has all nuclear DNA been replicated? If events have not been completed the control system receives signals and arrests the cell cycle.

The G1 checkpoint Timing: Towards the end of G1 phase. Controls: Entry to S phase (triggers the initiation of DNA replication). Assesses: Cell size and environmental conditions. Purpose: Ensures that sufficient cell growth has occurred and environmental conditions are favourable for proliferation.

What could happen to a yeast cell whose G1 checkpoint mechanism has been inactivated? Functioning checkpoints are not essential for progression through the cell cycle. Cells in which checkpoints are inactivated still progress through the cell cycle, even if the conditions required to progress are not met. An inactivated G1 checkpoint will not stop a cell a progressing to the S phase and the cell will proliferate as normal, providing that it continues to receive sufficient nutrients from its environment.

Cell size Time With nutritional cell cycle control Without nutritional cell cycle control Data is from normal yeast cells and those with the wee mutation. Time Nutrient supply reduced

In multicellular organisms the G1 checkpoint operates through intracellular and extracellular signals Fibroblast grown in culture with adequate nutrient supply and serum Fibroblast grown in culture with adequate nutrient supply and plasma To explain what ‘favourable environmental conditions’ means. http://commons.wikimedia.org/wiki/File:Fibroblast.jpg Fibroblasts are found in connective tissue and secrete collagen. They help to maintain a structural framework of tissues. Serum is prepared by letting blood clot then extracting the cell free liquid. Cell progresses through cycle and proliferates Cell cycle is arrested

Extracellular signal molecules with this function are called mitogens. Serum contains a protein that can bind to cells and stimulate them to progress through the cell cycle. Protein is platelet-derived growth factor (PDGF). PDGF is released from platelets that have formed blood clots. Its major role is likely to be to stimulate cell division during wound healing. http://commons.wikimedia.org/wiki/File:1PDG_Platelet-DerivedGrowthFactorBb.png Extracellular signal molecules with this function are called mitogens.

The most important decision Cells may either proliferate or leave the cell cycle. In the absence of mitogens cells enter a non-dividing state called the G0 phase. Cells can become terminally differentiated and remain in G0 permanently or re-enter the cell cycle when they receive appropriate signals.

G0 G1 M S G2 Cytokinesis Mitosis Reversibility depends on cell type Interphase M Cytokinesis Mitosis

Some types of cell can proliferate continuously Stem cells Tumour cells http://www.scientificamerican.com/media/inline/65A632E6-0816-AD4F-AE94975827A78F8E_1.jpg Human embryonic stem cells shown. Link to video clip of breast cancer cells in culture.

Most liver cells exist in a reversible G0 phase Normal hepatocyte: mitogenic signal absent Cell proliferation is stimulated by damage to liver G1 G2 S I M G0 G1 G2 S I M G0 Liver damage results in an increased production of hepatocyte growth factor. This protein stimulates liver cells to divide.

Red blood cells, neurons and skeletal muscle cells exist in a terminally differentiated G0 state http://commons.wikimedia.org/wiki/File:Redbloodcells.jpg http://commons.wikimedia.org/wiki/File:Pyramidal_hippocampal_neuron_40x.jpg http://commons.wikimedia.org/wiki/File:Skeletal_muscle_-_longitudinal_section.jpg

The G2 checkpoint Timing: End of G2 phase. Controls: Entry to M phase (triggers assembly of mitotic structures). Assesses: Completion of DNA replication. Purpose: Ensures that all DNA is replicated so that daughter cells can each receive a complete copy of the genome and function correctly.

The M checkpoint Timing: During metaphase. Controls: Exit from M phase (triggers anaphase and cytokinesis). Assesses: Attachment of all chromosomes to spindle fibres. Purpose: Ensures that each daughter cell receives the same chromosome complement as its parent when anaphase occurs.

The M checkpoint All chromosomes attached to spindle fibres One chromosome is not attached to spindle fibres Cell cycle progresses: cell enters anaphase Cell cycle arrested until all chromosomes are properly attached

http://upload. wikimedia http://upload.wikimedia.org/wikipedia/commons/5/5b/Mitosis-fluorescent.jpg Newt lung cell. Chromosomes stained blue, spindle fibres green.

Checkpoints operate through negative intracellular signals The presence of unattached chromosomes generates signals that stop the cell from progressing to anaphase. The same applies to unreplicated DNA. It is worth doing a demonstration of the advantage of negative regulation over positive regulation. For positive regulation: students are chromosomes, classroom is mitotic cell. Students move towards equator and shout ‘Go’ once they are in position. This makes unaligned chromosomes hard to detect. For negative regulation: students shout ‘Stop’ until they are aligned on the equator. It is now much easier to detect the attachment of the last chromosome.

The molecular mechanisms of cell cycle control

The cell cycle is controlled by the activity of cyclin-dependent kinases (Cdks) Cdk inactive G1 G2 S M Cdk active How can the activation pattern of Cdks be explained? Cdk inactive Cdk active

The cell cycle control system can be studied using model organisms Spisula: a mollusc used in the study of protein synthesis (eg of cyclins) in embryonic cells. Clam http://commons.wikimedia.org/wiki/File:Spisula_solida.jpg A to D show fertilisation and the first cell divisions of an embryo http://www.mbl.edu/news/press_releases/images/embryos.jpg

A time course of intracellular cyclin protein Relative level of cyclin protein Mitosis Mitosis Mitosis This graph helps us to explain why entry to the S phase only occurs when the cell has reached a certain size. Cyclins undergo a cycle of synthesis and degradation. Time

The activity of Cdks is regulated by cyclins Cyclin binding Cdk with protein kinase activity (cyclin–cdk complex) Inactive Cdk

Different cyclins bind to Cdks at different phases of the cell cycle The binding of G1-cyclins allows a cell to pass through the G1 checkpoint. The binding of S-cyclins allows a cell to initiate DNA replication in the S phase. The binding of M-cyclins promotes the events of mitosis. Mitosis

The activation of cyclin-Cdk complexes triggers cell cycle events G1-Cdk M-Cdk DNA replication triggered One type of Cdk in yeast cells. Four types of Cdks in vertebrate cells. S-Cdk Mitosis triggered A certain level of phosphorylation of target proteins results in the cell progressing to the next stage of the cycle.

Active retinoblastoma protein (Rb) inhibits cell cycle progression

Retinoblastoma is targeted by G1-Cdk Synthesis of S-cyclins Active G1-Cdk P P Active Rb Inactive Rb Active S-Cdk Depends on whether both copies of the gene are mutated and which type of cell the mutation occurs in. Other proteins are involved in controlling progression into the S phase. DNA replication What would be the consequence of a mutation to the gene that codes for the Rb protein?

The cell cycle has checkpoints for DNA damage Mutagen In which part(s) of the cell cycle would you expect these checkpoints to occur? What should a cell with damaged DNA do?

DNA damage prior results in the activation of the protein p53 1. Damaged DNA DNA damage prior results in the activation of the protein p53 2. Protein kinase activity triggered How could activated p53 bring about the inhibition of progress through the cell cycle? P Stable p53 Unstable p53

Cyclin–Cdk complex inactivated Active p53 can promote the transcription of genes that induce cell cycle arrest Regulatory DNA Expression of p21 gene S-Cdk or G1/S-Cdk p21 protein Cyclin–Cdk complex inactivated Cell arrested in G1

Active p53 can affect a cell in different ways Stimulates DNA repair Promotes transcription of genes that induce apoptosis Active p53 can affect a cell in different ways Promotes transcription of genes that induce cell cycle arrest In this section we have considered the control of cell proliferation. In the next section we look at the control of cell death. What would be the functional consequences of an inability to activate p53?

Ataxia telangiectasia: a genetic disease associated with an inability to activate p53 http://commons.wikimedia.org/wiki/File:Ataxia-telangiectasia2.gif Condition is autosomal recessive and is caused by a mutation to the ATM gene on chromosome 11. Symptoms of disease: neurodegeneration, particularly in the cerebellum (leading to ataxia: difficulty in coordinating muscle action), predisposition to cancer/hypersensitivity to mutagens, weakened immune system and genome instability. People with disease (1 in 40,000-100,000) have a greatly reduced life expectancy. What could cause the development of telangiectases (small clusters of enlarged blood vessels)?

Cell cycle review Interactive cell cycle animation. Control of cell cycle game on the Nobel Prize website (simulation). Animation of the action of the Rb and p53 proteins. This animation has the option of showing the checkpoints.

Control of the cell cycle LOs 1 ________ are critical control points where stop and go ahead signals regulate the cycle. For many cells the __ ________is the most important. If a go ahead signal is not reached at the G1 checkpoint the cell switches to a ___-_____ state called the __ phase. As the cell size increases during the G1 _____ proteins accumulate and combine with _____ to form regulatory protein molecules known as ____-______ _______ (____). Cdks cause the _________ of proteins that stimulate the cell cycle.

Control of the cell cycle LOs 2 If a sufficient threshold of _________ is reached the cell cycle moves on to the next stage. If an insufficient threshold is reached, the cell is held at a checkpoint. The __ ____ phosphorylates a transcription factor inhibitor, _________ (__) protein, allowing DNA replication in the __ phase. DNA damage triggers the activation of several proteins including ___ that can stimulate DNA repair, _____ the cell cycle or cause cell ____.

Control of the cell cycle LOs 1 Checkpoints are critical control points where stop and go ahead signals regulate the cycle. For many cells the G1 checkpoint is the most important. If a go ahead signal is not reached at the G1 checkpoint the cell switches to a non-dividing state called the G0 phase. As the cell size increases during the G1 cyclin proteins accumulate and combine with kinases to form regulatory protein molecules known as cyclin-dependent kinases (Cdks). Cdks cause the phosphorylation of proteins that stimulate the cell cycle.

Control of the cell cycle LOs 2 If a sufficient threshold of phosphorylation is reached the cell cycle moves on to the next stage. If an insufficient threshold is reached, the cell is held at a checkpoint. The G1 Cdk phosphorylates a transcription factor inhibitor, retinoblastoma (Rb) protein, allowing DNA replication in the S phase. DNA damage triggers the activation of several proteins including p53 that can stimulate DNA repair, arrest the cell cycle or cause cell death.