Lecture 6 The Cell Cycle

G0 RESTRICTION POINT The cell cycle can be divided into interphase and mitosis/cytokinesis. Interphase consists of the first gap phase – G1, the DNA synthesis phase – S, and the second gap phase – G2. The gap phases – G1 and G2 – are also called the growth phases of the cell cycle. G1 has what is known as the restriction point. This is the point at which the cell must decide if it will proceed with the remaining phases of the cell cycle, or go into the G0 phase – this is a non-dividing state of the cell. Many cells can re-enter the cell cycle from the G0 phase, but other do not. If the cell receives the OK to continue with the cell cycle – the cell proceeds to S, G2, and eventually mitosis and cytokinesis. These two daughter cells enter the G1 phase and reach the restriction point again. But what make the cell decide it is going to continue on to S phase, or any other phase of the cell cycle?

G0 G1 G1 S R R Rb represses gene expression P P Rb Rb relieves repression P Rb R growth factor G1 S In many cases, the decision to continue on to S phase depends on the presence of a growth factor signal. Last class – growth factors bind to receptor tyrosine kinases – see next slide…. and in this case, one response is to activate expression of the cyclin D gene cyclin D protein is made, binds to cdk4 protein already present (cycllin dependent kinase 4) and results in activation of its kinase activity cyclin D/cdk4 phosphorylates Rb protein – cells pass the restriction point this a protein that regulates expression of a variety of genes necessary for DNA synthesis one gene it activates is cyclin E, which activates its cdk partner, and that complex of cyclin E/cdk2 phosphorylates Rb at another amino acid, further changing it ability to regulate gene expression ultimate result is that cells enter S phase note: cyclin D and cyclin E proteins are targeted for destruction once cells enter S phase P R Rb P activate expression of cyclin D activate cdk2 P activate cdk4 Rb activate expression of cyclin E

growth factor binds to RTK activation of the receptor kinase activity transduction of the signal (go back) connective tissue digest the ECM with enzymes grow with PDGF or without with PDGF – cells divided signal transduction

S G2 M cyclin B cyclin A cdk1 cdk2 cyclin A/cdk2 activity necessary to progress through S need cyclin B/cdk1 to enter mitosis – part of MPF MPF will stimulate formation of the mitotic spindle checkpoints at G2 and M to make sure all is proceeding properly DNA damage checkpoints: at start of S at the end of S at the start of M Replicated DNA checkpoint at start of M Spindle assembly checkpoint during the beginning of M Segregated chromosomes checkpoint during late M

Cancer uncontrolled cell division genetic damage not corectable specific damage to genes that regulate cell division GF GF P P P P cancer cells – do not respond to normal mechanisms that control cell division density-dependent inhibition of growth anchorage dependence of growth they are also able to secrete signals that cause blood vessels to grow – gives them a nutrient source they can lose or destroy their attachment to other neighboring cells and breakdown their local area of the extracellular matrix result: the cells can move; can enter blood or lymphatic vessels in culture – cancer cells can grow without growth factors either make their own growth factors or there is an abnormality in the signaling pathway activation of cyclin D expression cell division

Mitosis and Cytokinesis mitosis: division of the nucleus and its chromosomes cytokinesis: division of the cytoplasm RESULT: one cell divides into 2 daughter cells

Phases of Mitosis Prophase Prometaphase Metaphase Anaphase Telophase

During S phase Going back to S phase – all DNA is replicated so that each chromosome becomes 2 sister chromatids connected by a centromere each of the two sister chromatids has a kinetochore – a protein-based structure that associates with specific portions of the centromere the two kinetochores face in oppsoite directions

Just outside the nucleus – two centromeres coordinating center for formation of the microtubules for the mitotic spindles when the cell knows that it will continue on with cell division, the centrosome is replicated the microtubules that make up the cytoskeleton partially break down and the assembly of the mitotic spindle begins an aster forms – a radial array of short microtubules the centrosomes push apart and will end up on opposite sides of the cell show video

Prophase – chromatin more tightly coiled each chromosome appears as two identical sister chromatids mitotic spindle begins to form centrosomes move away from each other Prometaphase – nuclear envelope begins to break down chromosomes form kinetochores chromosomes begin to interact with the microtubules begin the tug of war between the opposite poles asters begin to make associations with the plasma membrane the mitotic spindle continues to grow – microtubules that will not bind kinetochores reach out to each other and interact

Metaphase: centrosomes are at opposite sides of the cell chromosomes are attached to the spindle – such that the kinetochore of each sister chromatid is attached to a separate microtubule – one to a microtubule from one pole and the other to a microtubule from the other pole - will allow separation of the sister chromatids into the two separate cells later lined up at the metaphase plate Anaphase: the proteins holding the two sister chromatids together are inactivated two sister chromatids pull apart and move to opposite poles of the cell accomplish this through the kinetochore and motor proteins – uses ATP the microtubules not associated with chromosomes lengthen while the ones associated with chromosomes shorten (depolymerization of tubulin) Telophase: two new nuclear envelopes begin to form chromosomes become less condensed microtubules not bound to chromosomes continue to grow to push apart the two poles Cytokinesis: division of the cytoplasm

microtubles shorten as the chromosomes move towards their respective poles

In animal cells: cleavage furrow forms result of interactions between actin and myosin microfilaments – like muscle contraction tightening of a drawstring furrow deepens until the cell is pinched into 2 cells Plant cells: have a cell wall vessicles derived from the Golgi apparatus line up along the center of the cell using microtubules come together to form the cell plate vessicles carry material necessary to form a cell wall cell plate grows until it fuses with the existing plasma membrane

go over again rewatch movie?