Cell Division Why do cells divide? Reproduction Growth Repair & Replace

What is a genome? Eukaryotic Cell Linear many Circular single

How much DNA in a human cell? 2 meters # Genes? Fewer than 20,000 code for over 100,000 proteins

Chromatin DNA and proteins Protein function? Structure of DNA Control activity of genes

Heterochromatin vs. Euchromatin

Levels of DNA coiling and folding Nucleosome (10 nm in diameter) DNA double helix (2 nm in diameter) H1 Histone tail Histones Figure 16.21a Chromatin packing in a eukaryotic chromosome DNA, the double helix Histones Nucleosomes, or “beads on a string” (10-nm fiber)

Fig. 16-21b 30-nm fiber Looped domains (300-nm fiber) Chromatid (700 nm) 30-nm fiber Loops Scaffold 300-nm fiber Figure 16.21b Chromatin packing in a eukaryotic chromosome Replicated chromosome (1,400 nm) 30-nm fiber Looped domains (300-nm fiber) Metaphase chromosome

How many chromosomes in a cell? Somatic Cell 46 Sex Cell (Gamete) 23 Examples: Examples:

Parts of a Chromosome Cohesin proteins (not cohesion!) Chromatid= Most closely attached Cohesin proteins (not cohesion!) Chromatid= Replicated chromosome

Mitosis vs. cytokinesis nuclear division Cytoplasmic division

Comparing Mitosis & Meiosis Genetically identical Meiosis Genetic Variation Reduction Division 2n 2n 2n 2n n n n n n n

all subphases cell grows Produces proteins and all organelles Cell Cycle 2 phases: Interphase & mitotic phase all subphases cell grows Produces proteins and all organelles BOTH mitosis and cytokinesis 24 hrs 10 - 12 hours 5 - 6 hours 4- 6 hours 1 hour

Stages of Mitosis (Interphase) MITOSIS Prophase Metaphase Anaphase Telophase & Cytokinesis (I Probably Mixed All This up)

MTOC/Centrosome centrioles Aster Centrosome Microtubules Kineto- chores Centrosome

Interphase 90% of Cell Cycle G1, S, & G2 Nucleolus visible Chromosomes replicating Nuclear membrane intact Single Centrosome replicates in animal cells

When chromosomes replicate

Prophase Chromatin shortens and condenses Chromosomes appear as sister chromatids (X) Nucleolus disappears Nuclear membrane disappears Centrosomes (Centrioles) move to the poles Spindle fibers form

Metaphase Chromosomes meet @ equator/middle Centrioles are @ poles Spindle fibers attach to Kinetochores Longest phase

Anaphase Spindle fibers shorten and pull chromosome by centromere Chromosomes move apart/away Sister chromatids separate Shortest phase 2 sets of chromosomes!!

Telophase & Cytokinesis Chromosomes are @ poles Chromosomes lengthen & Nuclear envelope reforms Nucleoli reappear Cleavage furrow / Cell plate forms

What Phase is This?

Phases

Mitosis movie 3D tutorial cells alive mcgraw-hill.com

How Do Spindles Shorten?

Spindle fibers shortening

Cytokinesis Animal Cell Cleavage Furrow Plant Cell Cell Plate

G2 -> prometaphase

Metaphase -> cytokinesis

Animal Cell Division

Bacterial Cell Division Binary Fission

Frequency of cell division Frequency of cell division varies by cell type embryo cell cycle < 20 minute skin cells divide frequently throughout life 12-24 hours cycle liver cells retain ability to divide, but keep it in reserve divide once every year or two mature nerve cells & muscle cells do not divide at all after maturity permanently in G0 G2 S G1 M metaphase prophase anaphase telophase interphase (G1, S, G2 phases) mitosis (M) cytokinesis (C) C

Regulation of the Cell Cycle G1, G2, M checkpoints M checkpoint Chromosomes attached to spindles? Can sister chromatids separate correctly? G2 checkpoint DNA synthesis correct? Commitment to mitosis G1 checkpoint/ “Restriction point” Go / No Go can DNA synthesis begin? What cells never divide? G0

G0 phase G0 phase non-dividing, differentiated state most human cells in G0 phase liver cells in G0, but can be “called back” to cell cycle by external cues nerve & muscle cells highly specialized arrested in G0 & can never divide

Activation of cell division How do cells know when to divide? cell communication signals chemical signals in cytoplasm give cue (internal signals) Internal signals: promoting factors External signals: growth factors signals usually mean proteins activators inhibitors

When combined are now MPF complexes- mitosis promoting factor Regulatory molecules Protein kinases (Cdks- Cyclin dependant kinases) Activate or inactivate other proteins By phosphorylation G1 and G2 checkpoints Many are at a constant concentration But are in inactive form Cyclins Attach to kinases to activate them inactivated Cdk activated Cdk When combined are now MPF complexes- mitosis promoting factor

Cdks (Cyclin dependant kinases) and Cyclins ARE PROTEINS! Regulatory molecules Cdks (Cyclin dependant kinases) and Cyclins ARE PROTEINS! MPF complex

Cyclins & Cdks 1970s-80s | 2001 Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle Cyclin is synthesized in S phase and continues to increase during G2, falls during M Cyclin and Cdk form MPF complexes, MPF “mitosis promoting factor” activated at end of G2 MPF phosphorylates variety of proteins, initiates mitosis There are multiple cyclins, each with a specific role. Cyclins are unstable. Some are triggered for destruction by phosphorylation. Others are inherently unstable and are synthesized discontinuously during the cell cycle. Oscillations in the activities of cyclin-dependent kinases (CDKs) dictate orderly progression through the cell division cycle. In the simplest case of yeast, a progressive rise in the activity of a single cyclin-CDK complex can initiate DNA synthesis and then mitosis, and the subsequent fall in CDK activity resets the system for the next cell cycle. In most organisms, however, the cell cycle machinery relies on multiple cyclin-CDKs, whose individual but coordinated activities are each thought to be responsible for just a subset of cell cycle events.

Cyclins & Cdks 1970s-80s | 2001 Interaction of Cdk’s & different cyclins triggers the stages of the cell cycle Peaks during metaphase MPF cyclin component degrades during anaphase due to APC (anaphase promoting complex) Cdk is inactive There are multiple cyclins, each with a specific role. Cyclins are unstable. Some are triggered for destruction by phosphorylation. Others are inherently unstable and are synthesized discontinuously during the cell cycle. Oscillations in the activities of cyclin-dependent kinases (CDKs) dictate orderly progression through the cell division cycle. In the simplest case of yeast, a progressive rise in the activity of a single cyclin-CDK complex can initiate DNA synthesis and then mitosis, and the subsequent fall in CDK activity resets the system for the next cell cycle. In most organisms, however, the cell cycle machinery relies on multiple cyclin-CDKs, whose individual but coordinated activities are each thought to be responsible for just a subset of cell cycle events. bozeman 6:30

G2/ Cdk / cyclin synthesized M (Spindle) checkpoint G2 checkpoint Chromosomes attached at metaphase plate Replication completed DNA integrity Inactive Active Active G2/ Cdk / cyclin synthesized (MPF) Inactive M APC cytokinesis C G2 mitosis G1 S Cdk inactive / G1 cyclin degraded Inactive MPF = Mitosis Promoting Factor APC = Anaphase Promoting Complex Active G1 checkpoint Growth factors Nutritional state of cell Size of cell

Cyclin & Cyclin-dependent kinases CDKs & cyclin drive cell from one phase to next in cell cycle proper regulation of cell cycle is so key to life that the genes for these regulatory proteins have been highly conserved through evolution the genes are basically the same in yeast, insects, plants & animals (including humans) CDK and cyclin together form an enzyme that activates other proteins by chemical modification (phosphorylation). The amount of CDK molecules is constant during the cell cycle, but their activities vary because of the regulatory function of the cyclins. CDK can be compared with an engine and cyclin with a gear box controlling whether the engine will run in the idling state or drive the cell forward in the cell cycle.

External signals Growth factors (50 + different kinds) coordination between cells Is a protein signal released by body cells that stimulate other cells to divide density-dependent inhibition crowded cells stop dividing when cell binds a surface protein to another cell sends growth inhibiting signal to both cells anchorage dependence to divide cells must be attached to a substrate (extracellular matrix of a tissue “touch sensor” receptors

Growth factor signals growth factor cell division cell surface nuclear pore nuclear membrane P P cell division cell surface receptor Cdk protein kinase cascade P E2F P chromosome Rb P E2F cytoplasm Rb nucleus

Example of a Growth Factor Platelet Derived Growth Factor (PDGF) made by platelets in blood clots Bind to receptors on fibroblasts(type of connective tissue) binding of PDGF to cell receptors stimulates cell division in connective tissue heal wounds Erythropoietin (EPO): A hormone produced by the kidney that promotes the formation of red blood cells in the bone marrow. EPO is a glycoprotein (a protein with a sugar attached to it). The kidney cells that make EPO are specialized and are sensitive to low oxygen levels in the blood. These cells release EPO when the oxygen level is low in the kidney. EPO then stimulates the bone marrow to produce more red cells and thereby increase the oxygen-carrying capacity of the blood. EPO is the prime regulator of red blood cell production. Its major functions are to promote the differentiation and development of red blood cells and to initiate the production of hemoglobin, the molecule within red cells that transports oxygen. EPO has been much misused as a performance-enhancing drug (“blood doping”) in endurance athletes including some cyclists (in the Tour de France), long-distance runners, speed skaters, and Nordic (cross-country) skiers. When misused in such situations, EPO is thought to be especially dangerous (perhaps because dehydration can further increase the viscosity of the blood, increasing the risk for heart attacks and strokes. EPO has been banned by the Tour de France, the Olympics, and other sports organizations.

Example of a Growth Factor Erythropoietin (EPO): A hormone produced by the kidney that promotes the formation of red blood cells in the bone marrow. EPO is a glycoprotein (a protein with a sugar attached to it). The kidney cells that make EPO are specialized and are sensitive to low oxygen levels in the blood. These cells release EPO when the oxygen level is low in the kidney. EPO then stimulates the bone marrow to produce more red cells and thereby increase the oxygen-carrying capacity of the blood. EPO is the prime regulator of red blood cell production. Its major functions are to promote the differentiation and development of red blood cells and to initiate the production of hemoglobin, the molecule within red cells that transports oxygen. EPO has been much misused as a performance-enhancing drug (“blood doping”) in endurance athletes including some cyclists (in the Tour de France), long-distance runners, speed skaters, and Nordic (cross-country) skiers. When misused in such situations, EPO is thought to be especially dangerous (perhaps because dehydration can further increase the viscosity of the blood, increasing the risk for heart attacks and strokes. EPO has been banned by the Tour de France, the Olympics, and other sports organizations.

Growth Factors and Cancer Growth factors can create cancers proto-oncogenes (ch 18) normally activates cell division growth factor genes become oncogenes (cancer-causing) when mutated

Cancer & Cell Growth Cancer is essentially a failure of cell division control unrestrained, uncontrolled cell growth What control is lost? lose checkpoint stops

Immortal Cells

Tumors Mass of abnormal cells Benign tumor Malignant tumor abnormal cells remain at original site as a lump p53 has halted cell divisions most do not cause serious problems & can be removed by surgery Malignant tumor cells leave original site lose attachment to nearby cells carried by blood & lymph system to other tissues start more tumors = metastasis impair functions of organs throughout body

Traditional treatments for cancers Treatments target rapidly dividing cells high-energy radiation kills rapidly dividing cells chemotherapy stop DNA replication stop mitosis & cytokinesis stop blood vessel growth Taxol- freeze spindles microtubules

freeze spindles microtubules Taxol freeze spindles microtubules