Cell Division and Mitosis

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1 Chromatin-Uncoiled DNA during interphase Cell Division and Mitosis Chromosomes-Tightly coiled DNA Chromatid-One half of a duplicated chromosome. Each is identical and called sister chromatids Centromere-The region between sister chromatids. Centrosome-The main place where microtubules are organized. In animal cells, pairs of centrioles are found there Aster-Small microtubules that radiate from the centrosome. Found just before and during mitosis. Kinetochore-A complex of proteins associated with the centromere. It serves as the attachment site for spindle microtubules and the site at which motors generate forces to power chromosome movement. Unattached kinetochores are also the signal generators for the mitotic checkpoint, which arrests mitosis until all kinetochores have correctly attached to spindle microtubules Spindle Fiber (microtubule)- Spindle fibers form a protein structure that divides the genetic material in a cell. The spindle is necessary to equally divide the chromosomes in a parental cell into two daughter cells during both types of nuclear division: mitosis and meiosis Haploid-The number of chromosomes an organism has Diploid-The total number of chromosomes, meaning one copy from each parent.

2 What can be said about cell division of single celled organisms? Why must cells divide? What is the result of cell division? What is the exception? What is a genome? What is a chromosome? What is chromatin? How are somatic and gametes different?

3 Describe what a duplicated chromosome looks like. Eukaryotic cell division consists of two parts, and Cell Cycle consists of two parts, Interphase Mitosis Interphase is about % of the cell cycle and is divided into three parts G1 S-phase G2

4 Remember that the cell is continually growing M G o Phases of Mitosis-Typically divided into the five phases G 2 of Interphase Centrosomes (with centriole pairs) Chromatin (duplicated) E s Nucleolus Nuclear envelope Plasma membrane

5 Prophase Early mitotic spindle Aster Centromere e Chromosome, consisting of two sister chromatids Prometaphase Fragments of nuclear envelope Nonkinetochore microtubules Kinetochore Kinetochore microtubule Metaphase Metaphase plate Spindle Centrosome at one spindle pole

6 Anaphase Daughter chromosomes Telophase and Cytokinesis Cleavage furrow Nucleolus forming Nuclear envelope forming Cytokinesis Animal Cells

7 Plant Cells Binary Fission in Bacteria Prokaryotes (bacteria and archaea) reproduce by a type of cell division called In binary fission, the chromosome replicates (beginning at the ), and the two daughter chromosomes actively move apart The plasma membrane pinches, dividing the cell into two Figure Chromosome replication begins. Origin of replication Two copies of origin E. coli cell Cell wall Plasma membrane Bacterial chromosome 2 Replication continues. Origin Origin 3 Replication finishes. 4 Two daughter cells result.

8 Regulation of the Cell Cycle The frequency of cell division varies with the type of cell These differences result from regulation at the molecular level Cancer cells manage to escape the usual controls on the cell cycle Interesting Experiment EXPERIMENT Experiment 1 Experiment 2 S G 1 M G 1 RESULTS S S When a cell in the S phase was fused with a cell in G 1, the G 1 nucleus immediately entered the S phase DNA was synthesized. M M When a cell in the M phase was fused with a cell in G 1, the G 1 nucleus immediately began mitosis a spindle formed and chromatin condensed, even though the chromosome had not been duplicated. The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock The cell cycle control system is regulated by both internal and external controls The clock has specific checkpoints where the cell cycle stops until a go-ahead signal is received

9 For many cells, the checkpoint seems to be the most important If a cell receives a go-ahead signal at the G 1 checkpoint, it will usually complete the, G 2, and phases and divide If the cell does not receive the go-ahead signal, it will exit the cycle, switching into a nondividing state called the Figure G 1 checkpoint G 0 G 1 Control system S G 1 checkpoint M G 2 G 1 G 1 M checkpoint G 2 checkpoint (a) Cell receives a go-ahead signal. (b) Cell does not receive a go-ahead signal. The Cell Cycle Clock: Cyclins and Cyclin-Dependent Kinases Two types of regulatory proteins are involved in cell cycle control: cyclins and cyclin-dependent kinases (Cdks) Cdks activity fluctuates during the cell cycle because it is controlled by cyclins, so named because their concentrations vary with the cell cycle MPF (maturation-promoting factor) is a cyclin-cdk complex that triggers a cell s passage past the G 2 checkpoint into the M phase

10 Stop and Go Signs: Internal and External Signals at the Checkpoints An example of an internal signal is that kinetochores not attached to spindle microtubules send a molecular signal that delays anaphase Some external signals are growth factors, proteins released by certain cells that stimulate other cells to divide For example, platelet-derived growth factor (PDGF) stimulates the division of human fibroblast cells in culture A sample of human connective tissue is cut up into small pieces. Enzymes digest the extracellular matrix, resulting in a suspension of free fibroblasts. Petri dish Cells are transferred to culture vessels. Scalpels 4 PDGF is added to half the vessels. 10 µm Without PDGF With PDGF

11 clear example of external signals is density-dependent inhibition, in which crowded cells stop dividing Most animal cells also exhibit anchorage dependence, in which they must be attached to a substratum in order to divide Cancer cells exhibit neither density-dependent inhibition nor anchorage dependence Anchorage dependence Density-dependent inhibition Density-dependent inhibition 20 µm 20 µm (a) Normal mammalian cells (b) Cancer cells Loss of Cell Cycle Control Cancer Cancer cells do not respond normally to the body s control mechanisms Cancer cells may not need growth factors to grow and divide They may make their own growth factor They may convey a growth factor s signal without the presence of the growth factor They may have an abnormal cell cycle control system A normal cell is converted to a cancerous cell by a process called transformation Cancer cells that are not eliminated by the immune system form tumors, masses of abnormal cells within otherwise normal tissue If abnormal cells remain only at the original site, the lump is called a benign tumor

12 Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form additional tumors Summary Questions a. Which phase of the cell cycle does the cell spend most of its time? b. Loosely coiled DNA during interphase is referred to as. c. Why is the S phase important in creating identical daughter cells? d. Explain why chromosomes in prophase are double stranded. What are each of the strands referred to? What holds them together? e. Differentiate between mitosis and cytokinesis. How do these processes differ in plant and animal cells?