Cell Division and the Cell Cycle: A Comprehensive Overview

1. Chapter 12: The Reproduction of Cells Prokaryotic Cell Division: no nucleus Prokaryotes contain most vital genes in a singular circular chromosome-They reproduce through binary fission • Eukaryotic cells have a nucleus and divide via mitosis and cytokinesis

2. The Cell Cycle: I - Interphase includes a period of intense biochemical activity in preparation for cell division 1) G1 phase – first growth phase (G = “gap”) 2) S phase – synthesis phase when DNA is synthesized as chromosomes are duplicated 3) G2 phase – second growth phase

3. I I- M Phaseis the shortest part of the cell cycle and the phase during which the cell actually divides, includes: 1) Mitosis – division of the nucleus a)prophase b)prometaphase c)metaphase d) anaphase e)telophase 2) Cytokinesis – division of the cytoplasm

4. I. Interphase most proteins and other materials are synthesized throughout this phase G1 phase – growth and increase of enzymes for replication of DNA S phase – DNA replication G2 phase – protein synthesis and preparation for division -characterized by well defined nucleus and distinct nucleoli bound by a nuclear envelope, but duplicated chromosomes not distinguished individually yet

5. M – Phase (Mitosis) I.Prophase In the nucleus:  Nucleoli disappear  Chromatin fibers tightly coil and condense into discrete chromosomes that can be viewed with a light microscope  Each chromosome is composed of two identical sister chromatids joined at the centromere In the cytoplasm  Mitotic spindle fibers form (microtubules between two centrosomes) Centrosomes move apart, propelled along the surface of the nucleus

6. II. Prometaphase  Nuclear envelope dissolves, allowing microtubules to interact with the highly condensed chromosomes  Spindle fibers (bundles of microtubles) extend from each pole toward the cell’s equator  Each chromatid now has a specialized structure, the kinetochore, located at the centromere region Kinetochore microtubules become attached to the kinetochores and put the chromosomes into motion

7. III. Metaphase Centrosomes positioned at opposite poles of the cell  Chromosomes move to the metaphase plate, a line equidistant between the two spindle poles Centromeres of all chromosomes are aligned along the metaphase plate with help of kinetochore microtubules

8. IV. Anaphase  Anaphase is characterized by movement of chromosomes toward opposite poles  In order for this to happen, sister chromatids split apart into separate chromosomes  Because kinetochore fibers are attached to the centromeres, the chromosomes move “centromere first” in a V shape  Simultaneously, the poles of the cell move further apart,enlongating the cell  At the end of anaphase, the two poles have identical collections of chromosomes

9. V. Telophase  During telephase… - Daughter nuclei begin to form at the two poles - Nuclear envelopes form around the chromosomes - Nucleoli reappear - Chromatin fiber of each chromosome uncoils and the chromosomes become less distinct  At the end of telephase… - Mitosis, the division of one nucleus into two genetically identical nuclei, is complete - Cytokinesishas begun and the appearance of two separate daughter cells occurs shortly after mitosis is completed

10. M-PHASE (cytokinesis) Division of the cytosol a) involves a band of actin(microfilaments) in animal cells- forms a cleavage furrow b) involves a cell plate in plant cells

11. A Closer Look at the Mitotic Spindles:  The mitotic spindle is composed of microtubules and associated proteins. Its function is to help pull chromosomes toward the poles.  By late prometaphase, each chromatid of a replicated chromosome develops its own kinetochore (a protein structure on the centromere). 1) some spindle microtubules attach to the kineto- chores and are called kinetochore microtubules 2) some spindle microtubules are not directly attached to the chromosomes, but overlap at the metaphase plate, are called nonkinetochore microtubules

12. Control of Cell Division:(external and internal cues • Contents of the culture medium essential nutrients and special regulatory substances called growth factors • Cell density Crowding inhibits cell division density-dependent inhibition. cancer cells do not exhibit this.

13. G1 phase of the cell cyclerestriction point which occurs late in G1 can result in a nondividing state called the G0 phase (most human body cells Some cells such as nerve and muscle, stay in G0 and never divide. Other cells, such as the liver, can be induced by environmental cues (such as injury) to continue through the cell cycle and divide

14. Cell size The most important indicator -ratio of cytoplasmic volume to genome sizeprevents daughter cells from becoming progressively smaller with each cell cycle

15. 5) Cdks – cyclin dependent kinases a) kinases (proteins) are always in cell b) cyclins(proteins) fluctuate in concentration c) a kinase must join a cyclin to be activated Ex) MPF – M-phase promoting factor triggers passage through G2 checkpoint by phosphorylation (nuclear envelope)

16. Checkpoint Summary- not fully understood G1 checkpoint – cell size/ Cdks G2 checkpoint – Cdks (MPF) M checkpoint – Cdks and kinetochore attachment –triggers a cycle to destroy cyclins and the centromere proteins (APC-anaphase promoting complex)

17. Cancer Cells do not stop growing in response to cell density do not have a restriction point in G1 phase immortal (they continue to divide indefinitely); normal mammalian cells in culture only divide about 20 to 50 times

18. If abnormal cells evade destruction, they may gather to form a tumor (an unregulated growing mass of cells) • If the cells remain at this original site, the mass is benign can be removed by surgery If the cells can spread to other parts of the body, the tumor is called malignant. Only a malignant tumor is said to be cancer.