CHAPTER 12. THE CELL CYCLE. I. OVERVIEW. 1 . Cell division is defined as the reproduction of cells and is the characteristic that best distinguishes living things from nonliving matter. 2. In unicellular organisms, cell division reproduces an entire organism (Ex: bacteria)
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THE CELL CYCLE
1. Celldivisionis defined as the reproduction of cells and is the characteristic that best distinguishes living things from nonliving matter.
2. In unicellular organisms, cell division reproduces an entire organism (Ex: bacteria)
3. In multicellular organisms, cell division functions in:
4. Cell division is an integral part of the cell cycle, the life of a cell from formation to its own division.
a. Precise replication of DNA
b. Movement of this DNA to opposite ends of the cell
c. Separation into two daughter cells with identical genetic information, DNA
A. Cellular Organization of the Genetic Material
Somatic cells (nonreproductive cells) have two sets of chromosomes; produced by mitosis; genetically identical
A. Phases of the Cell Cycle
a. G1 Phase (first gap)—cell grows; first growth phase when the centrioles replicate
b. S Phase (synthesis)--DNA replicates
c. G2 Phase (second gap)—cell grows; second growth phase during which the cell is preparing for mitosis by producing proteins, ATP, etc.
1. Changes in nucleus:
--Composed of microtubules and associated proteins which are arranged between the two centrosomes (microtubule-organizing center)
--Apparently they are propelled along the surface of the nucleus by lengthening of microtubule bundles between them
1. Nuclear envelope fragments
2. Microtubules interact with chromosomes
3. Spindle fibers (bundles of microtubules) extend from each pole toward the equator (midpoint between the poles) (aka metaphase plate) of the cell
4. Each chromatid has akinetochore (specialized structure located at the centromere region)
5. Bundles of microtubules (kinetochore microtubules) attach to kinetochores and put chromosomes into agitated motion
6. Microtubules (nonkinetochore microtubules) radiated from each centrosome toward the metaphase plate (equator) without attaching to chromosome
1. Centrosomes positioned at poles (opposite
ends ) of the cell
2. Chromosomes move to metaphase plate (plane equidistant between the spindle poles)
3. Centromeres of all chromosomes aligned on metaphase plate
4. Kinetochore fibers of sister chromatids face opposite poles so identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell
5. Entire structure formed by nonkinetochore microtubules plus kinetochore microtubules called the spindle
1. Begins when paired centromeres of each chromosome move apart
2. Sister chromatids separate and are considered chromosomes
3. Spindle apparatus starts moving the separate chromosomes toward opposite poles
4. Chromosomes move in a V-shape because of attachment of kinetochore fibers to centromere
5. Kinetochore microtubules shorten at end attached to kinetochores
6. Poles of cell move farther apart slightly elongating the cell
7. At end of phase two poles have complete and equivalent collections of chromosomes
1. Nonkinetochore microtubules further elongate the cell
2. Daughter nuclei begin to form at the two poles
3. Nuclear envelop reforms from fragments of parent cell nuclear envelope and other portions of endomembrane system
4. Nucleoli reappear
5. Chromosomes uncoil and appear as chromatin
6. Cytokinesis occurs
7. At the end of the phase:
1. Important to events occurring in mitosis
2. Forms in cytoplasm during prophase
3. Composed of microtubules and associated proteins
4. Elements come from partial disassembly of the cytoskeleton
5. Elongation occurs by the addition of tubulin (protein) subunits at one end
6. Assembly begins in the centrosome(microtubule organizing center).
7. In animal cells, a pair of centrioles is in the center of the centrosome.
9. Parallel microtubules form bundles called spindle fibers.
10. Fibers elongate or shorten by adding and removing tubulin subunits from end away from centrosome.
11. 2 Types of Spindle Fibers:
a. kinetochore microtubules
-align chromosomes at cell’s midline
-to move chromosomes to poles
1. Process of cytoplasmic division that follows mitosis
2. Begins in telophase
3. Differs in plant and animal cells
4. Animal Cells
1. Cells grow to double their size and divide to form two cells
2. Prokaryotes (bacteria and cyanobacteria) reproduce by binary fission (division in half).
3. Have a single circular DNA molecule that carries most of the genes
4. Soon after chromosome replication begins, one copy of the origin of replication (first replicated region) begins to move toward the other end of the cell.
5. Replication continues and eventually one copy of the origin is at each end of the cell.
6. Plasma membrane grows inward, and new cell wall is deposited.
7. Two daughter cells
8. Single celled eukaryotes must still go through mitosis prior to division.
-human skin cells divide frequently
-liver cells divide to repair damaged cells
-nerve, muscle, and other specialized cells do not divide in mature humans
1. Cell cycle appears to be driven by specific chemical signals in the cytoplasm.
2. The sequential events of the cell cycle are directed by a distinct cell cycle control system, which is similar to a clock
7.For many cells, the G1 checkpoint seems to be the most important one
-Ex: a. muscle and nerve cells until death
b. liver cells until recruited back to cell cycle by such cues as growth factors
2. The activity of cyclins and Cdks fluctuates during the cell cycle
* cyclins – proteins that bind to and activate protein kinase
3. MPF (maturation-promoting factor) is a cyclin-Cdk complex that triggers a cell’s passage past the G2 checkpoint into the M phase
1. Cancer cells do not respond normally to the body’s control mechanisms
2. Cancer cells may not need growth factors to grow and divide:
3. A normal cell is converted to a cancerous cell by a process called transformation
4. Cancer cells form tumors, masses of abnormal cells within otherwise normal tissue
5. If abnormal cells remain at the original site, the lump is called a benign tumor
6. Malignant tumors invade surrounding tissues and can metastasize, exporting cancer cells to other parts of the body, where they may form secondary tumors
Describe the process of binary fission in bacteria and explain how eukaryotic mitosis may have evolved from binary fission