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الاحــياء - Biology الاحياء العامة

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قديم 03-01-2008, 07:20 PM   #1
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افتراضي cell biology

the sizei of cell
The head of a pin is about 2mm in diameter. Use this animation to compare the relative sizes of cells and organisms sitting on a pinhead. Nearly invisible without magnification, dust mites dwarf pollen grains and human cells. In turn, bacteria and viruses are even smaller.
Plant, Animal and Bacteria Cell Models
Living cells are divided into two types - procaryotic and eucaryotic (sometimes spelled prokaryotic and eukaryotic). This division is based on internal complexity. The following pages can provide graphic roadmaps to the organization of both of these cell types.

Eucaryotic: The cells of protozoa, higher plants and animals are highly structured. These cells tend to be larger than the cells of bacteria, and have developed specialized packaging and transport mechanisms that may be necessary to support their larger size. Use the Interactive animation of plant and animal cells to learn about their respective organelles.

Procaryotic: These cells are simple in structure, with no recognizable organelles. They have an outer cell wall that gives them shape. Just under the rigid cell wall is the more fluid cell membrane. The cytoplasm enclosed within the cell membrane does not exhibit much structure when viewed by electron microscopylll
Animal Cell Mitosis
This animation demonstrates the stages of mitosis in an animal cell. Use the control buttons in the upper left to run the complete animation. Click on any intermediate stage (for example, Anaphase), and see a representative still frame.
Events during Mitosis
Interphase: Cells may appear inactive during this stage, but they are quite the opposite. This is the longest period of the complete cell cycle during which DNA replicates, the centrioles divide, and proteins are actively produced. For a complete description of the events during Interphase, read about the Cell Cycle.

Prophase: During this first mitotic stage, the nucleolus fades and chromatin (replicated DNA and associated proteins) condenses into chromosomes. Each replicated chromosome comprises two chromatids, both with the same genetic information. Microtubules of the cytoskeleton, responsible for cell shape, motility and attachment to other cells during interphase, disassemble. And the building blocks of these microtubules are used to grow the mitotic spindle from the region of the centrosomes.

Prometaphase: In this stage the nuclear envelope breaks down so there is no longer a recognizable nucleus. Some mitotic spindle fibers elongate from the centrosomes and attach to kinetochores, protein bundles at the centromere region on the chromosomes where sister chromatids are joined. Other spindle fibers elongate but instead of attaching to chromosomes, overlap each other at the cell center.

Metaphase: Tension applied by the spindle fibers aligns all chromosomes in one plane at the center of the cell.

Anaphase: Spindle fibers shorten, the kinetochores separate, and the chromatids (daughter chromosomes) are pulled apart and begin moving to the cell poles.

Telophase: The daughter chromosomes arrive at the poles and the spindle fibers that have pulled them apart disappear.

Cytokinesis: The spindle fibers not attached to chromosomes begin breaking down until only that portion of overlap is left. It is in this region that a contractile ring cleaves the cell into two daughter cells. Microtubules then reorganize into a new cytoskeleton for the return to interphase.


Animal Cell Meiosis

Meiosis is important in assuring genetic diversity in sexual reproduction. Use this interactive animation to follow Meiosis I (reduction division) and Meiosis II in a continuous sequence or stop at any stage and review critical events. This animation is also available for offline use in single user and multi-user versions and on the CELLS alive! .

Events during Meiosis
Diploid Cell (2N): From a preceding mitotic division, the Oogonium (Spermatogonium) enters meiosis with DIPLOID (2N) chromosomes but TETRAPLOID (4N) DNA. Chromosomes then duplicate to produce SISTER CHROMATIDS (or HOMOLOGOUS DYADS).


Prophase I: Dyad pairs align to create "TETRADS", non-sister chromatids connect and trade sections at a "CHIASMA", a process called "CROSSING OVER".


Metaphase I: SPINDLE FIBERS attach to each dyad at the KINETOCHORE. Tension from spindle fibers aligns the tetrads at the cell equator.


Anaphase I: Chiasmata break apart and sister chromatids begin migrating toward opposite poles.


Telophase I: CLEAVAGE FURROW forms beginning the process of CYTOKINESIS (cell division). Resulting daughter cells are HAPLOID (1N).


Prophase II: Spindle formation begins and centrosomes begin moving toward poles.


Metaphase II: Tension from spindle fibers aligns chromosomes at the metaphase plate.


Anaphase II: CHROMATIDS separate and begin moving to the poles.


Telophase II: CLEAVAGE FURROW forms beginning CYTOKINESIS.


Gamete (1N): NUCLEAR ENVELOPES form and chromosomes disperse as CHROMATIN. Meiosis has produced 4 DAUGHTER CELLS, each with 1N chromosomes and 1N DNA. Later, in fertilization, male and female 1N gametes will fuse to form a 2N ZYGOTE.

The Cell Cycle
During development from stem to fully differentiated, cells in the body alternately divide (mitosis) and "appear" to be resting (interphase). This sequence of activities exhibited by cells is called the cell cycle.

Interphase, which appears to the eye to be a resting stage between cell divisions, is actually a period of diverse activities. Those interphase activities are indispensible in making the next mitosis possible.
Interphase: Interphase generally lasts at least 12 to 24 hours in mammalian tissue. During this period, the cell is constantly synthesizing RNA, producing protein and growing in size. By studying molecular events in cells, scientists have determined that interphase can be divided into 4 steps: Gap 0 (G0), Gap 1 (G1), S (synthesis) phase, Gap 2 (G2).
Gap 0 (G0): There are times when a cell will leave the cycle and quit dividing. This may be a temporary resting period or more permanent. An example of the latter is a cell that has reached an end stage of development and will no longer divide (e.g. neuron).

Gap 1 (G1): Cells increase in size in Gap 1, produce RNA and synthesize protein. An important cell cycle control mechanism activated during this period (G1 Checkpoint) ensures that everything is ready for DNA synthesis. (Click on the Checkpoints animation, above.)

S Phase: To produce two similar daughter cells, the complete DNA instructions in the cell must be duplicated. DNA replication occurs during this S (synthesis) phase.

Gap 2 (G2): During the gap between DNA synthesis and mitosis, the cell will continue to grow and produce new proteins. At the end of this gap is another control checkpoint (G2 Checkpoint) to determine if the cell can now proceed to enter M (mitosis) and divide.


Mitosis or M Phase: Cell growth and protein production stop at this stage in the cell cycle. All of the cell's energy is focused on the complex and orderly division into two similar daughter cells. Mitosis is much shorter than interphase, lasting perhaps only one to two hours. As in both G1 and G2, there is a Checkpoint in the middle of mitosis (Metaphase Checkpoint) that ensures the cell is ready to complete cell division. Actual stages of mitosis can be viewed at Animal Cell Mitosis.

Pumping Myocytes The Beat of a Single Cell
We may not immediately think of our heart as a collection of individual cells. But it is the complex interaction of numerous cell types that give the heart its ability to pump blood. Some cells form heart connective tissue, other cells grow into heart valves. And muscle cells give the heart its ability to beat and pump blood throughout the body.





You can dissolve an embryonic heart into its individual cell types with trypsin, an enzyme that destroys the protein glue between the cells. Plate these cells in a dish and you will see some cells - called myocytes - that beat independently. The cells shown here are from the chick embryo.

A single cell beats when a complex series of gates - called ion channels - open and close in an organized manner. Cell physiologists can measure how these ion channels work using a technique called the patch clamp.



Beating in Unison


As long as the beating cells do not touch one another, their beats are independent - some are faster, some are slower. But after two or three days, the myocytes form interconnected sheets of cells (monolayers, shown right) that beat in unison. Pores (gap junctions) open between adjacent touching cells, making their cytoplasms interconnected. It is these gap junctions that ensure that the connected cells work as one.

If the cells of the adult don't beat in unison, heart arrythmias can occur. Electronic pacemakers may sometimes be used in a patient whose heart doesn't beat in rhythm.

Apoptosis: Dance of Death
Human neutrophils are constantly produced in the marrow and released into the blood to search for invading pathogens. After only one day, they "commit suicide" and are replaced by younger cells. Beginning with the typical neutrophil shape (left panel, above) this apoptotic neutrophil undergoes a series of changes including violent membrane blebbing, called zeiosis, (animation to the right) and fragmentation of DNA creating a vacuolar nucleus. Apoptotic cells shrink in size, break into smaller pieces called apoptotic bodies that other body cells recognize and eat.



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قديم 03-26-2008, 06:17 PM   #2
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Umbrella رد: cell biology

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