Cell Biology - Cape Breton â€“ Victoria Regional School Board

page1

Cell Biology

The Cell Cycle & Mitosis

Site Index

Click on the page number to advance

The Cell Membrane Pages 1-- 2 -- 3 -- 4 -- 5

ER & the Golgi Apparatus Pages 6 -- 7 -- 8 -- 9 -- 10

Ribsomes Page 11

Mitochondria Pages 12 -- 13

Page 14 Nucelus Pages 15 -- 16

Plant Cell Pages 17 -- 18 -- 19 -- 20 21 -- 22 -- 23 -- 24

Cell Ttransport Page 25

Cell Diagram Pages 26 -- 27

Cell Theory Word Puzzle Page 28

The Cell Cycle & Mitosis Pages 29 -- 30 -- 31

Cell Reproduction Page 32

http://www.cbv.ns.ca/bec/science/cell/start.html [3/6/03 1:51:43 PM]

page1

Cell Biology Index | Next

The interiors of eukaryotic cells are subdivided by membranes. The DNA of the cell is packaged with protein and contained in the nucleus in units called chromosomes. Most eukaryotes possess cell walls, although cell walls are lacking in animals and some single celled organisms.

http://www.cbv.ns.ca/bec/science/cell/page1.html (1 of 2) [3/6/03 1:51:55 PM]

page1

Figure 5-9

A Plant Cell: Most mature plant cells contain large central vacuoles, which occupy a thin layer between vacuole and cell membrane. Within it are all of the cell's mitochondria and other organelles.

Index | Next


page1

Cell Biology Previous | Index | Next

Cell Structure: Chapter 1

Cell Outline

Cytoplasm Nucleus Plant CellCell Membrane Nuclear Membrane Cell WallEndoplasmic Reticulum Nucleolus VacuolesGolgi Apparatus Nucleoplasm PlastidsRibosomes ChromosomesLysosomesMicrotubules

Cell Membrane/Plasma Membrane

Light Microscope - merely a line

Modern Advances - 1970's Biochemistry & Electron Microscope. Pg. 146 - Foundations J.D. Robertson Original EM Photo

Unit Membrane Model


page1

Two layers of fat or phospholipids

Danielle & Dawson 3 Cells "Sandwich model" or Fluid Mosaic Model" (one thing, consisting of parts)

Previous | Index | Next


page1


Lipids:

Solid lipids = fats (e.g. butter)

Liquid lipid = oil (e.g. cooking oil)

"Differences in room temprature"

"The Fluid Mosaic Model of the Cell" or

The Sopa Bubble Model

Two layers of interchanging oils that move randomly.

Singer & NicolsonPhospholipid

A bihospholipid layer with proteins disperesed throughot whereever possible. The hollow protein acts as a pour for transport of other materials.

Because the individual molecules are frree to move about, the lipid bilayer is not a solid like a rubber balloon, but rather a liquid like the "shell" of a soap bubble. The bilayer itself is a fluid, with the vicosity of olive oil. Just as the surface tension holds the soap bubbkle together, even though it is made of a liquid, so the hydrogen bonding of water holds the membrane together.

All Membrane Within A Cell Are Interchangable

Endomembrane system functions in the concept of "Vesicle Transprot", i.e. how proteins are transported out of the cell by vesicles.

1. Endoplasmic Reticulum2. Golgi Apparatus

Types of membreane Protein in Cell membranes:

1. Glyco protein: contains special sugar molecules -- signatuere protein -- Identity Cells Blood type A &B


page1

2. Gate-keeper protein: opening & closing paths in the cell membrane3. Others act as receptor cites for hormones4. Carrier Protein:



Cell Biology -- Page 4


A Cell Membrane:

From Lipid Bilayers and Cell Membranes Avaliabe online:http://www.uic.edu/classes/phys/phys461/phys450/MARKO/N016.html


http://www.cbv.ns.ca/bec/science/cell/page4.html [3/6/03 1:52:03 PM]

http://www.uic.edu/classes/phys/phys461/phys450/MARKO/N016.html



A Cell Membrane:

The Fluid Mosaic Model of the Cell Membrane

Hollow Protien PoresPrevious | Index | Next




E.R. - Parallel membranes lying close to the nucleus. They do not extend to the cell membrane. It has the same structure as the cell membrane. Therefore, all membranes are the same.

The endoplasmic reticulum. weaving in sheets through the interior of the cell, creates a series of channels and interconnections between its membranes that isolate some spaces as membrane enclosed sacs called vesicles.

The endoplasmic reticulum (ER) is an extensive system of membranes that divides the interior of eukaryotic cells into the compartments and channels.

Golgi Apparatus:

It is the delivery system of the cell (packaging center). It collects, packages, modifies and makes carbohydrates and adds these to the protein.

Animal cells contain 10 to 20 Golgi bodies each.

Collectively, the Golgi are referred to as the Golgi complex.

It produces the vesicles to help transport materials out of the cell.Previous | Index | Next




Vesicle Transport:

When proteins are for export: Being passed out of the cell. The protein is constructed of amino acids on a ribosome. The ribosome is taken to the ER by special transporting protein known as a “Single Peptide,” the ER is the docking site for ribosomes.

The protein enters the ER and proceeds along its inside until it comes to the end. Then it “buds off” forming a vesicle that will be “transported to the Golgi bodies.”

Here the proteins are collected, packaged, and modified in special ways depending upon the ultimate destination. Vesicles containing these proteins then pinch off the Golgi bodies and head to the cell membrane.

These vesicles eventually fuse with the cell membrane releasing the protein. Where the protein goes and what it does depend upon the specific proteins.



If something is small enough, it will travel through a pore in the cell membrane. Substances can also enter by chemical reactions of proteins on the surface.

If substances are too large, in folding takes place (Endocytosis)













|

Nucleus

Figure 5 -18 How Proteins are secreted across membranes



A

A:Endoplasmic reticulum. The endoplasmic reticulum is continuous with the nuclear membrane and may appear as either rough or smooth endoplasmic reticulum. Ribosomes (black dots) are associated with only one side of the rough Endoplasmic reticulum; the other side bounds a separate compartment within the cell into0 which the ribosomes extrude newly synthesized proteins.

B B: Diagrammatic view of a ribosome on rough ER membrane. The ribosome is associated with a transmembrane protein channel through which the newly synthesized protein passes into the ER lumen. The signal sequence that labeled this pprotein as one to be exported was o9n the leading end of the protein but was clipped off before



the leading end entered the channel.

How the Endomembrane System Functions in Vesicle Transport Previous | Index | Next




Ribosomes are responsible for protein synthesis, i.e. where amino acids are assembled into proteins.

Ribosomes are originated in the nucleolus, i.e. the spot where chromosomes are making ribosomes (where thew DNA codes for ribosomes).

Nucleolus -- a ball of ribosomes

The ER is a "docking site" for ribosomes. The ribosome makes the protein then carries it to the ER by signal peptide. If a protein is for export it lands on the ER and passes out by Vesicle Transport.

If a protein intends to stay within the cell, ribosomes merely float around within the cell (not for export) and doesn't attach to the ER.

The endoplasmic reticulum, weaving in sheets through the interior of the cell, creates a series of channels and interconnections between its membranes that isolates some spaces as membrane-enclosed sacs called vesicles.

The surface of those regions of the ER devoted to the synthesis of transported proteins are heavily studded with ribosomes. When viewed with an electron microscope, their membrane surfaces appear pebbly, like the surface of sandpaper. Because of this "rocky beach" appearance, the regions of ER rich in bound ribosomes are often termed smooth ER. Regions of the endoplasmic reticulum with relatively few bound ribosomes are correspondingly called smooth ER.





2nd Theory of ER

The Rough ER are always rough, i.e., always full of ribosomes. Their function is to export protein. The smooth ER are always full of enzymes necessary to make carbohydrates and fats.

Mitochondria- site for cellular respiration (the release of energy from food moleules).

This is the cell's furnace, battery, or power house. Respiration does not always need oxygen. They are tubular or sausage like structures

Anaerobic Phase (No O2)

Aerobic Phase (O2)

Citric Acid Cycle Electron Transport System Approximately 36% of the energy from sugar molecules is converted into a molecule called adenosine triphosphate (ATP) , a chemical storage compound. Most of the remaining 64% is converted into thermal energy. This means that animals that maintain a constant body temperature use a great number of sugar molecules to keep warm.

Mitochondria have their own DNA, therefore it has its own genes, and therefore it makes its own RNA (Ribosomes) and can make its own protein.



Although it still depends on the nucleus

Autonomy -- owned and fully dependant.

Semi-Autonomy -- free, but still dependant (Mitochondria are semi-autonomous)

Lysomes -- Digestion and Recycling Centre; produced in the Golgi Apparatus.

They contain digestive enzymes called lysozymes. They are capable of digesting any cellular parts, harmful organisms, old cells, and proteins. The cellular parts can also be recycled into new parts for the cell. Proteins can also be recycled.









Autolysis- self-destruction of lysosomes causes the destruction of the cell.

When a tadpole grows into a frog, the tail is reabsorbed by autolysis of the cells in the tail. {cellular suicide} Lysosomes digest old Mitochondria in some cells and replaced every 10 days.

Vesicle- small containers or cellular components used to export/import materials. It contains proteins and such substances.

Vacuole-very large water containers in plant cells

Centrosomes -- responsible for cell division in animal cells.

Cell Division --



Microtubules -- (Cells skeleton) are long, hollow cylinders, composed of proteins. They influence cell shape, move the chromosomes in cell division, and provide the functional internal structure of cilia and flagella, as we discuss below.

Cytoskeleton, cellular skeleton. Hollow, tiny protein tubes, holding or supporting everything.







Microtubules also encircle the cell membrane (inner) and give it support and shape.

MTOC: Microtubule organizing centre: organize Microtubules.

Flagella (singular, flagellum) are fine, long, threadlike organelles protruding from the surface cells; they are used in locomotion and feeding.

Eukaryotic cells have a completely different kind of flagellum, based on a kind of cable made up of microtubules.

If there are many, short flagella organized in dense rows, they are called cilia, but cilia do not differ at all from flagella in structure

NOTE: Sperm cell -- only exampl;e of flagella in human biology.

The Nucleus

Nuclear Membrane Surrounds the nucleus. Same structure as the cell membrane.

"Before nucleus" -prokarvotes (cells)-Lack nuclear membrane. No nucleus.

"True nucleus" -eukaryotes {cells}-

Real, true nucleus with a membrane. -It is porous to allow material in and out. Also can be referred to as a nuclear envelope.

Nucleoplasm- The "cytoplasm, of the nucleus

Nucleolus-

A ball of ribosomes; the spot on the chromosomes (DNA) that codes for ribosomes (RNA) - ribosomes are subunits (smaller)



You have to break down the ribosomes in order to let them pass through the nuclear membrane. Ribosomes in the nucleus are really only parts of ribosomes. Once the ribosomes reach the outside, they reunite with ribosomal parts to form full ribosomes.

Ribosomes consist of ribosomal RNA









Chromosomes- chromatin (colored material). But now it's called DNA.

Chromosomes- DNA is coiled in the nucleus so tight, it's like a ball. When you look at it closely, it looks like a ball of string very tightly wound. When far away, it resembles a solid ball.

Chromosomes are tne cells way or packaging DNA ror tne purpose of cell division.

Plant Cells

Three major differences found in plant cells: 1. Cell Wall2. Vacuoles3. Plastids

Cell Wall

Plants grow towards the sun and toward the ground (roots) by adding cellulose.



Cell walls -- molecule by molecule The cell wall is the plant skeleton.

The cells share the wall with each other (rooms).

If the cell wants to transport substances, it uses the pores. Any highly toxic chemical will penetrate the cell wall and kill the cell.





The presence of additional chromosomes often produces widespread abnormalities. Many infants with such abnormalities are stillborn. Among those who survive, many are mentally deficient. In fact, studies of abnormalities in human chromosome number among living subjects are often carried out on patients in mental hospitals.

These patients frequently have abnormalities of the heart and other organs as well.

Down's Syndrome

One of the most familiar chromosomal abnormalities is the form of mental defciency known as Down's syndrome, after the physician who first described it. (the disease is also sometimes called mongolism, a name that derives from the characteristic appearance of the eyefold in these patients, which makes them look "foreign," or mongoloid, to Europeans.)

Down's syndrome usually involves more than one defect and so is referred to as a syndrome, a group of disorders that occur together. The syndrome includes, in most cases, not only mental deficiency but also a short, stocky body type with a thick neck and, often, abnormalities of other organs, especially the heart.

In about 95 percent of the cases of Down's syndrome, the cause of the genetic abnormality is nondisjunction of chromosome 21. This results in an extra chromosome 21 (Figure 18-lOb) in the cells of the defective child.

18 -10

(a) The normal diploid chromosome number of a human bting is 46. 22 pairs of autosomes and the 2 sex chromosomes.

In the karyotype below, the autosomes are grouped by size (A, B, C, etc.) and then the probable homologues are paired.

A normal woman has two X chromosomes and a normal man, shown here, an X and a Y. (b)

The karyotype of a male patient with Down's syndrome caused by nondisjunction. Note that there are three chromosomes 21.







Very large pores in plant cell walls allow the passage of molecules in and out, such as proteins, fats, and sugars. Things don't pass through the cell wall easily (cellulose). Vacuoles

Vacuoles are large water filled containers. from 60-90% of the cell in mature cells.

The vacuole creates pressure within the cell and plant. The vascular membrane is the same structure as cell membrane.

Vacuole - emptiness

1. 'l'urgor pressure holds up the plant. 2. Cells' garbage dump. 3. Poisonous Materials (Poison Ivy)' 4. Changing of color in the fall -- chemicals called tannins.

Plastids

Specialized plant containers.

The most important plastid is the chloroplast which contains chlorophyll.



(Like mitochondria).

The term plastid is a collective name for the type of specialized organelles found in plant cells.

Chloroplast- contains chlorophyll necessary for photosynthesis.

Grana- stacks of "dinner plates or coins with chlorophyll embedded between them.





The chloroplast has all the enzymes necessary for photosynthesis.

Two Reactions:

1. Light Reaction > Make > Converts

2. Dark Reaction > Glucose > Starch

* (Chlorophyll captures Light Energy from Sun = chemical) ( Energy in Glucose )

Chloroplasts have their own DNA, therefore, they have their own ribosomes and genes and can make their own protein. However, they still depend the nucleus for some proteins.

Plastids are Interchanqeable

Proplast forerunner for all developing plastids.

Chloroplast (Green) contain chlorophyll

Chromoplast (color) contain colored pigments- orange (carrots), (apples), yellow (banana), brown, and red (fall leaves), red (beets).

Amyloplasts contain starch "amylo" -greek for starch. -potatoes, apples, bananas.

These can switch back and forth.











Chloroplast – starch factory -- photosynthesis

Plastids – Collective name for specialized organ???? found in plants.

In addition to the nucleus there are other “rooms”. within cells, such as the mitochondrion. Mitochondria are found in animals, plants, fungi, and so on -only bacteria can get along without them. If you examine the upper micrograph on the facing page you will find some mitochondria. In addition to the mitochondria, there are other bodies which look rather similar, these are plastids. Look at the diagram of a plastid below. Can you tell the plastids and the mitochondria apart? The plastids are bigger and they contain layers of membranes, arranged like stacks of dinner plates.

Plastids are found only in plants and are of several kinds. For example, red and yellow colors in flowers and fruit are found in the chromoplasts and starch in potatoes is stored in amyloplasts. The most important plastid, however, is the chloroplast and this is the type shown in the micrograph. If we regard the mitochondrion as an oil refinery, then the chloroplast is the oil well.

Chloroplasts capture the energy of sunlight and use this energy to make sugar from water and carbon dioxide (a gas found in the air). This sugar is then turned into other products by the plant.

Almost all the energy used by living things on the earth, including that used by our bodies, is captured by chloroplasts. We get it by eating plants, or by eating animals that ate plants. Now look at the lower micrograph. It is not easy to interpret, but if you examine it carefully you should be able to find a stack of dinner plates (a granum) on the right.

If you look over to the left you might be able to see glimpses of the surfaces of the "dinner plates" (or membranes). Do you notice that on these surfaces there are several different kinds of little grains? Some are smaller some larger, some jumbled, and some orderly.



What are these little grains? Why should there be so much complexity?

Think about the job that the chloroplast does.

Would you expect it to be easy? Would you not expect that complicated machinery would be required for a complicated job?




Cell Biology

Color Commentary: Why Leaves

Change

From Canadian Gardening Previous | Index | Next

The brilliant display of autumn color many of us take for granted isn't a global phenomenon. In fact, relatively few areas around the world are so blessed- southern Manitoba, Ontario and Quebec, the northeast U.S., and a few locations in southwest Europe, eastern Asia and South America.

To produce brilliant fall foliage, a region first must grow trees capable of producing fall color. Second, the climate must be right: there must be adequate year-round precipitation, but relatively warm, dry, sunny autumn days combined with cool-but not freezing-nights (below 8 degrees C/45 degrees F). If any of these conditions varies in the extreme, fall color is influenced.

During its growing season, a tree conducts the life sustaining business of photosynthesis: using sunlight as energy, leaves absorb water and carbon dioxide from the air to produce vital plant sugars, releasing oxygen back into the air. But photosynthesis occurs only when auxin, the main growth hormone of plants, is present; as long as it's produced, leaves stay green and cling to the branches.



Once a critical temperature is reached, trees stop producing auxin, which signals the leaves to move the sugars they've been manufacturing into the stems, trunk and roots to be stored for winter.

Since autumn temperatures vary widely, nature provides a second trigger, a metabolic process called photoperiodism, which reacts to changes in day length. Photoperiodism makes lilacs and tulips bloom in May rather than August, and in fall it signals deciduous trees-via growth-inhibiting hormones-to slow down photosynthesis, move nutrients out of the leaves and begin shedding them to protect the vulnerable parts of the tree from winter temperatures that kill.

As nights lengthen, photosynthesis ceases: leaves stop producing chlorophyll, the ingredient that makes leaves green. As chlorophyll decomposes, the underlying pigments in the leaves of some species become visible for the first time-carotin (orange) and xantophyll (yellow). Sometimes they mix on the same tree, making a spectacular show. A different process causes red and scarlet fall colors. During the warm,sunny days of fall, leaves are busy manufacturing sugars to send through the trunk for storage in the roots. As nights cool and days shorten, the tree's photoperiod triggers a thick, corky area known as the abscission lone in the leaf petioles (the stalk that attaches a leaf to a branch or stem). This lone prevents sugars from moving, trapping some in the leaves. Trapped sugars are converted into a third pigment, anthocyanin, which yields the scarlet. red and purple-red found in Certain species. During an autumn with lots of sunshine, and therefore high levels of sugars, the anthocyanin produces more intense scarlet and red foliage.

Gradually, as nights cool, growth-inhibiting hormones signal a tree to create enzymes that digest the cells of the abscission layers, and leaves begin to fall. The fallen leaves eventually break down, and the remaining nutrients are absorbed into the soil and recycled to feed the tree's roots. And the process begins again -- the days of spring lengthen and photoperiodism lets the tree know it's time to begin the entire process anew.





Animal Cell

Plant Cell







Membrane Transport Process

Process Energy Source Description Examples

Passive processes Simple diffusion Kinetic energy

Kinetic energy

Net movement of particles (ions. molecules. etc.) from an area of their higher concentration to an area of their lower concentration. that is. along their concentration gradient

Movement of fats, oxygen, carbon dioxide through the lipid portion of the membrane, and ions through protein channels under certain conditions

Osmosis Kinetic energy

Simple diffusion of water through a selectively permeable membrane

Movement of water into and out of cells via membrane pores

Facilitated diffusion

Kinetic energy

Same as simple diffusion. but the diffusing substance is attached to a lipid-soluble membrane carrier protein

Movement of glucose into cells



Filtration Hydrostatic pressure

Movement of water and solutes through a semipermeable membrane from a region of higher hydrostatic pressure to a region of lower hydrostatic pressure that is along a pressure gradient

Movement of water, nutrients, and gases through a capillary wall; formation of kidney filtrate

Active processes Active transport (solute pumping)

ATP (cellular energy)

Movement of a substance through a membrane against a concentration (or electrochemical) gradient: requires a membrane carrier protein

Movement of amino acids and most ions across the membrane

Bulk transport Exocytosis

ATP Secretion or ejection of substances from a cell: the substance is enclosed in a membranous vesicle. which fuses with the plasma membrane and ruptures. Releasing the substance to the exterior

Secretion of nurotransmitters, hormones, mucus, etc; ejection of cell wastes



Phagocytosis (endocytosis)

ATP 'Cell eating" A large external particle (proteins. bacteria. dead cell debris) is surrounded by a "seizing foot" and becomes enclosed in a plasma membrane sac

In the human body, occurs primarily in protective phagocytes (some white blood cells, macrophages)

Pinocytosis (endocytosis)

ATP 'Cell drinking" Plasma membrane sinks beneath an external fluid droplet containing small solutes: membrane edges fuse. forming a fluid- filled vesicle

Occurs in most cells; important for taking in solutes by absorptive cells of the kidney and intestine

Receptor-mediated endocytosis

ATP Selective endocytosis process. external substance binds to membrane receptors. and coated pits are formed

Means of intake of some hormones, cholesterol, iron, and other molecules

Bulk Transport: Large particles and macromolecules are transported through plasma membranes by bulk transport. Like solute pumping. bulk transport is energized by ATP. The two kinds of bulk transport are

oxocytosis and endocytosis.



Exocytosis: (ek"-so-si-to'-sis) is the mechanism by l whIch substances are moved from the cell interior into the extracellular space. It accounts for hormone secretion. Neurotransmitter release, mucus secretion. and. in some cases. the ejection of wastes. In exocytosis. the substance or cell product to be released is first enclosed within a membranous sac.





Animal Cell





Plant Cell





Development of the Cell Theory



Across Down1 - The area of protoplasm surrounding the nucleus 2 - All the material within a cell

5 - Calls that do not have a true nucleus 3 - The condition in which all acting influences are balanced

7 - Responsible for photosynthesis 4 - Organelles that may function as factories or storage areas in plants

10 - The control center of a cell 6 - Contain the genetic material of cells

12 - Water pressure extended on cell walls 8 - Higher concentration of solutes than surrounding solution

15 - Solutions that are equal in concentration 9 - A cell that has a true nucleus

16 - Protein factories found in the cytoplasm 11 - This type of transport consumes cellular energy

17 - A solution in which the solutes are in lower concentration

13 - The movement of water molecules through a semi permeable membrane

19 - Molecular movement form areas of high to low concentration

14 - Protein catalysts speed up reactions in cells

20 - Outer layer of a cell 18 - Lens closest to the eye in a microscope



The Cell Cycle & Mitosis Tutorial

Previous | Index | Next Please go to http://www.biology.arizona.edu for the complete site

The Biology Project and also for updates to any information found on this page


The Cell Cycle

Stages of the cell cycle

The cell cycle is an ordered set of events, culminating in cell growth and division into two daughter cells. Non-dividing cells not considered to be in the cell cycle. The stages, pictured to the left, are G1-S-G2-M. The G1 stage stands for "GAP 1". The S stage stands for "Synthesis". This is the stage when DNA replication occurs. The G2 stage stands for "GAP 2". The M stage stands for "mitosis", and is when nuclear (chromosomes separate) and cytoplasmic (cytokinesis) division occur. Mitosis is further divided into 4 phases, which you will read about on the next page.

Regulation of the cell cycle

How cell division (and thus tissue growth) is controlled is very complex. The following terms are some of the features that are important in regulation, and places where errors can lead to cancer. Cancer is a disease where regulation of the cell cycle goes awry and normal cell growth and behavior is lost.

Cdk (cyclin dependent kinase, adds phosphate to a protein), along with cyclins, are major control switches for the cell cycle, causing the cell to move from G1 to S or G2 to M.

MPF (Maturation Promoting Factor) includes the CdK and cyclins that triggers progression through the cell cycle.

p53 is a protein that functions to block the cell cycle if the DNA is damaged. If the damage is severe this protein can cause apoptosis (cell death).


http://www.biology.arizona.edu/


1. p53 levels are increased in damaged cells. This allows time to repair DNA by blocking the cell cycle.

2. A p53 mutation is the most frequent mutation leading to cancer. An extreme case of this is Li Fraumeni syndrome, where a genetic a defect in p53 leads to a high frequency of cancer in affected individuals.

p27 is a protein that binds to cyclin and CdK blocking entry into S phase. Recent research (Nat. Med.3, 152 (97)) suggests that breast cancer prognosis is determined by p27 levels. Reduced levels of p27 predict a poor outcome for breast cancer patients.

The Biology Project The University of Arizona Thursday, April 24, 1997

Contact the Development Team

http://www.biology.arizona.edu All contents copyright © 1997. All rights reserved.



http://www.biology.arizona.edu/the_biology_project/the_biology_project.html

http://www.biology.arizona.edu/contact/default.html


Previous | Index | Next Please go to http://www.biology.arizona.edu for the complete site



DNA Basics

What is DNA and where is it stored?

The nucleus is a membrane bound organelle that contains the genetic information in the form of chromatin, highly folded ribbon-like complexes of deoxyribonucleic acid (DNA) and a class of proteins called histones.

When a cell divides, chromatin fibers are very highly folded, and become visible in the light microscope as chromosomes. During interphase (between divisions), chromatin is more extended, a form used for expression genetic information.

The DNA of chromatin is wrapped around a complex of histones making what can appear in the electron microscope as "beads on a string" or nucleosomes. Changes in folding between chromatin and the mitotic chromosomes is controlled by the packing of the nucleosome complexes.

DNA or deoxyribonucleic acid is a large molecule structured from chains of repeating units of the sugar deoxyribose and phosphate linked to four different bases abbreviated A, T, G, and C. We will later show how the simple structure of DNA contains the information for specifying the proteins that allow life. The process of mitosis is designed to insure that exact copies of the DNA in chromosomes are passed on to daughter cells.

The Biology Project The University of Arizona

Friday, May 9, 1997 Contact the Development Team











Please go to http://www.biology.arizona.edu for the complete site



Mitosis

What is (and is not) mitosis?

Mitosis is nuclear division plus cytokinesis, and produces two identical daughter cells during prophase, prometaphase, metaphase, anaphase, and telophase. Interphase is often included in discussions of mitosis, but interphase is technically not part of mitosis, but rather encompasses stages G1, S, and G2 of the cell cycle.

Interphase & mitosis

Interphase The cell is engaged in metabolic activity and performing its prepare for mitosis (the next four phases that lead up to and include nuclear division). Chromosomes are not clearly discerned in the nucleus, although a dark spot called the nucleolus may be visible. The cell may contain a pair of centrioles (or microtubule organizing centers in plants) both of which are organizational sites for microtubules.

Prophase Chromatin in the nucleus begins to condense and becomes visible in the light microscope as chromosomes. The nucleolus disappears. Centrioles begin moving to opposite ends of the cell and fibers extend from the centromeres. Some fibers cross the cell to form the mitotic spindle.

Prometaphase The nuclear membrane dissolves, marking the beginning of prometaphase. Proteins attach to the centromeres creating the kinetochores. Microtubules attach at the kinetochores and the chromosomes begin moving.




Metaphase Spindle fibers align the chromosomes along the middle of the cell nucleus. This line is referred to as the metaphase plate. This organization helps to ensure that in the next phase, when the chromosomes are separated, each new nucleus will receive one copy of each chromosome.

Anaphase The paired chromosomes separate at the kinetochores and move to opposite sides of the cell. Motion results from a combination of kinetochore movement along the spindle microtubules and through the physical interaction of polar microtubules.

Telophase Chromatids arrive at opposite poles of cell, and new membranes form around the daughter nuclei. The chromosomes disperse and are no longer visible under the light microscope. The spindle fibers disperse, and cytokinesis or the partitioning of the cell may also begin during this stage.

Cytokinesis In animal cells, cytokinesis results when a fiber ring composed of a protein called actin around the center of the cell contracts pinching the cell into two daughter cells, each with one nucleus. In plant cells, the rigid wall requires that a cell plate be synthesized between the two daughter cells.

Mitosis

animation

(480 k)

The Biology Project The University of Arizona Thursday, April 24, 1997

Contact the Development Team




javascript:movie()

javascript:movie()

http://www.apple.com/quicktime/download/index.html



Cell Reproduction

Cell Reproduction.

The Cell Cycle and Mitosis Previous | Index

IndexA. The Cell Cycle B. Phases of Mitosis C. Prophase D. Metaphase

E. Anaphase F. Telophase G. Cytokinesis H. The Mitotic Spindle

The Cell Cycle

A. The cell cycle is the well ordered sequence of events between the time a cell divides to form two daughter cells and the time those daughter cells divide.

1. Includes a doubling of a cell's cytoplasms, precise duplication of DNA < mitosis and Cytokinesis. 2. Each daughter cell contains a single, intact nucleus and some surrounding cytoplasm. 3. Duration of cell cycle varies with the type of cell. Some cells divide each hour, others take more than 24 hours. 4. Some cell types, for example, nerve and muscle cells, never or rarely divide once they are formed

5. Two phases a. M phase (mitotic phase) = Mitosis + Cytokinesis (usually). b. Interphase = Remainder of cell cycle excluding M phase

i. Interphase comprises about 90% of the cell cycle and includes most of a cells growth and metabolic activities with a very high degree of biochemical activity. ii. Many components are made continuously throughout Interphase although DNA synthesis occurs only during a limited time.


Cell Reproduction

iii. Interphase includes G 1 phase, S phase (DNA synthesis), and G2 phase. iv. In brief, what we see in late Interphase is:

❍ The nucleus is well defined and bounded by the nuclear envelope.

❍ One of more nucleoli are present.❍ .Two pairs of centrioles are adjacent to nucleus

(formed earlier by replication of original pairs).❍ Around each pair of centrioles, microtubules

form in a radial array, called as aster.❍ Chromosomes are duplicated (occurred in S

phase), but cannot be distinguished individually due to loosely packed chromatin fiber arrangement.

index B. Phases of Mitosis

1. Mitosis is unique to eukaryotes and may be an evolutionary adaptation for distributing alarge amount of genetic material.2. Details may vary, but overall process is similar in most eukaryotes.3. It is a reliable process with experimental evidence showing only one error per 100,000 cell divisions.4. Mitosis and cytokinesis form a continuum, but for ease of description, mitosis is usually divided into four stages: prophase, metaphase, anaphase and telophase.

index C. Prophase

1. During prophase, changes occur in both the nucleus and cytoplasm.2 In the nucleus

a. Nucleoli disappear. :b. Chromatin fibers become tightly coiled and folded into discrete, observablechromosomes.


Cell Reproduction

c. Each duplicated chromosome is composed of two identical sister chromatids joined at the centromere.

3. In the cytoplasm:

a. Mitotic spindle forms. It is composed of microtubules and associated proteins which are arranged between the centriole pairs.b. Centriole pairs move apart, apparently propelled along the surface of the nucleus bylengthening of the microtubule bundles between them.c. Nuclear envelope fragments in late prophase.d. Absence of nuclear envelope allows microtubules to interact with the more highlycondensed chromosomes.e. Polar fibers (bundles of microtubules) extend from each pole toward the equator of the cell.

4. Each chromatid new has a specialized structure called the kinetochore located at the centromere region.

5. Bundles of micro tubules (kinetochore fibers) are attached and interact with the polar fibers of the spindle to put the chromosomes into agitated motion.

index D. Metaphase

1. During metaphase:a. Centriole pairs are positioned at opposite ends (pole) of the cellb. Chromosomes move to the metaphase plate, the plane equidistant between the spindle poles.c. Centromeres of all chromosomes are aligned on the metaphase plate. '\d. The long axis of each chromosome is roughly at a right angle to the spindle axis.e. Kinetochore fibers of sister chromatids face


Cell Reproduction

opposite poles, so identical chromatids are attached to kinetochore fibers radiating from opposite ends of the parent cell.f. Entire structure formed by polar fibers plus kinetochore fibers is called the spindle.

index E. Anaphase

1. Anaphase begins when paired centromeres of each chromosome move apart.

a. Sister chromatids separate and are considered chromosomes.b. The spindle apparatus starts moving the separate chromosomes (once joined as sister chromatids) toward opposite poles of the cell. Due to the attachment of the kinetochore fibers to the centromeres, the chromosomes move in a "V" shape.c. The kinetochore fibers begin to shorten at the chromosomes near the poles.d. The poles of the cell start to move farther apart slightly elongating the cell.

2. At the end of anaphase, the two poles have complete and equivalent collections ofchromosomes.

index F. Telophase

1. The main events during telophase:

a. Polar fibers further elongate the cell.b. Daughter nuclei begin to form at the two poles.c. Nuclear envelopes are formed around the chromosomes from fragments of the parent cell's nuclear envelope and other portions of the endomembrane system.d. Nucleoli reappear.e. Chromatin fiber of each chromosome uncoils and the chromosomes become less distinct.


Cell Reproduction

2. The spindle poles move away from each other as a cell elongates during mitosis, and. different hypotheses have been formulated to explain this:

a. Some of the movement may be due to addition of subunits to the polar fibers and their resulting elongation.b. The poles may be pushed apart by interdigitating polar fibers that slide past each other in the equatorial region of spindle overlap. According to this hypothesis, cell elongation is powered by ATP hydrolysis catalyzed by an ATPase related to dynein.

3. Sister chromatid separation and movement by kinetochore microtubules during anaphase is not fully understood:

a. Dynein and ATP do not appear to be involved.b. Low-energy dissociation of kinetochore microtubules into their protein subunits may play a role in moving chromosomes.c. One hypothesis holds that the polar ends of the microtubules depolymerize. As the tubulin subunits dissociate, the shortening microtubules with their attached chromosomes are thus drawn to the poles.d. A second model is based on evidence that dissociation of microtubules occurs at their kinetochore ends. This model suggests that depolymerization of microtubules at this end is an exerbonic reaction that provides the energy for the kinetochore to move poleward along the remaining tip of the microtubule ahead of the depolymerization

index G. Cytokinesis

1. Cytokinesis means movement of the cytoplasm.

a. Important changes can be observed in the cytoplasm during late anaphase or early telophase.


Cell Reproduction

b. These changes result in division of a cell into two cells.

2. Cytokinesis occurs as cleavage in animal cells. a. First indication of cleavage is the formation of a cleavage furrow which forms as a shallow groove in the cell surface near the old metaphase plate.b. A contractile ring of micro filaments composed on the protein actin forms on the cytoplasmic side of the furrow.c. These micro filaments contract, reducing the diameter of the contractile ring.I d. This reduction continues, deepening the cleavage furrow, until the parent cell is pinched in two.e. The remains of the mitotic spindle, which is the last connection between the daughter cells, breaks and the two cells are completely separate.

3. Cytokinesis in plant cells is very different due to the presence of cell walls.

a. No cleavage furrow formsb. Across the midline of the parent cell (old metaphase plate), a structure called the cell plate formsc. This cell plate forms from the coalescing of vesicles derived from the Golgi apparatus.

4. By the end of telophase

a. Mitosis, the equal division of one nucleus into two genetically identical nuclei, is complete.b. Cytokinesis has begun and the appearance of two separate daughter cells occurs shortly after mitosis is completed.

index H. The Mitotic Spindle

1. The mitotic spindle is important to the events occurring in mitosis.


Cell Reproduction

a. It forms in the cytoplasm during prophase and is composed of fibers formed from microtubules and associated proteins. b. Microtubules of the cytoskeleton are partially disassembled during assembly of the mitotic spindle

Previous | Index


Documents

Cell Biology - Cape Breton â€“ Victoria Regional School Board