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 Introduction
 Cell contents
 DNA basics
 Mitosis
 The theories of cell origin
 Types of cells
 Cell Components
 Cell Cycle

INTRODUCTION
Cells are the fundamental unit of living things--they are the smallest structures that show all the features of living things. All organisms consist of small cells, typically too small to be seen by a naked eye, but big enough for an optical microscope . Each cell is a complex system consisting of many different building blocks enclosed in membrane bag. There are unicellular (consisting only of one cell) and multicellular organisms. Bacteria and baker’s yeast are examples of unicellular organisms - any one cell is able to survive and multiply independently in appropriate environment.

There are estimated about 6x1013 cells in a human body, of about 320 different types. For instance, there are several types of skin cells, muscle cells, brain cells (neurons), among many others. The number of cell types is not well-defined, it depends on the similarity threshold (what level of detail we would like to use to distinguish between the cell types, e.g., it is unlikely that we would be able to find two identical cells in an organism if we count the number of their molecules). The cell sizes may vary depending on the cell type and circumstances. For instance, a human red blood cell is about 5µ (0.005 mm) in diameter, while some neurons are about 1 m long (from spinal cord to leg). Typically the diameter of animal and plant cells are between 10 and 100 microns.

Viruses are not quite living organisms, but when inside a living host cell they show some features of a living organism. Viruses are too small to be seen in an optical microscope, but are big enough to reveal their structure in an electron microscope (the characteristic size of the virus is about 0.05-0.1µ, while the wavelength of green light is about 0.5µ).

Back to the top! The theories of cell origin

Before the invention of microscopes there was no way to observe the small-scale structure of living things. Starting in the 17th century technology began to develop to enable scientists to see the cells of microbes, and to see the cellular structure of larger animals. Robert Hooke (1665) coined the term "cell" while looking at the nonliving tissue known as cork. In 1675, Anton van Leeuwenhoek, a Dutch lens maker, first reported observations of living cells which he called "animicules". Robert Brown (1831) was the first to report the discovery the nucleus of the cell. The understanding that larger animals were multicellular was relatively slow in develop. It was not until the 19th century that the idea that all animals and plants were constructed of cells, and that cells have a kind of life-cycle of their own was thought of. Matthias Jakob Schleiden (1838), a German Botantist, after extensive studies reported that all plants were composed of fundamental units known as cells. In 1839, Theodore Schwann, a German Zoologist, after extensive studies reported that all animals were composed of fundamental units known as cells. Rudolf Virchow (1858) proposed that all cells came from pre-existing cells. The basic tenets of the cell theory:

  • All organisms are composed of cells
  • Cells are the basic unit of life
  • Cells arise only from other pre-existing cell

Back to the top! Types of Cells

There are two types of organisms - eukaryotes and prokaryotes, and two types of cells respectively. The term "eukaryote" means "true nucleus" while "prokaryote" means "before the nucleus". This emphasizes the central importance of the nucleus to the eukaryotic cell, and suggests that prokaryotes are more primitive organisms, and that eukaryotes evolved from them by, among other things, acquiring a true nucleus. The both do have DNA for genetic material, have a exterior membrane, have ribosomes, accomplish similar functions, and are very diverse. For instance, there are over 200 types of cells in the human body, that very greatly in size, shape, and function.

Prokaryotes
Eukaryotes
  • Unicellular
  • Nucleus absent
  • Genetic material is not enclosed within a membrane
  • Do not engulf solids
  • Do not have centrioles or asters
  • Do not have internal membrane-bound organelles
  • Smaller than eukaryotes (diam 1µ)
  • E.g. bacteria and cyanophyte
  • Unicellular or multicellular
  • Nucleus present
  • Genetic material is surrounded by a membrane
  • Engulf solids
  • Have centrioles or asters
  • Do not have internal membrane-bound organelles
  • Larger than prokaryotes
  • E.g. humans, algae, protazoa

Bacteria belong to the prokaryotes. However, most organisms which we can see, such as trees, grass, flowers, weeds, worms, flies, mice, cats, dogs, humans, mushrooms and yeast are eukaryotes. The distinction between eukaryotes and prokaryotes is rather important, because many of the cellular building blocks and life processes are quite different in these two organism types. This is believed to be the result of different evolutionary paths.

Prokaryotes are the simplest cells. Prokaryotic cells are smaller than eukaryotic cells (a typical size of a prokaryotic cell is about 1 micron in diameter) and have simpler structure (e.g., they do not have any inner cellular membranes that are always present in Eukaryotes). Prokaryotes are single cellular organisms, but note that being a single cell does not mean that an organism is a prokaryote. Being smaller than eukaryotes does not mean that prokaryotes are any less important – for instance it is quite likely that the number of bacteria living in the mouth and digestive tract of a human are larger than the number of eukaryotic cells in the same individual and many of these bacteria are necessary for a human being to live a normal life (these numbers are rather difficult to estimate, rather a hypothesis). Prokaryotes are sometimes also known as microbes.

  • Prokaryotic cells have the simplest overall structure.
  • The prokaryotic cell is bounded by a lipid bilayer membrane, but does not contain any internal membrane-bound organelles. It contains a region rich in DNA called a nucleoid.
  • The nucleoid contains a circular molecule of DNA which is the cells genetic material, or genome.
  • Surrounding the nucleoid is a region of cytoplasm rich in ribosomes, small protein-RNA structures which do the job of synthesizing proteins. Finally, surrounding the plasma membrane is a cell wall.
  • Prokaryotes can have surface appendages which do particular jobs. Flagella are used for locomotion. Pili are used for sexual reproduction (mating)
  • One of the more surprising aspects of cell structure is the way that DNA molecules are packaged into cells.
  • The genome of a common bacterium like the gut bacterium Escherichia coli is about 1.5 millimeters in length, but the length of the bacterium is about 1000 times shorter.
  • In the nucleoid, the DNA is both compacted so as to fit within the bacterium, and yet is still accessible to the machinery necessary both to read the genetic instructions (synthesize RNA) and to copy the DNA (replicate).

Eukaryotic cells are much more complex.

  • By contrast to prokaryotic cells, eukaryotic cells are much more complicated. Much of the cell is taken up by subcellular organelles bound by their own membranes (analogous to the plasma membrane surrounding the cell)
  • Most importantly, eukaryotic cells have a nucleus which contains all of the cell's DNA. In fact, .
  • The problem of packaging DNA is greater in eukaryotes. A human genome would stretch about 1 meter, but must fit in a cell 50,000 times smaller.
  • However, eukaryotic cells (which included plants, animals, and fungi) include many other subcellular structures with important roles in cellular metabolism
Back to the top! Cell Contents

Cells are 90% fluid (cytoplasm) which consists of free amino acids, proteins, glucose, and numerous other molecules. The cell environment (ie. the contents of the cytoplasm, and the nucleus, as well as, they way the DNA is packed) affect the gene expression/regulations, and thus are very important parts of inheritance, below are approximations of other components:

Elements:

  • 59% Hydrogen (H)
  • 24% Oxygen (O)
  • 11% Carbon (C)
  • 4% Nitrogen (N)
  • 2% Others - Phosphorus (P), Sulphur (S), etc.

As far as molecules that make up the cell:

  • 50% protein
  • 15% nucleic acid
  • 15% carbohydrates
  • 10% lipids
  • 10% Other

What is inside the cell is the cytoplasm which is:

  • Cytosol - a lot of water - all except the organelles.

Organelles (which also have membranes) in 'higher' eukaryote organisms:

  • Nucleus (in eukaryotes) - where genetic material (DNA) is located, RNA is transcribed. Nucleolus is rich in RNA and site of ribosomal RNA synthesis and assembly.
  • Endoplasmic Reticulum (ER) - Membranous structure important for protein synthesis. It is a transport network for molecules destined for specific modifications and locations. There are two types:
    • Rough ER - Has ribosomes, and tends to be more in 'sheets'. Synthesis of exported proteins.
    • Smooth ER - Does not have ribosomes and tends to be more of a tubular network. Synthesis of lipids.
  • Ribosomes - Half are on the Endoplasmic Reticulum, the other half are 'free' in the cytosol, this is where the RNA goes for translation into proteins.
  • Golgi Apparatus - Membranous structure important for glycosylation and secretion. Intermediate in protein export.
  • Lysosomes - Digestive sacks - the main point of digestion, these are only found in animal cells.
  • Peroxisomes - Use oxygen to carry out catabolic reactions, in both plant and animals.
  • Microtubules - Made from tubulin, and make up centrioles,cilia,etc.
  • Cytoskeleton - Comprises microtubules, actin and intermediate filaments; supports internal structures and gives shape to the cell.
  • Mitochondria - convert foods into usable energy. (ATP production) A mitochondrion does this through aerobic respiration. They have 2 membranes, the inner membranes shapes differ between different types of cells, but they form projections called cristae. The mitochondrion is about the size of a bacteria, and it carries its own genetic material and ribosomes.
  • Vacuoles - Storage organ for storage of nutrients & water. More commonly associated with plants which commonly have large vacuoles.

 

 

Found in Plants and not in animals:

  • Plastids - Specialised organelles found only in Algae and Plants; include chloroplasts, chromoplasts and amyloplasts. Chloroplasts contains stacks of flattened membranes specialized in photosynthesis. Chromoplasts store pigments which color plant parts (e.g. petals). Plastids which store starch (e.g. in roots) are the amyloplasts, which are also called leukoplasts.
  • Cell Wall - found in prokaryotic plants and it provides structural support and protection

A model of an eukaryotic cell
  • The most important difference between plant and animal cells is the fact that plant cells do not have the capacity for movement.
  • Movement of animal cells is important in normal cell function (e.g., phagocytic cells). Movement is critical in development.
  • Animal cells are also not able to trap energy from light since they lack chloroplasts. They are required to get energy necessary to life by eating other living things.
COMPONENTS OF THE CELL
Back to the top! Cell Membrane
The cell membrane separates the cell from its surroundings and comprises a double molecular layer.It has an "oily" interior while both surfaces are attracted water. The cell membrane is effectively continuous with the endoplasmic reticulum and the nuclear membrane. The cell membrane at the cell surface has many proteins associated with it. Some proteins act as receptors for molecules outside the cell, whilst others transport materials across the membrane.
Name(s): cell membrane, plasmalemma, plasma membrane
Location: the outer surface of the cell but frequently convoluted and is effectively continuous with ER and nuclear envelope.
Appearance: double molecular layer.
Size: about 6-7nm in depth (excluding associated proteins).
Function: segregation of cell from surroundings and control of traffic in and out of the cell.
Back to the top! Nucleus, Nucleolus, Nuclear Envelope and Nuclear Pores
The nucleus sits roughly in the middle of the cell and contains the cell's genetic information encoded in DNA. The nucleus is demarked by a double membrane, the nuclear envelope, which segregates the nuclear contents from the rest of the cell. Molecular portals, called nuclear pores, permit certain traffic in and out of the nucleus.
Name(s): nucleus
Location: approx centre of cell
Appearance: usually spherical or ovoid but may be lobed
Size: usual range: 5 - 10 micrometers
Function: contains the genetic information of the cell coded in DNA
The nucleolus is where the components of ribosomes are manufactured. These ribosomal components exit through the nuclear pores and enter the cytoplasm where they assemble into ribosomes.
Name(s): nucleolus
Location: roughly in centre of nucleus.
Appearance: approximately spherical but with an ill-defined edge
Size: about 1 micrometer in diameter
Function: production of robosomal components
The nuclear envelope is a double-layered membrane that separates the interior of the nucleus from the rest of the cell. It is bridged by numerous nuclear pores and its outer layer is continuous with the membrane of the rough endoplasmic reticulum.
Name(s): nuclear envelope, nuclear membrane
Location: surrounds nucleus
Appearance: double membrane, punctuated by numerous nuclear pores and with attached ribosomes
Size: total about 40nm in depth
Function: segregates nucleus from rest of cell
The nuclear pores are the rosette-like purple structures that are scattered over the nuclear envelope.
Name(s): nuclear pore (the actual 'gap"), nuclear pore complex (gap + surrounding protein machine)
Location: all over nuclear envelope.
Appearance: flower-like on surface. 8-fold structure. Complex structure in and beneath nuclear envelope.
Size: about 120nm in diameter.
Function: controls transport in and out of nucleus.


Back to the top! DNA BASICS

The nucleus 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. 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.
Back to the top! Mitochondria
A mitochondion appears as an elongate ovoid body. The mitochondrion's double membrane can just be discerned. The vertical bulkhead-like structures are cristae, formed from infoldings of the inner membrane. These cristae are the site of much enzymic activity involved in energy production for the cell. Mitochondria are thought to have been free-living organisms that became incorporated into cells early on in evolution. They even contain their own DNA, just like a separate life-form. Such mitochondrial DNA is very useful in constructing family trees.
Name(s): mitochondrion
Location: scattered throughout cytoplasm
Appearance: long ovoid with a double membrane, the inner one infolds to create bulkhead-like structures called cristae.
Size: variable in range of several micrometers
Function: energy metabolism
Back to the top! Endoplasmic reticulum

The rough endoplasmic reticulum is seen as stack of cisternae connected by bridges. It has an interior space or "lumen" which is continuous with the lumen of the nuclear envelope. The surface is covered by ribosomes, which impart the "rough" appearance. Proteins synthesised by these attached ribosomes pass into the lumen. Small transitional vesicles bud from the roughER which carry proteins destined for processing by the Golgi. As well as connecting to the nuclear envelope, the rough ER is continuous with the smooth endoplasmic reticulum.
Name(s): rough endoplasmic reticulum, rough ER
Location: throughout cytoplasm
Appearance: sacs
Size: about 100nm in depth
Function: used in protein synthesis.
The smooth endoplasmic reticulum is seen as a tangle of interconnected tubules. Unlike the rough ER, it lacks ribosomes. The smooth ER is continuous with the rough ER but has different functions.
Name(s): smooth endoplasmic reticulum, smooth ER, tubular endoplasmic reticulum, agranular endoplasmic reticulum
Location: in cytoplasm
Appearance: interconnected and ramifying tubules; no ribosomes
Size: tubules are about 150nm in diameter (variable)
Function: used in lipid synthesis, detoxification and other metabolic processes.

Back to the top! Golgi Complex
Golgi complex is seen as a stack of membranous sacs tilted at 45 degrees and receives proteins synthesised in the rough endoplasmic reticulum. These are ferried to the Golgi by transfer vesicles. The proteins are then processed by the Golgi for export, membrane use or for inclusion in lysosomes.
Name(s): Golgi complex, Golgi apparatus, Golgi stack, Golgi body, Golgi
Location: variable
Appearance: stack of discoid saccules but may form complex network. Surrounded by vesicles.
Size: variable but approx 2500nm across
Function: processing of proteins
Back to the top! Centrioles
The centrioles are two tube-like objects seen very close to the nucleus. Centrioles are bundles of microtubules that sit in a grainy region called the centrosome. From the edge of this region, microtubules assemble and project in ray-like fashion to the edges of the cell. Their central location gives the centrioles (and centrosome) their names.
Name(s): centrioles
Location: near nucleus at approximate centre of cell.
Appearance: tubular bundle of microtubules. There are 2 centrioles in the centrosome.
Size: about 500nm in length
Function: associated with the centrosome that generates microtubules. During cell division, the centrioles go to opposite sides of the cell and organise the microtubules that drag the chromosomes apart (so that one set each of the duplicated chromosomes end up in each daughter cell).
Back to the top! Microtubules
Microtubules are spoke-like structures that originate near the centre of the cell at the centrosome. The centrosome contains two centrioles, which are bundles of microtubules. Microtubules are protein polymers which are relatively rigid and afford the cell some strength. Microtubules also act in a funicular-like way to help objects move around in the cell.
Name(s): microtubule
Location: throughout cell radiating from centromere
Appearance: slender, individual, spoke-like.
Size: about 25nm in diameter
Function: cell support, movement of cell structures (including the pulling apart of daughter chromosomes in cell division).

Back to the top! Lysosomes
Lysosomes originate from the trans surface of the Golgi and they contain a variety of powerful enzymes (hydrolases) that break down material. Prions (the rogue proteins associated with Mad Cow Disease) are resistant to degradation by lysosomal enzymes and so accumulate in the cell.
Name(s): lysosome
Location: usually towards periphery of cell near Golgi.
Appearance: approximately spherical with a single membrane.
Size: variable; generally in the range: 200 - 400 nm
Function: contain powerful digestive enzymes that are used to destroy invading matter and unwanted cellular material.
Back to the top! Ribosomes
Ribosomes are bead-like objects that are attached to the exterior (cytoplasmic side) of the rough endoplasmic reticulum. Ribosomes are concerned with protein synthesis. Stringing them together is messenger RNA which directs protein manufacture as it passes through the ribosomes. The growing proteins project into the cavity (lumen) of the rough ER. Other ribosomes are free in the cytoplasm. Such membrane-bound ribosomes impart a beaded (rough) appearance to endoplasmic reticulum when it is inspected by an electron microscope.
Name(s): bound ribosomes, attached ribosomes
Location: the outer (cytosol) surface of the rough endoplasmic reticulum.
Appearance: approximately spherical bodies often arranged in strings or spirals (because they are strung together by messenger RNA).
Size: about 25nm in diameter
Function: synthesis of proteins
Polysomes are variable in length and are strings of ribosomes joined by messenger RNA. As the mRNA feeds through these ribosomes, so proteins are synthesised. The proteins synthesised by these free ribosomes pass into the cytoplasm. (Proteins that are destined for the interior of the rough endoplasmic reticulum have a lead sequence that binds to the roughER. This links their associated ribosomes to the surface of the rER.
Name(s): polysomes, polyribosomes
Location: free in cytoplasm
Appearance: approximately spherical bodies often arranged in strings or spirals (because they are strung together by messenger RNA).
Size: ribosomes are about 25nm in diameter
Function: synthesis of proteins
Back to the top! Peroxisomes
Unlike lysosomes, peroxisomes do not bud from the Golgi but seem to originate independently. Like mitochondria, they may once have been separate organisms that became incorporated into cells early on in evolution. They oxidise various materials and then dispose of the hydrogen peroxide that results using an enzyme called catalase.
Name(s): peroxisome
Location: in cytoplasm
Appearance: variable; approx spherical with a single membrane and a granular or crystalline-like interior in some cases.
Size: variable (illustrated at about 700nm in diameter).
Function: oxidise materials and then catalyse the destruction of the resulting hydrogen peroxide.
Back to the top! 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 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.

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

Back to the top! 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
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.


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